[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.

Abstract. We present a statistical study of the low (<1 Hz) frequency electric and magnetic field spectral densities observed by Cluster spacecraft in the high altitude cusp and mantle region. At the O + gyrofrequency (0.02-0.5 Hz) for this region the electric field spectral density is on average 0.2-2.2 (mV m −1 ) 2 Hz −1 , implying that resonant heating at the gyrofrequency can be intense enough to explain the observed O + energies of 20-1400 eV. The relation between the electric and magnetic field spectral densities results in a large span of phase velocities, from a few hundred km s −1 up to a few thousand km s −1 . In spite of the large span of phase velocity, the ratio between the calculated local Alfvén velocity and the estimated phase velocity is close to unity. We provide average values of a coefficient describing diffusion in ion velocity space at different altitudes, which can be used in studies of ion energization and outflow. The observed average waves can explain the average O + energies measured in the high altitude (8-15 R E ) cusp/mantle region of the terrestrial magnetosphere according to our test particle calculations.

Abstract. A Monte Carlo simulation was used to study the effects of wave-particle interactions (WPI) on ion outflow at high latitudes (the auroral region and the polar cap). As the ions drift upward along the geomagnetic field lines, they interact with the electromagnetic turbulence and, consequently, get heated in the direction perpendicular to the geomagnetic field. The mirror force converts some of the gained ion energy in the perpendicular direction into parallel kinetic energy.These effects combine to form an ion-conic distribution. Previous studies of WPI in the auroral region neglected the body forces (i.e., gravitational and polarization electrostatic) and the altitude dependence of the spectral density. In contrast, this work includes the effect of body forces and an altitude-dependent spectral density. The ion distribution function, the profiles of ion density, drift velocity, and parallel and perpendicular temperatures are presented for both H + and O + ions.These results are compared with the ones corresponding to polar wind conditions. The main conclusions are as follows: (1) the effect of body forces is more important in the polar wind case and for the O + ions than it is for the auroral region and the H + ions, respectively; (2) the O + ions are preferentially energized in both regions; (3) both ions (H + and O +) are more energetic in the auroral region at most altitudes; and (4) the results of the Monte Carlo simulations agree with the "analytical" results of the mean particle theory.

[1] Cluster observations of oxygen ion outflow and low-frequency waves at high altitude above the polar cap and cold ion outflow in the lobes are used to determine ion heating rates and low-altitude boundary conditions suitable for use in numerical models of ion outflow. Using our results, it is possible to simultaneously reproduce observations of high-energy O + ions in the high-altitude cusp and mantle and cold H + ions in the magnetotail lobes. To put the Cluster data in a broader context, we first compare the average observed oxygen temperatures and parallel velocities in the high-altitude polar cap with the idealized cases of auroral (cusp) and polar wind (polar cap) ion outflow obtained from a model based on other data sets. A cyclotron resonance model using average observed electric field spectral densities as input fairly well reproduces the observed velocities and perpendicular temperatures of both hot O + and cold H + , if we allow the fraction of the observed waves, which is efficient in heating the ions to increase with altitude and decrease toward the nightside. Suitable values for this fraction are discussed based on the results of the cyclotron resonance model. Low-altitude boundary conditions, ion heating rates, and centrifugal acceleration are presented in a format suitable as input for models aiming to reproduce the observations.

The polar wind is an ambipolar plasma outflow from the terrestrial ionosphere at high latitudes. As the ions drift upward along geomagnetic flux tubes, they move from collision‐dominated (ion barosphere) to collisionless (ion exosphere) regions. A transition layer is embedded between these two regions where the ion characteristics change rapidly. A Monte Carlo simulation was used to study the steady‐state flow of H+ ions through a background of O+ ions. The simulation domain covered the collision‐dominated, transition, and collisionless regions. The model properly accounted for the divergence of magnetic field lines, the gravitational force, the electrostatic field, and H+‐O+ collisions. The H+ velocity distribution, f(H+), was found to be very close to Maxwellian at low altitudes (deep in the barosphere). As the ions drifted to higher altitudes, f(H+) formed an upward tail. In the transition layer, the upward tail evolved into a second peak with a kidney bean shape, and hence, f(H+) developed a double‐humped shape. The second peak grew with altitude and eventually became dominant as the ions reached the exosphere. This behavior is due to the interplay between the electrostatic force and the velocity‐dependent Coulomb collisions. Moreover, the H+ heat flux, q(H+), was found to change rapidly with altitude in the transition layer from a positive maximum to a negative minimum. This remarkable feature of q(H+) is closely related to the coincident formation of the double‐humped structure of f(H+). The double‐hump distribution might destabilize the plasma or, at least, cause enhanced thermal fluctuations. The double‐hump f(H+), and the associated wave turbulence, have several consequences with regard to our understanding of the polar wind and similar space physics problems. The plasma turbulence can significantly alter the behavior of the plasma in and above the transition region and, therefore, should be considered in future polar wind models. The wave turbulence can serve as a signature for the formation of the double‐hump f(H+). Also, more sophisticated (than the existing bi‐Maxwellian 16‐moment) generalized transport equations might be needed to properly handle problems such as the one considered here.