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Astronomical Time and Atomic Time

Basically there are two types of time standards. One is based on astronomy and is called "Astronomical Time". The other is based on the frequency of atomic oscillations and is called "Atomic Time".

Basically there are two types of time standards.  One is based on astronomy and is called "Astronomical Time".  The other is based on the frequency of atomic oscillations and is called "Atomic Time".Astronomical Time is based on the repetition of astronomical events for setting frequency standards.  For example, the occurrence of day and night is caused by the rotation of the Earth, and seasons by the Earth revolving around the Sun.  In the past, people used sundial to indicate the time derived from the sun's apparent position, and was referred to as "Apparent Solar Time".  However, as the Earth revolves around the Sun in an elliptical orbit, and due to the inclination of the Earths rotation axis to the orbital plane, the speed of the apparent motion of the Sun varies through the year and "Apparent Solar Time" is as such non-uniform.  A more uniform time standard called "Mean Solar Time" is defined by the uniform motion of a fictitious Sun on the celestial sphere that agrees with the averaged "Apparent Solar Time".  "Universal Time" (UT0) is the "Mean Solar Time" at the Greenwich meridian (longitude 0 degree), also known as "Greenwich Mean Time" (GMT).  With emergence of better clocks, astronomers began to notice a discrepancy in UT0 measured at different locations.  This was later found to be caused by a wobble in the axis of the Earth.  After detailed measurements at various observatories around the world, the discrepancy was corrected by the introduction of a new time designation called UT1."Atomic Time" is based on the frequency of atomic oscillations.  Research on atomic frequency standards revealed that the frequency of atomic oscillations of caesium-133 atom was very stable and even, and a definition of the "second" was adopted based on such characteristics.  In 1967, the Thirteenth General Conference of Weights and Measures adopted a resolution to replace the definition of the "second" based on astronomical observation by that of the atomic oscillation of caesium-133 atom.  The International Atomic Time (TAI) became the international reference of atomic time based on the "second" as defined above.Affected by the atmospheric circulation and other geophysical phenomena, the Earth's rotation is uneven and on the whole slowing down gradually.  As a result, the "second" as defined by astronomical time is slightly longer than that defined by atomic time.  Since the beginning of 1972, to reconcile the two time scales, the Coordinated Universal Time (UTC) was adopted.  It primarily follows the atomic time, but with a leap second introduced when necessary such that the difference between UTC and the astronomical time is kept to less than 0.9 seconds (Figure 1). UTC is the common time standard for civilian use, maintained by the Bureau International des Poids et Mesures (BIPM) based on the weighted average of the times kept by some 300 atomic clocks in over 50 national time institutes and laboratories around the world (Figure 2).On 1 January 1972, Hong Kong adopted UTC as the official time standard.  The Hong Kong Observatory is the official timekeeper in Hong Kong, providing the Hong Kong Standard Time using a caesium beam atomic clock (Figure 3) with a daily difference of less than a millionth second.  Through a high-accuracy time transfer system and making use of the Global Positioning System common-view method, time information of the Observatory's atomic clock is also relayed to BIPM for maintaining the Hong Kong Standard Time, as well as contributing to the determination of UTC.

September 2013

https://www.hko.gov.hk/en/education/astronomy-and-time/time-service/00409-astronomical-time-and-atomic-time.html

en

What year is it today?

The Gan-Zhi year denotation has been used for more than two thousand years. Although it attracts less interest in modern society, it still generates a lot of attention and discussion in the Chinese community around the time of Chinese New Year. Whether the changeover of Gan-Zhi should happen on the day of "Spring Commences" or on "Lunar New Year's Day"?

On 4 February this year, the day of "Spring Commences", a friend asked: "What year is it today?" My simple and quick response was "2018", an answer that did not quite meet his expectation. He actually wanted to test my knowledge in the details of using "Heavenly Stems and Earthly Branches" (also known as Celestial Stems and Terrestrial Branches, or simply "Gan-Zhi") to denote the year in Chinese calendar. The concept was presented by my colleague Mr Wong Wai-kwong through his web article "Changeover of Gan-Zhi in Chinese Calendar" [1] published early this year. Here, I extracted some main points from his article with supplementary information added.There are 10 heavenly stems, beginning with "Jia" and ending at "Gui"; and 12 earthly branches following the sequence from "Zi" to "Hai" [2]. Each heavenly stem is paired with an earthly branch to form the Gan-Zhi sexagenary cycle that starts with Jia-Zi and ends at Gui-Hai. Each year is therefore represented by a Gan-Zhi pair and the cycle is repeated every 60 years.The Gan-Zhi year denotation has been used for more than two thousand years. Although it attracts less interest in modern society, it still generates a lot of attention and discussion in the Chinese community around the time of Chinese New Year. For example, in 2018, the solar term "Spring Commences" fell on 4 February while the first day of the first Chinese calendar month (commonly known as "Lunar New Year's Day" in Hong Kong) was on 16 February. The question then was whether the changeover of Gan-Zhi should happen on the day of "Spring Commences" or on "Lunar New Year's Day"?The Hong Kong Observatory, being the authority responsible for the compilation of calendars in Hong Kong, has long been using the first day of the first Chinese calendar month to mark the changeover of Gan-Zhi. Such a practice has been described in the chapter on astronomy in the ancient book "Records of the Grand Historian", in which the first day of the first Chinese calendar month was defined as the beginning of a year and "Spring Commences" signaled the start of the four seasons.Let me give a brief explanation here about the relationships among year, month and solar terms in the Chinese calendar. Based on changes in the moon phase, each Chinese calendar month has 29 or 30 days, approximately the number of days between two successive occurrences of new moon or full moon. This comes from the lunar calendar (Yinli) and provides a convenient way to count the days by observing the moon phases (i.e., new moon -> first quarter -> full moon -> last quarter -> new moon), even in the absence of advanced time instruments and clocks. Comparatively speaking, the cycle of a Chinese calendar year is a bit more complicated, each comprising 12 Chinese calendar months in a normal year and 13 months in a leap year, and the extra month is called a "leap month". The 24 solar terms come from the solar calendar (Yangli) and are defined based on the revolution of the Earth around the Sun. Each cycle of the 24 solar terms corresponds exactly to one year, or more precisely, one “tropical year”. Starting with “Spring Commences”, it effectively describes the seasonal cycle and provides guidance for farming and daily routines in the ancient times. One can see that there is no direct relationship between Yinli and Yangli and one tropical year is actually longer than 12 Chinese calendar months combined. To harmonize the two calendars, the day of the second new moon of the Chinese calendar month (leap month not to be counted) after “Winter Solstice” is defined as the Lunar New Year's Day, i.e. the beginning of the Chinese calendar year, and the Metonic cycle of 7 leap months in 19 years is adopted.For more than a century, the Gregorian calendar has been widely adopted and there has been a lack of detailed authoritative guidelines for compiling the Chinese calendar. This has led to the emergence of popular Chinese folk calendars compiled by different users in accordance with their own specific needs or purposes. For instance, fortune tellers or fortune-telling enthusiasts tend to use "Spring Commences" as the time mark for the changeover of Gan-Zhi, in contrast to the common practice adopted for the official calendars. It should be noted, however, that the Chinese Government has since issued a standards document "Calculation and promulgation of the Chinese calendar" dated 12 May 2017. The guidelines for compilation of Chinese calendar and the definition of the beginning of the Chinese calendar year given in the document are consistent with the Observatory's practice. The Observatory has compiled a Gregorian-Chinese Calendar Conversion Table covering the period from 1901 to 2100 for general reference at https://www.hko.gov.hk/en/gts/time/conversion.htm

April 2018

https://www.hko.gov.hk/en/education/astronomy-and-time/time-service/00506-what-year-is-it-today.html

en

The Chinese Agricultural Calendar Explained

A calendar is a system of arranging days according to astronomical events for regulating everyday life. The traditional Chinese calendar is known as the Agricultural Calendar or Nongli, as the calendar divides the year into seasons for agriculture, which is the principal economy of the country.

