Abstract
Abstract. Recent Cluster observations of the vicinity of the high latitude magnetopause indicate the presence of beams of singly charged oxygen ions, which are of ionospheric origin. In this paper we examine the role of magnetic turbulence combined with a dc electric field across the magnetopause in causing the cross field transport of protons and of singly charged oxygen ions, by means of a kinetic test particle simulation. We find that the observed values of magnetosheath turbulence and electric fields can produce a substantial escape of the oxygen ions relative to protons. By varying the magnetic turbulence level in the simulation, we find that the number of O+ crossing the magnetopause grows with δB/B0, and that very few ions can cross the magnetopause for δB/B0=0. The ion temperature also grows with δB/B0, showing that magnetic turbulence is effective in thermalizing the kinetic energy gain due to the cross-magnetopause potential drop. We suggest that this mechanism can help to explain Cluster observations of energetic oxygen ions during a high-latitude magnetopause crossing.
Highlights
It was suggested by Haerendel et al (1978) that the magnetospheric regions above the polar cusps would correspond to strong perturbations of the magnetic and velocity fields
The strong turbulence observed could be due either to magnetic reconnection occurring at the high latitude magnetopause (Scudder et al, 2002), or to instabilities related to the shear flows in the magnetosheath, or to kinetic instabilities at frequencies around the local proton gyrofrequency (Sahraoui, 2006; Nykyri et al, 2006; Zimbardo, 2006)
Are computed on a 3-dimensional grid in space (Veltri et al, 1998; Greco et al, 2003). It was shown by Taktakishvili et al (2003) and by Greco et al (2003), that this magnetic turbulence allows the flow of ions across the magnetopause, for magnetic fluctuations levels δB/B0≥0.3
Summary
It was suggested by Haerendel et al (1978) that the magnetospheric regions above the polar cusps would correspond to strong perturbations of the magnetic and velocity fields. The proposed energization mechanisms range from magnetic reconnection either at low or at high latitudes, to resonant heating by electric field fluctuations (Andre and Yau, 1997; Bogdanova et al, 2004), to centrifugal acceleration of the outflowing ions (Nilsson et al, 2006). The main idea is that magnetic turbulence provides ions with the mobility across the magnetopause In this case they may be accelerated by the quasi-dc potential drop, as well as by other mechanisms. 3 we give the numerical results, showing the differences between proton and oxygen behaviour, the dependence of the results on the magnetic turbulence level, and the influence of varying the cross-magnetopause electric field.
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