Analysis and Mitigation of Sun Signatures in TWINS Images

The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission has been stereoscopically observing Low Altitude Emissions (LAEs) since June 2008, during the generally low level of geomagnetic activity that has prevailed for the extended solar minimum. The orbits and orientations of the two separate TWINS 1 and TWINS 2 spacecraft are such that the Sun is typically in the field of view of at least one TWINS instrument for several hours every day, and this condition persists for a span of approximately six months for each spacecraft. The flux from the Sun signature is typically two orders of magnitude above the flux from the geophysical LAE signature, and therefore dominates the image. This paper discusses efforts to mitigate this Sun signal by removing affected pixels from TWINS images. To remove the Sun signal from an image, a combination of the satellite's position and instrument pointing information is used to calculate the look-direction of the individual image pixels, and pixels within a specified angular separation from the Sun direction are removed (blacked out). This analysis technique is demonstrated on images obtained from a small geomagnetic storm on 11 October 2008, during which the TWINS 1 and TWINS 2 spacecraft observed LAE signatures during 0900-1100UT, but TWINS 1 images exhibit significant sunlight contamination. The analysis successfully mitigates the Sun signal in TWINS 1 images.