THEMIS analysis of observed equatorial electron distributions responsible for the chorus excitation

A statistical survey of plasma densities and electron distributions (0.5-100 keV) is performed using data obtained from the Time History of Events and Macroscale Interactions During Substorms spacecraft in near-equatorial orbits from 1 July 2007 to 1 May 2009 in order to investigate optimum conditions for whistler mode chorus excitation. The plasma density calculated from the spacecraft potential, together with in situ magnetic field, is used to construct global maps of cyclotron and Landau resonant energies under quiet, moderate, and active geomagnetic conditions. Statistical results show that chorus intensity increases at higher AE index, with the strongest waves confined to regions where the ratio between the plasma frequency and gyrofrequency, f(pe)/f(ce), is less than 5. On the nightside, large electron anisotropies and intense chorus emissions indicate remarkable consistency with the confinement to 8 R-E. Furthermore, as injected plasma sheet electrons drift from midnight through dawn toward the noon sector, their anisotropy increases and peaks on the dayside at 7 < L < 9, which is well correlated with intense chorus emissions observed in the prenoon sector. These statistical results are generally consistent with the generation of both lower-band and upper-band chorus through cyclotron resonant interactions with suprathermal electrons (1-100 keV). Two typical events on the nightside and dayside are studied in greater detail and additional interesting features are identified. Pancake distributions of electrons with energy below similar to 2 keV, which could be responsible for the excitation of upper-band chorus, are observed at lower L shells (< 7) on the nightside and at larger L shells (>6) on the dayside. In addition, very isotropic distributions at a few keV, which may be produced by Landau resonance and contribute to the formation of the typical gap in the chorus spectrum near 0.5 f(ce), are commonly observed on the dayside.