An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract It is possible to measure surface radiant temperature fields of subpixel spatial resolution from satellites which contain more than one channel in the thermal infrared spectral region. Because of the different response of the Planck function at different wavelengths, the radiant temperatures measured in two channels may be expressed in terms of contributions from two temperature fields, each occupying a portion of the pixel, where the portions are not necessarily contiguous. The resulting simultaneous nonlinear equations may be solved for the complimentary portions of the pixel occupied and one unknown temperature. In two adjacent pixels which can be assumed to have the same target temperatures and same background temperatures, both unknown temperatures may be found. <s> BIB001 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract The aim of this study is to estimate the feasibility of monitoring the surface temperature of an active volcano, using satellite thermal infrared data. For this, the data collected by a radiometer on board the NOAA polar-orbiting satellites over Mt. Etna were processed (geometrical and radiometric corrections) and merged with a Digital Terrain Model in order to correct for the variations of mean ground temperature and atmospheric absorption with altitude. Four pictures, or scenes, subsequent to the eruption of 17th March 1981, have been interpreted; and they display two thermal anomalies: one, near the summit zone; and the other, on the NNW slope where lava flows took place. Some elementary models of thermal evolution are proposed. <s> BIB002 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> It is a remarkable fact that we know much more about the highest volcano on Mars than we do about the highest volcano on earth. Olympus Mons is well known to planetary scientists, has been the principal subject of a number of papers, and has been mentioned in innumerable other papers and articles. By contrast, the highest volcano on earth, Nevado Ojos del Salado (on the frontier between Chile and Argentina at latitude 27°S) is so poorly known that there are no papers about it, and even its height (∼6800 m) is not well determined. Ojos del Salado is only one of the many volcanos in the great volcanic province of the central Andes, which extends between 14° and 29°S. The existing Catalogue of Active Volcanoes of the World (Including Solfatara Fields, Part XV, Chilean Continent, by L. Casertano, International Association of Volcanology, Naples, Italy, 1963) lists only 16 volcanos in the region as being “active,” but it is obvious after even the briefest visit that there are dozens of others that should be regarded as active. Many of these are over 6000 m high, and they include several of the earth's greatest volcanos. <s> BIB003 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Data from remote sensing satellites operating in the short-wavelength infrared can be used to measure temperatures of about 150°C and above. The gain settings of the Landsat Thematic Mapper (TM) sensors are such that to record pixel-integrated temperatures from 150°C to over 1000°C requires the use of several spectral bands. If the radiant source occupies less than a whole pixel, as is the case for fumaroles or a crusted lava surface with incandescent cracks, then the pixel-integrated temperature is less than the actual surface temperature of the hot areas and appears to be different at each wavelength. By determining pixel-integrated temperature in two spectral bands we can estimate both the temperature and size of hot areas which occupy less than a complete pixel. In some cases our observations provide the only available data on the state of activity of a particular volcano; in others they support and refine field observations. Our Landsat TM data reveal otherwise unobserved precursor and subsequent activity related to the September 1986 eruption of Lascar volcano (Chile) and supplement other data on the March 1986 eruption of Augustine volcano (Alaska). They also indicate the continued presence of lava lakes at Erta 'Ale (Ethiopia) in the absence of any ground-based observations and document minor eruptive activity at Mount Erebus (Antarctica). Landsat Multispectral Scanner data have been used to measure the variation in temperature along a lava flow on Sierra Negra (Galapagos Islands) which occurred in 1979. <s> BIB004 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Remote sensing in the short wavelength infrared (SWIR) has the potential for providing a great deal of information about activity at volcanoes, by virtue of the radiation emitted by hot surfaces. This can be used to give warning of imminent eruptions, monitor their progress and model volcanic processes. The Landsat TM is the only operational instrument which returns suitable data. This is particularly valuable because it incorporates two SWIR bands enabling constraints to be put on the sub-pixel structure of the image. Current orbital themial infrared imagery (eg AVHRR, TM band 6) cannot resolve most volcanic thermal anomalies because the pixel size is too coarse. Moreover, the gains of operational thermal infrared sensors make them unsuitable for measuring volcanic temperatures. <s> BIB005 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> ACTIVE volcanoes transfer both material and heat to the Earth's surface, but their thermal budgets are more difficult to monitor than their eruption rates, or the output of volatiles. Short-wavelength infrared data from the Landsat Thematic Mapper have been used to measure temperatures1,2, and the logical next step is to measure the total radiant energy flux, Q, in order to monitor changes in thermal budgets with time, and to allow the activity of different volcanoes to be compared. Here we report Thematic Mapper measurements of Q at Lascar volcano, north Chile, for December 1984 that are consistent with the earlier suggestion1 that a lava lake was the source of the anomaly, and values for 1985–86 that are much lower, suggesting that fumarolic activity was then a more likely heat source. Our results show that satellite remote sensing may be used to monitor the activity of a volcano quantitatively, in a way not possible by conventional ground studies, and may provide a method for predicting eruptions. <s> BIB006 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract A study of volcanic activity and its effects on the atmosphere is one of 28 interdisciplinary investigations, for the Earth Observing System (EOS), due to be launched in 1997 and 1999. The volcanology investigation will include long- and short-term monitoring of selected volcanoes, the detection of precursory activity associated with unanticipated eruptions, and the detailed study of on-going eruptions. The data collected will allow us to address two aspects of volcanism: volcanic padforms and the atmospheric effects of eruptions. A variety of instruments on the two NASA EOS platforms, together with supplemental data from the Japanese and European platforms, will enable the study of local- to regional-scale thermal and deformational features of volcanoes, and the chemical and structural features of volcanic eruption plumes and aerosols. This investigation fits well within the overall goal of the EOS Project, which is to study the regional and global interrelationships between components of the Earth System, because it specifically investigates the links between volcanism, atmospheric chemistry and short-term (1–3 year) climate change. <s> BIB007 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Surface temperatures of a 1989 andesite lava flow at Volcan Lonquimay, Chile, derived from an analysis of Landsat Thematic Mapper (TM) infrared data, show a progressive cooling of crusted lava from 250°C to 170°C down a 1.5 km segment of the flow. These results are obtained by solving “dual-band” equations for the two short-wavelength infrared TM bands, assuming that the lava surface can be represented by two thermal components, exposed core and chilled crust, and unlike previous investigations, by predetermining the temperature of the hot component rather than the cool component. This approach is consistent with both observational data and theoretical models, and shows that any exposed core of the flow occupies tiny fractions, possibly less than 0.03%, of the surface area. As a result, the crust made by far the greatest contribution to radiative heat losses, estimated at between 1 and 3 MW per 30×30 m pixel, along most of the flow's length. Convective heat transfer into the air above the flow is about two thirds the corresponding radiative flux. These estimated surface heat losses suggest thicknesses of less than 1 m for a conductive thermal boundary layer component of the crust. Radiance values in the thermal infrared band 6 of the TM image are consistent with temperatures derived from the short-wavelength infrared data and reveal the thermal manifestation of a topographic step beneath the flow. <s> BIB008 