Green, J. L., S. A. Boardsen, and S. F. Fung, Angular distribution of
non-thermal continuum radiation, presented at the American Geophysical
Union Fall Meeting, San Francisco, December 15-19, 1996.

The Angular Distribution of the Non-Thermal Continuum Radiation

James L. Green, NASA GSFC, Scott Boardsen, HSTX, Shing Fung, NASA GSFC

   Four years of plasma wave observations at 31.1 kHz from the 
VLF electric field instrument on the Hawkeye spacecraft revealed the 
latitudinal distribution of the Earthıs non-thermal continuum 
radiation using a wave intensity map technique.  The wave intensity 
map clearly identified the source region and propagation extent of the
non-thermal continuum. The source region is confirmed to be near 
dawn just outside the plasmasphere. The Hawkeye observations 
clearly show that the non-thermal continuum radiation is primarily 
observed at relatively low latitudes and is beamed outward from the 
dawn source region.
   The observed angular distribution of the non-thermal 
continuum radiation is modeled by ray tracing calculations in a 
realistic model magnetosphere. The ray tracing calculations at 31 
kHz showed both trapped and escaping components.  Rays generated 
in the sunward direction from a dawn source and within less than 
10o of the equator reflect many times between the magnetopause and 
plasmapause. These rays may undergo more than 20 reflections.  
Rays generated at higher latitudes either pass directly through the 
magnetopause or are reflected at the magnetopause and are 
preferentially beamed tailward due to the curvature of the 
magnetopause and escape from the magnetosphere.  In order for rays 
to pass through the magnetosphere the local plasma frequency at the 
magnetopause must be lower than the ray frequency. 
	These results clearly show that the non-thermal continuum 
radiation has a distinct angular distribution with the source at low 
latitudes on the morning side just outside the plasmasphere, and 
that trapped and escaping components can exist simultaneously at 
the same frequency. In addition, the observations and the ray tracing 
calculations show that the trapped continuum radiation component 
does not completely fill the magnetospheric cavity as some have 
previously believed but is confined to mostly the equatorial plane.