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Frequently Asked Questions about Solar Indices
What is solar wind? Solar matter ejected from the Sun becomes the solar wind. It flies outward in all directions and includes charged particles -- electrons and protons. On a quiet solar day, the speed of the solar wind heading toward Earth averages about 400 km per second. That's about 900,000 miles per hour. The Sun is some 93 million miles from earth so it takes a particle in the solar wind around 103 hours to arrive at Earth. That's about four days.
What are solar flares? Most of the disturbances to radio signal propagation on Earth come from solar flares and coronal mass ejections (CMEs) from the Sun. For instance, x-ray flares affect our radio propagation. Their wavelength ranges from 1 - 8 Angstroms. X-ray flares are classified as:
C class, the smallest, usually have a minimal impact on propagation.
M class, medium size, have a progressively adverse impact to propagation. Their electromagnetic radiation can cause the loss of all propagation on the Sunlit side of Earth due to increased absorption in the D region of the planet's atmosphere.
X class, the biggest, also have a progressively adverse impact. Their electromagnetic radiation also can cause the loss of all propagation on the Sunlit side of Earth. Big X class flares can emit very energetic protons that are guided into Earth's polar cap by the our planet's magnetic field. That brings on a polar cap absorption event (PCA), with high D region absorption along radio signal paths passing over the polar areas of the Earth.
What causes radio blackouts? Scientists wonder about the relationship between solar flares and CMEs. They are known to happen together or separately. The time to affect Earth differs. The electromagnetic radiation from a big solar flare, traveling at the speed of light, can cause short radio blackouts on the Sunlit side of the Earth within about 10 minutes of the eruption on the Sun. On the other hand, the energetic particles ejected by a solar flare and the shock wave from a CME can take four days to arrive at Earth and disrupt propagation.
What is solar flux? Since they are counted when seen, sunspots are a subjective measurement. To have a more objective measurement of the Sun's output, scientists use the 10.7 cm solar flux -- a general measurement of the activity of the Sun. The 10.7 cm solar flux doesn't describe the formation of the ionosphere, because energy at a wavelength of 10.7 cm is insufficient to cause ionization. The useful correlation is between the smoothed 10.7 cm solar flux and the smoothed sunspot number. Smoothed numbers are used as correlating daily values or monthly averages is not useful.