Global properties of magnetotail current sheet flapping: THEMIS perspectives

A Runov,D Larson,J Mcfadden,U Auster,I Mann,V Angelopoulos,K.-H Glassmeier,V A Sergeev

doi:10.5194/angeo-27-319-2009

Abstract

Abstract. A sequence of magnetic field oscillations with an amplitude of up to 30 nT and a time scale of 30 min was detected by four of the five THEMIS spacecraft in the magnetotail plasma sheet. The probes P1 and P2 were at X=−15.2 and −12.7 RE and P3 and P4 were at X=−7.9 RE. All four probes were at −6.5>Y>−7.5 RE (major conjunction). Multi-point timing analysis of the magnetic field variations shows that fronts of the oscillations propagated flankward (dawnward and Earthward) nearly perpendicular to the direction of the magnetic maximum variation (B1) at velocities of 20–30 km/s. These are typical characteristics of current sheet flapping motion. The observed anti-correlation between ∂B1/∂t and the Z-component of the bulk velocity make it possible to estimate a flapping amplitude of 1 to 3 RE. The cross-tail scale wave-length was found to be about 5 RE. Thus the flapping waves are steep tail-aligned structures with a lengthwise scale of >10 RE. The intermittent plasma motion with the cross-tail velocity component changing its sign, observed during flapping, indicates that the flapping waves were propagating through the ambient plasma. Simultaneous observations of the magnetic field variations by THEMIS ground-based magnetometers show that the flapping oscillations were observed during the growth phase of a substorm.

Highlights

Flapping motion of the magnetotail current sheet manifests as variations of the magnetic field with an amplitude of up to several tens of nT, often with a change of the magnetic field polarity, indicating the current sheet crossing
In this paper we report on the THEMIS major conjunction event study, providing an example of magnetotail current sheet flapping as seen by the tail-elongated fleet with the separation of ∼10 RE
In this paper we report on the observation of large amplitude magnetic field variations with a quasi-period of 20– 40 min observed by four THEMIS probes situated in the plasma sheet at −8>X>16 RE within −6.5>Y >−7.5 RE

Summary

Introduction

Flapping motion of the magnetotail current sheet manifests as variations of the magnetic field with an amplitude of up to several tens of nT, often with a change of the magnetic field polarity, indicating the current sheet crossing. On a statistical basis as well as in the event analyzes, it was found that the magnetic field variations with an amplitude of δB>15 nT and the characteristic time scale of 30–300 s, referred to as flapping motion, are due to corrugation of the current sheet surface, i.e., surface waves, propagating flankward from the near-midnight sector of the magnetotail at a velocity of 30– 100 km/s (Zhang et al, 2002; Sergeev et al, 2004; Runov et al, 2005). Analysis of the magnetic field configuration on the strongly tilted flapping fronts reveals that they are formed by a vertical slippage of a certain volume of the plasma sheet with respect to neighboring ones (Petrukovich et al, 2003, 2006) This complex geometry of the flapping current sheet is in agreement with results of several theoretical models of flapping waves (Golovchanskaya and Maltsev, 2004; Erkaev et al, 2008). In this paper we report on the THEMIS major conjunction event study, providing an example of magnetotail current sheet flapping as seen by the tail-elongated fleet with the separation of ∼10 RE

Data analysis

Findings

Summary and discussion

Concluding remarks