Contribution of Lightning Whistlers to the Plasmaspheric Hiss Spectrum

James L. Green, Shing F. Fung, Scott Boardsen, and Leonard N. Garcia,


   The classic theoretical work by Kennel and Petschek, [1966] held that, under
certain circumstances, whistler mode waves increase in energy from a
gyroresonance interaction with radiation belt electrons causing the electrons to
change pitch angle and precipitate (pitch angle scattering). Lyons et al. [1972]
and Abel and Thorne [1998a,b] showed that plasmaspheric hiss would be the
dominant whistler mode wave responsible for this scattering thereby maintaining
the electron slot region between the inner and outer electron belts. Therefore,
understanding the origin of hiss is of fundamental importance in understanding
the distribution and dynamics of the electron radiation belts.
   The origin of plasmaspheric hiss is still somewhat controversial as either
generated by a gyroresonance process [see for example: Thorne et al., 1973;
Huang et al., 1983; Church and Thorne, 1983] or by lightning [Sonwalker and
Inan, 1989; Draganov et al., 1992] or both but the relative contribution from
these two sources is unknown even though the literature in this field is
extensive. Thorne et al. [1979] suggested that plasmaspheric hiss would only
grow in intensity from the background thermal noise to its observed intensity
from gyroresonance acceleration as the whistler mode wave returned through the
equator repeatedly. Based on limited data, Solomon et al. [1988] have shown that
amplification of background noise to observed hiss intensities is possible. In
addition, from ray tracing calculations of magnetospherically reflected
whistlers, Thorne and Horne [1994] concluded that lightning generated whistlers
could not be the source of plasmaspheric hiss because they are subject to
significant damping due to Landau resonant interactions with suprathermal
electrons with energies greater than about 100 eV.
   Using observations from the low frequency linear wave receiver on DE, Sonwalker
and Inan [1989] have shown that lightning-generated whistlers often triggers
plasmaspheric hiss.  These in situ observations were the first to demonstrate
that lightning could be the original source of plasmaspheric hiss.  Ray tracing
calculations by Draganov et al. [1992] demonstrated that the refraction of the
plasmasphere on lightning whistlers (higher frequencies waves move to higher L
shells) produced a natural way to obtain a hiss like spectrum on lower L shells.
In addition, Draganov et al. [1992] determined that the total wave energy from
lightning whistlers may maintain the experimentally observed levels of
plasmaspheric hiss.
   The long-standing issues surrounding the origin of plasmaspheric hiss,
especially with regard to lightning whistlers, and their contributions to
slot-region electron losses are explored in this analysis by 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 suggests that lightning may be the dominant source for
plasmaspheric hiss, which has been regarded as the primary agent for creating
the slot region in the radiation belts.

References:
Abel, B., and R. M. Thorne, Electron scattering loss in EarthÕs inner
magnetosphere: 1. Dominant physical processes, J. Geophys. Res., 103, 2385-2396,
1998a.

Abel, B., and R. M. Thorne, Electron scattering loss in EarthÕs inner
magnetosphere: 2. Sensitivity to model parameters, J. Geophys. Res., 103,
2397-2407, 1998b.

Church, S. R., and Thorne, R. M., On the origin of plasmaspheric hiss: Ray path
integrated amplification, J. Geophys. Res., 88,7941-7957, 1983.

Draganov, A. B., U. S. Inan, V. Sonwalkar, and T. F. Bell, Magnetospherically
reflected whistlers as a source of plasmaspheric hiss, Geophys. Res. Lett., 19,
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Huang, C. Y., C. K. Goertz, and R. R. Anderson, A theoretical study of
plasmaspheric hiss generation, J. Geophys. Res., 88, 7927-7940, 1983.

Kennel, C. F., and H. E. Petschek, Limit on stably trapped particle fluxes, J.
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Lyons, L. R., R. M. Thorne, and C. F. Kennel, Pitch-angle diffusion of radiation
belt electrons within the plasmasphere, J. Geophys. Res., 77, 3455-3474, 1972.

Solomon, J., N. Cornilleau-Wehrlin, A. Korth, and G. Kremser, An experimental
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Sonwalker, V. S., and U. S. Inan, Lightning as an embryonic source of VLF hiss,
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Thorne, R. M., E. J. Smith, R. K. Burton, and R. E. Holzer, Plasmaspheric hiss,
J. Geophys. Res., 78, 1581-1595, 1973.

Thorne, R. M., and R. B. Horne, Landau damping of magnetospherically reflected
whistlers, J. Geophys. Res., 99, 17249-17258, 1994.

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2nd Kanazawa Workshop on Waves in Plasmas and Electromagnetic Applications, 
Kanazawa University, Kanazawa, Japan, 23-24 March 2006