A calendar is a system of arranging days according to astronomical events for regulating everyday life. The traditional Chinese calendar is known as the Agricultural Calendar or Nongli, as the calendar divides the year into seasons for agriculture, which is the principal economy of the country.Calendars that are based on the moon's orbit around the Earth are known as lunar calendars (Yinli). Solar calendars (Yangli) are another category of calendars that are based on the positions of the Sun through the seasons. The Agricultural Calendar is an integrated lunar-solar calendar (Yinyangli) as it embraces the movement of the moon as well as that of the Sun."Tropical year" and "synodic month" are the basic elements of the Agricultural Calendar. A tropical year is the time from a vernal equinox to the next, which is 365.2422 days (365 days 5 hours 48 minutes and 46 seconds). The time between two successive occurrences of new moon or full moon is called a synodic month, and equals 29.5306 days (29 days 12 hours 44 minutes 3 seconds).In ancient time, observation of moon phases (i.e., new moon > first quarter > full moon > last quarter > new moon) is a convenient way to count the days. Months in the Agricultural Calendar start with a new moon, which occurs when the moon and the Sun move to the same longitude on the ecliptic. Solar eclipses always fall on the first day of a month in the Agricultural Calendar. Lunar eclipses always coincide with the full moon phases when the longitudes of the moon and the Sun on the ecliptic differ by 180 degrees.There are only 354.3672 days in 12 synodic months, more than 10 days shorter than a tropical year (365.2422 days). The difference accumulates to give a leap month. Back to 5 or 6 century B.C., the Metonic cycle of 7 leap months in 19 years was already adopted in the Agricultural Calendar.

https://www.hko.gov.hk/en/education/astronomy-and-time/time-service/00413-the-chinese-agricultural-calendar-explained.html

en

A Brief History about the Authoritative Organizations for Compiling Calendar in China

The Agricultural Calendar, which has been in use for a long time in China, is a combination of two calendar systems. The Agricultural Calendar contains the 24 solar-terms and solar year of the solar calendar system as well as the four major moon phases. In the history of China, the authoritative organization had the responsibility of fixing all these major time marks in the annual compilation of the Agricultural Calendar.

Compiling the official calendar has a long history in China. Though the names were different, the functions of the authoritative organizations for compiling the calendar were more or less the same in different dynasties: "Taishiling" in the Qin and Han Dynasties, "Taishijian" in the Sui Dynasty, "Taishiju / Sitiantai" in the Tang Dynasty, "Sitianjian" in the Song Dynasty, "Taishiyuan" in the Yuan Dynasty, and the most commonly known "Qintianjian"[1] in the Ming and Qing Dynasties. Johann Adam Schall von Bell, a German Jesuit, was one of the most famous officers in charge of the Qintianjian in the Qing Dynasty. He conducted a major reform of the calendar used in the Qing Dynasty that provided a solid foundation (Figure 1) for the compilation of the Agricultural Calendar thereafter.The Agricultural Calendar, which has been in use for a long time in China, is a combination of two calendar systems, viz. the solar calendar system based on the positions of the Sun through the seasons and the lunar calendar system following the moon’s orbit around the Earth. Hence, the Agricultural Calendar contains the 24 solar-terms and solar year of the solar calendar system as well as the four major moon phases, namely "new moon", "first quarter", "full moon" and "last quarter", of the lunar calendar system. In the history of China, the afore-mentioned authorities had the responsibility of fixing all these major time marks in the annual compilation of the Agricultural Calendar.Nowadays, "Observatory" is the term commonly used for official organizations responsible for the compilation of calendars in many places, e.g. the Purple Mountain Observatory in Nanjing, China and, of course, the Hong Kong Observatory in the Hong Kong Special Administrative Region. The Hong Kong Observatory compiles and publishes annually the "Hong Kong Observatory Almanac" (Figure 2) and the "Hong Kong Observatory Calendar" (Figure 3), providing useful information relating to astronomical events, tides and climatology for general references by members of the public.

October 2015

https://www.hko.gov.hk/en/education/astronomy-and-time/time-service/00470-a-brief-history-about-the-authoritative-organizations-for-compiling-calendar-in-china.html

en

Space Weather - An Introduction

What is "space weather"? What is a "solar cycle"? What are "sunspots"? What is a "solar flare"? What is a "solar wind"? What is a "geomagnetic storm"? What are "auroras"? How is space weather monitored?

What is "space weather"?Space weather originates from the Sun. It generally refers to all solar activities such as sunspots and solar flares, and the effects they may have on the Earth. The intensity varies from time to time, sometimes strong and sometimes weak. "Good weather" means a calm period of solar activities, while "bad weather" is a period of frequent and disturbed activities which may affect telecommunications, navigation and power systems on Earth and the operations of satellites or spacecraft.What is a "solar cycle"?Solar activity is periodic. A solar cycle (also called a sunspot cycle) is an approximately 11-year period with increasing and decreasing sunspot numbers. Each cycle starts from the time of minimum activity. The cycle numbering system dates back to the eighteenth century and the current solar cycle is cycle 23.What are "sunspots"?Sunspots are dark areas on the Sun's surface with relatively low temperatures (compared with other parts of the Sun) and strong magnetic fields. They normally appear in groups. The number of sunspots is usually taken as an indicator of solar activity. It increases significantly and can reach hundreds at the peak of a solar cycle.What is a "solar flare"?Solar flare is a violent solar activity. Its occurrence is related to a sudden burst of electromagnetic waves and a vast amount of charged particles (mostly electrons) from the Sun. These electromagnetic waves can affect telecommunications, radio broadcast and navigation system on Earth. Sometimes, charged particles can also endanger the operations of spacecraft and satellites in space, and expose astronauts to higher amounts of radiation.What is a "solar wind"?The Sun releases tremendous energy. Solar wind refers to this released energy in the form of charged particles at high speed, reaching several hundred kilometres per second. While the magnetic field on the Earth (which traps charged particles encircling the Earth into radiation belts called the Van Allen belts) normally protects our planet from the solar wind, it may be deformed in the event of violent solar wind, resulting in a geomagnetic storm on Earth.What is a "geomagnetic storm"?A geomagnetic storm occurs when a violent solar wind hits the Earth and severely distorts the Earth's magnetic field. Geomagnetic storms can seriously affect radio broadcasts and navigation systems, and paralyze electric grids on Earth. During the great geomagnetic storm in 1989, electricity was suspended for 9 hours in Quebec and a number of American satellites were taken out of service. The last significant geomagnetic storm occurred in October 2003 when intense solar flares erupted from the Sun. Among other incidents, the geomagnetic storm caused a Japanese communications satellite to shut down temporarily.What are "auroras"?Auroras are most commonly visible at high latitudes. They are associated with geomagnetic activity brought about by solar wind. Auroras occur as a result of charged particles (mostly electrons) from the Sun colliding with gas particles in the Earth's atmosphere, producing a glow in different colours. The patterns and shapes of the aurora are related to the flow of charged particles and the magnetic fields. In respect of the auroral colour, oxygen molecules produce green and red colours while nitrogen molecules produce purplish-red and blue colours.How is space weather monitored?As space weather originates from the Sun, it is monitored by international space weather centres using special satellites or telescopes. We will discuss more about this in the next issue.Other related websites [1] National Satellite Meteorological Center, China [2] Space Weather Prediction Center, United States

September 2004

https://www.hko.gov.hk/en/education/space-weather/general/00415-space-weather-an-introduction.html

en

Why can the sun persistently produce energy for the stable output of light and heat?

The sun contains massive hydrogen that serves as a lasting supply of fuel for the generation of large amount of energy through persistent nuclear fusion in the solar core region. The sun acts as if it is a self-regulating nuclear power reactor. The force of gravity, gas pressure and radiation pressure of the sun interact to maintain a state of dynamic equilibrium.

