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, 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 linear or non-linear
Z-to-O mode conversion process at the Òradio windowÓ. New observations of NTC
show that the emission is associated with density ducts in the plasmapause
region. Both the plasmapause and density ducts concentrate Z-mode waves thereby
enhancing the Z-to-O mode conversion for the generation of NTC. The Radio Plasma
Imager (RPI) on the IMAGE spacecraft frequently detects radiation patterns of
NTC as it crosses the magnetic equator. 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 hollowed
out appearance with the radiation intensity diminishing around the magnetic
equator consistent with off- equatorial beaming from the source, while others do
not show such a diminishing in intensity around 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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Global Aspects of Magnetosphere-Ionosphere Coupling, 2006 Yosemite Workshop, 
Yosemite National Park, CA, USA, 7-10 February 2006