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Centrifugal acceleration at high altitudes above the polar cap: A Monte Carlo simulation
Department of Physics, Al-Quds University, Jerusalem.
Department of Physics, Al-Quds University, Jerusalem.
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Rymdteknik.ORCID-id: 0000-0002-2347-1871
Swedish Institute of Space Physics / Institutet för rymdfysik.
2015 (engelsk)Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 8, 6409-6426 s.Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

A Monte Carlo simulation was used to study the outflow of O+ and H+ ions along three flight trajectories above the polar cap up to altitudes of about 15 RE. Barghouthi (2008) developed a model on the basis of altitude and velocity-dependent wave-particle interactions and a radial geomagnetic field which includes the effects of ambipolar electric field and gravitational and mirror forces. In the present work we improve this model to include the effect of the centrifugal force, with the use of relevant boundary conditions. In addition, the magnetic field and flight trajectories, namely, the central polar cap (CPC), nightside polar cap (NPC), and cusp, were calculated using the Tsyganenko T96 model. To simulate wave-particle interactions, the perpendicular velocity diffusion coefficients for O+ ions in each region were determined such that the simulation results fit the observations. For H+ ions, a constant perpendicular velocity diffusion coefficient was assumed for all altitudes in all regions as recommended by Nilsson et al. (2013). The effect of centrifugal acceleration was simulated by considering three values for the ionospheric electric field: 0 (no centrifugal acceleration), 50, and 100 mV/m. It was found that the centrifugal acceleration increases the parallel bulk velocity and decreases the parallel and perpendicular temperatures of both ion species at altitudes above about 4 RE. Centrifugal acceleration also increases the temperature anisotropy at high altitudes. At a given altitude, centrifugal acceleration decreases the density of H+ ions while it increases the density of O+ ions. This implies that with higher centrifugal acceleration more O+ ions overcome the potential barrier. It was also found that aside from two exceptions centrifugal acceleration has the same effect on the velocities of both ions. This implies that the centrifugal acceleration is universal for all particles. The parallel bulk velocities at a given value of ionospheric electric field were highest in the cusp followed by the CPC followed by the NPC. In this study a region of no wave-particle interaction was assumed in the CPC and NPC between 3.7 and 7.5 RE. In this region the perpendicular temperature was found to decrease with altitude due to perpendicular adiabatic cooling.

sted, utgiver, år, opplag, sider
2015. Vol. 120, nr 8, 6409-6426 s.
HSV kategori
Forskningsprogram
Atmosfärsvetenskap
Identifikatorer
URN: urn:nbn:se:ltu:diva-13104DOI: 10.1002/2015JA021325Lokal ID: c483ad54-d6cd-4d41-818c-aaf8235b4994OAI: oai:DiVA.org:ltu-13104DiVA: diva2:986055
Merknad
Validerad; 2015; Nivå 2; 20150910 (ninhul)Tilgjengelig fra: 2016-09-29 Laget: 2016-09-29 Sist oppdatert: 2017-10-19bibliografisk kontrollert

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