The Characteristics and Evolution of Chorus on the Ground and in Space During Geomagnetic Storms

Whistler mode waves, predominantly in the form of ELF/VLF chorus, are known to be generated by electrons injected into the magnetosphere during geomagnetic storms. It has been suggested that this chorus may play a role in accelerating electrons to MeV energies. Ground observations provide a complementary perspective to those from satellite-borne receivers. This paper uses a decade (1992-2001) of nearly continuous (>95%) VLF/ELF observations from the VELOX instrument at Halley station, Antarctica (76S 27W, L=4.3). These comprise 1s resolution measurements of ELF/VLF wave power, arrival azimuth, polarisation and peak-mean-minimum ratios, in eight frequency bands from 500 Hz to 10 kHz. To provide a background reference, the mean wave power and standard deviation have been computed for each frequency, hour of local time, and month of the year. The variations in chorus activity during several storms, including the well-studied Bastille Day (14 July 2000) and 31 March 2001, have been compared with these reference levels. Chorus is often observed lasting many hours, beginning a day or so after the storm main phase, and reaching high intensities and also high VLF frequencies (up to ~5kHz); the latter implies a source region which has moved relatively close (L<4) to the Earth. Modulation on minute time scales can result from fluctuations in solar wind pressure and precipitation. During the initial phase of the storm, waves intensities may be suppressed by strong and sustained ionospheric absorption. A superposed epoch analysis using 300 storms with minimum Dst < -50 nT has been done to show the characteristic ground chorus storm response. The results are compared with electron flux and chorus observations made in space, and the predictions of chorus mediated acceleration theory.