Substorm dependence of chorus amplitudes: Implications for the acceleration of electrons to relativistic energies

Intense interest currently exists in determining the roles played by variou s wave-particle interactions in the acceleration of electrons to relativist ic energies during/following geomagnetic storms. Here we present a survey o f wave data from the CRRES Plasma Wave Experiment for lower band (0.1-0.5f( ce)) and upper band (0.5-1.0f(ce)) chorus, f(ce) being the electron gyrofre quency, to assess whether these waves could play an important role in the a cceleration of a seed population of electrons to relativistic energies duri ng and following geomagnetic storms. Outside of the plasmapause the chorus emissions are largely substorm-dependent, and all chorus emissions are enha nced when substorm activity is enhanced. The equatorial chorus (/ lambda (m ) / < 15 degrees) is strongest in the lower band during active conditions ( AE > 300 nT) with average amplitudes typically >0.5 mV m(-1) predominantly in the region 3 < L < 7, between 2300 and 1300 magnetic local time (MLT). T his is consistent with electron injection near midnight and subsequent drif t around dawn to the dayside. The high-latitude chorus (/X/ > 15 degrees) i s strongest in the lower band during active conditions, with average amplit udes typically >0.5 mV m(-1) in the region 3 < L < 7 over a range of local times on the dayside, principally in the range 0600-1500 MLT, Consistent wi th wave generation in the horns of the magnetosphere. An inner population o f weak, substorm-independent emissions with average amplitudes generally < 0.2 mV m(-1) are seen in both bands largely inside L = 4 on the nightside d uring quiet (AE < 100 nT) and moderate (100 nT < AE < 300 nT) conditions. T hese emissions lie inside the plasmapause and are attributed to signals fro m lightning and ground-based VLF transmitters. We conclude that the signifi cant increases in chorus amplitudes seen outside of the plasmapause during substorms support the theory of electron acceleration by whistler mode chor us in that region. The results suggest that electron acceleration by whistl e mode chorus during/following geomagnetic storms can only be effective whe n there are periods of prolonged substorm activity following the main phase of the geomagnetic storm.