Scattering of field-aligned beam ions upstream of Earth's bow shock

A Kis,E A Lucek,H Rème,M Scholer,H Kucharek,B Klecker

doi:10.5194/angeo-25-785-2007

Abstract

Abstract. Field-aligned beams are known to originate from the quasi-perpendicular side of the Earth's bow shock, while the diffuse ion population consists of accelerated ions at the quasi-parallel side of the bow shock. The two distinct ion populations show typical characteristics in their velocity space distributions. By using particle and magnetic field measurements from one Cluster spacecraft we present a case study when the two ion populations are observed simultaneously in the foreshock region during a high Mach number, high solar wind velocity event. We present the spatial-temporal evolution of the field-aligned beam ion distribution in front of the Earth's bow shock, focusing on the processes in the deep foreshock region, i.e. on the quasi-parallel side. Our analysis demonstrates that the scattering of field-aligned beam (FAB) ions combined with convection by the solar wind results in the presence of lower-energy, toroidal gyrating ions at positions deeper in the foreshock region which are magnetically connected to the quasi-parallel bow shock. The gyrating ions are superposed onto a higher energy diffuse ion population. It is suggested that the toroidal gyrating ion population observed deep in the foreshock region has its origins in the FAB and that its characteristics are correlated with its distance from the FAB, but is independent on distance to the bow shock along the magnetic field.

Highlights

A well-known and long investigated feature of the Earth’s bow shock is the presence and behavior of energetic ions in the foreshock region
The simultaneous observation of the toroidal gyrating and the diffuse ions leads to the conclusion that we deal with two totally different ion populations: while the diffuse ions are coming from the quasi-parallel side of the bow shock, our results show that the toroidal gyrating ions are former field-aligned beam ions and originate on the quasiperpendicular side
The upstream event observed by spacecraft 1 (SC1) between 11:00– 14:00 UT on 18 February 2003, provides the possibility to analyze in detail the spatio-temporal evolution of the field-aligned beam (FAB) ion population in front of the Earth’s quasi-parallel bow shock

Summary

Introduction

A well-known and long investigated feature of the Earth’s bow shock is the presence and behavior of energetic ions in the foreshock region. Meziane et al (2004) have reported observations of gyrophase bunched ions in the close vicinity of the quasi-parallel bow shock These distributions are probably due to modulation of specularly reflected ions by the monochromatic low frequency upstream waves in this region of the bow shock. Burgess (1987) used one-dimensional hybrid simulations combined with the test particle technique to analyze the density of beam ions as a function of shock angle and shock Mach number He predicted that the density of reflected ions decreases with increasing Mach number and increasing Bn. An alternative mechanism, proposed by Edmiston et al (1982) and Tanaka et al (1983), considers the leakage of part of the heated downstream plasma as a mechanism to produce upstream field-aligned beams. The simultaneous observation of the toroidal gyrating and the diffuse ions leads to the conclusion that we deal with two totally different ion populations: while the diffuse ions are coming from the quasi-parallel side of the bow shock, our results show that the toroidal gyrating ions are former field-aligned beam ions and originate on the quasiperpendicular side

Energetic ion distributions

Relation to bow shock geometry

Findings

Discussion