Modeling of the Atmosphere Magnetosphere Ionosphere System (MAMI)

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Isolating the Aurora in a DE-1 image

Images that are obtained by the scanning imager on the DE-1 satellite do not only show aurora, but also all other emissions that originate from the atmosphere. Depending on the seleted wavelength, the scattered solar light may obscure part of the aurora. The analysis that we apply to obtain energy and energy flux of the auroral precipitating particles can only use auroral light emissions. We thus need to seperate the emissions of the aurora from the scattered sunlight.

raw image
The ability of the atmosphere to scatter sunlight is wavelength dependent. One bright auroral feature, the first negative emission from the nitrogen ion, is also very bright from scattered sunlight. This image was taken with a filter to pass only the first negative emission at 391.4 nm wavelength. The dayside atmosphere is so bright, that it was blocked out to protect the detector in the instrument. This is the black area. The aurora is visible as a section of a circular band around the magnetic north pole. But even in the dark nightside, sunlight scattered at high altitudes still obscures much of the aurora.

The first step in the image analysis is to find only those pixels of the image that actually originate in the earth's atmosphere. We eliminate all pixels where the instrument was looking tangentially through the upper atmosphere along the limb. Only those parts of the image remain where the instrument looked down to the earth.
usable pixels

terminator cut-off
Then we calculate the position of the terminator at a given altitude in the atmosphere. In this case we set the terminator at a solar depression angle of 17 degrees, which relates to an altitude of the shadowheight of about 300 km. Portions of the image on the sunward side of that line are discarded. This procedure eliminates most of the sunlit part of the image, but there is still scattered sunlight in the same place where we see the aurora.

The solar contribution to the brightness in this area of the image is more or less proportional to the amount of illuminated atmosphere. This can be determined from the shadowheight or the solar depression angle. We divide the image into strips that contain only pixels of almost equal solar depression angle, find the average brighness of these strips, and substract this average from that area of the image. Working our way through the entire image, we thus generate a "clean" image of the aurora.
This final image is then used in the analysis with the AURORA model to obtain ionospheric parameters from the auroral brightness.
cleaned image

This sequence of images and the successive proccessing can also be viewed in form of an animation. If you have a QuickTime display tool (e.g. xanim) you can use that to view the animation. Special thanks to Bill who put the QT animation together.

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Last updated: 2 Feb 96
Questions? Comments? Please send E-mail to lumm@gi.alaska.edu

	The ability of the atmosphere to scatter sunlight is wavelength dependent. One bright auroral feature, the first negative emission from the nitrogen ion, is also very bright from scattered sunlight. This image was taken with a filter to pass only the first negative emission at 391.4 nm wavelength. The dayside atmosphere is so bright, that it was blocked out to protect the detector in the instrument. This is the black area. The aurora is visible as a section of a circular band around the magnetic north pole. But even in the dark nightside, sunlight scattered at high altitudes still obscures much of the aurora.
The first step in the image analysis is to find only those pixels of the image that actually originate in the earth's atmosphere. We eliminate all pixels where the instrument was looking tangentially through the upper atmosphere along the limb. Only those parts of the image remain where the instrument looked down to the earth.
	Then we calculate the position of the terminator at a given altitude in the atmosphere. In this case we set the terminator at a solar depression angle of 17 degrees, which relates to an altitude of the shadowheight of about 300 km. Portions of the image on the sunward side of that line are discarded. This procedure eliminates most of the sunlit part of the image, but there is still scattered sunlight in the same place where we see the aurora.
The solar contribution to the brightness in this area of the image is more or less proportional to the amount of illuminated atmosphere. This can be determined from the shadowheight or the solar depression angle. We divide the image into strips that contain only pixels of almost equal solar depression angle, find the average brighness of these strips, and substract this average from that area of the image. Working our way through the entire image, we thus generate a "clean" image of the aurora. This final image is then used in the analysis with the AURORA model to obtain ionospheric parameters from the auroral brightness.