Evidence for transient, local ion foreshocks caused by dayside magnetopause reconnection

Yann Pfau-Kempf,Urs Ganse,Heli Hietala,Sanni Hoilijoki,Sebastian Von Alfthan,Liisa Juusola,Minna Palmroth,Steve E Milan

doi:10.5194/angeo-34-943-2016

Abstract

Abstract. We present a scenario resulting in time-dependent behaviour of the bow shock and transient, local ion reflection under unchanging solar wind conditions. Dayside magnetopause reconnection produces flux transfer events driving fast-mode wave fronts in the magnetosheath. These fronts push out the bow shock surface due to their increased downstream pressure. The resulting bow shock deformations lead to a configuration favourable to localized ion reflection and thus the formation of transient, travelling foreshock-like field-aligned ion beams. This is identified in two-dimensional global magnetospheric hybrid-Vlasov simulations of the Earth's magnetosphere performed using the Vlasiator model (http://vlasiator.fmi.fi). We also present observational data showing the occurrence of dayside reconnection and flux transfer events at the same time as Geotail observations of transient foreshock-like field-aligned ion beams. The spacecraft is located well upstream of the foreshock edge and the bow shock, during a steady southward interplanetary magnetic field and in the absence of any solar wind or interplanetary magnetic field perturbations. This indicates the formation of such localized ion foreshocks.

Highlights

The super-Alfvénic solar wind impinging upon the geomagnetic field is slowed down and diverted around the Earth by the bow shock which forms upstream of our planet
We propose a scenario by which dayside magnetopause reconnection generates flux transfer events (FTEs), which in turn cause steepening fast magnetosonic bow and stern waves to propagate throughout the magnetosheath
We trace the observed magnetic field to find the locus on the bow shock surface where θB−n = 60◦, and we trace the trajectories of ions reflected from there with twice the solar wind inflow speed in the solar wind rest frame

Summary

Introduction

The super-Alfvénic solar wind impinging upon the geomagnetic field is slowed down and diverted around the Earth by the bow shock which forms upstream of our planet. Most of the plasma is abruptly compressed and heated by the shock while being transported downstream into the magnetosheath There, it flows along the magnetopause surface, which delimits the magnetosphere, that is, the magnetic cavity in which the Earth is situated. Fluid theories such as ideal magnetohydrodynamics imply that no wave or matter can travel upstream from a shock. Greenstadt et al, 1980; Schwartz et al, 1983), a fraction of the incoming ions is reflected by the shock surface and streams back along the magnetic field direction The region where such a backstreaming ion population exists is called the ion foreshock. The value of θB−n allowing the reflection of particles is dependent on several factors, Published by Copernicus Publications on behalf of the European Geosciences Union

Methods

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Discussion

Conclusion