Volcanic emissions seen from space
During a volcanic eruption, large amounts of ash and gases, especially sulphur dioxide (SO2), are ejected into the atmosphere.
Because eruptions can happen at any time, it’s vital to have an uninterrupted global monitoring system.
Knowing where a plume is helps forecasters at the Volcanic Ash Advisory Centres (VAACs). They issue the volcanic ash advisories – explaining where ash is and how high into the atmosphere it goes – for different areas around the world. The warnings issued by the VAACs are important for air travel, allowing pilots to avoid areas where ash in the atmosphere could be a hazard.
The eruption of Eyjafjallajökull in Iceland in 2010 caused severe disruption to air traffic around Europe.
While the impacts of volcanic emissions on aircraft are still not fully understood and remain under investigation, there have been incidents when the engines on aeroplanes have stopped working after travelling through volcanic clouds.
A number of different elements of a volcanic eruption can be seen from space, depending on the instruments and data being used.
The SEVIRI imaging radiometers on Meteosat-9 and 10 have proved very useful for identifying volcanic ash clouds. The instruments each have 12 channels, at visible and infrared wavelengths, and experts are able to construct red-green-blue (RGB) images which clearly show where the ash cloud/plume is.
This RGB composite image shows the ash plume from Grímsvötn in Iceland, and spreading south in May 2011.
Forecasters can use this information, combined with meteorological data, to predict where the ash cloud is likely to go.
The spread of the volcanic ash in the atmosphere is shown in pink in this video capturing the afternoon of 23 May to morning of 24 May 2011.
The Infrared Atmospheric Sounding Interferometer (IASI) instrument on Metop provides data on the concentration of particles within the cloud. This is particularly important for aviators, who need to know the density of the cloud.
Sulphur dioxide (SO2)
SO2 emission can precede an eruption, providing an important clue for vulcanologists and emergency planners needing advance warning.
Sulphur dioxide often, but not always, travels with the ash cloud. The GOME-2 (Global Ozone Monitoring Experiment-2) scanning spectrometer on Metop can monitor SO2 using the specific absorption features of this gas in the ultraviolet region of the electromagnetic spectrum.
Measurements by IASI are also sensitive to SO2. IASI measures a portion of the infrared spectrum emitted by the Earth and atmosphere. Parts of the spectrum are sensitive according to the amount of SO2 in the atmosphere. By measuring the spectrum precisely, good estimates of the SO2 in the atmosphere can be retrieved.
Here are images from the overpass of Metop-B’s IASI sensor seven hours after the start of the eruption of the Indonesian volcano Sangeang Api, in May 2014.
Using simple spectral-line differences (absorption on/off), the presence of SO2 and ash from the eruption can be seen clearly.
This video shows the ash/sulphur dioxide (SO2) cloud emitted from Mount Etna in Sicily on 26 October 2013 (00:00-23:45 UTC) as observed by EUMETSAT’s Meteosat-10 satellite and shown in the RGB Dust Product.
In the RGB Dust product SO2 is represented by the green colour.
The SEVIRI 3.9 micron (near-infrared) channel is very sensitive to the temperatures often found in fires. This can be used to detect fire and lava flow from volcanoes.
It is more complicated when the ash cloud is over the fire as during the day the 3.9 channel is also very sensitive to the small particles of ash.
Combining all the data from Metop and Meteosat Second Generation satellites gives experts a more complete picture of exactly what is happening in atmosphere following a volcanic eruption.
The biggest step forward in volcanic ash forecasting will be when the Meteosat Third Generation (MTG) satellites are launched. In addition to improved imagery every ten minutes, the provision of data from the MTG infrared and ultraviolet/visible instruments will be crucial for volcanic ash modelling.