Estimation of the Electromagnetic Energy Carried by the Alfven waves Traveling in the Plasma Sheet

It is not still very clear how the charged particles in the geotail plasma sheet are energized. Terasawa et al. (1997) showed that the temperature and density of the plasma in the plasma sheet depends on the kinetic energy and the density of the solar wind as well as the orientation of the interplanetary magnetic field (IMF). It suggests that the heating process controlled by the solar wind condition would energize the particles entering into the plasma sheet from the solar wind. Magnetic reconnection is considered to be a potential process to energize the plasma sheet particles. However, it is difficult to consider that the particles in the plasma sheet are energized just by the reconnection because the reconnection is a discontinuous process occurring in very limited region while energetic particles are observed always in the geotail plasma sheet. We are considering that Alfven wave travelling in the plasma sheet is another potential process to energize the particles in the plasma sheet. We discovered large-amplitude Alfven waves often appear in the geotail plasma sheet in the Geotail observations. Large fraction of the Alfven-wave events coincides with (1) ion beams or (2) dipolarization. It suggests that Alfven waves are excited by cyclotron instability or the change of the magnetospheric shape. Large Poynting flux events (less than 0.04 W/m2) are always found with ion beams. Since the probability of Alfven waves with large Poynting flux decreases with approaching to the earth, the wave energy is suggested to dissipate in the short time after the decay of the sunward ion beam. The plasma sheet may be heated by the Alfven waves. The energy carried by the Alfven waves, Poynting flux, flows along the ambient magnetic field. To estimate the total energy carried by the Alfven waves in the plasma sheet, it is necessary to estimate the scale of the waves in the direction perpendicular to the ambient magnetic field. This scale can be estimated from the deviation of the electric to magnetic field amplitude ratio from the local Alfven speed (Wygant et al., 2002). On the other hand, the inertia of the electron and the heavy ions may cause the deviation of the E to B amplitude ratio. Therefore, electron temperature and the ion composition should be examined as well to estimate the scale of the Alfven waves in the plasma sheet.