The low-density tail lobes in the nightside magnetotail connect magnetically to the high-latitude polar caps at one end and to the interplanetary field at the other end. The effects of the strong dipole are not seen tailward of about 20 RE. Beyond that distance, the fields in the northern and southern tail lobes are nearly antiparallel and have an almost constant intensity of about 20 nT over a long range of distances from the Earth. The plasma sheet separating the northern and southern lobes hosts densities of the order of 1 cm−3 and partially very low magnetic field values reaching only a few nT. Thus, while the plasma beta (ratio of plasma and magnetic pressures, β = 2μ0p∕B2) is very low in the tail lobes, it generally exceeds unity at the center of the field reversal region. The inner part of the plasma sheet with its weak field and at times intense cross-tail current sheet is highly variable with bursts of fast flows, magnetic reconnection, and large-scale reconfiguration events. In the inner magnetosphere, particles can become trapped on closed orbits drifting around the Earth guided by the quasi-dipolar intrinsic field. The ring current located roughly at 4 – 6 RE radial distance encircles the Earth with highly variable intensity also modulated by the level of geomagnetic activity.
The magnetospheric structure is maintained by intense electric current systems at the magnetospheric boundaries, across the tail plasma sheet, and parallel to the magnetic field lines connecting the ionosphere with various parts of the magnetosphere (Figure 3). These field-aligned currents mediate a strong coupling between the ionosphere and the magnetosphere: The plasma sheet is magnetically connected to the auroral ovals, which encircle the magnetic poles and host continuous, diffuse auroral precipitation in addition to the bright auroral displays associated with events of geomagnetic activity. The geocentric solar magnetospheric coordinate system (GSM) is used in the following discussion. In that system, the x-axis points Sunward along the Sun-Earth line, the z-axis is in the plane containing the Sun-Earth line and the dipole axis and points northward, and the y-axis completes the right-handed triad pointing duskward.
The solar wind motional electric field in the Earth’s frame of reference (E = −Vsw × BIMF) imposes a large-scale convection pattern within the magnetosphere and ionosphere (Dungey, 1961). Dayside reconnection allows solar wind plasma and field entry to the dayside magnetosphere, from where the plasma convects across the polar cap and tail lobes to another reconnection location in the distant tail. At the distant tail reconnection region, plasmas are accelerated toward and away from the reconnection region such that within the tail plasma sheet, the flows are Earthward on the Earthward side of the reconnection region and away from the Earth in the tailward side of the reconnection region (Lyons and Williams, 1984). In the ionosphere, this large-scale convection pattern induces antisunward flow across the polar cap and sunward plasma flow in the auroral region ionosphere (Heelis et al., 1982). This basic flow pattern (in highly variable forms) can always be found underlying the temporal changes associated with the space weather events.
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