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Volcanoes are often remote, and have footprints that may extend across many hundreds or thousands of square kilometres. They are generally inaccessible during eruption, and may continue to be inaccessible for extended periods of time after eruption, while their products can be scattered or dispersed over regional or global scales. Consequently, since direct measurement can only provide us with part of the picture of many volcanic processes, remote sensing is now playing an increasingly important role in advancing understanding of the science underlying volcanic behaviour, on this planet and beyond (e.g. Mouginis-Mark et al. 2000; Sparks et al. 2012). Satellite, airborne and ground-based remote sensing are increasingly vital tools for monitoring active or potentially active volcanoes, and assessing their likely, real-time or time-averaged impact (Fig. 1). At the same time, the synoptic-scale surveys that are often well suited to remote-sensing techniques allow us to address questions about the fundamental processes that control volcano behaviour in a way that is not necessarily possible from individual case studies. New research is often driven by technological advancements in the development of novel sensors or launching of new platforms, meaning that the space agencies are increasingly involved in identifying scientific questions and priorities (e.g. Ferruci et al. 2012). Multiple and complementary data streams are increasingly being used both to monitor active volcanoes and advance volcanological science. The key geophysical parameters that comprise monitoring data streams typically include the categories of (i) seismicity, (ii) surface deformation, (iii) thermal measurements and (iv) gas flux and composition data as major components. While measurements of seismicity remain the domain of ground-based seismometers, remote-sensing techniques have made major contributions in each of the others. This Special Publication volume is concerned with the use of remote sensing at volcanoes. It is split into three parts, roughly arranged from the subsurface, to the surface, and then further afield as volcanic products are injected and dispersed into the atmosphere. The papers span a range of applications of remote-sensing techniques to monitor and understand (a) surface deformation, (b) surface thermal anomalies and (c) gas fluxes, as well as tracking ash and gas plumes from eruptions to gain insights into the extent of a volcano’s impacts. Volcanology is driven, in part, by the operational concerns surrounding volcano monitoring and hazard and crisis management but the goal of volcanological science is, at its heart, to understand the processes that underlie volcanic activity. This Special Publication is also concerned with how we go from observations to this deeper understanding, including the progress that can be made by integrating observations and modelling. While this volume focuses mainly on satellite-based remote sensing, integrating datasets from different platforms is also of vital importance, and so papers on airborne remote sensing and measurement from both manned and unmanned aircraft are also included.

Measuring volcano deformation is key to understanding the behaviour of erupting vol- canoes and detecting those in periods of unrest. Satellite techniques provide the opportunity to do so on a global scale but, with some notable exceptions, the deformation of volcanoes in the tropics has been understudied relative to those at higher latitudes, largely due to technical difficulties in applying Interferometric Synthetic Aperture Radar (InSAR). We perform a systematic survey of the Central American Volcanic Arc to investigate the appli- cability of Interferometric Synthetic Aperture Radar (InSAR) to volcanoes in the tropics. Volcano characteristics that may prevent InSAR measurement include: (1) dense vegetation cover; (2) per- sistent activity; and (3) steep slopes. Measurements of deformation are further inhibited by atmos- pheric artefacts associated with: (4) large changes in topographical relief. We present a systematic method for distinguishing between water vapour artefacts and true deformation. Our data show a linear relationship (c .2 cm /km) between the magnitudes of water vapour artefacts and volcano edifice height. For high relief volcanoes (e.g. Fuego, Guatemala, 3763 m a.s.l. (above sea level)) errors are of the order of 4-5 cm across the volcano's edifice but are less than 2 cm for lower relief (e.g. Masaya, Nicaragua, 635 m a.s.l.). Examples such as Arenal, Atitlan and Fuego illustrate that satellite acquisition strategies incorporating ascending and descending tracks are particularly important for studying steep-sided volcanoes. Poor coherence is primarily associated with temporal decorrelation, which is typically more rapid in southern Central America where Evergreen broadleaf vegetation dominates. Land-use classification is a better predictorof decorrelation rate thanvegetationindex.Comparisonof coher- ence for different radar wavelengths match expectations; high resolution X-band radar is best suited to local studies where high-resolution digital elevation models (DEMs) exist, while L-bandwavelengths are necessary for regional surveys.However, this is the first time that relation- ships between phase coherence and time, perpendicular baseline, radar wavelength, and land use have been quantified on the scale of a whole volcanic arc.

The Ozone Monitoring Instrument (OMI) is a satellite-based ultraviolet (UV) spectrometer with unprecedented sensitivity to atmospheric sulphur dioxide (SO2) concentrations. Since late 2004, OMI has provided a high-quality SO2 dataset with near-continuous daily global coverage. In this review, we discuss the principal applications of this dataset to volcano monitoring: (1) the detection and tracking of large eruption clouds, primarily for aviation hazard mitigation; and (2) the use of OMI data for long-term monitoring of volcanic degassing. This latter application is relatively novel, and despite showing some promise, requires further study into a number of key uncertainties. We discuss these uncertainties, and illustrate their potential impact on volcano monitoring with OMI through four new case studies. We also discuss potential future avenues of research using OMI data, with a particular emphasis on the need for greater integration between various monitoring strategies, instruments and datasets.