Abstract
Abstract. Higher order moments, e.g., perpendicular and parallel heat fluxes, are related to non-Maxwellian plasma distributions. Such distributions are common when the plasma environment is not collision dominated. In the polar wind and auroral regions, the ion outflow is collisionless at altitudes above about 1.2 RE geocentric. In these regions wave–particle interaction is the primary acceleration mechanism of outflowing ionospheric origin ions. We present the altitude profiles of actual and "thermalized" heat fluxes for major ion species in the collisionless region by using the Barghouthi model. By comparing the actual and "thermalized" heat fluxes, we can see whether the heat flux corresponds to a small perturbation of an approximately bi-Maxwellian distribution (actual heat flux is small compared to "thermalized" heat flux), or whether it represents a significant deviation (actual heat flux equal or larger than "thermalized" heat flux). The model takes into account ion heating due to wave–particle interactions as well as the effects of gravity, ambipolar electric field, and divergence of geomagnetic field lines. In the discussion of the ion heat fluxes, we find that (1) the role of the ions located in the energetic tail of the ion velocity distribution function is very significant and has to be taken into consideration when modeling the ion heat flux at high altitudes and high latitudes; (2) at times the parallel and perpendicular heat fluxes have different signs at the same altitude. This indicates that the parallel and perpendicular parts of the ion energy are being transported in opposite directions. This behavior is the result of many competing processes; (3) we identify altitude regions where the actual heat flux is small as compared to the "thermalized" heat flux. In such regions we expect transport equation solutions based on perturbations of bi-Maxwellian distributions to be applicable. This is true for large altitude intervals for protons, but only the lowest altitudes for oxygen.
Highlights
Non-Maxwellian ion velocity distributions have been observed at different high altitudes and high latitudes (e.g., Slapak et al, 2011; Waara et al, 2010; Huddleston et al, 2000; Winningham and Burch, 1984)
The obtained behavior of the ion heat flux has been discussed in terms of the ion potential energy due to gravity and the polarization electric field, perpendicular adiabatic cooling, and wave–particle interactions
Our main findings are the following: 1. In the polar wind region and for H+ ions, the total heat flux is always positive. This means that the flow of energy is upward along the geomagnetic field lines, and energy is going from low altitudes to higher altitudes
Summary
Non-Maxwellian ion velocity distributions have been observed at different high altitudes and high latitudes (e.g., Slapak et al, 2011; Waara et al, 2010; Huddleston et al, 2000; Winningham and Burch, 1984). Different theoretical studies have discussed the ion outflows in the polar wind and auroral regions either by using kinetic models (e.g., Lemaire and Scherer, 1973, and for more details see the review by Tam et al, 2007), transport theory approach (e.g., Schunk and Watkins, 1982; Demars and Schunk, 1989, 1992, 1994; Khazanov et al, 1984; Ganguli et al, 1987, Ganguli and Palmadesso, 1987) or the Monte Carlo method (e.g., Barakat and Schunk, 1983; Barghouthi et al, 1993, 2001; Barakat et al, 1995) These models have investigated the ion outflows either in the collision-dominated region or collisionless regions or in both regions. In these studies the ion motion was followed along geomagnetic field lines under the effects of external forces (gravity and polarization electric field) and the interactions between the ion and the electromagnetic waves observed in those regions They obtained altitude profiles for ions density, drift velocity, and parallel and perpendicular temperatures and the ion velocity distribution functions. This paper is organized as follows: following a description of the theoretical formulations (Sect. 2), we present H+ and O+ ion velocity distributions (Sect. 3), compute and discuss H+ and O+ ion heat fluxes (Sect. 4), compare the actual H+ and O+ ion heat fluxes and H+ and O+ ion “thermalized” heat fluxes (Sect. 5), discuss the applicability of our results (Sect. 6), and draw some general conclusions based on our results (Sect. 7)
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