How can the sun produce so much energy?  Although the sun is so far away from the earth, its continuous emission of light and heat enables living things to sustain and evolve on earth for billions of years. The sun is a huge volume of gases that are mostly hydrogen. The massive hydrogen provides billion of years of incessant fuel for solar nuclear fusion to continuously produce a huge amount of energy.What is nuclear fusion? Nucleus fusion is also called the nucleosynthesis or thermonuclear reaction. It is a process of nuclear reaction in which two or more light atomic nuclei are combined into a heavier atomic nucleus. Since a small part of the mass is converted into energy in the course of nuclear fusion, the mass of resulting heavier atomic nucleus is slightly less the original total mass of the lighter atomic nuclei. Therefore, nuclear fusion can produce a huge amount of energy by virtue of the mass - energy equivalence (E=mc2) proposed by the famous physicist, Albert Einstein. Hydrogen bomb, which releases a huge amount of energy from nuclear fusion during detonation, is a good example of converting mass into energy.  What is the mode of nuclear reaction in the sun's energy production?  The American physicist, Hans Bethe (born in Germany), based on his research results, pointed out that the most important nuclear reaction in a bright star is a "carbon - nitrogen cycle". However, the nuclear reactions in comparatively dimmer stars such as our sun, are mainly the "proton-proton (hydrogen nucleus - hydrogen nucleus) chain reaction". Because of Hans Bette's contributions in the nuclear reaction theory and particularly the discovery of stellar energy generation, he received the Nobel Prize for Physics in 1967.Why nuclear fusion does not occur readily?  Since atomic nuclei consist of positively charged protons, they repel each other by virtue of electrostatic force. Under high temperature condition, particles possess sufficient kinetic energy to overcome the electrostatic repulsion and approach each other. The subsequent collisions among atomic nuclei provide the opportunities of nuclear fusion to occur. However, particles can recoil and separate without undergoing nuclear fusion after collision (called elastic collision). In fact, many collisions among atomic nuclei are elastic in nature without any resultant nuclear fusion.  George Gamow proposed that for any given temperature, there is a narrow range of energies known as the "Gamow window" where nuclear fusion is most likely to occur.What are the conditions favourable for the occurrence of nuclear fusion?  Besides a suitably high temperature, the number or chance of mutual collisions among atomic nuclei will increase when the gaseous density is high. Since only a small portion of the collisions can result in nuclear fusion, both high temperature and high density are necessary conditions for nuclear fusion to occur.Is the entire sun undergoing nuclear fusion? The sun is a huge volume of gas and its total mass is very great. As a result of gravitational force, the pressure will be greater when it is closer to the center of the sun. Therefore, the solar core is a highly compressed region in which nuclear fusion can occur under the high-density, high-temperature conditions.  On the other hand, both the density and the temperature are much lower in the sun's outer layer where occurrence of nuclear fusion is unfavourable. Hence, nuclear fusion mainly occurs and persists deep inside the sun near its center - the solar core region (Fig. 1).  In other words, nuclear fusion does not occur in the entire sun.Why can the sun produce an enormous amount of energy in the form of light and heat? The sun contains massive hydrogen that serves as a lasting supply of fuel for the generation of large amount of energy through persistent nuclear fusion in the solar core region.The sun acts as if it is a self-regulating nuclear power reactor.  The force of gravity, gas pressure and radiation pressure of the sun interact to maintain a state of dynamic equilibrium (Fig. 1). For example, when the nuclear reaction slows down, less energy is produced and the temperature decreases. The sun's great volume of gas will shrink, resulting in an increase of density and temperature in the solar core region, which in turn speeds up the reaction of nuclear fusion. On the other hand, if the nuclear reaction becomes faster, more energy is produced. The sun's great volume of gas will expand, resulting in a decrease of density and temperature in the solar core region. The reaction of nuclear fusion will eventually slow down.To summarize, the sun's nuclear fusion reaction occurs in an orderly manner.  In the process, hydrogen is consumed, energy is produced, and there is a steady release of light and heat.

September 2010

https://www.hko.gov.hk/en/education/space-weather/general/00418-why-can-the-sun-persistently-produce-energy-for-the-stable-output-of-light-and-heat.html

en

The relationship between sunspot activity and the reversal of the sun's magnetic polarity

The magnetic north and south poles of the sun reverse once in about 11 years. The magnetic north pole at the beginning will become the magnetic south pole later. Similarly, the magnetic south pole at the beginning will become the magnetic north pole later. The sun's magnetic north and south poles will reverse twice and resume to their original positions in about 22 years.

What are sunspots? Sunspots are areas on the photosphere (the visible surface of the sun), which appear darker than the surrounding surface of the sun by virtue of their relatively lower temperature.  But in absolute terms, they are actually very bright. Sunspots can last from hours to months, which are carried around the solar surface by the sun's rotation. Sunspots often appear in pairs or clusters. The formation of sunspots is associated with solar magnetic disturbances.What is a sunspot cycle? The number of sunspots changes periodically. In the early stage of a sunspot cycle, the number of sunspots increases when the solar state changes from less active to more active. The period or year with the most sunspots is called "solar maximum", it is also the time when the solar activity is most active and solar magnetic storms often occur. When the solar state becomes less active, the number of sunspots decreases in the later stage of the sunspot cycle. The period (in year) with the least sunspots is called "solar minimum", during which the solar activity is most quiescent. The period of a sunspot cycle usually refers to the duration starting at its solar minimum and ending at the next solar minimum. The whole period of sunspot cycle lasts about 11 years. What is the reversal cycle of the sun's magnetic polarity (the solar magnetic activity cycle)? The magnetic north and south poles of the sun reverse once in about 11 years. The magnetic north pole at the beginning will become the magnetic south pole later. Similarly, the magnetic south pole at the beginning will become the magnetic north pole later. The sun's magnetic north and south poles will reverse twice and resume to their original positions in about 22 years. So, the reversal period of the sun's magnetic poles or magnetic field is about 22 years.Why does the sun's magnetic field change?  When the sun spins, the lines of magnetic force beneath the sun's surface will also rotate. The sun is a fluid. Unlike the earth having a crust, the sun does not have a hard shell. The sun's spinning speed decreases as its latitude increases (i.e., differential rotation). The sun rotates relatively slowly near the poles. The sun's rotational period at the equator is about 25 days. The sun's rotational period at latitude 60 degrees is about 29.3 days, which is relatively long. Because the sun has differential rotational speeds, the embedded lines of magnetic force beneath sun's surface near the equator rotate fastest. The early longitudinal lines of magnetic force in the sun will become curly and twist gradually. Hence, the magnetic fields gradually change.What are the development characteristics of sunspot activity? Sunspots often appear near 30-35 degrees North and South of the sun's hemisphere at the beginning of the sunspot cycle. The sunspots tend to appear in lower latitudes later, and can appear near 5 degrees North and South of the sun's hemisphere near the end of the sunspot cycle.The sunspots usually appear in a pair or a group on sun's surface inside a bipolar magnetic region (BMR) with strong magnetic field, where the lines of magnetic force are twisted. The pair of sunspots shows opposite magnetic polarity. If one of the two sunspots is a magnetic south pole, the other sunspot will be a magnetic north pole. Annular curved lines of strong magnetic force connect the two sunspots. At the early stage, the axis joining the two sunspots with "preceding" (p) and "following" (f) members is roughly parallel to the equator. The preceding (relative to the direction of the sun's rotation) sunspot is usually larger and more obvious with relatively longer lifetime. In the sun's northern hemisphere, the preceding sunspot will tend to be closer to the solar equator. The following sunspot will tend to be closer to the sun's north magnetic pole. Similarly, in the sun's southern hemisphere, the preceding sunspot will tend to be closer to the solar equator. The following sunspot will tend to be closer to the sun's south magnetic pole. The number of sunspots will increase as the sun activity becomes active.Joy's law and Hale's law Sunspot pairs or groups are tilted with the leading sunspots closer to the equator than the following sunspots. The tilting angle increases with latitude. The tilting phenomenon of sunspot pair is known as Joy's law.The sunspots of a new sunspot cycle after a minimum of solar activity, appearing in high latitudes, are of opposite magnetic polarity in the northern and southern hemispheres. As the cycle progresses, the mean latitude of sunspots in each hemisphere steadily decreases and the sunspots will appear closer to the equator without change in their magnetic polarities. However, the magnetic polarities of sunspots forming in the next sunspot cycle reverse. This phenomenon of sunspot polarity reversal was discovered by the American astronomer, G.E. Hale, and is known as Hale's law.What is the relationship between sunspot activity and the reversal of the sun's magnetic polarity? The relationship is explained by the Babcock's model of the Sun's magnetic cycle (Fig.1). During the period of peak solar activity (solar maximum), the outward projected annular lines of magnetic force between two sunspots will grow up, which will later rupture and separate from the sunspots. The detached annular lines of magnetic force will then get in touch and join with the nearby lines of magnetic force outside the sun, forming an independent loop of lines of magnetic force outside the sun. On the other hand, some lines of magnetic force in the sun will neutralize the lines of magnetic force from some groups of sunspots, weaken, re-connect and organize into a new solar magnetic field with reversed magnetic north and south poles gradually. So, sunspot activity and the reversal of the sun's magnetic polarity are related. If the change of solar magnetic field is also taken into consideration, the period of a sunspot cycle should be about 22 years. 

