Ground observations of chorus following geomagnetic storms

Abstract

It has been suggested that whistler mode chorus waves play a role in acceleration and loss of radiation belt electrons during geomagnetic storms. In this paper we present data from a complete solar cycle (1992–2002) of nearly continuous (>95%) VLF/ELF observations from the VLF/ELF Logger Experiment (VELOX) instrument at Halley station, Antarctica (76°S, 27°W, L = 4.3), to determine whether there is statistical evidence for enhanced whistler mode chorus waves during geomagnetic storms. The data comprise 1 s resolution measurements of ELF/VLF wave power in eight frequency bands from 500 Hz to 10 kHz. The variations in chorus activity during several storms, including the well‐studied Bastille Day event (14 July 2000), show enhanced wave power but are variable from event to event. The average behavior has been found from a superposed epoch analysis using 372 storms with minimum Dst less than −50 nT, including 82 large storms with minimum Dst less than −100 nT. Compared with average prestorm levels, the chorus intensity decreases in the storm main phase but is enhanced in the recovery phase, typically maximizing a day after the storm onset. At 1 kHz the enhancement is independent of storm severity, suggesting a saturation effect, whereas larger storms produce larger wave intensities at higher frequencies in the chorus band (e.g., 3 kHz), which is interpreted as the effect of a chorus source region located on lower L shells than for weaker storms. The storm chorus enhancement maximizes at postdawn local times, leading to a 24 hour recurrence effect. A long‐enduring depression in wave intensities, of 10 days or more, is found near the top of the normal chorus band (∼5 kHz). We suggest that this is due to precipitation from enhanced relativistic particle fluxes affecting the subionospheric propagation of spherics from nearby thunderstorm regions across the L = 2–4 zone.