3-D force-balanced magnetospheric configurations

S Zaharia,C Z Cheng,K Maezawa

doi:10.5194/angeo-22-251-2004

Abstract

Abstract. The knowledge of plasma pressure is essential for many physics applications in the magnetosphere, such as computing magnetospheric currents and deriving mag-netosphere-ionosphere coupling. A thorough knowledge of the 3-D pressure distribution has, however, eluded the community, as most in situ pressure observations are either in the ionosphere or the equatorial region of the magnetosphere. With the assumption of pressure isotropy there have been attempts to obtain the pressure at different locations,by either (a) mapping observed data (e.g. in the ionosphere) along the field lines of an empirical magnetospheric field model, or (b) computing a pressure profile in the equatorial plane (in 2-D) or along the Sun-Earth axis (in 1-D) that is in force balance with the magnetic stresses of an empirical model. However, the pressure distributions obtained through these methods are not in force balance with the empirical magnetic field at all locations. In order to find a global 3-D plasma pressure distribution in force balance with the magnetospheric magnetic field, we have developed the MAG-3-D code that solves the 3-D force balance equation computationally. Our calculation is performed in a flux coordinate system in which the magnetic field is expressed in terms of Euler potentials as . The pressure distribution, , is prescribed in the equatorial plane and is based on satellite measurements. In addition, computational boundary conditions for ψ surfaces are imposed using empirical field models. Our results provide 3-D distributions of magnetic field, plasma pressure, as well as parallel and transverse currents for both quiet-time and disturbed magnetospheric conditions. Key words. Magnetospheric physics (magnetospheric configuration and dynamics; magnetotail; plasma sheet)

Highlights

The magnetospheric plasma pressure is a quantity essential to many physical processes
We present several computed quiet- and active-time 3-D magnetospheric quasi-equilibria, in which the magnetic fields are in force balance with different observed pressure distributions: the pressure given by the so-called Spence-Kivelson formula (Spence and Kivelson, 1993), plasma sheet pressure from the GEOTAIL satellite, as well as anisotropic pressure profiles based on observations close to Earth by AMPTE/CCE (Lui and Hamilton, 1992; De Michelis et al, 1999)
In addition to knowing the magnetic field vector, the knowledge of the 3-D magnetospheric plasma pressure distribution is needed for many physical applications

Summary

Introduction

The magnetospheric plasma pressure is a quantity essential to many physical processes. Among the magnetospheric field models developed, probably the most popular are the empirical models, in which one postulates the structure of the magnetospheric currents, and the model parameters are obtained by fitting the model field to an array of observations. Those observations represent data collected by many spacecraft at different locations and at different times, and the empirical models describe large-scale time-averaged magnetospheric states rather than instantaneous “snapshots” of the magnetospheric field. As opposed to the rather extensive observations and studies of the magnetic field, the magnetospheric plasma pressure is much less known. One way of obtaining such a global distribution, starting from scarce pressure data, will be presented in this study

Results

Discussion

Conclusion