The longitudinal dependence of whistler and chorus characteristics observed on the ground near L = 4

Abstract

Whistler activity at L ≃ 4 is known to be a function of longitude, peaking in the Weddell Sea sector of Antarctica; a combination of source and propagation factors, the latter possibly partly associated with the South Atlantic geomagnetic anomaly, is believed to be responsible. There is evidence, for example from satellite surveys, that chorus and hiss activity may also be longitude dependent. To investigate this further, we have compared VLF data from four L ≃ 4 Antarctic stations from a 2‐day period in June 1982. Siple, Halley, and Sanae form a closely spaced (∼20°–0° geomagnetic longitude) triplet, while Kerguelen is ∼120° (geomagnetic) to the east, on the opposite side of the anomaly. To a large extent there was a repeatable diurnal variation in activity at all stations on the two days. Events observed at Siple tended to be similar to those observed ∼9 hours earlier (the same MLT) at Kerguelen on the same day. There was a very marked drop‐off in both whistler and VLF emission activity between Siple and Halley on the one hand and Sanae on the other. The reason for this is not clear; it may be either a source effect such as the lower occurrence of lightning over eastern North America compared to the adjacent Atlantic Ocean, or else a wave‐particle interaction effect whereby the conditions for wave growth or amplification are more favorable, or substorm particle injections penetrate the magnetosphere more deeply, at the longitude of Siple than further east. Comparison of the spectral forms of whistler mode activity at neighboring stations suggests that wave generation occurs simultaneously over relatively wide longitude (or local time) sectors (≳30° or 2 hours). Individual interaction regions are smaller than this, ≲5° in longitude, comparable with the previously inferred sizes of whistler ducts. Data sets with longer time coverage and better spatial resolution are required to answer some of the problems raised by this limited study.