Hot and cold ion outflow: Spatial distribution of ion heating

Abstract

[1] Ions apparently emanating from the same source, the ionospheric polar cap, can either end up as energized to keV energies in the high-altitude cusp/mantle, or appear as cold ions in the magnetotail lobes. We use Cluster observations of ions and wave electric fields to study the spatial variation of ion heating in the cusp/mantle and polar cap. The average flow direction in a simplified cylindrical coordinate system is used to show approximate average ion flight trajectories, and discuss the temperatures, fluxes and wave activity along some typical trajectories. It is found that it is suitable to distinguish between cusp, central and nightside polar cap ion outflow trajectories, though O + heating is mainly a function of altitude. Furthermore we use typical cold ion parallel velocities and the observed average perpendicular drift to obtain average cold ion flight trajectories. The data show that the cusp is the main source of oxygen ion outflow, whereas a polar cap source would be consistent with our average outflow paths for cold ions observed in the lobes. A majority of the cusp O + flux is sufficiently accelerated to escape into interplanetary space. A scenario with significant oxygen ion heating in regions with strong magnetosheath origin ion fluxes, cold proton plasma dominating at altitudes below about 8 R E in the polar cap, and most of the cusp oxygen outflow overcoming gravity and flowing out in the cusp and mantle is consistent with our observations.

Relative outflow enhancements during major geomagnetic storms – Cluster observations

“…However, during storm time, accelerated O + was observed in the cusp. Nilsson et al (2012) showed consistent results with a little acceleration in the polar cap and lobes but significant heating and subsequent acceleration in the cusp and plasma mantle. They did however not divide their data according to geomagnetic activity.…”

Seasonal variations and north–south asymmetries in polar wind outflow due to solar illumination

“…There are three main regions of outflow at high latitudes: the auroral oval, the cusp, and the polar cap. The auroral oval and the cusp are regions of intense ion outflow in response to strong energy inputs like Poynting flux, particle precipitation, and the work done by strong field-aligned electric fields accelerating ions upwards (Lockwood et al, 1985;Zheng et al, 2005;Moore and Khazanov, 2010;Nilsson et al, 2012). In the absence of such energy inputs, the main source of energy for ion outflow in the polar cap is solar illumination.…”

“…But the ions with the lowest energies may be convected across the polar cap and mix with the polar wind ions (e.g. Green and Waite Jr., 1985;Nilsson et al, 2012), so that it is not always possible to discern polar wind ions from the ions originating in the cusp. This also provides another possible explanation for the O + observed above the polar caps and in the lobes.…”

Outflow of low-energy O<sup>+</sup> ion beams observed during periods without substorms

“…If only the field-aligned portion is included, the temperature will be reduced to < 100 eV. Note also that because of mirror force, the temperature perpendicular to the magnetic field may indicate some heating (see also Nilsson et al, 2012, for Cluster studies of ion heating throughout the polar cap magnetosphere).…”