On the occurrence and spatial extent of electron precipitation induced by oblique nonducted whistler waves

Abstract

Two different 4-hour sequences of subionospheric Very Low Frequency (VLF) signal perturbations are examined to characterize electron precipitation induced by nonducted obliquely propagating whistlers. These lightning-induced electron precipitation (LEP) events are typically associated with cloud-to-ground lightning discharges. The temporal and spatial signatures of LEP events associated with two disparate storms occurring over two 4-hour periods are examined. A distributed set of VLF observing sites, known as the Holographic Array for Ionospheric and Lightning Research (HAIL), captures the full latitudinal extent of the events, providing evidence that 90% of the precipitation occurs over a region 8° ± 1° and 9° ± 1° in latitudinal extent for the two time periods. The measured peak of the precipitation is poleward displaced (6°45′ ± 30′ and 7°45′ ± 30′ for the two case studies) from the causative discharge. Analysis indicates that the onset delay and the duration of precipitation steadily increase with increasing L-value, while the signal recovery time is independent of L-value for the LEP events associated with both storms. The causative lightning discharges associated with the two storms were located at different latitudes. For lightning occurring in the storm at higher latitudes, the associated LEP events are of longer duration and exhibit precipitation in a smaller area displaced less from the causative discharge. The onset delays and event durations increase more rapidly with increasing L-value for events associated with lightning occurring in the storm at higher latitudes. The general spatial and temporal signatures are consistent with those expected for LEP events induced by nonducted whistlers. Copyright 2004 by the American Geophysical Union.