June 2010

https://www.hko.gov.hk/en/education/space-weather/general/00424-the-relationship-between-sunspot-activity-and-the-reversal-of-the-suns-magnetic-polarity.html

en

Body Checking the Sun - 3-D Images

The National Aeronautics and Space Administration (NASA) released the first 3-D images of the Sun in late April. The pictures were taken by a twin spacecraft called Solar Terrestrial Relations Observatory or STEREO. Taking 3-D images of the Sun is like having a CT scan of the Sun itself. It helps scientists understand the complicated solar physics and make more accurate and timely space weather forecasts.

The National Aeronautics and Space Administration (NASA) released the first 3-D images of the Sun in late April. The pictures were taken by a twin spacecraft called Solar Terrestrial Relations Observatory or STEREO. Taking 3-D images of the Sun is like having a CT scan of the Sun itself. It helps scientists understand the complicated solar physics and make more accurate and timely space weather forecasts.STEREO were launched to space in late October 2006. Their orbits follow that of the Earth, with one spacecraft running ahead of the Earth, and the other behind. They separate from one another by about 45 degrees. Similar to how human eyes operate, such a setting provides a perception of depth making objects look 3-dimensional.To view the 3-D images taken by the spacecraft, you need a pair of special glasses like those used for 3-D movies. In the NASA webpage, there is an instruction for making a pair of 3-D glasses. Go and give it a try!To view 3-D images of the Sun: https://www.nasa.gov/mission_pages/stereo/news/stereo3D_press.htmlTo know more about STEREO: https://www.nasa.gov/stereoTo make your own 3-D glasses: https://stereo.gsfc.nasa.gov/classroom/glasses.shtml

June 2007

https://www.hko.gov.hk/en/education/space-weather/general/00416-body-checking-the-sun-3d-images.html

en

Origin of Solar Flare

Lines of magnetic field extend from the surface into the corona displaying arching shapes in broad loops. These loops trap extremely hot gas, where the temperature reaches 10 to 40 million degrees causing confined gas to emit extreme ultraviolet radiation and x-rays. But how does the energy trapped in the magnetic field turn into heat and light?

Solar flares occasionally erupt from the Sun's surface. A solar flare is a violent solar activity that is related to a sudden burst of electromagnetic waves and a large release of charged particles (mostly electrons) from the Sun. So, what is the physical mechanism behind the eruption?Solar activities have been closely monitored since the development of advanced solar instruments. Scientists found that solar flares normally erupt near sunspots where comparatively stronger magnetic field is located. They generally agree that the energy released in a flare has been stored in the Suns magnetic fields. Lines of magnetic field extend from the surface into the corona displaying arching shapes in broad loops. These loops trap extremely hot gas, where the temperature reaches 10 to 40 million degrees causing confined gas to emit extreme ultraviolet radiation and x-rays. But how does the energy trapped in the magnetic field turn into heat and light?The breakthrough in our understanding came in 2002, when NASA launched the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) that captures detailed images of solar flares. Dr. Gordon D Holman, astrophysicist in NASA, revealed in his research that solar flare is the consequence of magnetic reconnection, a phenomenon of sudden rearrangement of magnetic field lines. After examining pictures taken by RHESSI, Dr. Holman and his colleague discovered that "bubbles" of plasma emanated from the Sun at the time when broken magnetic field lines reconfigured themselves. The bright light was actually x-rays coming from overheated plasma and acceleration of electrons into space.

June 2006

https://www.hko.gov.hk/en/education/space-weather/general/00420-origin-of-solar-flare.html

en

Spectacular Fireworks Show on the Sun

A single, unusual, transient but spectacular fireworks event on the Sun's surface was recorded by NASA's Solar Dynamics Observatory (SDO) on 7 June 2011. It was an eruption on the solar surface after the flash of a solar flare.

Introduction to a space fireworks eventA single, unusual, transient but spectacular fireworks event on the Sun's surface was recorded by NASA's Solar Dynamics Observatory (SDO) on 7 June 2011. It was an eruption on the solar surface after the flash of a solar flare. This article is based on the information provided by the NASA Science News of 11 July 2011.Why were the fireworks "dark"? The power of the eruption ejected a large volume of plasma into the coronal space (the solar atmosphere). The plasma spread out over the coronal space and fell back to the solar surface. It was like an intense volcanic eruption with ashes and lava falling back. Since the plasma was cooler than the rest of the solar surface, it appeared darker than the surrounding. What caused the eruption? The blast or solar flare was triggered by an unstable magnetic filament, loaded with relatively cool plasma drawn from regions below the Sun's surface. The energy stored in the magnetic filament was released when it broke. The large amount of energy released caused the explosion of plasma in spray of dark blobs and streamers.What else could we see? The plasma blobs were as big as planets, and many were larger than Earth. They rose and fell ballistically under the influence of the Sun's gravity. Since the plasma blobs consisted of charged particles and were "caught" by the Sun's magnetic fields, some of them were funnelled towards the vicinity of sunspot groups hundreds of thousands of kilometres away like guided missiles. The plasma blobs exploded like bombs when they hit the solar surface. SDO also detected a shock wave originating from the blast site of the solar flare. It was like a solar tsunami propagating more than halfway across the sun, churning up magnetic filaments and loops en route.Had the event caused any abnormal space weather to Earth? The blast propelled a significant coronal mass ejection (CME) from the Sun. CME was essentially a magnetized cloud of ionized gas consisting of high speed energetic particles. Since its path of propagation had missed the Earth, there was no adverse space weather affecting the safety of astronauts or operation of artificial satellites, telecommunication systems and terrestrial power supply.

https://www.hko.gov.hk/en/education/space-weather/general/00421-spectacular-fireworks-show-on-the-sun.html

en

Shock Wave

Solar wind usually travels at a supersonic speed of several hundred kilometers per second. Solar wind impacts on a planet like the Earth producing a bow-shaped shock wave near the Earth. It is known as bow shock.

A bullet travels at speed higher than that of sound. A bow-shaped shock wave will form at the nose of the bullet (Figure 1). Similarly, an aircraft flying at supersonic speed will also produce a shock wave. It results a sonic boom which can be heard. A yacht or kayak traversing a lake also produces a bow-shaped shock wave (Figure 2), known as bow wave. The bow wave gets larger as the yacht moves faster.By the same principle, a high-speed current approaching an object can also produce a shock wave. Here is an example observed in space.Solar wind usually travels at a supersonic speed of several hundred kilometers per second.  Figure 3 shows solar wind impacting on a planet like the Earth producing a bow-shaped shock wave near the Earth. It is known as bow shock. The intense solar wind causes the Earth's magnetosphere to be deformed, resulting in a tail-like feature  (see figure 4).Another example of shock wave involves charged particle moving faster than the speed of light in a medium such as water. The light emitted is known as Cerenkov Radiation which is a photonic shock wave. [see "Nature's Wonder - What is Cerenkov Radiation?"]