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Fifteen Landsat Thematic Mapper (TM) images of Lascar volcano (Chile), recorded between December 1984 and April 1992, document the evolution of a lava dome within the summit crater. Four of the scenes were acquired at night. In every image, the two short-wavelength infrared bands, 5 and 7, have detected thermal radiation from the volcano. As a consequence of the Planck distribution function, the relative response of these two channels depends on the proportions of very hot (> 600°C) surfaces occupying tiny pixel areas and broader regions at moderate temperatures (< 280°C). Intercomparison of bands 5 and 7 thereby provides a means for interpreting TM thermal anomalies even in the absence of ground observations. Pronounced changes in the configuration and intensity of the Lascar anomaly suggest that the volcano has experienced at least two cycles of lava dome activity since 1984. The first of these progressed through a “cooling” period, possibly reflecting a reduced flux of magmatic volatiles at the surface, and culminated in an explosive eruption on September 16, 1986, which appears to have completely destroyed the inferred lava dome. The TM data indicate that a new dome had been emplaced by November 1987, more than 15 months before it was first discovered by local observers. Lascar's style of cyclical effusive and explosive activity is typical of many volcanoes, and the remote sensing techniques presented herein could be applied elsewhere. <s> BIB009 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract Abstract. Many volcanic eruptions go essentially unmonitored. Potentially the Advanced Very High Resolution Radiometer (AVHRR), with its global coverage, frequent return period, and sensitivity in the thermal infrared, represents a data source capable of monitoring surface volcanic activity unrecorded by ground observations or other satellite sensors. In this study an attempt is made to demonstrate this potential by extracting information for the 1984 eruption at Krafla, Iceland. Seven cloud-free AVHRR images were available for the 14 day period of eruptive activity. The surface activity was detectable as a major thermal anomaly in all three of the longer wavelength channels and was vigorous enough during one night-time pass to be detectable in the near-infrared channel (0.725-1.1μm). Channel 2 and 4 radiance data were used to calculate the size and temperature of sub-pixel heat sources within the lava flow field, and a heat source at 1050° C was estimated as occupying an area of approximately 24... <s> BIB010 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Lascar Volcano (22°22'S, 67°44'W) is the most active volcano of the central Andes of northern Chile. Activity since 1984 has been characterised by periods of lava dome growth and decay within the active crater, punctuated by explosive eruptions. We present herein a technique for monitoring the high-temperature activity within the active crater using frequent measurements of emitted shortwave infrared (SWIR) radiation made by the spaceborne along-track scanning radiometer (ATSR). The ATSR is an instrument of low spatial resolution (pixels 1 km across) that shares certain characteristics with the MODIS instrument, planned for use as a volcano monitoring tool in the NASA EOS Volcanology Project. We present a comprehensive time series of over 60 cloud- and plume-free nighttime ATSR observations for 1992–1995, a period during which Lascar experienced its largest historical eruption. Variations in short wavelength infrared flux relate directly to changes in high-temperature surfaces within the active crater. From these data, interpretations can be made that supplement published field reports and that can document the presence and status of the lava dome during periods where direct, ground-based, observations are lacking. Our data agree with less frequent information collected from sensors with high spatial resolution, such as the Landsat thematic mapper (Oppenheimer et al. 1993) and are consistent with field observations and models that relate subsidence of the dome to subsequent explosive eruptions (Matthews et al., 1997). Most obviously, Lascar's major April 1993 eruption follows a period in which the magnitude of emitted shortwave infrared radiation fell by 90%. At this time subsidence of the 1991–1992 lava dome was reported by field observers and this subsidence is believed to have impeded the escape of hot volatiles and ultimately triggered the eruption (Smithsonian Institution 1993a). Extrapolating beyond the period for which field observations of the summit are available, our data show that the vulcanian eruption of 20 July 1995 occurred after a period of gradual increase in short wavelength infrared flux throughout 1994 and a more rapid flux decline during 1995. We attribute this additional, otherwise undocumented, cycle of increasing and decreasing SWIR radiance as most likely representing variations in degassing through fumaroles contained within the summit crater. Alternatively, it may reflect a cycle of dome growth and decay. The explosive eruption of 17 December 1993 appears to have followed a similar, but shorter, variation in SWIR flux, and we conclude that large explosive eruptions are more likely when the 1.6-μm signal has fallen from a high to a low level. The ATSR instrument offers low-cost data at high temporal resolution. Despite the low spatial detail of the measurements, ATSR-type instruments can provide data that relate directly to the status of Lascar's lava dome and other high-temperature surfaces. We suggest that such data can therefore assist with predictions of eruptive behaviour, deduced from application of physical models of lava dome development at this and similar volcanoes. <s> BIB011 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> The 1991–1993 eruption of Mount Etna, Sicily built a 7.2 km² aa lava flow field within the Valle del Bove. This letter provides estimates of the power losses associated with various cooling mechanisms operating during the development of the flow field, derived from time-series analysis of nighttime satellite remote sensing data. Unlike sensors used in many previous remote sensing studies of active lava, the ERS-1 Along Track Scanning Radiometer (ATSR) provides unsaturated measurements of spectral radiance from even very hot surfaces. This allows for more accurate solution of the lava flow endmember thermal components. We find that radiative and conductive processes dominated, with our model suggesting energy losses of similar magnitude. Radiative losses peaked about one month into the eruption, when lava flowed in open channels. Conductive losses peaked after two months, with a marked decline caused by a significant decrease in the horizontal spreading rate of lava. By the end of the eruption the 235 × 106 m³ of erupted lava is estimated to have lost only 28% of its initial thermal energy. A further 6.5 × 1017 J must be lost before the lava reaches ambient temperature. <s> BIB012 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> We present a thermal model to calculate the total thermal flux for lava flowing in tubes, on the surface, or under shallow water. Once defined, we use the total thermal flux to estimate effusion rates for active flows at Kilauea, Hawaii, on two dates. Input parameters were derived from Landsat Thematic Mapper (TM), field and laboratory measurements. Using these parameters we obtain effusion rates of 1.76±0.57 and 0.78±0.27 m3 s–1 on 23 July and 11 October 1991, respectively. These rates are corroborated by field measurements of 1.36±0.14 and 0.89±0.09 m3 s–1 for the same dates (Kauahikaua et al. 1996). Using weather satellite (AVHRR) data of lower spatial resolution, we obtain similar effusion rates for an additional 26 dates between the two TM-derived measurements. We assume that, although total effusion rates at the source declined over the period, the shut down of the ocean entry meant that effusion rates for the surface flows alone remained stable. Such synergetic use of remotely sensed data provides measurements that can (a) contribute to monitoring flow-field evolution, and (b) provide reliable numerical data for input into rheological and thermal models. We look forward to being able to produce estimates for effusion rates using data from high-spatial-resolution sensors in the earth observing system (EOS) era, such as Landsat 7, the hyperspectral imager, the advanced spaceborne thermal emission spectrometer, and the advanced land imager. <s> BIB013 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Satellite monitoring of remote volcanoes is greatly benefitting the Alaska Volcano Observatory (AVO), and last year's eruption of the Okmok Volcano in the Aleutian Islands is a good case in point. The facility was able to issue and refine warnings of the eruption and related activity quickly, something that could not have been done using conventional seismic surveillance techniques, since seismometers have not been installed at these locations. ::: ::: AVO