National Weather Service United States Department of Commerce

Satellite perspectives of an Aleutian Chain volcanic eruption:

A volcanic eruption occurred on December 21, 2016 on Bogoslof Island, 60 miles northwest of Unalaska. The first image is a high-resolution infrared image taken from the AVHRR instrument on the NOAA-19 polar-orbiting satellite.  The bright pink/purple on the image indicates temperatures of the top of the vapor plume; in the -50C to -60C range. 


The next loop again shows the vapor plume in infrared, this time from the Himiwari-8 geostationary satellite.  A plot of lightning strikes from a ground-based detection system is shown near the eruption. You may notice in this image, the plume is not directly over the volcano.  The  gap between Bogoslof Island and where the satellite observes the plume is due to a viewing angle problem called parallax.  Parallax is defined as “the effect whereby the position or direction of an object appears to differ when viewed from different positions, e.g., through the viewfinder and the lens of a camera.”



The issue of parallax can be illustrated here, the previous image has been re-mapped to view as if looking overhead. In reality the viewing angle is much more extreme. Here is what the satellite is actually “seeing;" it is centered over the equator at 140.7 E longitude. The volcanic plume is very near the edge of what the satellite can discern due to the curvature of the earth.

A zoomed-in loop of the visible channel from Himiwari (credit: RAMMB/CIRA): 


The next loop is a view from GOES-West. Again, lightning strikes are plotted (showing the general eruption location) on an infrared image.  The plume is much closer to reality in this image, however, because GOES-W orbits over the equator, the plume is depicted further northward than the actual position. The issue of parallax affects us here in Alaska on a regular basis due to our high latitude, because geostationary satellites are centered over the equator.