Ring Current Behavior as Revealed by Energetic Proton Precipitation

Abstract

The precipitation of energetic ions and electrons into the upper atmosphere is a direct manifestation of their acceleration and pitch angle scattering in the magnetosphere. Electric fields inject/convect the particles from the tail plasma sheet towards the earth, and when closer to the Earth they spread in local time due to magnetic field forces. The electrons drift towards the morning sector and the ions towards the evening sector thus creating the ring current. Certain aspects of the ring current behavior can be revealed by the precipitating energetic protons. From these particles a proxy for the energy injection rate into the ring current can be estimated, and a ring current index which correlates highly with the pressure corrected D st * can be calculated. The pitch angle distribution of the precipitating ring current protons is either isotropic with a filled loss cone, or anisotropic with an almost empty loss cone. In the isotropic zone the ring current protons are stable to wave growth. In the anisotropic zone, however, the protons are unstable to wave growth. Thus, there exists a fairly wide L-value interval equatorward of the isotropic zone with ample conditions for EMIC (electromagnetic ion-cyclotron) wave generation. In the anisotropic zone a number of wave-particle phenomena linked to the precipitating protons take place: enhanced proton pitch angle scattering manifested as intensity peaks at mid-latitudes, SAR arc formation, Pcl and IPDP wave generation, and increased loss of relativistic electrons. An important decay process for the ring current protons is through charge exchange. The ENAs (Energetic Neutral Atoms) from this process create a well defined belt or region of ENA and protons observed at low altitudes along the geomagnetic equator. This belt reveals important aspects of the ring current such as: the ring current injection region, the drift of ring current particles, and convection losses of the ring current particles to the dayside magnetopause, and its asymmetric and symmetric behavior.