Nonlinear Energization of Ionospheric Ions by Electrostatic Fields

Summary form only given. There are two observations that are particularly ubiquitous in the auroral ionospheric plasmas: intense electric fields in density depleted regions, and transverse energization of ions emanating from such regions. The observed gain in ion energies is sufficiently large to allow the ions to gravitationally escape the ionosphere and populate the magnetosphere. The gain in energy cannot be explained by a linear model of wave-particle interaction between the ions and the observed electrostatic fields. We have been studying the nonlinear interactions of ionospheric ions with electrostatic fields to understand the observed energization of ions. The electrostatic fields are assumed to be represented either by a set of plane waves propagating obliquely to the geomagnetic field, or localized field structures with transverse spatial dimensions that are small compared to the thermal Larmor radii of the ions. The dynamics of ions is compared and contrasted for the two cases. For obliquely propagating waves, nonlinear, coherent transverse energization of low energy ions is possible for certain conditions that depend on the frequency spectrum, wave numbers, and relative obliqueness of the waves. The interaction of low energy ions with localized electrostatic field structures is significantly different from their interaction with waves. We find that the phase space of the energized ions is chaotic and, for long-time interactions, the ions can undergo large energy gains akin to Levy flights. Detailed analytical and numerical results comparing the energization of ions by localized field structures with energization by plane waves will be discussed. From our studies we conclude that nonlinear wave-particle interactions are needed to explain the observed gains in ion energies