F. El-Lemdani Mazouz

Wave-particle interactions in the equatorial source region of whistler-mode emissions

Wave-particle interactions can play a key role in the process of transfer of energy between different electron populations in the outer Van Allen radiation belt. We present a case study of wave-particle interactions in the equatorial source region of whistler-mode emissions. We select measurements of the Cluster spacecraft when these emissions are observed in the form of random hiss with only occasional discrete chorus wave packets, and where the wave propagation properties are very similar to previously analyzed cases of whistler-mode chorus. We observe a positive divergence of the Poynting flux at minima of the magnetic field modulus along the magnetic field lines, indicating the central position of the source. In this region we perform a linear stability analysis based on the locally measured electron phase space densities. We find two unstable electron populations. The first of them consists of energy-dispersed and highly anisotropic injected electrons at energies of a few hundreds eV to a few keV, with the perpendicular temperature more than 10 times higher than the parallel temperature with respect to the magnetic field line. Another unstable population is formed by trapped electrons at energies above 10 keV. We show that the injected electrons at lower energies can be responsible for a part of the waves that propagate obliquely at frequencies above one half of the electron cyclotron frequency. Our model of the trapped electrons at higher energies gives insufficient growth of the waves below one half of the electron cyclotron frequency and a nonlinear generation mechanism might be necessary to explain their presence even in this simple case.

Equatorially confined warm trapped ions at around 100 eV near the plasmapause

Near the equatorial plasmapause at around 4-5 Earth radius (RE) geocentric distance, pancake distributed tens of eV ions are sometimes found, as previously reported by Olsen et al. (1987). Cluster CIS data during perigee traversals in 2001-2002 (nearly 200 traversals) revealed new features of these equatorially-trapped warm ions. (1) The characteristic energy of He+ is often higher than that of H+. (2) Some events show non-thermal ring distribution for He+ rather than superthermal pancake distribution. H+ can also have the ring distribution in such events. (3) While majority of the events are dispersion-free, some events show energy-time dispersion, indicating drifts from different local times. (4) The time scale of the development is about an hour, which is much shorter than the drifting time of these ions around the Earth. Cluster statistics also confirmed some results from the previous studies: (5) These ions are confined within a few degrees of latitudinal range near the equator, and have nearly 90{degree sign} pitch angles. (6) At a geocentric distance of about 4-4.5 RE where Cluster traversed the equator during its perigee, the probability of observing clear events is about 40-45% in the noon and dusk sectors and about 20-25% in the night-to-dawn sector. (7) They are dominated by H+ with variable content of He+. The He+/H+ ratio is much less than 5% for the majority of the cases.