Modeling of the Atmosphere Magnetosphere Ionosphere System (MAMI)

Removing the Solar Contribution from an Auroral Image

This example demonstrates the process which we apply to isolate the auroral brightness from the scattered sunlight.

The image on the top left shows the raw image from UVI using the LBH-long filter. The entire auroral zone in in bright sunlight. The sun is illuminating the earth from the top right side, the terminator on the ground (solar zenith angle of 90 degrees) is in the left lower corner. Superimposed on the image is a black and white strip which identifies those image pixels that have a common solar zenith angle (70 to 72 degrees). The white part of the strip identifies pixels that also have a geomagnetic latitude between 62 and 77 degrees, the approximate position of the auroral zone. We determine this strip of common solar zenith angle for all angles, as shown in the animation (440 kB) of this image.

The line plot at the bottom shows the count rates of all pixels in this strip as a function of position along the strip. The title indicates the solar zenith angle that is currently displayed. The red line shows the level of the solar contribution to the image brightness, which we determined from the average of all pixels in each strip excluding those which are in the auroral zone. One can see how the scattered sunlight decreases with solar zenith angle by the motion of the red line downwards in the course of the animation.

The brightness of the pixels above the red line is due to the aurora. Subtracting the scattered sunlight contribution individually for each solar zenith angle strip, we are left with an image that includes only the aurora. This is shown in the upper right panel.

This particular example is taken from an auroral substorm on May 19, 1996 (20:30 UT), which was of course not visible from the ground, since it was obscured by bright sunlight everywhere.