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

Abstract

Intense interest currently exists in determining the roles played by various wave‐particle interactions in the acceleration of electrons to relativistic energies during/following geomagnetic storms. Here we present a survey of wave data from the CRRES Plasma Wave Experiment for lower band (0.1–0.5fce) and upper band (0.5–1.0fce) chorus, fce being the electron gyrofrequency, to assess whether these waves could play an important role in the acceleration of a seed population of electrons to relativistic energies during and following geomagnetic storms. Outside of the plasmapause the chorus emissions are largely substorm‐dependent, and all chorus emissions are enhanced when substorm activity is enhanced. The equatorial chorus (|λm| < 15°) 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). This is consistent with electron injection near midnight and subsequent drift around dawn to the dayside. The high‐latitude chorus (|λm > 15°) is strongest in the lower band during active conditions, with average amplitudes 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 with wave generation in the horns of the magnetosphere. An inner population of 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 during quiet (AE < 100 nT) and moderate (100 nT < AE < 300 nT) conditions. These emissions lie inside the plasmapause and are attributed to signals from lightning and ground‐based VLF transmitters. We conclude that the significant increases in chorus amplitudes seen outside of the plasmapause during substorms support the theory of electron acceleration by whistler mode chorus in that region. The results suggest that electron acceleration by whistle mode chorus during/following geomagnetic storms can only be effective when there are periods of prolonged substorm activity following the main phase of the geomagnetic storm.