High altitude ion heating observed by the Cluster spacecraft

Detta är en avhandling från Umeå : Umeå universitet, Institutionen för fysik

Sammanfattning: This thesis deals with heating of outowing oxygen ions at high altitude above the polar cap using data from the Cluster spacecraft. oInospheric plasma may flow up from the ionosphere but at velocities which are low enough that the ions are still gravitationally bound. For the ions to overcome gravity, further acceleration is needed. The cusp/polar cap is an important source of outowing oxygen ions. In the cusp/polar cap, transverse heating is more common than eld-aligned acceleration through a magnetic eld-aligned electric eld. It is thus believed that transverse heating of ions is important for ion outow and one of the probable explanations for transverse heating is wave-particle interaction.A general conclusion from our work on high altitude oxygen ion energization is that ion energization and outow occur in the high altitude cusp and mantle. The particles are often heated perpendicularly to the geomagnetic eld and resonant heating at the gyrofrequency is most of the time intense enough to explain the observed O+ energies measured in the high altitude (8 { 15 Earth radii, RE ) cusp/mantle region of the terrestrial magnetosphere. The observed average waves can explain the observed average O+ energies. At lower altitude only a few percent of the observed spectral density around the oxygen gyrofrequency needs to be in resonance with the ions to obtain the measured O+ energies. A difference as compared to low altitude measurements is that we must assume that almost all wave activity is due to waves which can interact with the ions, and of these we assume 50 % to be left-hand polarized. We also have shown a clear correlation between temperature and wave intensity at the gyrofrequency at each measurement point. We have described the average wave intensity and corresponding velocity diffusion oeffcients as a function of altitude in a format convenient for modelers.Furthermore we have shown that the wave activity observed in this high altitude region is consistent with Alfven waves, and inconsistent with static structures drifting past the spacecraft. We have also shown how large the variability of the observed spectral densities is, and how sporadic the waves typically are. Based on three cases we have found that the regions with enhanced wave activity and increased ion temperature are typically many ion gyro radii in perpendicular extent.