Magnetospheric plasmas

3.2 Magnetospheric plasmas

While the magnetospheric field in general excludes the solar wind plasma from the magnetosphere, at times the solar wind can gain entry into the magnetosphere through the magnetopause. This occurs when the IMF and the magnetospheric fields are antiparallel and magnetic reconnection creates an “open magnetosphere” configuration with a field component normal to the boundary (Dungey, 1961

). The plasma entry is strongly modulated by the orientation of the interplanetary magnetic field, and thus enhanced during periods of southward IMF (Russell and McPherron, 1973 ). While the plasma enters mostly through the dayside magnetopause, a portion of the particles gain entry through the magnetopause boundary layers at low latitudes, well tailward of the terminator (Paschmann, 1997 ). During northward IMF, high-latitude reconnection over the magnetic poles allows for particle entry to the high-latitude plasma mantle. A minor portion of the energy, estimated to be about 10%, enters via viscous interaction processes (Axford and Hines, 1961

). The solar wind plasma, consisting mostly of protons and a small percentage of doubly charged helium and other heavier ions, populates much of the outer magnetosphere through these processes.

In addition to the solar wind flow, the atmosphere contributes both to the neutral and charged particle density in the magnetosphere. The neutral exosphere, or the upward extension of the atmosphere, fills the near-Earth space with an exponentially decreasing density (Chamberlain, 1963 ; Tinsley et al., 1986 ). While the neutral atoms and molecules are not influenced by the electromagnetic forces, the geocorona is significant to space physics processes in at least two ways: Coulomb collisions and charge-exchange processes with neutral atoms are significant loss mechanisms for energetic charged particles in the inner magnetosphere (Fok et al., 1995 ), and thus contribute to the recovery of quiet conditions after a magnetic storm. Furthermore, the slow neutral atoms charge-exchanging with fast charged particles create a population of high-speed neutrals that propagate linearly away from the collision site. During active times, when the flux of energetic charged particles is sufficiently high, this provides (the only) means to image the inner parts of the magnetosphere (Bertaux et al., 1989 ; Williams et al., 1992 ).

Above about 80 km altitude, the solar ultraviolet radiation ionizes a small portion of the atmospheric gas creating what is known as the ionosphere. The ionospheric plasma processes couple both to the neutral atmosphere below and to the magnetospheric processes above. From space weather point of view, the ionosphere is significant both because of its effects on radio waves and because it hosts strong electric currents whose effects extend to the ground. The solar wind electric field drives a global convection pattern in the ionosphere, which produces convection electrojets carried by drift currents flowing eastward on the duskside and westward on the dawnside of the ionosphere (Weimer, 1995 ). Consequently, the plasma flows from the dayside toward the nightside over the polar cap, returning to the dayside along the lower latitudes, consistent with the Dungey cycle (Dungey, 1961 ). The ionosphere is coupled to the magnetosphere by highly structured and dynamic field-aligned currents, which on an average sense form a pair of large-scale field-aligned current sheets (see Figure 3 ). The more poleward current Region 1 currents couple to the magnetotail and magnetospheric boundaries, whereas the more equatorward Region 2 currents couple to the ring current in the inner magnetosphere (Iijima and Potemra, 1976 ). The region encircling the magnetic poles where the current systems flow is also called the auroral oval, as this is the region where particles precipitating from the plasma sheet and colliding with atmospheric atoms and molecules create visible auroral light (Lui and Anger, 1973 ). During active magnetospheric processes, auroral precipitation is enhanced, localized, and highly structured, especially in the nightside oval.

The upward extension of the ionosphere, the plasmasphere, is a torus filled with low-energy (around 1 eV), dense (10 – 1000 cm⁻³) plasma originating from the ionosphere. The plasmasphere consists mostly of protons, with singly charged helium accounting for about 20% of the number density. The radius of the torus is variable, but typically the plasmasphere extends close to geostationary orbit, being more compressed during geomagnetically active times and more extended during long periods of magnetic quiescence (Grebovsky, 1970 ). The Earth’s rotation sets up an electric field, which drags the cold plasma into a corotational motion. The interplay between the solar wind-imposed electric field and the corotation electric field creates a boundary inside of which particles are trapped on closed orbits around the Earth. In the vicinity of this boundary, the plasma density has a sharp gradient; this is known as the plasmapause (Goldstein et al., 2003 ). Particles inside the trapping boundary remain on closed drift paths around the Earth while those outside the trapping boundary drift under the dawn-to-dusk electric field to the dayside boundary and are lost in the outer magnetosphere and eventually to the solar wind.

The ring current encircling the Earth roughly in the region 2 – 7 R_E from the Earth consists of plasmas originating both from the solar wind and the ionosphere (Daglis et al., 1999 ). The ring current typically consists of ions in the energy range from a few tens of keV to several hundred keV, and has a highly variable intensity controlled by the level of geomagnetic activity. During magnetically active times, ion outflow from the ionosphere is greatly enhanced, and consequently the ring current composition changes from being dominated by solar wind protons and doubly charged helium to consisting large percentages (up to dominating the mass and energy density) of ionospheric oxygen and to lesser amounts of singly charged helium (Daglis et al., 1999 ). The ring current decays via charge exchange processes and Coulomb collisions with the exospheric particles.

The outer van Allen radiation belt consists mainly of electrons in the energy range from hundreds of keV to several MeV. The electrons reside colocated with the ring current and the plasmasphere in the inner magnetosphere, roughly from 3 R_E to slightly beyond geostationary orbit. These relativistic electron fluxes show sharp dropouts and enhancements in response to the varying geomagnetic activity (Baker et al., 2001 ), and their dynamics is key to space weather, as these electrons pose a hazard to satellites in Earth orbit. Their rapid drift motion around the Earth is largely controlled by the magnetic field geometry, in contrast to the cold particles that are guided both by the electric and magnetic fields. The electron acceleration and loss processes are strongly dependent on the electric field structure and the wave-particle interactions in the inner magnetosphere (Friedel et al., 2002 ).

The variety of processes bringing particles into the inner magnetosphere, the fact that the quasi-dipolar field allows trapping of the particles to closed drift orbits, and the relatively low collision frequencies that keep the system from obtaining thermal equilibrium all contribute to the complexity and variability of the inner magnetosphere plasma populations. It is important to note that the dynamic processes all occur on time scales that are short compared to collision times, which means that the plasma populations can retain their characteristics without being thermalized, and that the plasma distribution functions can significantly deviate from simple Maxwellians. Figure 4 shows the key plasma populations in the magnetosphere.

Figure 4:

Inner magnetosphere plasma populations. The outer van Allen belt, the ring current, and the plasmasphere are colocated within the inner magnetosphere trapping region. The plasma sheet in the closed field line region extends to the dayside magnetopause and to the distant nightside tail.