Magnetic field fluctuations across the Earth’s bow shock

A Czaykowska,R A Treumann,W Baumjohann,T M Bauer

doi:10.5194/angeo-19-275-2001

Abstract

Abstract. We present a statistical analysis of 132 dayside (LT 0700-1700) bow shock crossings of the AMPTE/IRM spacecraft. We perform a superposed epoch analysis of low frequency, magnetic power spectra some minutes up-stream and downstream of the bow shock. The events are devided into categories depending on the angle θBn between bow shock normal and interplanetary magnetic field, and on plasma-β. In the foreshock upstream of the quasi-parallel bow shock, the power of the magnetic fluctuations is roughly 1 order of magnitude larger (δB ~ 4 nT for frequencies 0.01–0.04 Hz) than upstream of the quasi-perpendicular shock. There is no significant difference in the magnetic power spectra upstream and downstream of the quasi-parallel bow shock; only at the shock itself, is the magnetic power enhanced by a factor of 4. This enhancement may be due to either an amplification of convecting upstream waves or to wave generation at the shock interface. On the contrary, downstream of the quasi-perpendicular shock, the magnetic wave activity is considerably higher than upstream. Down-stream of the quasi-perpendicular low-β bow shock, we find a dominance of the left-hand polarized component at frequencies just below the ion-cyclotron frequency, with amplitudes of about 3 nT. These waves are identified as ion-cyclotron waves, which grow in a low-β regime due to the proton temperature anisotropy. We find a strong correlation of this anisotropy with the intensity of the left-hand polarized component. Downstream of some nearly perpendicular (θBn ≈ 90°) high-β crossings, mirror waves are identified. However, there are also cases where the conditions for mirror modes are met downstream of the nearly perpendicular shock, but no mirror waves are observed.Key words. Interplanetary physics (plasma waves and turbulence) – Magnetospheric physics (magnetosheath; plasma waves and instabilities)

Highlights

Since the beginning of the space age, the Earth’s bow shock is of particular interest because it serves as a unique laboratory for the study of shock waves in collisionless plasmas
We investigate the average behaviour of the low frequency magnetic wave power as measured by AMPTE/IRM during 132 bow shock crossings
The nice new result of the present statistical study is the good correlation of the ratio of left-to-right hand polarized components of the magnetic wave activity and the anisotropy of the proton temperature

Summary

Introduction

Since the beginning of the space age, the Earth’s bow shock is of particular interest because it serves as a unique laboratory for the study of shock waves in collisionless plasmas. The reflected ions gyrate back to the shock and enter the downstream region, where their presence causes a high perpendicular temperature anisotropy (Sckopke et al, 1983). This anisotropy leads to the generation of ioncyclotron and mirror waves (e.g. Price et al, 1986; Gary et al, 1993). Haviour of the low frequency magnetic wave power as measured by AMPTE/IRM during 132 bow shock crossings We show that these waves behave differently across quasiperpendicular and quasi-parallel shocks.

Data description and method of analysis

Comparison of quasi-perpendicular and quasi-parallel bow shock crossings

Comparison of quasi-perpendicular low-β and high-β bow shock crossings

Findings

Conclusions