Hot and cold ion outflow: Observations and implications for numerical models

Abstract

[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.

Relative outflow enhancements during major geomagnetic storms – Cluster observations

“…In such a case we may not see a strong difference in the plasma beta, but the scaled outflow enhancement observed would be even more significant and our observations would be an underestimation of the actual enhancement. For exam- ple, the year of our most extreme geomagnetic storm, the Halloween storm, has a scaled O + outflow of approximately 10 11 m −2 s −1 (see Table 1), typical for the polar cap (see also Nilsson et al, 2013, for typical fluxes in different regions). There was an insignificant amount of data points in the cusp and plasma mantle (β > 0.1) also for the storms; therefore, the amount of data that could be located outside the intended magnetospheric region does not affect the statistics.…”

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

“…We have mapped the footprint of Cluster 3 using Tsyganenko 89 and 96 models to show that the spacecraft are located in the auroral oval. By comparing our results with those in Nilsson et al (2010) and with the discussion in Nilsson et al (2013), one may find that not much heating and centrifugal acceleration is expected along these outflow paths in the near-Earth lobes to the plasma sheet. The auroral ions have similar and different features from the ions in the polar wind, upwelling ions (cleft ion fountain) and polar cap (Yan and Maggiolo et al, 2006Maggiolo et al, , 2011Nilsson et al, 2006).…”

Oxygen ion energization by waves in the high altitude cusp and mantle

“…This agrees with the results of Waara et al (2011) where it was found that the average wave activity could explain the average ion temperatures only for geocentric distances above 8 R E . In more recent work, we have found that the fraction of the wave activity, which is efficient in heating the ions, must be lower at lower altitudes (Nilsson et al, 2012). This is likely the explanation for the overestimate of the temperature for low spectral density which mostly corresponds to low altitude.…”