monitors about 100 active volcanoes in the North Pacific (NOPAC) region, but only a handful are observed by costly and logistically complex conventional means. The region is remote and vast, about 5000 × 2500 km, extending from Alaska west to the Kamchatka Peninsula in Russia (Figure 1). Warnings are transmitted to local communities and airlines that might be endangered by eruptions. More than 70,000 passenger and cargo flights fly over the region annually, and airborne volcanic ash is a threat to them. Many remote eruptions have been detected shortly after the initial magmatic activity using satellite data, and eruption clouds have been tracked across air traffic routes. Within minutes after eruptions are detected, information is relayed to government agencies, private companies, and the general public using telephone, fax, and e-mail. Monitoring of volcanoes using satellite image data involves direct reception, real-time monitoring, and data analysis. Two satellite data receiving stations, located at the Geophysical Institute, University of Alaska Fairbanks (UAF), are capable of receiving data from the advanced very high resolution radiometer (AVHRR) on National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellites and from synthetic aperture radar (SAR) equipped satellites. <s> BIB014 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Long-term thermal modeling of volcanoes using satellite imagery provides an effective tool for monitoring remote yet dangerous volcanoes in the North Pacific. This region includes volcanoes in Alaska, the Aleutian Islands, and the Kamchatkan Peninsula. Thermal infrared data collected multiple times per day from weather satellites show distinct signatures for three different types of volcanic activity at different volcanic centers. Near real-time automated techniques are being developed to monitor relative changes in radiant temperature at volcanoes in this region. Radiant temperature values as a function of time are extracted and compared to background values for a series of active volcanoes. By establishing a long-term thermal record for these volcanoes, significant deviations indicative of an impending eruption can be detected. This tool is used to search for precursors to explosive eruptions in order to increase warning times and hazard mitigation for these potentially catastrophic events. A six year archive of satellite imagery has been compiled for this active region, and is available for study. <s> BIB015 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> The synoptic coverage offered by satellites provides unparalleled opportunities for monitoring active volcanoes, and opens new avenues of scientific inquiry. Thermal infrared radiation can be used to monitor levels of activity, which is useful for automated eruption detection and for studying the emplacement of lava flows. Satellite radars can observe volcanoes through clouds or at night, and provide high-resolution topographic data. In favorable conditions, radar inteferometery can be used to measure ground deformation associated with eruptive activity on a centimetric scale. Clouds from explosive eruptions present a pressing hazard to aviation; therefore, techniques are being developed to assess eruption cloud height and to discriminate between ash and meterological clouds. The multitude of sensors to be launched on future generations of space platforms promises to greatly enhance volcanological studies, but a satellite dedicated to volcanology is needed to meet requirements of aviation safety and volca... <s> BIB016 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Volcanic eruptions can present unpredictable hazards to populations living within regions containing potentially active volcanoes and for people traveling in jet aircraft that intersect with ash-laden eruption clouds. Methods of monitoring volcanic activity include searching for variations in the thermal signal from active fumaroles, lava domes, lava lakes, flows, and other features. Over many active volcanoes in the Western Hemisphere, low spatial resolution (4 km/pixel) weather satellite data acquired every 15 min are used to identify changes in eruptive activity, but are of insufficient spatial resolution to map active volcanic features. The Enhanced Thematic Mapper Plus (ETM+) on Landsat 7 can be used to monitor active volcanoes at a higher spatial resolution (15- to 60-m pixels). ETM+ also offers improvements over its predecessor, the Thematic Mapper of Landsats 4 and 5, by way of a 15-m panchromatic band and higher spatial resolution (60 m/pixel) thermal infrared (IR) band. With higher spatial resolution panchromatic data, we are able to map lava flow fields, trace very high temperature lava channels, and, at Lascar volcano, identify an arcuate feature associated with a collapsed crater floor, a phenomenon that may precede explosive activity. With improved spatial resolution in the thermal IR, we are able to map the bifurcation and braiding of underground lava tubes at Kilauea. Identifying tube locations and tracking their extension are important because, for a given volumetric lava production (effusion) rate, tube-fed flows can extend a much greater distance than surface flows. At both Kilauea and Etna, we are able to use the thermal data to estimate effusion rates, an important parameter for assessing how far a flow is likely to extend and therefore the hazard it poses. An improved ETM+ data collection and distribution system includes a well-formulated and -executed Long-Term Acquisition Plan and less expensive data that is available much faster than previously possible. <s> BIB017 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> The dual-band method has been widely used as the basis for determining lava surface temperatures from infrared satellite data; the method is based on the assumption that such surfaces can be described in terms of two end-member thermal components—hot cracks within a thermally homogeneous crust. The recent launch of the first orbiting hyperspectral imaging system, Hyperion, on- board the National Aeronautics and Space Administration Earth Observing-1 (EO-1) satellite heralds a new era of space-based hy- perspectral data collection that will allow more detailed models of lava-flow surface temperatures to be developed and parameterized. To this end, we have analyzed thermal images of active pahoehoe lava flows collected on Kilauea volcano, Hawaii, by using a forward- looking infrared (FLIR) 595 PM ThermaCAM, in order to assess the number of thermal components required to characterize the surface temperatures of active lava-flow surfaces. The FLIR images show that an active lava-flow surface comprises a contin- uum of temperatures that define distinctive temperature distribu- tions. Numerical model results reveal that the two-component dual- band method fails to resolve any of the major properties of the temperature distributions contained in these data (i.e., mode, skew, range, or dispersion). However, modeling five to seven thermal components allows all significant properties of the subpixel tem- perature distributions contained within the FLIR images to be de- termined. Thus, although the hyperspectral data provided by the EO-1 Hyperion yield as many as 66 wavebands in the 0.5-2.5 mm atmospheric window, useable data in 9-13 of these should be suf- ficient to perform accurate temperature characterization of active lava-flow surfaces from space. <s> BIB018 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> MODVOLC is a non-interactive algorithm developed at the Hawaii Institute of Geophysics and Planetology (HIGP) that uses low spatial resolution (1-km pixel-size) infrared satellite data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) to map the global distribution of volcanic thermal anomalies in near-real-time. MODVOLC scans the Level-1B MODIS data stream, on a pixel-by-pixel basis, for evidence of pixel and sub-pixel-sized high-temperature radiators. Once a hot spot has been identified its details (location, emitted spectral radiance, time, satellite observation geometry) are written to ASCII text files and transferred via FTP to HIGP, from where the results are disseminated via the internet http://modis.higp.hawaii.edu). In this paper, we review the underlying principles upon which the algorithm is based before presenting some of the results and data that have been obtained since its inception. We show how MODVOLC reliably detects thermal anomalies at a large number of persistently and sporadically active volcanoes that encompass the full range of common eruptive styles including Erebus (Antarctica), Colima (Mexico), Karymsky (Kamchatka), Popocatepetl (Mexico), Etna (Italy), and Nyiragongo (Democratic Republic of Congo), amongst others. We also present a few cautionary notes regarding the limitations of the algorithm and interpretation of the data it provides. <s> BIB019 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract An innovative, Advanced Very High Resolution Radiometer (AVHRR)-based technique for improved