December 2009

https://www.hko.gov.hk/en/education/space-weather/general/00419-shock-wave.html

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Effects of Space Weather

Effects on Satellites and Spacecraft, Effects on human body, Effects on Communication and Navigation Systems, Effects on Electric Power and Pipelines

Space weather (see Introduction) can cause a wide range of effects on human activities. Some are discussed below.Effects on Satellites and SpacecraftSatellites and spacecraft are vulnerable to space weather.Increased radiation from the Sun and occurrence of geomagnetic storms (i.e. severe distortion of geomagnetic field of the Earth) may cause disturbances such as increases in density in the Earth's upper atmosphere. These result in a greater drag on the movement of satellites and spacecraft, causing a slow-down or even a change in orbit. Their useful life may also be reduced. Operators of these facilities are highly concerned because of these high operation and maintenance costs.Charged particles from the Sun may strike the spacecraft and satellites and make direct physical damage to the equipment. Also, charge accumulation may occur on the electronics on board satellites and spacecraft, resulting in a build-up of the electric field. When an electrical discharge eventually happens, the electronics may be damaged, hence affecting the operation of the satellites and spacecraft.In May of 1998, intense electron fluxes were released from the Sun. During that time, an American communication satellite, Galaxy-4, failed, leading to the suspension of paging service for 45 million people. It was believed that electrical discharge between electronic components onboard the satellite was the main cause of the failure.Effects on human bodyThe intense radiation from violent space weather can affect human DNA or cell replications. Astronauts in space, if not properly protected from the dangerous radiation, may be adversely affected. Flight at high altitudes may also result in a higher dose of radiation at the height of solar activity.Effects on Communication and Navigation SystemsTo enable long-distance communication, many telecommunication systems transmit radio signals via the ionosphere. Electromagnetic waves produced by solar flares may disturb the ionosphere and interfere with radio signals, resulting in degraded communication quality.Maritime and aviation navigation systems use low frequency signals to locate vessels or aircraft. Intense solar activity may generate inaccuracy in information in these systems, causing errors in the positioning of vehicles.Effects on Electric Power and PipelinesOccurrence of geomagnetic storms (i.e. severe distortion of geomagnetic field of the Earth) can induce electric current in oil pipelines and electric cables. Flow meters may be affected, giving incorrect reading. The rate of corrosion of pipeline may also increase. The abnormal current induced in power grids may damage transformers because of overheating. During the great geomagnetic storm in 1989, transformer failures resulted in electricity suspension in Quebec for 9 hours.Please click here to view "Space Weather (Part I) - An Introduction"

December 2004

https://www.hko.gov.hk/en/education/space-weather/effects-of-space-weather/00425-effects-of-space-weather.html

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Journey to Mars and Cosmic Radiation

The analysis of the radiation data from NASA's 2013 Curiosity rover and various radiation standards will help to evaluate the radiation challenges that humans may encounter during a Mars landing. The emerging technologies for radiation dose measurement and protection will play a crucial role in overcoming these challenges.

From "Red Planet" to "The Martian", the barren and desolate red planet besides the Earth has always been a world full of fantasies for sci-fi movie lovers. Many of them wish that one day they could visit Mars in person. Their dreams seem getting closer to reality following numbers of successful unmanned Mars exploration missions commissioned by China, the United States, and European Space Agency in recent years. People all over the world are looking forward to having first human landing on Mars. For those having a more ambitious mind set, they have even been exploring the possibility of having large-scale migration to Mars. Could Mars exploration in fact be carried out smoothly as we think?Unlike our mother Earth, which is well protected by thick atmosphere and a charge-shielding magnetic field against cosmic radiation, staying at Martian surface for some time for exploration is highly challenging and a very difficult task for human beings. Assessment based on radiation measurement data reveals that the average daily dose rate of cosmic radiation on Martian surface is 0.67 mSv. In addition, the average cosmic radiation dose during the journey to Mars is 1.8 mSv per day, which is close to the annual radiation dose from natural background radiation (~ 2 mSv) received on the Earth. NASA's Spacecraft ‘Curiosity’ has once measured the total radiation dose during the three phases of its mission, including the 180-day journey to Mars, 500-day surface survey, and 180-day return journey. It showed that the total amount of radiation dose at the three phases was similar (figure 1). However, the study had not taken into account the possibilities of other radiological events in Space such as solar particle events (figure 2) or radiological variations on Martian surface (figure 3). The high radiation dose could pose significant threat to astronauts’ health and safety.Sci-fi Movie ‘The Wandering Earth’ manifested the health threat of cosmic radiation to the astronaut. Once an astronaut embarks on a Space journey, he/she is exposed to different ionizing radiation from various sources, including cosmic radiation from the Milky Way, Protons and Electrons emitted by the Sun, protons, electrons and neutrons as trapped by the Earth's magnetic field in the Van Allen belts . Based on the estimation of Space journey duration, the cumulative radiation dosage for a single mission could exceed 100mSv, which can have long term health impact. One may wonder: How should human beings cope with such radiation threat brought by Space travel?With the advancement and application of measurement technology of radiation dose in human organs of Space travel, this can help establish more comprehensive radiation safety assessment methods for the development of new protection technologies to safeguard the health and safety of astronauts. New technological means, such as Phantom Torso (see figure 4), ray tracing technology, polyethylene-based nanocomposites and water walls, have shown great potential in the reduction of radiation exposure. These advancements in radiation protection technologies give us hope for the realization of Space travel to Mars one day in the future.

July 2023

https://www.hko.gov.hk/en/education/space-weather/effects-of-space-weather/00703-Journey-to-Mars-and-Cosmic-Radiation.html

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Space Travel and Cosmic Radiation

This article introduce the radiation dose of astronauts during space missions.

With the advancement in technology, commercial space travel is no longer out of reach. One day, you and I may have the opportunity to become astronauts. Before we step into the outer space, let's understand the effects of cosmic radiation on astronauts.On the ground, Earth's magnetic field and atmosphere protect us from direct exposure of solar particles or other high-energy particles emitted from distant supernovas (“cosmic radiation”). As the spacecraft lifts off, the atmosphere becomes thinner and the Earth's magnetic field gradually weakens, and astronauts have to rely on the radiation protective materials installed on the spacecraft for protection. However, due to weight and fuel constraints, there is no way for a spacecraft to install large amounts of shielding, such as high-density metal lead. Furthermore, regarding high-energy cosmic radiation, the lead shielding can only to some extent attenuate the radiation and cannot completely absorb it. Therefore, the radiation level inside the space capsule will be several times to dozens of times higher than that at the earth's surface. In addition, when performing spacewalk in an extravascular activity, to allow flexible movement, the radiation shielding of the space suit will be less and the astronauts will be exposed to even more radiation.In the 1960s and 1970s, the total duration of the Apollo missions, the lunar surface duration, and the average radiation dose of astronauts were all recorded by NASA (see Table 1 for details) [1]. From the public data of six successful moon landings, it can be seen that the average radiation dose of an astronaut performing a ten-day space mission and a fifty-hour lunar surface mission is 5.4 mSv, in which mSv is a unit of effective radiation dose for evaluation of the effect of radiation on the human body. It is worth noting that the radiation doses astronauts received from different missions are diverse. Taking Apollo 14 as an example, although the lunar surface duration was not the longest, the average radiation dose of astronauts during the mission was particularly high, reaching 11.4 mSv. Among space missions, the radiation dose of the Apollo missions is still not the highest. Table 2 lists the radiation dose of some other space missions. Among them, the radiation dose of the astronauts who stayed in the Skylab 4 for 87 days was fifteen times higher than that of the Apollo 14 astronauts.Although there are only limited research data on the effects of cosmic radiation on the human body, as cosmic radiation is more penetrating than the generally artificially produced radiation, it may have a greater impact on the human body. Therefore, scientists need to continue the hard work to find more ways to protect the astronauts.

July 2022

https://www.hko.gov.hk/en/education/space-weather/effects-of-space-weather/00676-Space-travel-and-cosmic-radiation.html

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Effect of solar activities to the Earth's atmosphere

If solar activities become active, what are the effects to the Earth's atmosphere? When the sun is active, would the Earth be heated up and lead to temperature rise? In the long run, is there any effect to the Earth's climate? Is there any effect to the weather of a small city like Hong Kong?

Q: If solar activities become active, what are the effects to the Earth's atmosphere?  A: When the sun becomes active, the occurrences of phenomena such as solar flares, coronal mass ejection become frequent.  The amount of high-energy particles and extreme shortwave radiation released, such as X-rays and UV, will also increase.  These phenomena affect the ionosphere of the Earth's atmosphere the most.  It can disturb Earth's magnetic field, affect communications and create auroras.Q: When the sun is active, would the Earth be heated up and lead to temperature rise? A: The atmosphere of the Earth can absorb and scatter the solar radiation.  In simple words, the atmosphere can absorb radiation of different wavelengths of the solar spectrum.  However, it absorbs the radiation mainly in the long wave and extreme short wave regimes.  The energy released from solar activities is mainly extreme short wave radiation; and will be absorbed by the gases in the upper atmosphere.  The extra amount of radiation reaching the ground level will be minimal.  Thus the effect to the temperature at ground level is negligible.Q: In the long run, is there any effect to the Earth's climate? A: A research suggested that the particles from cosmic rays in outer space can reach the Earth easily during a weak solar activity period.  The particles can act as condensation nuclei and cloud forms easier.  Hence, the effect is the decrease in temperature and increase in rainfall.  However, take Hong Kong as an example, years with above-average rainfall seem to be unrelated to the strength of the solar activity.  In addition, the issue of anthropogenic climate change in the past years complicated the problem.  As such, there is not enough evidence to prove that solar activities can affect the long term climate of the Earth.Q: Is there any effect to the weather of a small city like Hong Kong? A: The short-term weather of a city should not be related to the activities of the Sun.  Weather only relates to the weather system in different scales, such as ridges of high pressure, troughs of low pressure, typhoons, etc.

