Average characteristics of the midtail plasma sheet in different dynamic regimes of the magnetosphere

N P Dmitrieva,M A Shukhtina,V A Sergeev

doi:10.5194/angeo-22-2107-2004

N P Dmitrieva, M A Shukhtina + Show 1 more

Open Access

https://doi.org/10.5194/angeo-22-2107-2004

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Abstract

Abstract. We study average characteristics of plasma sheet convection in the middle tail during different magnetospheric states (Steady Magnetospheric Convection, SMC, and substorms) using simultaneous magnetotail (Geotail, 15-35 RE downtail) and solar wind (Wind spacecraft) observations during 3.5 years. (1) A large data set allowed us to obtain the average values of the plasma sheet magnetic flux transfer rate (Ey and directly compare it with the dayside transfer rate (Emod for different magnetospheric states. The results confirm the magnetic flux imbalance model suggested by Russell and McPherron (1973), namely: during SMC periods the day-to-night flux transport rate equals the global Earthward plasma sheet convection; during the substorm growth phase the plasma sheet convection is suppressed on the average by 40%, whereas during the substorm expansion phase it twice exceeds the day-to-night global flux transfer rate. (2) Different types of substorms were revealed. About 1/3 of all substorms considered displayed very weak growth in the tail lobe magnetic field before the onset. For these events the plasma sheet transport was found to be in a balance with the day-to-night flux transfer, as in the SMC events. However, the lobe magnetic field value in these cases was as large as that in the substorms with a classic growth phase just before the onset (both values exceed the average level of the lobe field during the SMC). Also, in both groups similar configurational changes (magnetic field stretching and plasma sheet thinning) were observed before the substorm onset. (3) Superimposed epoch analysis showed that the plasma sheet during the late substorm recovery phase has the characteristics similar to those found during SMC events, the SMC could be a natural magnetospheric state following the substorm.

Highlights

Magnetospheric convection is the basic phenomenon, which determines the strength and consequences of the externallydriven disturbances in the magnetosphere
The results confirm the magnetic flux imbalance model suggested by Russell and McPherron (1973), namely: during Steady Magnetospheric Convection (SMC) periods the day-to-night flux transport rate equals the global Earthward plasma sheet convection; during the substorm growth phase the plasma sheet convection is suppressed on the average by 40%, whereas during the substorm expansion phase it twice exceeds the day–to-night global flux transfer rate
The enhanced intensity of the tail current is still a prerequisite for the expansion onset to occur. This is clear when comparing the average value of the lobe field BL(20) at T=T0 with its value computed based on solar wind conditions using the statistical relationship obtained for substorm onset epoch in Paper 1

Summary

Introduction

Magnetospheric convection is the basic phenomenon, which determines the strength and consequences of the externallydriven disturbances in the magnetosphere. The return convection from the tail reconnection region to the dayside magnetopause (TD segment) has different properties It has to pass through the high β plasma sheet where the pressure gradients building up during convection may modify (even stop or locally reverse) the return flow (Erickson, 1992). This can temporarily lead to the imbalance between DT and TD flux transport rates. The B value from the strong-B region near the magnetopause is expected to be mapped to the ionosphere, where it can be observed as the potential drop across the polar cap, P C This quantity is possible to measure with the radars and spacecraft instruments.

Data base and event selection

Findings

Concluding remarks