automatic detection of volcanic hotspots and thermal anomalies is proposed in this paper. It is mainly based on a multitemporal analysis of historical, long-term satellite records. Such a technique basically rests on the Robust AVHRR Techniques (RAT) approach, which has been already successfully applied to several natural and environmental emergencies (e.g., fires, floods, earthquakes). In this work, the proposed technique has been tested on an extended set of eruptive events of Mt. Etna and Stromboli volcanoes. Results achieved, in terms of reliability (low false alarm rate) as well as of effectiveness (detection sensitivity), are described in detail. Moreover, the potential in low-level thermal anomaly detection, as possible pre-eruptive thermal signs, is also addressed and preliminary results obtained for a couple of events, discussed. The study cases here presented show the benefits of such a technique especially when different observational conditions (time/season of pass, atmospheric moisture content, solar illumination, satellite angles of view, etc.) are considered, making such a method globally applicable. The future prospects, also in terms of possible operational scenarios, coming from the implementation of such an approach on the new generation of satellite sensors (such as SEVIRI aboard Meteosat Second Generation) are also discussed. <s> BIB020 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract The first recorded eruption of Anatahan began at approximately 17:00 local time on May 10, 2003. Here, we present observations made by a suite of Earth-orbiting satellites of the heat, ash and gas emitted from the volcano before, during and after the eruption. No thermal or sulphur dioxide emissions are apparent in MODIS (Moderate Resolution Imaging Spectroradiometer), AIRS (Atmospheric Infrared Sounder), EP TOMS (Earth Probe Total Ozone Mapping Spectrometer) and ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) satellite images acquired prior to the eruption. However, within 24 h of eruption onset, the University of Hawaii's near-real-time satellite thermal monitoring system ‘MODVOLC’ detected the eruption and confirmed Anatahan's eastern caldera as the center of the activity. Although the eruption was initially phreatic, it quickly transitioned into a magmatic phase that culminated in the emplacement of a dacitic lava dome. The onset of the magmatic phase is recorded by MODVOLC as an order of magnitude increase in the heat flux from the volcano after May 16, indicative of fresh lava at the surface, relative to low values (50–90 MW) observed during the initial phreatic phase. MODVOLC detected thermal emission from the volcano on a further 22 occasions during the next 2 weeks, allowing us to quantitatively document temporal variations in thermal output during the eruption. We use MODIS, EP TOMS and AIRS data to document ash and sulphur dioxide emissions from Anatahan covering the period May 10–June 13, 2003. Using daily satellite observations in conjunction with ground-based estimates, we arrive at a total SO 2 discharge of ∼ 0.51 Mt for the ∼ 1 month of activity, of which ∼ 0.11 Mt was emitted during activity on May 10–12. Decreases in measured SO 2 flux prior to the observation of a lava dome on June 4 indicate that the emerging dome may have blocked the upper conduit and inhibited degassing during this period. A new episode of dome growth was detected by MODVOLC on April 12, 2004. So far, the level of thermal emission during the recent period of activity is substantially lower than that observed during the May–June 2003 eruption. <s> BIB021 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract The ability to monitor and rapidly react to remote detection of volcanic activity has been greatly improved through use of the Autonomous Sciencecraft Experiment (ASE), an advanced software application installed on a spacecraft in Earth orbit. ASE is a NASA New Millennium Program experiment demonstrating science-driven autonomous command and control of a spacecraft. Flying on the Earth Observing-1 ( EO-1 ) spacecraft, ASE successfully detected thermal emission from the Mt. Erebus lava lake on 7 May 2004, having analyzed a Hyperion hyperspectral data product on board the spacecraft. EO-1 was re-tasked by ASE to obtain a follow-up observation 7 h later and sent a notification of detection of volcanic activity to the ground. The entire process was carried out autonomously. Initial acquisition to receipt on the ground of the positive detection took less than 3 h, a process that without ASE would have taken weeks. The ASE Thermal Classifier has detected several styles of effusive volcanic activity: active lava lakes, pahoehoe flow fields, open channel flows and lava domes. ASE successfully demonstrated that science-driven spacecraft operation greatly enhances science return per returned byte through the identification of the most valuable data, allowing prioritization of downlink products and the discarding of null data sets. This technology has applications on missions elsewhere in the solar system. Modified thermal classifiers can be used for detecting and monitoring active volcanism on the jovian satellite Io, the neptunian moon Triton, and searching for active volcanism on Mars and icy satellites. The success of ASE is an incentive for future instrument and mission designers to consider on-board data-processing requirements (especially data storage capacity, number of processors and processor speed, and RAM) in order to take advantage of this flight-proven technology. <s> BIB022 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract Kliuchevskoi volcano, located on the Kamchatka peninsula of eastern Russia, is one of the largest and most active volcanoes in the world. Its location and diversity of eruption styles make satellite-based monitoring and characterization of its eruptive activity essential. In 2005, the Kamchatka Volcano Emergency Response Team (KVERT) first reported that seismic activity of Kliuchevskoi increased above background levels on 12 January (Kamchatka Volcanic Eruption Response Team (KVERT) Report, 2005. Kliuchevskoi Volcano, 14 January through 13 May 2005. ( http://www.avo.alaska.edu/activity/avoreport.php?view=kam info&id=&month=January&year=2005 ). Cited January 2007). By 15 January Kliuchevskoi entered an explosive–effusive phase, which lasted for five months and produced basaltic lava flows, lahar deposits, and phreatic explosions along its northwestern flank. We present a comparison between field observations and multispectral satellite image data acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument in order to characterize the eruptive behavior. The ASTER instrument was targeted in an automated urgent request mode throughout the eruption timeline in order to collect data at the highest observation frequency possible. Brightness temperatures were calculated in all three ASTER wavelength regions during lava flow emplacement. The maximum lava flow brightness temperatures, calculated from the 15 m/pixel visible near infrared (VNIR) data, were in excess of 800 °C. The shortwave infrared (SWIR) data were radiometrically and geometrically corrected, normalized to the same gain settings, and used to estimate an eruptive volume of 2.35 × 10 − 2 km 3 at the summit. These data were also used to better constrain errors arising in the thermal infrared (TIR) data due to sub-pixel thermal heterogeneities. Based on all the ASTER data, the eruption was separated into three phases: an initial explosive phase (20 January–31 January), an explosive–effusive phase (1 February–8 March), and a subsequent cooling phase. Decorrelation stretch (DCS) images of the TIR data also suggested the presence of silicate ash, SO 2 , and water vapor plumes that extended up to 300 km from the summit. The ASTER rapid-response program provided important multispectral, moderate spatial resolution information that was used to detect and monitor the eruptive activity of this remote volcano which can be applied to other eruptions worldwide. <s> BIB023 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Autonomous onboard processing of data allows rapid response to detections of dynamic, changing processes. Software that can detect volcanic eruptions from thermal emission has been used to retask the Earth Observing 1 spacecraft to obtain additional data of the eruption. Rapid transmission of these data to the ground, and the automatic processing of the data to generated images, estimates of eruption parameters and maps of thermal structure, has allowed these products to be delivered rapidly to volcanologists to aid them in assessing eruption risk and hazard. Such applications will enhance science return from future Earth-orbiting spacecraft and also from spacecraft exploring the Solar System, or beyond, which hope to image dynamic processes. Especially in the latter case, long communication times between the spacecraft and Earth exclude a rapid response to what may be a transient process — only using onboard autonomy can the spacecraft react quickly to such an event. <s> BIB024 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Analysis of thermally emissive volcanic features using satellite infrared remote sensing has been conducted over recent decades, primarily using shortwave and thermal infrared (SWIR; TIR) radiance data. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), mounted on the Earth Observation System (EOS) Terra satellite, offers an advance on earlier instruments, having more bands covering the SWIR atmospheric window and offering a wider dynamic range. This paper compares methods used to analyse ASTER SWIR imagery of active volcanoes, using both simulated cases and actual ASTER imagery of Lascar Volcano, and focuses on radiative power estimates. Those based on the Oppenheimer approach are found to be most reliable for simulated surfaces, with the Lombardo and Buongiorno and Dozier retrievals having larger uncertainties in most cases. However, the Dozier Method results in the highest proportion of successful retrievals, the reliability of which is influenced by factors including band comb... <s> BIB025 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> The surface temperatures of active lavas relate to cooling rates, chemistry, and eruption style. We analyzed 61 hyperspectral satellite images acquired by the National Aeronautics and Space Administration's Earth Observing-1 (EO-1) Hyperion imaging spectrometer to document the surface temperature distributions of active lavas erupted at 13 volcanoes. Images were selected to encompass the range of common lava eruption styles, specifi cally, lava fountains, fl ows, lakes, and domes. Our results reveal temperature distributions for terrestrial lavas that correlate with composition (i.e., a statistically signifi cant difference in the highest temperatures retrieved for mafi c lavas and intermediate and felsic lavas) and eruption style. Maximum temperatures observed for mafi c lavas are ~200 °C higher than for intermediate and felsic lavas. All eruption styles exhibit a low-temperature mode at ~300 °C; lava fountains and 'a' a flows also exhibit a higher-temperature mode at ~700 °C. The observed differences between the temperatures are consistent with the contrasting rates at which the lava surfaces are thermally renewed. Eruption styles that allow persistent and pervasive thermal renewal of the lava surface (e.g., fractured crusts on channel-fed 'a' a flows) exhibit a bimodal temperature distribution; eruption styles that do not (e.g., the continuous skin of pahoehoe lavas) exhibit a single mode. We conclude that insights into composition and eruption style can only be gained remotely by analyzing a large spatio-temporal sample of data. This has implications for determining composition and erup- tion style at the Jovian moon Io, for which no in situ validation is available. <s> BIB026 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> [1] There are many reports of land surface temperature (LST) anomalies appearing prior to large earthquakes. A number of methods have been applied in hindcast mode to identify these anomalies, using infrared datasets collected from Earth-orbiting remote sensing satellites. Here we examine three such methods and apply them to six years (2001–2006) of MODIS LST data collected over the region of the 2001 Gujarat (India) earthquake, which previous studies have identified as a site exhibiting possible pre-seismic and post-seismic thermal anomalies. Methods 1 and 2 use an LST differencing technique, while Method 3, the Robust Satellite Technique (RST), has been developed specifically for the identification of thermal anomalies within spatio-temporal datasets. In relation to the Gujarat Earthquake, results from Methods 1 and 2 (LST differencing) indicate that changes previously reported to be potential precursory thermal ‘anomalies’ appear instead to occur within the range of normal thermal variability. Results obtained with Method 3 (RST) do appear to show significant ‘anomalies’ around the time of the earthquake, but we find these to be related to positive biases caused by the presence of MODIS LST data gaps, attributable to cloud cover and mosaicing of neighboring orbits of data. Currently, therefore, we find no convincing evidence of LST precursors to the 2001 Gujarat earthquake, and urge care in the use of approaches aimed at identifying such seismic thermal anomalies. <s> BIB027 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> We developed and tested an automated algorithm that analyzes thermal infrared satellite time series data to detect and quantify the excess energy radiated from thermal anomalies such as active volcanoes. Our algorithm enhances the previously developed MODVOLC approach, a simple point operation, by adding a more complex time series component based on the methods of the Robust Satellite Techniques (RST) algorithm. Using test sites at Anatahan and Kīlauea volcanoes, the hybrid time series approach detected ~15% more thermal anomalies than MODVOLC with very few, if any, known false detections. We also tested gas flares in the Cantarell oil field in the Gulf of Mexico as an end-member scenario representing very persistent thermal anomalies. At Cantarell, the hybrid algorithm showed only a slight improvement, but it did identify flares that were undetected by MODVOLC. We estimate that at least 80 MODIS images for each calendar month are required to create good reference images necessary for the time series analysis of the hybrid algorithm. The improved performance of the new algorithm over MODVOLC will result in the detection of low temperature thermal anomalies that will be useful in improving our ability to document Earth’s volcanic eruptions, as well as detecting low temperature thermal precursors to larger eruptions. <s> BIB028 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> [1] Between 24 March and 5 June 2010, the Hyperion hyperspectral imager and Advanced Land Imager (ALI) on NASA's Earth Observing 1 (EO-1) spacecraft obtained an unprecedented sequence of 50 observation pairs of the eruptions at Fimmvorðuhals and Eyjafjallajokull, Iceland. This high acquisition rate was possible only through the use of data flow streamlined by using the autonomously operating NASA Volcano Sensor Web (VSW). The VSW incorporates notifications of volcanic activity from multiple sources to retask EO-1 and process Hyperion data to extract eruption parameters from high spatial and spectral resolution visible and short-wavelength infrared data. Physical changes in eruption style and magnitude were charted as the eruptions ran their course. Rapid data downlink and automatic data-processing algorithms generated a variety of products which are compared with estimates from ground-based observations and post-eruption in situ measurements. Estimates of effusion rate from heat loss measurements underestimate actual effusion rate (while still following broad eruption rate trends) but are closer to in situ estimates for effusive eruptions (Fimmvorðuhals) than explosive, ash-rich eruptions (Eyjafjallajokull). During the later stages of the 2010 eruption, VSW-generated products were rapidly delivered to end-users in Iceland to aid in the assessment of risk and hazard. The success of the VSW led to Icelandic Meteorological Office (IMO) in situ sensors being incorporated into the VSW, and in May 2011 an IMO seismic alert autonomously triggered EO-1 observations of a new eruption at Grimsvotn volcano. Finally, the VSW demonstrates an autonomy-driven, multi-asset, spacecraft retasking and data processing system that maximizes science return, a desirable capability for future NASA missions. <s> BIB029 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract A combination of satellite and airborne high-resolution visible and thermal infrared (TIR) image data detected and measured changes at Redoubt Volcano during the 2008–2009 unrest and eruption. The TIR sensors detected persistent elevated temperatures at summit ice-melt holes as seismicity and gas emissions increased in late 2008 to March 2009. A phreatic explosion on 15 March was followed by more than 19 magmatic explosive events from 23 March to 4 April that produced high-altitude ash clouds and large lahars. Two (or three) lava domes extruded and were destroyed between 23 March and 4 April. After 4 April, the eruption extruded a large lava dome that continued to grow until at least early July 2009. <s> BIB030 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> article i nfo The investigation of high-temperature natural phenomena, such as wildland fires and active lava flows, is a primary science objective for the proposed Hyperspectral Infrared Imager (HyspIRI) mission. Current planning for HyspIRI includes a mid-infrared (MIR) channel centered at 4 μm that will allow measurement of radiance emitted from high-temperature targets. In this paper we present the results of a study to specify the saturation temperature for the MIR channel. This study was based on reviews of the literature, together with case studies of