Whistlers observed by the Cluster spacecraft outside the plasmasphere

MANUEL PLATINO; UMRAN INAN; TIMOTHY BELL; DONALD GURNETT; JOLENE PICKETT; PATRICK CANU; PIERRETTE DÉCRÉAU

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 2005 vol. 110 p. 1 - 16

0148-0227

Lightning generated whistlers are ubiquitous within the plasmasphere at both high and low altitudes, and these waves propagate efficiently in both ducted and nonducted modes. 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 for the lack of these waves at higher altitudes are not well understood. In the present paper we use data from the Wide Band Plasma (WBD) instruments on the four Cluster spacecraft to study the characteristics of lightning-generated whistlers observed on 4 separate days in 2001 at L shells ranging from L = 4 to L = 5, magnetic latitudes ranging from -20º to 10º, and Kp indices ranging from 3 to 6. The propagation paths of the lightning-generated whistlers are determined using a two-dimensional ray-tracing model to calculate the ray paths and group delays from the lower ionosphere to each of the four Cluster spacecraft over a range of frequencies (1 kHz < f < 8 kHz). The electron density distributions used for the ray-tracing calculations are derived from measurements with the Whisper relaxation sounder instrument. Our new results indicate that whistlers are observed outside the plasmasphere in the low-density regions only in the presence of large-scale irregularities within which the waves are "ducted". This conclusion is sustained by an exhaustive search of whistlers outside the plasmasphere using all the Cluster passes during 2001 and 2002. In all the cases we found that dispersion characteristics are matched by ray-tracing simulations only if the whistlers are ducted. In some cases, whistler wave energy injected by an individual lightning discharge appears with significant smearing in time. The new results presented in this paper support a possible explanation of why whistlers outside the plasmasphere are rarely observed, based on wave conversion electromagnetic whistler mode to quasi-electrostatic lower hybrid mode (Bell et al., 2004).