Contribution of Lightning Whistlers to the Plasmaspheric Hiss Spectrum

James L. Green, Shing F. Fung, Scott Boardsen, Leonard N. Garcia, Xi Shao1, 
and Robert F. Benson


The long-standing issues surrounding the origin of plasmaspheric hiss,
especially with regard to lightning whistlers, and their contributions to
slot-region electron losses will be explored using observations from the
Dynamics Explorer-1 (DE) and IMAGE spacecraft. DE and IMAGE are polar orbiting
satellites with plasma-wave receivers that measure low-frequency equatorial
electromagnetic emissions (below about 0.3 kHz), plasmaspheric hiss (0.3-3 kHz),
and VLF ground-transmitter radiation (10-50 kHz) within the plasmasphere. Since
whistler-mode waves have Òfield-line likeÓ trajectories, or are completely
field-aligned when traveling in plasmaspheric density ducts, only polar orbiting
missions are able to survey their entire spatial domain and provide a unique
perspective that cannot be obtained from equatorial missions. In this study,
plasmaspheric hiss has been identified as having the same frequency and spatial
distribution as the electromagnetic wave observations over the range of ~ 330 Hz
to 3 KHz within the plasmasphere. Geographic control of plasmaspheric hiss,
along with similar local time and seasonal variations between the distribution
of lightning and hiss within the plasmasphere, are compelling indicators of a
lightning origin. From an exhaustive analysis of wave observations from DE and
IMAGE it is found that, over a significant portion of the wave spectrum, the
distribution of the broad-band hiss intensity is similar to the distribution of
lightning: stronger over continents than over oceans, stronger in the summer
than in the winter, and stronger on the dayside than on the nightside.
Simultaneous observations of lightning by the TRMM satellite and by IMAGE show
that the plasmaspheric whistler-mode spectrum can extend up to frequencies
greater than 50 kHz over regions of lightning. These frequencies far exceed what
has been considered the upper frequency of the hiss spectrum of ~3 kHz. This
investigation supports lightning as the dominant source for plasmaspheric hiss,
which has been regarded as the primary agent for creating the slot region in the
radiation belts.

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URSI National Radio Science Meeting, University of Colorado at Boulder
January 4-7, 2006