airborne and satellite-based data acquired over high-temperature targets. The spatial resolution of MIR radiance measurements is an important consideration in the remote sensing of high-temperature phenomena, due to thepresence ofmaterialsatdifferent temperatures withinthe areacoveredbyanimage pixel.The HyspIRI MIR channel will provide a spatial resolution of 60 m, which is ~40 times finer (in terms of area) than the finest spatial resolution provided by heritage instruments (370 m). This fine spatial resolution will increase the prob- ability that high-temperature targets fill an image pixel and, therefore, the HyspIRI MIR channel will require a saturation temperature 2 to 4 times higher than the saturation limits of heritage instruments. Based on our study, we recommend a saturation temperature of 1200 K (927 °C). This recommendation accounts for the high temperatures expected for natural phenomena, expected performance of the MIR channel, and overlap in sensitivity between the MIR and thermal infrared (7.5-12 μm) HyspIRI channels. <s> BIB031 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract The Vanuatu archipelago consists of 80 islands and hosts 5 subaerial volcanoes (Yasur, Lopevi, Ambrym, Aoba and Gaua) that have shown sign of activity during the past decade. In this contribution we provide a 15 years-long datasets (2000–2015) of the thermal activity recorded at these active volcanoes by means of MIROVA (Middle InfraRed Observation of Volcanic Activity) a new volcanic hotspot detection system based on MODIS data. The analyzed volcanoes are characterized by a spectrum of volcanic activities whose thermal signature has been tracked and carefully analyzed. These include strombolian–vulcanian explosions at Yasur, lava flows at Lopevi, lava lakes at Ambrym, surtseyan-type eruptions within the Voui crater lake of Aoba and ash-dominated eruptions with strong degassing at Gaua. The collected data reveal several details of the long term eruptive dynamics at single sites such as a monthly long pulse in thermal emissions at Yasur volcano as well as at the two active craters of Ambrym (Benbow and Marum). Heating cycles within Aoba crater lake and intermittent pressurized eruptions at Lopevi volcano has also been detected and shed light in the eruptive dynamics of the analyzed volcanoes. In addition we were able to track a two years long intensification of thermal output at Benbow crater (Ambrym) that preceded the occurrence of the first intra-caldera eruptions of this volcano since 1989. We emphasize how the data provided by MIROVA represent a new, safe and affordable method for monitoring in near-real time a large spectrum of volcanic activities taking place at Vanuatu and other volcanic areas. <s> BIB032 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract Merapi is one of Indonesia's most active and dangerous volcanoes. Prior to the 2010 VEI 4 eruption, activity at Merapi during the 20th century was characterized by the growth and collapse of a series of lava domes. Periods of very slow growth were punctuated by short episodes of increased eruption rates characterized by dome collapse-generated pyroclastic density currents (PDCs). An eruptive event of this type occurred in May–June, 2006. For effusive eruptions such as this, detailed extrusion rate records are important for understanding the processes driving the eruption and the hazards presented by the eruption. We use thermal infrared (TIR) images from the Moderate Resolution Imaging Spectrometer (MODIS) instrument on NASA's Aqua and Terra satellites to estimate extrusion rates at Merapi Volcano during the 2006 eruption using the method of Harris and Ripepe (2007). We compile a set of 75 nighttime MODIS images of the eruptive period to produce a detailed time series of thermal radiance and extrusion rate that reveal multiple phases of the 2006 eruption. These data closely correspond to the published ground-based observational record and improve observation density and detail during the eruption sequence. Furthermore, additional analysis of radiance values for thermal anomalies in Band 21 (λ = 3.959 μm) of MODIS images results in a new framework for detecting different styles of activity. We successfully discriminate among slow dome growth, rapid dome growth, and PDC-producing dome collapse. We also demonstrate a positive correlation between PDC frequency and extrusion rate, and provide evidence that extrusion rate can increase in response to external events such as dome collapses or tectonic earthquakes. This study represents a new method of documenting volcanic activity that can be applied to other similar volcanic systems. <s> BIB033 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Infrared Remote Sensing of Volcanoes: A History <s> Abstract Time-series analysis techniques are being increasingly used to process satellite observations of volcanic gas emissions and heat flux, with the aim of identifying cyclic behaviour that could inform hazard assessment or elucidate volcanic processes. However, it can be difficult to distinguish cyclic variations due to geophysical processes from those that are artefacts of the observation technique. Here, we conduct a comprehensive investigation into the origin of cyclicity in volcanic observations by analysing daily, global satellite measurements of volcanic SO2 loading by the Ozone Monitoring Instrument (OMI) and thermal infrared anomalies detected by the Moderate Resolution Imaging Spectroradiometer (MODIS). We use a fast Fourier Transform (FFT) multi-taper method (MTM) to analyse multiple phases of activity at 32 target volcanoes, utilising measurements obtained from three NASA satellite instruments (Aura – OMI, Aqua – MODIS and Terra – MODIS), and identify a common cycle (period of ~ 2.3 days), which is not considered to be of volcanic origin. Based on the presence of this cycle in multiple satellite datasets, we attribute it to variations in viewing angle during the 16-day orbit repeat cycle of sun-synchronous satellites maintained in Low Earth Orbit (LEO). A 5-point averaging correction procedure is tested on satellite observations from Kilauea volcano, Hawaii, and is found to reduce the impact of higher frequency cycles and reveal the presence of longer-period geophysical signals. In addition to the identification of a signal common to different measurement techniques, an underlying cyclical pattern was found in the OMI SO2 observations (periods of ~ 7.9 and 3.2 days) generated by the OMI Row Anomaly (ORA). We conclude that identification of the presence and magnitude of non-geophysical cyclic behaviour, which can suppress natural cycles in time-series data, and implementation of appropriate corrections, is crucial for accurate interpretation of satellite observations. The use of data averaging to suppress non-geophysical cycles imposes limits on the length of natural cycles that can be confidently identified in moderate resolution satellite observations from polar-orbiting spacecraft. <s> BIB034
Given the previous detail concerning the infrared sensors used for monitoring volcanic activity, it will be evident that the discipline has a history of over half a century. As technology has progressed, so have the applications to which the discipline has been applied  , and these will be highlighted here. Early infrared radiometer remote sensing surveys were airborne, as opposed to in Earth orbit, with  describing this application to Hawaiian volcanoes and producing more detailed and accurate thermal maps than had been possible from land-based observations alone. Later,   presented radiance maps and detailed how thermal emissions varied spatially at the new Icelandic volcanic islands of Surtsey and Jolnir. Cassinis and Lechi   also describe aerial volcanic surveys in Italy and at Mount St. Helens, respectively. Gawarecki et al.  was the first study to use satellite-acquired infrared data in the detection and monitoring of volcanic activity. However, despite such early experimental endeavours, it remains the case that even today, no satellite sensor has been launched with the sole aim of volcanic infrared monitoring. Volcanologists have been inventive and made use of sensors launched for purposes other than volcanic observation, and unfortunately, nothing seems set to alter this requirement in the near future. Bonneville et al. BIB002 stated, in relation to volcanic remote sensing, that "infrared remote-sensing methods remain of limited use" (p. 293), and at the time, this will have done little to encourage take-up of the discipline. This assessment was set to change however, as the discipline soon began to burgeon. Francis and McAllister BIB003  showed how thermal activity at remote Andean volcanoes could be identified without the need for costly and dangerous fieldwork, while BIB004 took a more global approach, highlighting the general utility of satellite infrared monitoring of volcanoes. Qualitative studies gradually became quantitative, with BIB006 , for example, becoming the first study to consider volcanic surface power emissions as