Global Imaging of Earth's Plasmasphere

Bill R. Sandel

Global imaging of Earth's magnetosphere has been recognized as an
important goal for at least two decades, but has been beyond our
grasp until now. NASA's IMAGE, launched in late March, is the
first mission devoted to investigation of the magnetosphere by
imaging. This satellite is equipped with instruments to study the
distributions of plasmas in the magnetosphere by imaging photons
in the extreme and far ultraviolet, by imaging energetic neutral
atoms, and by imaging using radio sounding.

I will focus on the first-mentioned instrument, the Extreme
Ultraviolet Imager (EUV), which is directed toward study of the
plasmasphere. Earth's plasmasphere forms because low-energy ions,
mainly H+ and He+, that escape the ionosphere are trapped in the
geomagnetic field, where they form a doughnut-shaped cloud of
radius several RE. The outer edge of the plasmasphere is marked by
a sharp density gradient called the plasmapause. The position and
shape of the plasmapause, and other features within the
plasmasphere, depend on geomagnetic activity. EUV measures the
brightness of 30.4-nm sunlight resonantly scattered by He+ in the
plasmasphere.

The challenges in imaging this region include detecting weak
emissions using a camera on a spinning satellite, assuring time
resolution adequate to track spatial features that change on time
scales of tens of minutes, and providing the wide field of view
(nearly 90 degrees) needed to encompass the plasmasphere in a 
single exposure. At the target wavelength of 30.4 nm, transmitting 
optics cannot be used, and traditional reflective optics offer
efficiencies near 1%. In spite of these obstacles, EUV is now
recording the first images of the plasmasphere.

With time resolution of 10 minutes and spatial resolution of 0.6 
degrees, these images and movies made from them reveal a host of 
features, some expected and some new, that show how the 
plasmasphere responds to geomagnetic activity.

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Colloquium at Rice University, 18 October 2000