Chorus, Energetic Electrons and Magnetospheric Substorms

Abstract

The origin(s) of whistler mode chorus in the outer region of the terrestrial magnetosphere has been investigated using simultaneous measurements of chorus, energetic (79 ± 23 keV) electron fluxes and pitch angle distributions and ambient magnetic fields obtained with OGO 5. It is found that chorus occurring within 15° of the magnetic equator (equatorial chorus) was detected during magnetospheric sub-storms and is closely related to enhanced, anisotropic fluxes of energetic electrons. The observations are consistent with wave generation by a loss-cone instability associated with freshly injected 10–100 keV electrons. Chorus observed at higher latitudes appear to have several causes. 1) Nightside emissions are substorm related and are observed when the magnetic field changes from a tail-like to a more dipolar configuration. Possible explanations are an onset of chorus growth due to a change in the electron pitch angle distribution or a decrease in the Landau damping of the waves as they propagate to higher latitudes. 2) Dayside high latitude emissions, which are sub-storm related, are found to be correlated with high fluxes of energetic electrons. Local, high latitude generation in “minimum B pockets” or equatorial generation and subsequent propagation to higher latitudes by wave ducting can both occur. It is possible to determine the proper mechanism for each individual case if the measurements of the value f/f_c (f_c is local electron gyrofrequency) and the ambient plasma densities can be made. 3) High latitude chorus also occurs during prolonged geomagnetic quiet but is not well understood. These emissions were not related to features in the electron flux or pitch angle distribution. Solar wind pressure fluctuations and magnetic flux cutting were ruled out as major causes of these emissions. Several possible generation mechanisms are proposed. One possibility is that lower energy electrons (E < 55 keV) are responsible. Another possible generation mechanism requires the existence of boundary layer plasma. This enhanced, thermal plasma could lower the local wave phase velocity, leading to enhanced wave-particle interactions and to chorus growth. Further study is necessary to test the latter hypothesis.