September 2010

https://www.hko.gov.hk/en/education/space-weather/effects-of-space-weather/00426-effect-of-solar-activities-to-the-earths-atmosphere.html

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Atmospheric drag on the motion of satellites

Atmospheric drag could slow down the motion of a satellite when its orbit is low enough to be affected by the friction of Earth's atmosphere. A low Earth-orbiting satellite is often placed just above the Earth's atmosphere, where there is almost no air to drag on the satellite and reduce its speed.

Atmospheric drag could slow down the motion of a satellite when its orbit is low enough to be affected by the friction of Earth's atmosphere. A low Earth-orbiting satellite is often placed just above the Earth's atmosphere (Fig.1), where there is almost no air to drag on the satellite and reduce its speed. For example, the Hubble Space Telescope (HST) has a low altitude orbit and operates at an altitude of 610 km with an orbital period of 97 minutes.The sunspot cycle 24 has begun in January 2010 and the solar activity will increase gradually. When there are intense solar activities bringing intense solar winds towards the earth, the upper atmosphere of the earth will heat up and expand to enclose the orbits of some low Earth-orbiting satellites in it (Fig.2). It will produce greater frictional drag against the motion of the satellite.The drag would slow down the orbiting speed of the satellite. It will cause the satellite to de-orbit, decrease in altitude and eventually burn up in the atmosphere through its voyage back to the earth by gravitational force. Re-boosts in advance to maintain its orbit could extend the operational period of a satellite if it has enough fuel.Furthermore, the atmospheric density and composition in the lower atmosphere were also dependent on local time besides the solar activity. Scientists found that the atmospheric density below 200km was higher at nighttime. Thus, satellites operating at very low altitude orbits would experience even more frictional drag at nighttime.

March 2010

https://www.hko.gov.hk/en/education/space-weather/effects-of-space-weather/00427-atmospheric-drag-on-the-motion-of-satellites.html

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Space Weather and Polar Flights

Flights at very high latitudes (>78°N) are called polar flights. In this part of the world, they are mainly long-haul flights between Asia and the North America, flying over the Arctic starting from the late 1990s. Intense solar activities produces a higher dose of radiation in the upper atmosphere over the polar regions.

Flights at very high latitudes (>78oN) are called polar flights. In this part of the world, they are mainly long-haul flights between Asia and the North America, flying over the Arctic starting from the late 1990s. The number of polar flights has dramatically increased over the last decade.Polar flights not only save time but also fuel, hence lowering the operating cost. During intense solar activities, approaching charged particles in the solar wind can be trapped by the Earth's geomagnetic field and guided towards the geomagnetic polar regions. This sometimes creates the beautiful phenomenon of aurora. However, it also produces a higher dose of radiation in the upper atmosphere over the polar regions. Precautionary measures are necessary for pilots, crews, as well as passengers who travel often. [see "Radiation Tidbits - Air Travel and Cosmic Radiation" for related topics.

June 2009

https://www.hko.gov.hk/en/education/space-weather/effects-of-space-weather/00429-space-weather-and-polar-flights.html

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The possible effects on earth's climate by the solar spectral change in a solar cycle

The amount of ultraviolet radiation was much smaller than expected and the amount of visible light reaching the Earth increased during the period. The visible light for warming the Earth's surface increased even during the declining phase of solar activity. The spectral changes seemed to have altered the distribution of ozone in the atmosphere.

The discrepancy between intuition and research findingsIntuitively, most people tend to believe that the Earth would be warmed by more light and heat radiated from the sun during its active stage at the period of solar maximum, and that decreasing solar activities would reduce the total solar energy output and cool down the Earth. However, atmospheric physicists, Joanna Haigh and her colleagues at the Imperial College London recently found that solar radiation affects Earth's surface climate in the opposite way, at least during the few years (2004-2007 during sunspot cycle 23) of solar activity recession before the solar minimum in 2008 (Figure 1). Their findings are published in Nature, October 2010.Solar spectral changes They found that the amount of ultraviolet radiation was much smaller than expected and the amount of visible light reaching the Earth increased during the period. The visible light for warming the Earth's surface increased even during the declining phase of solar activity. The spectral changes seemed to have altered the distribution of ozone in the atmosphere. They showed that the stratospheric ozone concentration decreased below an altitude of 45 kilometres above sea level during the period 2004-2007, but the ozone concentration increased above this altitude.Haigh has also explained how the above response of ozone to changes in solar ultraviolet (UV) radiation modulates the "radiative forcing" of sunlight. An increase in ozone will reduce the flux of solar ultraviolet radiation reaching the tropopause and increase the flux of infrared radiation in stratosphere. The net effect would be a small increase in the net downward flux of solar radiation.What is "radiative forcing"?"Radiative forcing" is the change in the net vertical (downward minus upward energy flow) flux of solar radiation at the tropopause due to a change in the strength of an external driver for climate change, such as a change in the concentration of carbon dioxide or the radiation output of the Sun. A positive value of "radiative forcing" (more incoming energy) denotes warming on the earth surface and in lower atmosphere and vice versa.In the "Intergovernmental Panel on Climate Change" (IPCC)'s report, "radiative forcing" is further defined as the change relative to the year 1750 (the pre-industrial era). It often refers to a global and annual average value.Besides "radiative forcing", what is the possible climatic effect associated with the "ozone response"? Ozone helps to heat up the stratosphere both by absorbing the sun's ultraviolet radiation and by absorbing infrared radiation from the lower atmosphere (troposphere) as a greenhouse gas. The less the ozone, the cooler the stratosphere (Figure 2) becomes. These changes in the stratosphere may have indirect downward impact on the tropospheric circulations, resulting in climate change.Possible effects of producing weather anomalies on the Earth's climate? Haigh's findings have drawn other scientists' attention and aroused discussions. If the warming and cooling effects on earth could be out of phase with the solar activity and the ozone-related "radiative forcing", in what ways would the processes in the troposphere and stratosphere modulate the Earth's climate? In response to Haigh's findings, another scientist, Quirin Schiermeier, has reiterated some meteorologists' belief that "blocking events" may occur more frequently during phases of low solar activity. What is a "blocking event"? Figure 3 shows a typical "blocking event", during which there is a "blocking system" consisting of a blocking high (on the pole-ward side) and a cut-off low. The pattern of the associated height field nearby the blocking high resembles the shape of a Greek letter [ (Omega). A cut-off low is a closed cold core low completely detached from the upper-air westerly air-stream. A blocking system is a slow moving or nearly stationary large weather system. The upper-air westerly air-stream (in east) ahead of it forms a significant wavy flow or deep trough as being distorted by the blocking system. The dry, rainy, snowy or cold weather in different regions affected by such kind of large quasi-stationary weather system and the associated deep trough could persist for many days. Furthermore, the collapse of a blocking system could also release a large pool of extreme cold air southward. If "blocking event" and collapse of "blocking system" occur frequently, the possibility of regional extreme weather like snow damage and very cold weather will increase. So, "Blocking events" are unusual patterns of upper-air (e.g. on 500 hPa level) westerly jet stream in the upper troposphere that can cause intervals of cold spell and abnormal weather. Since solar activity is cyclic in about every 11 years, it should have no long-term impact on climate. Haigh's findings may provide some clues on understanding and quantifying natural climatic fluctuations. Exceptional cold weather events occur albeit average global temperatures is rising In addition to the more than expected decline of solar ultraviolet flux, ozone reduction at an altitude of the stratosphere below about 45 km above sea level might cool down part of the stratosphere above the tropopause (Figure 2). Would it imply subsequent interactions between the stratosphere and the troposphere to cause abnormal upper-air jet streams, leading to "blocking situations" pushing the polar cold air to persistently move southward (Figure 3), resulting in regional extreme weather on earth surface? The solar activity was low during the minimum sunspot period in recent years. Exceptional cold weather had occurred in winter of some regions albeit average global temperatures is rising. For example in early 2008, south China suffered from a severe snowstorm while Hong Kong experienced the longest cold spell since 1968 (Figure 4). Since 2008 is a year of solar minimum, could it have any relationship with the exceptional cold weather event? Or, could the La Nina event that occurred between late 2007 and mid 2008 be an alternative cause for the cold weather event? ... To better understand and answer these questions, scientists need more data for further studies.

