Whistlers observed by the Cluster spacecraft outside the plasmasphere.

Abstract : Lightning generated whistlers are ubiquitous within the plasmasphere at both highand low altitudes, and these waves propagate efficiently in both ducted and nonductedmodes. On the other hand, in the magnetospheric region outside the plasmasphere,lightning-generated whistlers are commonly observed at low altitudes (up to 6000 km)but only rarely at higher altitudes near the magnetic equatorial plane. The reasons forthe lack of these waves at higher altitudes are not well understood. In the presentpaper we use data from the Wide Band Plasma (WBD) instruments on the four Clusterspacecraft to study the characteristics of lightning-generated whistlers observed on 4 separatedays in 2001 at L shells ranging from L =4toL = 5, magnetic latitudes ranging from20 to 10, and Kp indices ranging from 3 to 6. The propagation paths of thelightning-generated whistlers are determined using a two-dimensional ray-tracing modelto calculate the ray paths and group delays from the lower ionosphere to each of thefour Cluster spacecraft over a range of frequencies (1 kHz < f < 8 kHz). The electrondensity distributions used for the ray-tracing calculations are derived frommeasurements with the Whisper relaxation sounder instrument. Our new resultsindicate that whistlers are observed outside the plasmasphere in the low-density regionsonly in the presence of large-scale irregularities within which the waves are ‘‘ducted.’’This conclusion is sustained by an exhaustive search of whistlers outside theplasmasphere using all the Cluster passes during 2001 and 2002. In all the cases wefound that dispersion characteristics are matched by ray-tracing simulations only if thewhistlers are ducted. In some cases, whistler wave energy injected by an individuallightning discharge appears with significant smearing in time. The new resultspresented in this paper support a possible explanation of why whistlers outside theplasmasphere are rarely observed, based on wave conversion electromagnetic whistlermode to quasi-electrostatic lower hybrid mode (Bell et al., 2004).
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M. Platino, U.S. Inan, T.F. Bell, D.A. Gurnett, J.S. Pickett, et al.. Whistlers observed by the Cluster spacecraft outside the plasmasphere.. Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2005, 110 (A3), pp.A03212. ⟨10.1029/2004JA010730⟩. ⟨hal-00153623⟩



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