opposed to values of radiance. Harris   provides detailed chronologies of the development of the discipline that cannot be bettered here, but in summary, it is shown that during the 1990s, the discipline expanded significantly. The work of Mouginis-Mark et al. BIB007 is perhaps responsible for stimulating most interest, by promoting the volcanic utility of the forthcoming NASA Earth Observation System. The initial priority during this period, according to  , was the identification of 'hotspots' and following this, the isolation of temperature distributions (e.g.,  BIB009  ). Once perfected in their observations, such studies became more quantitative in their focus and increasingly attempted to relate remote observations to actual phenomena on the ground. Oppenheimer et al. BIB011 BIB009 , for example, were able to identify eruptive periods from a time series of satellite data for Lascar Volcano, Chile, while BIB015 BIB013 showed how infrared observations could be used to estimate lava effusion rates. It was slowly becoming evident that a reliable and direct relationship existed between the radiant heat emitted from a volcano and the type and intensity of activity displayed at the surface. During this period, the potential seen in the discipline was highlighted in a review paper entitled: 'Remote Sensing of Active Volcanoes' BIB016 . With the small volumes of data coming from the handful of early infrared remote sensors, the early process of detecting the thermal signatures of volcanic activity was largely manual, but between 1995 and 2010, many automated algorithms were developed to detect volcanic thermal anomalies and to avoid false-alarms  . Harris et al. BIB010 , for example, developed an algorithm for AVHRR MIR observations, which identified hotspots within a uniformly warm surface, by harnessing the radiance temperature difference between Bands 3 (3.8 µm) and 4 (10.8 µm). The concept was that, given that MIR channels (i.e., AVHRR Band 3) are more sensitive to surfaces of magmatic temperature than to those at terrestrial temperatures (see Figure 1) , where this temperature difference exceeded a given threshold (applied as 10 K), the surface could be reliably interpreted as displaying a sub-pixel hotspot, which was contributing to the Band 3 signal. Automated processes using such algorithms rapidly developed, including some that were location-specific, such as the Okmok algorithm, which was named after an Alaskan volcano by the same name BIB015 BIB014 ; the Okmok algorithm functioned operationally with AVHRR data, by comparing the temperature of a potentially anomalous pixel to the temperatures of pixels around it; where this temperature difference was shown to be significant, it was automatically determined as anomalous  . Steps in automation were also facilitated by improvements in data distribution and storage, with one of the possibilities raised being the monitoring and observation of volcanic activity over wide areas, as opposed to at just one location. Dehn et al. BIB015 showed how routine AVHRR observations, and the Okmok algorithm, could be used to monitor all volcanoes in the North Pacific, and soon after this, one of the most pervasive and widely-referenced volcanic detection systems was developed called MODVOLC   BIB019 . This system, which remains operational, works on similar principles to other algorithms: that hotter surfaces will produce a higher spectral radiance at shorter wavelengths, and vice versa. For each MODIS pixel of data, the MODVOLC algorithm calculates a normalized thermal index (NTI) value by differencing the spectral radiant detection between the MIR (Band 22: 3.9 µm) and TIR (Band 32: 12.00 µm) bands and then normalizing the result by the sum of these radiances: where R 22 and R 32 refer to the spectral radiance (W m −2 sr −1 m −1 ) in MODIS Bands 22 and 32, respectively. If Band 22 is saturated, data from the equivalent, low-gain Band 21 can be used instead. A greater difference between Bands 22 and 32 indicates the presence of a hotter surface, and a threshold is used to identify potential volcanic hotspots BIB019 . The principle of differencing MIR and TIR signals was also utilised in a later, AATSR-bespoke volcano monitoring system: "Volcanoes Monitoring by Infra-Red", or VoMIR  . Arguably the most sophisticated automated system of the period was the "Autonomous Sciencecraft Experiment System" (later named the "Volcano Sensor Web") developed for the sensors of the EO-1 spacecraft BIB022 BIB029 . This system functioned by processing Hyperion SWIR data; if a potential hot surface was identified, then a trigger was generated to re-point the Hyperion sensor appropriately. The system could also be triggered by other sources, such as MODIS, or on-the-ground observations, which re-tasked the satellite to make follow-up observations within two days of notification BIB022 BIB024 . The effectiveness of this system was demonstrated during the 2010 Eyjafjallajökull eruption, when 50 such observations were made of the erupting volcano over a period of 74 days, ultimately providing data to stakeholders in Iceland to assist in risk and hazard assessments BIB029 . Despite early recognition of their utility in observing active volcanoes (e.g., BIB005 BIB004 ), few detection algorithms were ever developed for SWIR data. This is because of the greater availability of regular MIR and TIR observations and because MIR and TIR observations were usually available at moderate to low spatial resolution. This latter point meant that large volumes of frequent, wide-area coverage data were obtained that required automated processing, as opposed to more localised, and often ad hoc, SWIR observations  . One issue that had to be dealt with from the outset of the discipline was that volcanic surfaces may vary by several-hundred degrees over just a few centimetres BIB009 , and given that 30 m (and often cruder) has been the highest spatial resolution of data available in the infrared, techniques were required to ensure that the data obtained were as accurate as possible. Take a 30-m ASTER or Landsat-8 SWIR pixel for example: the situation on the ground could feasibly match the simulated pixel area of Figure 7 , consisting of two (or more) surfaces of different temperature (and consider the possibilities within the 1000 m × 1000 m pixels of AVHRR or MODIS). As such, while radiance detected within one pixel might give an approximate indication of activity on the ground, such a pixel integrated value also hides much information. Given the varying sensitivity of different bands to surfaces of different temperature, any pixel integrated value detected will also be dependent on the band used to image it. For example, again taking the pixel modelled in Figure 7 , if viewed in ASTER's SWIR Band 4, the radiance temperature detected would be 615 K, as compared with 505 K if imaged with ASTER's Band 9 (see Figure 8 ). This varying sensitivity is further demonstrated by the wider area of detection in Figure 9 where the Bands 4 and 9 signal detections are compared for the same volcanic surface. pixel integrated value also hides much information. Given the varying sensitivity of different bands to surfaces of different temperature, any pixel integrated value detected will also be dependent on the band used to image it. For example, again taking the pixel modelled in Figure 7 , if viewed in ASTER's SWIR Band 4, the radiance temperature detected would be 615 K, as compared with 505 K if imaged with ASTER's Band 9 (see Figure 8 ). This varying sensitivity is further demonstrated by the wider area of detection in Figure 9 where the Bands 4 and 9 signal detections are compared for the same volcanic surface. pixel integrated value also hides much information. Given the varying sensitivity of different bands to surfaces of different temperature, any pixel integrated value detected will also be dependent on the band used to image it. For example, again taking the pixel modelled in Figure 7 , if viewed in ASTER's SWIR Band 4, the radiance temperature detected would be 615 K, as compared with 505 K if imaged with ASTER's Band 9 (see Figure 8 ). This varying sensitivity is further demonstrated by the wider area of detection in Figure 9 where the Bands 4 and 9 signal detections are compared for the same volcanic surface. The difference in band sensitivity can be harnessed for volcano monitoring, and this was first documented in BIB004 , using a technique developed in ground breaking studies from 1981: BIB001  . These studies had recognised the distinction between those scenarios in which a whole pixel's field of view was filled by a thermal anomaly and those in which only a portion of the field of view was thermally anomalous. In the former case, the pixel would measure the full radiance from the surface (e.g., a volcano), and the average temperature of the surface could be calculated; in the latter case, where the anomalous surface consisted of only a portion of the area, the radiance measured would be reduced by this portion, and this would be the case for all