Comment on “On the origin of whistler mode radiation in the plasmasphere” by Green et al.

Abstract

[1] Green et al. [2005] have recently performed a statistical analysis of the global distribution of whistler-mode radiation in the plasmasphere. They use the results to draw inferences on the origin of different emissions and the potential impact of such waves on scattering loss of resonant electrons from the radiation belts. Specifically, they present evidence that the average intensity of waves at frequencies near 3.0 kHz exhibits a spatial distribution in both MLT and geographical longitude similar to the distribution of lightning. Green et al. [2005] claim that the properties of such waves are representative of plasmaspheric hiss, an important magnetospheric emission, which has previously been shown to be a major scattering agent in the slot region between the inner and outer radiation belts [Lyons and Thorne, 1973; Abel and Thorne, 1998]. We disagree with both the interpretation made by Green et al. [2005] of the nature of the reported waves and their conclusion that lightning is ultimately responsible for radiation belt loss in the slot region between the inner and outer belts. Because their work has been made public via a NASA press release and reported in popular science news articles, we feel it is important to voice our strong disagreement with their conclusions, which give an erroneous impression about the role of lightning in the loss of radiation belt electrons. [2] Plasmaspheric hiss is a broadband whistler-mode emission, primarily confined within the high-density plasmasphere, with peak power spectral intensity near a few hundred Hz [Thorne et al.,1973]. The intensity of plasmaspheric hiss falls rapidly above 1 kHz, and the waves are rarely seen above 3 kHz. Plasmaspheric hiss is easily identified on high time resolution spectrograms and clearly distinguishable from more discrete signals originating from lightning, which can occur over a similar frequency range, but which generally extend to higher frequency. Lightningassociated emissions that enter the magnetosphere are strongest near a few kHz [Edgar, 1976] and become very weak below 1 kHz, where plasmaspheric hiss emissions have their peak spectral intensity. A second distinguishing characteristic is the pronounced dependence of the intensity of plasmaspheric hiss on both the flux of resonant electrons [Cornilleau-Wehrlin et al., 1985], and on the level of geomagnetic activity [Smith et al., 1974; Thorne et al., 1976; Meredith et al., 2004]. Discrete whistlers originating from lightning, and magnetospherically reflected whistlers that merge into a broadband distribution following many internal reflections [Bortnik et al., 2003], show little dependence on geomagnetic activity and their power spectral intensity is usually much weaker than hiss [e.g., RisticDjurovic et al., 1998]. [3] Careful examination of the average latitudinal distribution of dayside 1.2 kHz waves in Figure 2 of Green et al. [2005] reveals two distinct populations; one peaked at high latitude (which may best be interpreted as lightning-generated emissions propagating away from the atmosphere) and a second population distributed around the equator, which is remarkably similar to the modeled distribution of obliquely propagating magnetospherically reflected (MR) whistlers [Thorne and Horne, 1994], which also originate from lightning and tend to settle, after several magnetospheric reflections, around a field line where the wave frequency is comparable to the equatorial lower hybrid frequency [Bortnik et al., 2003]. As a consequence, MR whistlers occur in a band, which moves to higher frequency at lower L [Bortnik et al., 2003], while plasmaspheric hiss tends to occur in a broad band which is relatively independent of L. Green et al. [2005] identify both reported populations of 1.2 kHz waves as plasmaspheric hiss, but we assert that they are simply the direct result of lightning emissions. Although the waves have an MLT morphology similar to the distribution of plasmaspheric hiss [Meredith et al., 2004], the average power spectral intensity reported by Green et al. [2005] is more than an order of magnitude smaller than the intensity of lower-frequency plasmaspheric hiss, under moderately active geomagnetic conditions, reported by Meredith et al. [2004]. Because the rate of electron pitchangle scattering is proportional to the power of resonant waves [Lyons et al., 1972], the waves reported by Green et al. [2005] are relatively insignificant in causing electron loss from the outer plasmasphere, where scattering by the more intense lower frequency (300–700 Hz) plasmaspheric hiss is dominant [Abel and Thorne, 1998]. Nevertheless, these weaker lightning-generated emissions could still contribute to electron loss in the inner portion of the plasmasphere, where hiss becomes weaker [Thorne et al., 1973], and where resonance with energetic electrons becomes less effective [Abel and Thorne, 1998]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, A09210, doi:10.1029/2005JA011477, 2006 Click Here for Full Article