Abstract
<p>Wave-particle interaction is a very important mechanism in describing the outflow of ions at high latitudes and high altitudes. Quasi-linear perpendicular velocity diffusion coefficients are used to describe the effect of wave-particle interactions, therefore it is essential to determine the correct diffusion coefficients that must be used to model the outflow of ions. In this study a Monte Carlo method is used to assess the role of different diffusion coefficients for O+ and H+ ions at high altitudes above the polar cap. Two different sets of diffusion coefficients obtained from Barghouthi [1997]; Barghouthi et al. [1998] and Nilsson et al. [2013] are used. Barghouthi [1997]; Barghouthi et al. [1998] used spectral density measurements from Dynamic Explorer 1 spacecraft (DE-1) observations to calculate the diffusion coefficients, while Nilsson et al. [2013] used spectral density measurements from the Cluster spacecraft to obtain the diffusion coefficients. It was found that diffusion coefficients from Barghouthi [1997]; Barghouthi et al. [1998] in the cusp (aurora) and central polar cap (polar wind) respectively, describe well the observations of ion outflow at altitudes lower than 5 RE, but yield unreasonably high parallel velocities and temperatures at higher altitudes. Also diffusion coefficients from Cluster spectral density measurements produce reasonable results for high altitudes and unreasonably low parallel velocities and temperatures for the low altitude region. Therefore it is suggested that a combination of these diffusion coefficients is used where the diffusion coefficients given by Barghouthi [1997]; Barghouthi et al. [1998] are used at low altitudes and the diffusion coefficients obtained from Cluster measurements are used at high altitudes.</p>
Highlights
The outflow of plasma from ionospheric origin along open geomagnetic field lines above the polar cap is subject to intense investigation
Using the diffusion coefficients for O+ ions obtained from Nilsson et al [2013], the diffusion coefficients based on spectral densities measured on board Cluster spacecraft are given by for the central polar cap and for the cusp with a value of h = –21 for both regions
Diffusion coefficients by Nilsson et al [2013] in the central polar cap (CPC) and cusp given by Equations (6) and (7) respectively and the results due to these diffusion coefficients will hereinafter be denoted by NilP and NilA 1 (NilA) respectively
Summary
The outflow of plasma from ionospheric origin along open geomagnetic field lines above the polar cap is subject to intense investigation. It is well observed that ions have high perpendicular temperatures at high altitudes despite the presence of perpendicular adiabatic cooling [Huddleston et al 2000, Bouhram et al 2004, Nilsson et al 2004, Nilsson et al 2006] This is an indication of a perpendicular heating mechanism that acts at high altitudes. Arvelius et al [2005] showed using the Cluster CIS/CODIF instrument that at 8-12 RE geocentric distance the distribution of O+ outflows cannot be explained by velocity filter dispersion only. This is a clear suggestion of further additional energization processes for outflowing O+ ions at high altitudes and high latitudes in the dayside polar region. Field aligned acceleration mechanisms such as field aligned electric fields [Lundin et al 1995, Maggiolo et al 2006] and centrifugal acceleration [Cladis 1986, Horwitz et al 1994, Demars et al 1996, Nilsson et al 2008, Nilsson et al 2010] were investigated to explain the energization up to kiloelectronvolt energies of O+ ions above the polar cap
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