Ground observations of chorus following geomagnetic storms

Details Ground observations of chorus following geomagnetic storms Ground observations of chorus following geomagnetic storms

Feb 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004jgra..10902205s&link_type=abstract

Journal of Geophysical Research, Volume 109, Issue A2, CiteID A02205

Physics

10

Magnetospheric Physics: Storms And Substorms, Magnetospheric Physics: Plasma Waves And Instabilities, Magnetospheric Physics: Energetic Particles, Trapped, Magnetospheric Physics: Energetic Particles, Precipitating

Scientific paper

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.

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