Solar wind control of plasma number density in the near-Earth plasma sheet: three-dimensional structure

D. Nagata,S. Ohtani,T. Mukai,S. Machida,Y. Saito

doi:10.5194/angeo-26-4031-2008

Abstract

Abstract. The plasma number density in the near-Earth plasma sheet depends on the solar wind number density and the north-south component of interplanetary magnetic field (IMF Bz) with time lag and duration of several hours. We examined the three-dimensional structure of such dependences by fitting observations of plasma sheet and solar wind to an empirical model equation. Analyses were conducted separately for northward and southward IMF conditions. Effects of solar wind speed and IMF orientation were also examined by further subdivision of the dataset. Based on obtained results, we discuss (i) the relative contribution of the ionosphere and solar wind to plasma sheet mass supply, (ii) the entry mechanisms for magnetosheath particles, and (iii) the plasma transport in the plasma sheet. We found that solar wind number density dependence is weaker and IMF Bz dependence is stronger for faster solar wind with southward IMF, which suggests the contribution of ionospheric particles. Further from the Earth, different interplanetary conditions result in different structures of solar wind dependence, which indicate different solar wind entry mechanisms: (1) southward IMF results in a strong dependence on solar wind number density in the flank high-latitude region, (2) slow solar wind with northward IMF leads to lower-latitude peaks of solar wind number density dependence in the flank region, (3) fast solar wind with northward IMF results in a strong dependence on solar wind number density at the down-tail dusk flank equator, and (4) solar wind number density dependence is stronger in the downstream of quasi-parallel bow shock. These features are attributable to (1) low-latitude dayside reconnection entry, (2) high-latitude dayside reconnection entry, (3) entry due to decay of Kelvin-Helmholtz vortices, and (4) diffusive entry mediated by kinetic Alfven waves, respectively. Effect of IMF Bz and its time lags show plasma sheet reconfiguration associated with enhanced convective transport under southward IMF. Duration of IMF Bz effect under northward IMF is interpreted in terms of turbulent diffusive transport.

Highlights

The sources of particles for the terrestrial plasma sheet are the fast flowing interplanetary medium, solar wind, and the terrestrial ionosphere
We examined the effects of solar wind number density, IMF Bz, and its time lag simultaneously by fitting solar wind and plasma sheet observations to a local analytic function (Nagata et al, 2007)
We examined three-dimensional structures of dependences of plasma sheet number density on solar wind number density and IMF Bz by fitting the observations to an empirical model

Summary

Introduction

The sources of particles for the terrestrial plasma sheet are the fast flowing interplanetary medium, solar wind, and the terrestrial ionosphere. Both sources are known to be able to supply sufficient amount of particles to the plasma sheet (e.g. Chappell, et al, 1987; Walker et al, 2003; Moore et al, 2005). Subsequent supply of thermal ions from the high-latitude ionosphere to the central plasma sheet was estimated by particle tracing in a model electromagnetic field and in situ observations at low altitudes Correlation between solar wind parameters and plasma sheet number density has been examined for the various solar wind conditions to examine the solar wind particle supply

Methods

Results

Discussion

Conclusion