Geomagnetic activity as a reflection of processes in the magnetospheric tail: 2. Plasma convection model

Abstract

Here, we attempt to develop a simple model describing the geomagnetic activity averaged over time and use it to identify the abovementioned depen� dence. Here, our main principle is to simplify the model as much as possible and get rid of unnecessary details. In line with this, we try to rely less on observational data and prefer to obtain the required parameters from gen� eral considerations. The main simplifications of the model are the following. The geomagnetic activity largely depends on solar wind parameters, especially the IMF orientation. This is related to the occurrence of magnetic distur� bances—periods with highly increased activity. The model is designed for relatively quiet periods. Due to time averaging when all fluctuations are suppresses, the model parameters can be taken as constant. Accordingly, the model will be applicable to the description of the stationary state of the system only. The solar wind plasma and, accordingly, the plasma in the body of the geomagnetic tail will be assumed to be an ideal conductor, and the dissipation of energy will be received only in the area of the day� side magnetopause and in the remote tail zone, as well as in areas of particle dropout in the polar ionosphere. We will postulate that the electron and ion tempera� tures in the tail plasma are equal. The geometry of the magnetosphere and its mag� netic sheets in the areas of the near and distant tail is assumed to be the most simplified. The area in the dis� tant tail, where the magnetic field is recombined and plasma convection directed to the Earth emerges— the socalled Х�point—is determined from the condi� tion of hydrodynamic instability of the flow. The resulting turbulence provides the required dissipation of the magnetic field in this zone. The dropout of particles into the polar ionosphere is considered with the effect of electron cooling: drop� out of fast electrons and backflow of slow electrons from the ionosphere. It will be shown that this effect makes it possible to overcome the socalled "convec� tion crisis" (Erickson and Wolf, 1980) because it sig� nificantly reduces the dropout time. Abstract—A model of plasma convection in the magnetospheric tail was developed. Although highly simpli� fied, the model adequately describes the main characteristics of the process. We have calculated the physical parameters characterizing the magnetotail, as well as described the convection of fluxtubes in it and the pro� cess of electron dropout. The model explains the semiannual variation in magnetic activity.