observing wavelengths. If the ratio of two pixel values was then taken, it would remain the same in both cases, effectively peak picking the radiance received by the pixel and allowing for an estimation of the sub-pixel two-component thermal structure. The difference in band sensitivity can be harnessed for volcano monitoring, and this was first documented in BIB004 , using a technique developed in ground breaking studies from 1981: BIB001  . These studies had recognised the distinction between those scenarios in which a whole pixel's field of view was filled by a thermal anomaly and those in which only a portion of the field of view was thermally anomalous. In the former case, the pixel would measure the full radiance from the surface (e.g., a volcano), and the average temperature of the surface could be calculated; in the latter case, where the anomalous surface consisted of only a portion of the area, the radiance measured would be reduced by this portion, and this would be the case for all observing wavelengths. If the ratio of two pixel values was then taken, it would remain the same in both cases, effectively peak picking the radiance received by the pixel and allowing for an estimation of the sub-pixel two-component thermal structure. To characterise a pixel area into two components, non-linear, simultaneous equations, termed the 'dual-band' equations, can be used: and: where Rx and Ry = radiance detected in bands x and y (W m −2 sr −1 m −1 ), adjusted for atmospheric effects and surface emissivity; Ph = proportion of the pixel occupied by the hotspot; Lx(Th) and Ly(Tc) = spectral radiance (L, W m −2 sr −1 m −1 ) emitted in a particular band by a surface at temperature Th (the hot component) or Tc (the cooling component), as determined by the Planck function (Equation (1)). As the 'dual-band' method relies on two equations and as there are three unknowns (Th, Tc and Ph), it requires the assumption of either Th, Tc or Ph. Volcanologically, Th may relate to the temperature of recently erupted lava or an exposed lava lake, while Tc may relate to the temperature of the surrounding surface unaffected by volcanic processes; both of these can be estimated; Ph, in contrast, will usually be impossible to estimate accurately and has to be derived. This assumption of two discrete temperature regions will only be appropriate in some situations and in most cases, will only To characterise a pixel area into two components, non-linear, simultaneous equations, termed the 'dual-band' equations, can be used: and: where R x and R y = radiance detected in bands x and y (W m −2 sr −1 m −1 ), adjusted for atmospheric effects and surface emissivity; P h = proportion of the pixel occupied by the hotspot; L x (T h ) and L y (T c ) = spectral radiance (L, W m −2 sr −1 m −1 ) emitted in a particular band by a surface at temperature T h (the hot component) or T c (the cooling component), as determined by the Planck function (Equation (1)). As the 'dual-band' method relies on two equations and as there are three unknowns (T h , T c and P h ), it requires the assumption of either T h , T c or P h . Volcanologically, T h may relate to the temperature of recently erupted lava or an exposed lava lake, while T c may relate to the temperature of the surrounding surface unaffected by volcanic processes; both of these can be estimated; P h , in contrast, will usually be impossible to estimate accurately and has to be derived. This assumption of two discrete temperature regions will only be appropriate in some situations and in most cases, will only approximate reality. Furthermore, the technique requires the pixels from which the data are extracted to be completely aligned, therefore precluding use of the technique with data from different sensors. Determining the temperature and corresponding areas of thermal activity at the surface is useful, but what is arguably more useful from a quantitative point of view is the overall radiant power (i.e., energy) emission. The advantage of an overall power value per pixel is that this value can be summed for all imaging pixels to derive an overall power emission value for the volcanic surface, thus providing a means of monitoring volcanic activity over time and of comparing the activity at different volcanoes. Glaze et al. BIB006 was one of the first pieces of research to undertake such analysis, based on dual-band equation retrievals, thus setting a standard procedure that was to be reproduced in subsequent studies (e.g., BIB017 BIB009 BIB008 BIB012 ). The radiant power emission, or heat-loss, for an assumed two-component pixel (such as that in Figure 7 ) can be calculated using the Stefan-Boltzmann law and by calculating the contribution of each component to the overall radiant heat-flux of the pixel: where Q = radiant power emission (W), σ = Stefan-Boltzmann constant (5.67 × 10 −8 W m −2 K −4 ), ε = surface emissivity (unit-less) and A = pixel surface area (m 2 ). As multi-spectral infrared imaging capabilities became more widespread, and with higher spatial resolution, the possibility of viewing the surface in more detail, while also allowing for the differentiation of different features within the same surface, became more realistic. Wooster and Rothery BIB011 showed this by comparing detailed JERS OPS observations of Lascar Volcano, Chile, with those that a corresponding 1000 m × 1000 m ATSR pixel would observe. Similar enhancements in observational detail were also demonstrated by BIB017 in which it was shown that features, such as Hawaiian lava-flow braiding, could be discerned using multispectral and high resolution Landsat ETM+ data. Experimentations were conducted into using more than two bands of data to better characterize a volcanic surface, with BIB018 , for example, showing that in relation to an active lava flow, a continuum of temperatures is displayed and that as such, a lava-flow surface would be better characterised with 5-7 components. Despite this, and because its outputs are more accurate than assuming one uniform temperature, use of the dual-band technique has persisted. Blackett and Wooster BIB025 also showed that despite the availability of multiple bands of data, such as the six-SWIR bands of ASTER, the settings of such bands were often sub-optimal for volcanic observational purposes. In the early 2000s, two data sources remained unexploited: those from the NASA EO-1 satellite's ALI and Hyperion sensors. Their impressive complement of bands meant that these sensors had the potential to discern numerous sub-pixel surfaces for fine and detailed surface characterisations. Wright et al. BIB026 , for example, later demonstrated the utility of such hyper-spectral imagery by using Hyperion data to distinguish various expressions of thermal activity, such as lava lakes, flows and fountains and, indeed, even different lava types, at various volcanoes globally. As time progressed, long datasets of remotely-sensed infrared imagery had accumulated, extending back to early AVHRR and Landsat observations and coming forward to the NASA-EOS sensors. This provided the possibility of determining the "average", or baseline radiant emissions, for a particular volcano, and of identifying potential periodicity in these signals and/or chronologies of particular eruptive events. Pergola et al. BIB020 harnessed the long AVHRR time series for the purpose determining an average infrared signal, both spatially and temporally in volcanic regions, against which volcanic thermal anomalies could be compared and/or identified. The algorithm used to calculate this average was called the Robust AVHRR Technique, and while this technique has been criticised BIB027 , it was later harnessed by BIB028 , but this time with MODIS data, to develop an even more sensitive hot-spot detection algorithm. Wright et al.  analysed the entire MODVOLC database, identifying different eruption types and cyclical activity at various volcanoes and calculating the overall heat-flux from all volcanoes active during the operational period of the system. Chronologies of specific eruptive episodes have also been presented, from BIB021 , which used various sensors to derive a 2003 eruption chronology of Anatahan, Indonesia, to BIB030 BIB023  , each of which document eruption chronologies using infrared data for volcanoes in the North Pacific. Since the end of the last decade, the number of new publications in the discipline has been declining, although the discipline has certainly not stopped making progress. Coppola et al.  BIB032 , for example, document and validate a new and more sensitive volcanic activity and real-time detection algorithm called MIROVA which uses MODIS MIR data. Additionally, BIB033 used MODIS TIR data to identify distinct phases of lava dome growth and collapse at Merapi Volcano, Indonesia, and to determine the varied volcanic causes of thermal anomalies detected. Now is perhaps a period of reflection as scientists review data from the past (e.g.,  BIB034 ) and offer suggestions for the design of future infrared imaging satellites (e.g., BIB031 ).