Magnetospheric structure

3.1 Magnetospheric structure

The intrinsic geomagnetic field of the Earth has an intensity of about 50,000 nT in the polar regions and about 30,000 nT at the equator. The size of the magnetospheric cavity is determined by the magnetic pressure of the internal field on one hand, and on the solar wind dynamic pressure on the other: The magnetospheric boundary, the magnetopause, forms at a location where the solar wind and magnetospheric plasmas and magnetic fields are in pressure balance. Typical solar wind conditions give a standoff distance of the magnetopause at about 10 R_E (Earth radius = 6370 km) upstream of the Earth, but under strong solar wind driving the magnetopause can be pushed well inside the geostationary orbit (at 6.6 R_E where a satellite orbiting around the Earth has a 24-hour rotation period and thus remains at a constant longitude above the Earth). In the antisunward direction, the solar wind flow deforms the dipolar magnetic field to a cometary taillike shape where the magnetopause is on average about 30 R_E from the Sun-Earth line, depending on the solar wind pressure. The magnetotail extends far beyond lunar orbit at least a few hundred R_E in the antisunward direction. As the solar wind flow past the magnetosphere is both supersonic and super-Alfvénic, a bow shock is formed upstream of the magnetopause, and the flow is decelerated within the magnetosheath between the shock and the magnetopause (see Figure 2

Figure 2:

Basic structure of the magnetosphere. The faint blue arrows show the magnetic field direction toward the Earth in the northern tail lobe, away from the Earth in the southern lobe, and northward at the dayside magnetosphere. The red regions in the inner magnetosphere contain both the ring current and the outer van Allen belt, where the ions and electrons are trapped on closed drift paths. Direction of the Sun is to the left (courtesy ESA).

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 R_E. 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 R_E 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.

Figure 3:

Large-scale current systems in the magnetosphere. An enlargement of the Earth showing the auroral oval, auroral electrojet currents, and the large-scale Region 1 (more poleward) and Region 2 (more equatorward) currents bounding the high-latitude polar cap is shown in the background (Figure: Teemu Makinen/Finnish Meteorological Institute).

The solar wind motional electric field in the Earth’s frame of reference (E = −V_sw × B_IMF) 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.