December 2010

https://www.hko.gov.hk/en/education/space-weather/effects-of-space-weather/00430-the-possible-effects-on-earths-climate-by-the-solar-spectral-change-in-a-solar-cycle.html

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Extreme Space Weather Events in History

A solar storm brought some chaos to the telegraph system in 1859. On 1 September that year, English astronomer Richard Carrington observed one very extreme explosion of a group of sunspots through the image projected from his telescope.

Space weather is a current hot topic as we expect to approach a solar maximum this year (2013), but just what does the condition of the Sun mean to us here on Earth? In this series, we shall take a look at some of the severe solar storms, and examine how they affected our society at the time.Part 1First of all, let us go through some basics of solar activity. The Sun is essentially a ball of boiling plasma, in which convection is driven by heat at the core. From there, heated particles (above 5000°C) would rise to the surface, losing energy to the outer Space during the process, and subsequently sink back to the core. Although these sinking particles would be cooled down to at most 3000°C, they are still much cooler than those rising particles. Therefore they would appear much dimmer. The dark spots that show up on the Sun's surface are where these sinking particles concentrate in and they are what we call sunspots.As a sunspot marks where the sinking branches of multiple convective cells meet, a large number of charged particles are brought together creating a strong magnetic field. When the Sun is particularly active, the vigorous convective motion could twist the field lines so much that they have to be reconnected. During the reconnection process, those particles originally trapped in the magnetic field lines would get flung out, releasing a huge amount of radiation.1859: Carrington EventA solar storm brought some chaos to the telegraph system in 1859. On 1 September that year, English astronomer Richard Carrington observed one very extreme explosion of a group of sunspots through the image projected from his telescope (Figure 2).This solar flare hurled at Earth a huge cloud of charged particles that arrived in just 17 hours and greatly interfered with the geomagnetic field. The resulting fluctuations induced large currents in telegraph wires, and sparks were reported at stations in Paris and Washington DC. Most lines had to be closed in response.Since the telegraph was practically the only electrical system back then, the impact was relatively limited, but the solar storm itself is in fact one of the most powerful storms on record. Energetic particles from the Sun could trigger a chain of chemical reactions that result in nitrates being deposited onto the Earth's surface and preserved in ice cores, and samples corresponding to 1859 have been analysed to show the highest concentration of the compound in 500 years. The Carrington Event is therefore believed to be the largest over these past centuries, and is often portrayed as the worst-case scenario. If a solar storm of such magnitude were to happen today, the impacts would certainly be much more serious.

March 2013

https://www.hko.gov.hk/en/education/space-weather/space-weather-events/00433-extreme-space-weather-events-in-history.html

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Intense Solar Activity in September 2005

In early to mid September 2005, sunspots (number 808) came into our view with the Sun's rotation. Solar activity subsequently became active. Several intense solar flares erupted from 7 to 17 September. Among them, a violent one occurred on 7 September, making it one of the largest flares in this solar cycle (Cycle 23).

In early to mid September 2005, sunspots (number 808) came into our view with the Sun's rotation (Figure 1). Solar activity subsequently became active. Several intense solar flares erupted from 7 to 17 September. Among them, a violent one occurred on 7 September, making it one of the largest flares in this solar cycle (Cycle 23).The Solar X-ray Imager onboard of the GOES satellite started encountering problem on 11 September. Before the instrument went down, it captured a movie of the Sun between 1 and 11 September (Source/Credits: National Geophysical Data Center). Pay special attention to the images on 7 September, when spectacular solar flares started to emerge from the left to the centre of the Sun's disk, following its rotation. Space weather was originally forecast to worsen as the sunspots moved in to face the Earth. Fortunately, the sunspot activity weakened somewhat and did not produce much effect on the Earth.

September 2005

https://www.hko.gov.hk/en/education/space-weather/space-weather-events/00435-intense-solar-activity-in-september-2005.html

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From Space weather to Earth's weather - A discussion of the effects of sunspots on the unusually cold winter of 2010-2011

At the beginning of 2011, unusual weather appears in many regions of the world. Unusually freezing cold weather affected China, Japan and Korea with record-breaking low temperatures. In addition, there were droughts in Russia as well as floods in Canada and Australia.

The coldest January since 1977 in Hong Kong  Owing to the frequent replenishments of the winter monsoon, the monthly mean temperature of 13.7 degrees for the first month of 2011 was 2.4 degrees below normal, making it the coldest January since 1977.The freezing weather adversely affected the harvest of crops At the beginning of 2011, unusual weather appears in many regions of the world. Unusually freezing cold weather affected China, Japan and Korea with record-breaking low temperatures. In addition, there were droughts in Russia as well as floods in Canada and Australia. These weather hazards brought about the underproduction of crops and in turn the rise of the price of wheat. These remind people of the following events.Sunspots and the earth climate  .In 1801, British scientist, William Herschel, studied the change of number of sunspots during 1650 to 1800 and its effects. According to his analysis, rainfall became less when sunspots were sparse. So much so that sunspots determined indirectly the wheat prices in London. Hence, he attempted to correlate the yearly sunspot numbers and the prices of the grain. He proposed that an anti-correlation existed between the number of sunspots visible on the sun and the wheat prices. That was to say when there were little or no sunspots, the harvest of wheat was low and its price rose. In the contrary, when solar activity was active, the harvest of the grain was high and its price was low. He convinced himself that there might be a causal relationship between the sunspot cycle and the Earth's climate. This speculation matched with the improvement of agricultural production in the United Kingdom when the sunspot number rose during 1798 to 1804. Economists who were involved with the stock market followed up his study later on. They, however, found that the long-term results did not always show this anti-correlation.The research results by the University of Reading at the beginning of 2010  Mike Lockwood is a professor of space environment physics at the University of Reading, UK. Professor Lockwood and his research team compared the levels of solar activity with the past 351-year temperature record in Central England. These data provided information that traced back to the beginning of the Maunder Minimum (see para. 5),Mike Lockwood opined that solar activity seemed able to modulate blocking events. When the solar activity was low, blocking events became more stable. In the winter between 2009 and 2010, a prolonged blocking situation of distorted upper-air westerly jet stream caused the long spell of freezing weather in Europe. The possible effect of "blocking events" on the Earth's climate was discussed in "The possible effects on earth's climate by the solar spectral change in a solar cycle".Very few sunspots appeared in the period of "Maunder Minimum"The period from 1645 to 1715 is often called the "Maunder Minimum" (Fig.1), when the solar activity was extremely inactive with only a few or even no sunspots for a long time. Europe experienced a series of severely cold winter in this period - the "Little Ice Age". "Little Ice Age" happened in the period approximately between 1550 (or perhaps as early as 1300) and 1850 A.D. During this period, it was generally colder than nowadays in Northern Hemisphere, especially in Europe. Since the temperature was relatively low, mountain glaciers advanced in many parts of the world. Some rivers froze and snow covered low altitude areas besides high mountains throughout the years. Origin and southward advancement of cold air Since there is a continuous cooling period of approximately half a year without sunshine on the Earth's north and south poles every year, a large amount of cold air accumulates near the polar regions. Figure 2 shows a blocking situation on the 500 hPa upper-air weather chart on 20 January 2011. The westerly jet was blocked and appeared in an unusually and extremely distorted form such that a large amount of extremely cold arctic air was guided to move southward, resulting in very cold weather in the vicinity. In the north, the regions affected by cold continental air-streams were cold and dry. When the cold air-stream met the moist air mass in the south, the weather could become snowy or freezing and extremely cold in some places. In the 24th solar cycle, will the warming trend of the earth slow down? In 2001, a computer climate model of NASA (National Aeronautics and Space Administration of the United States) had reinforced a notion that existed for a long time. It indicated that low solar activity could have changed the atmospheric circulation in the Northern Hemisphere and triggered a "Little Ice Age" in several regions including North America and Europe during 1400s to 1700s. Mike Lockwood also studied solar activities with data for over 9,000 years. Solar activity tended to increase quite slowly and took more than 300 years from low (quiescent) to high (active). However, solar activity often dropped rapidly to fall from high to low in a little more than 100 years. The present decline in solar activity started in 1985 (Fig.1) and is now about "half way to a Maunder Minimum condition".The sunspot numbers were lower in the early half of the past 120 years (Fig.1). During the early half of the past 120 years, it was colder in Hong Kong than the later half (Fig.3). The positive correlation between the air temperatures of Hong Kong and the sunspot numbers also exists around 1970s. Although Fig.1 and Fig.3 show some degree of positive correlation between the air temperatures of Hong Kong and the sunspot numbers, greenhouse effect is still an overriding factor for the warming trend.Also, unusually cold winters occurred occasionally in various regions of the world in the past few years. During this period, very few sunspots were observed as it was not far from the last solar minimum in 2008. The present sunspots in the 24th solar cycle are less than anticipated. Furthermore, NASA has recently predicted that the sunspot number would be much smaller than the sunspot numbers of previous several solar cycles (by comparing Fig.1 and Fig.4). If this situation persists, will the trend of global warming be slowed down in the 24th solar cycles? To provide a more complete and convincing answer, subsequent observations of sunspots and more research are crucial.It must be pointed out that the cold winters discussed above refer to certain parts of the northern hemisphere.  For the Earth as a whole, the warming trend is unequivocal.  In fact, 2010 was the warmest year since instrumental record of temperature began more than 100 years ago.

