Source Region Characteristics of Non-thermal Continuum Radiation

J. L. Green, S. A. Boardsen, L. Garcia, S. Fung, P.M.E. Decreau, S. Grimald, 
K. Hashimoto, H. Matsumoto, and B. W. Reinisch 


Non-thermal continuum radiation (NTC) is a fundamental inner magnetospheric
radio emission observed in every magnetosphere in the solar system. For the
Earth, there are three main types of NTC that are distinguished by their
frequency range and source location. The normal continuum radiation (also
referred to as the trapped and escaping continuum) is typically in the 5 to 100
kHz frequency range generated at the plasmapause in the morning sector. The
continuum enhancement is observed from 10-100 kHz coming from night-side
plasmapause source regions. Kilometric continuum is also generated at the
plasmapause, near the magnetic equator, deep in notch structures of the
plasmasphere over a frequency range from 100 to 800 kHz. Observations of NTC
from IMAGE, Cluster, and Geotail are revealing important new characteristics in
and near the source region that will place additional constraints on any
generation theory. NTC is believed to be generated by the Z to O-mode conversion
process at the Òradio windowÓ. These radiation patterns often take on a
'Christmas tree' like appearance in frequency-time spectrograms, with the lower
frequency components more spread out in magnetic latitude about the equator than
that of the higher frequency components. Some of these patterns have a two-cone
structure with the radiation intensity diminishing around the magnetic equator
consistent with off- equatorial beaming from the source, while others do not
show a one-cone structure about the equator. One possible explanation of this
radiation pattern is a very sharp plasmapause coupled with an off-equatorial
beaming angle that is dependent upon the ratio of fce/fpe as predicted by many
theories. Some of these patterns appear to be qualitatively consistent with
theories like linear mode conversion that predict off equatorial beaming, while
other patterns seem to be inconsistent.

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Radio Science Symposium for a Sustainable Humanosphere, Kyoto University, 
Kyoto, Japan, 20-21 March 2006