Role of Density Depletion Ducts on the Generation of Continuum Radiation

Green, J.L., Boardsen, S.A., Garcia, L.N., Fung, S.F. 


There are three main types of non-thermal continuum (NTC) radiation 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 frequency range
coming from night-side plasmapause source regions. Kilometric continuum is
generated at the plasmapause, near the magnetic equator, deep in notch
structures of the plasmasphere over a frequency range from 100 to 800 kHz. On a
number of occasions it has been found that IMAGE/Radio Plasma Imager (RPI)
observes density depletion ducts near the plasmapause associated with NTC. In
addition, IMAGE/RPI observes density depletion ducts preferentially on the late
morning side consistent with the location of the normal non-thermal continuum
source region. NTC is believed to be generated by the linear or non-linear
Z-to-O mode conversion process at the "radio window". Ray tracing calculations
are used to investigate Z-mode propagation characteristics to the point of
conversion to the escaping O mode within a density depletion duct. Preliminary
results show that the density duct concentrates the Z-mode waves thereby
enhancing the Z-to-O mode conversion for the generation of NTC.

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Fall Meeting, American Geophysical Union, San Francisco, U.S.A., 
5-9 December 2005