March 2011

https://www.hko.gov.hk/en/education/space-weather/space-weather-events/00432-from-space-weather-to-earths-weather-a-discussion-of-the-effects-of-sunspots-on-the-unusually-cold-winter-of-20102011.html

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The Revelation of a Thinning Martian Atmosphere

People have long been speculating about the possibility of life on Mars. Mars has polar ice caps which may contain water, and there are also features resembling valleys carved by rivers on its surface. The early Martian climate might be warmer and wetter, possessing the essential ingredients for supporting life. This was very different from Mars today as it is now basically a cold and arid planet.

The National Aeronautics and Space Administration of the United States launched the "Mars Atmosphere and Volatile Evolution Mission" (MAVEN) in November 2013 with an objective of studying the losses of the Martian atmosphere and water to outer space. After entering orbit around Mars in September 2014, the MAVEN's operating spacecraft found that the Martian atmosphere was eroded by the solar wind. Relevant measurements indicated that solar wind stripped away gases at a rate of about 100 gram per second[1]. This atmospheric erosion was even more serious during solar storms (Fig. 1).People have long been speculating about the possibility of life on Mars. Mars has polar ice caps which may contain water, and there are also features resembling valleys carved by rivers on its surface. The early Martian climate might be warmer and wetter, possessing the essential ingredients for supporting life. This was very different from Mars today as it is now basically a cold and arid planet. The MAVEN project revealed that as Mars lacked a global magnetic field, it was exposed to the invasion of solar wind which crashed into the Martian atmosphere and stripped away its gases. The surface pressure of Mars is less than 10 hPa[2],[3] which is about one hundredth of the Earth's atmospheric pressure at the mean sea level. Since the Martian atmosphere became thinner, any liquid water on the surface would then be lost.Although our living Earth has a global magnetic field that protects us from the impact of solar wind, studies showed that the Earth's magnetic field could reverse, so that the positions of magnetic north and magnetic south would interchange. This "geomagnetic reversal" phenomenon was discovered in the 1960s when scientists studied the seafloor magnetism and noticed normal and reversed polarity of magnetic stripes alternating across the mid-ocean ridge on the oceanic crust[4]. The stripes emerged symmetrically on two sides of the mid-ocean ridge[5] (Fig. 2). During seafloor spreading, volcanic activity erupted new volcanic rocks near the crest of the mid-ocean ridge and they gradually moved away from the crest on both sides of the ridge. The fresh rocks would be magnetized with a polarity that followed the Earth's magnetic field at the time. The magnetic stripes showing normal and reversed polarity reflect the change of geomagnetic polarity with time.Based on paleomagnetic research, some scientists estimated that polarity transition could take place in a long time span from around one to ten thousand years, but some others also suggested that the duration of this process could be much shorter[6]. There is also a hypothesis that the Earth's magnetic field will weaken substantially during reversals, such that it's atmosphere may be invaded by solar wind just like the situation in Mars. High energy particles from solar wind may bring climate change and damages to lives on the Earth. Yet there is no concrete evidence to indicate a direct link between geomagnetic reversal and mass extinctions. What are the causes of geomagnetic reversal and what are its impacts on the biosphere? To answer these challenging questions, more in-depth researches by scientists have yet to be done.

July 2016

https://www.hko.gov.hk/en/education/space-weather/space-weather-events/00485-the-revelation-of-a-thinning-martian-atmosphere.html

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Space Weather Alert Categorization

In general, geomagnetic storms affect the people on Earth most. Solar radiation storms pose threats mainly to those flying in space and high altitudes, while radio blackouts mainly affect those operations involving navigation or radio communications.

Solar storms pose hazards to the near-Earth space environment. After studying these storms, scientists categorize their effects into three main types. These are listed in decreasing severity, as follows: 1. Geomagnetic storms - the Earth's geomagnetic field is distorted due to the strong solar wind. Through electromagnetic induction, transient electric surges can damage transformers, electronic instruments and navigation equipment. 2. Solar radiation storms - streams of energetic particles and plasma pose threats to astronauts in space and to passengers and crews on polar flights. 3. Radio blackouts - intense X-rays with ionizing power upset the Earth's ionosphere, producing signal scintillations (which mean noise, distortion and attenuation) and disrupting radio communication. In general, geomagnetic storms affect the people on Earth most.  Solar radiation storms pose threats mainly to those flying in space and high altitudes, while radio blackouts mainly affect those operations involving navigation or radio communications.The Space Weather Prediction Center (SWPC) of the U.S. National Oceanic and Atmospheric Administration (NOAA) uses the above categorization when issuing alerts and warnings of space weather. This webpage aims at conveying these alerts and warnings to the general public in Hong Kong.

December 2009

https://www.hko.gov.hk/en/education/space-weather/monitoring-and-warnings/00436-space-weather-alert-categorization.html

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How is Space Weather Monitored?

Observation from the ground is affected by the Earth's atmosphere and weather. Observation from space does not have such a limitation. In recent years, satellites and spacecraft have been used to monitor the space weather.

Solar phenomena, such as the sun spots and solar flares, provide good indications of space weather. In particular, the intensity of x-ray and ultraviolet radiation from the Sun provide valuable information for space weather forecasting.Space weather can be observed from the ground and from space. On the ground, telescopes and optical instruments are used to record the number of sun spots, as well as to observe aurora, which provide evidence of geomagnetic storms.Observation from the ground is affected by the Earth's atmosphere and weather. Observation from space does not have such a limitation. In recent years, satellites and spacecraft have been used to monitor the space weather. These include: Advanced Composition Explorer (ACE), Solar and Heliospheric Observatory (SOHO), Transition Region And Coronal Explorer (TRACE), and Geosynchronous Operational Environmental Satellite (GOES). Details are as follows.ACE was launched in 1997 by the United States. It is capable of measuring the composition of charged particle from the Sun and the interplanetary magnetic field. It gives us important information on the materials released during solar events.A cooperation between European Space Agency (ESA) and National Aeronautics and Space Administration (NASA), SOHO was launched in 1995 to study solar wind and the internal structure of the Sun.TRACE was launched in 1998 by the United States. It aims at studying the evolution of magnetic field structures of the Sun.The GOES Solar X-ray Imager operated by the United States is capable of monitoring solar flare events. An explosive development of solar flare on 20 January 2005 can be seen below.Knowledge of the Sun's structure and the solar system's formation gives us clues to space weather. The Genesis spacecraft was launched in 2001 to collect the solar wind particles. Its sample-collection capsule returned to the Earth on 8 September 2004. Despite a crash due to failure of the parachute system, a significant amount of the samples were recovered. Study of the solar wind particles will help our understanding of the origin and evolution of the solar system.

March 2005

https://www.hko.gov.hk/en/education/space-weather/monitoring-and-warnings/00437-how-is-space-weather-monitored-the-latest-prediction-of-soloar-activity.html

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