Two processes have been invoked to explain the storm-time ring ring current. Some researchers attribute its current to injections of plasma into the inner magnetosphere during the expansion phase of magnetospheric substorms . The driving mechanism in this case is the explosive release of energy extracted from the solar wind/magnetosphere interaction that has been stored in the magnetotail. Other researchers contend that storm-time ring current growth is directly driven by the interaction of the solar wind with the magnetosphere. According to this view, extended periods of a strong southwardly directed interplanetary magnetic field (IMF) intensify the Earth's dawn-dusk convection electric field, which leads to the increased convective transport of charged particles from the nightside plasma sheet deep (L < 4) into the inner magnetosphere. Fluctuations in the intensity of the field play an important role in this process, by trapping the freshly injected particles in the inner magnetosphere. For longer and more intense storms, with main phases lasting 6 hours or more, the radial diffusion of previously trapped higher-energy particles across L-shells also contributes to ring current growth. The current understanding of ring current formation tends to favor the enhanced convection model over the substorm plasma injection model. However, even if substorms are not the primary driver of storm-time ring current formation, they are nonetheless thought to contribute significantly to it in two ways: first, by inducing some of the fluctuations in the global convection electric field responsible for the trapping of freshly injected particles; and second, through their role in injecting ionospheric ions into the plasma sheet, thus enhancing this ring current plasma source.
What we learn from HENA and MENA images about particle acceleration and injection will allow us to resolve the controversy about the role of substorms (impulsive electric fields) and storms (enhancement of the dawn-dusk convection electric field) in the build-up of the storm-time ring current.
Following the storm's main phase, the ring current begins to decay, returning to its pre-storm state in two to three days. During the storm recovery phase, particle transport into the ring current slows, allowing various loss processes to reduce ring current particle fluxes to their quiet-time level. The primary loss process during both the main and recovery phases is charge exchange with the neutral hydrogen atoms in the Earth's geocorona. In this process, a singly charged energetic ion trapped on a geomagnetic field line collides with a geocoronal neutral hydrogen atom and acquires its electron. This interaction produces a low-energy H+ ion and an energetic neutral atom, which, no longer trapped by the Earth's magnetic field, travels in a straight line either earthward or out into space. (This is the source of the energetic neutral atoms that IMAGE's neutral atom imagers will detect).
A second loss process, affecting principally low-energy (~10- 30 keV) ring current ions, involves Coulomb collisions with the thermal ("cold," ~1 eV) plasma of the plasmasphere. Collisions with plasmaspheric electrons result in the energy degradation of ring current ions and the formation of a population of low-energy (< 500 eV) ions inside the plasmasphere. In addition to their role in ring current energy decay, Coulomb collisions between the ring current ions and the plasmasphere have important plasmaspheric and ionospheric effects, heating the plasmasphere and providing the major energy source for stable auroral red (SAR) arcs (broad diffuse bands of atomic oxygen emissions at 630 nm occurring during the storm recovery phase in the mid-latitude ionosphere).
The third process thought to contribute to ring current decay is the precipitative loss of ring current particles into the atmosphere as a result of wave-particle interactions. The role of this loss process in the evolution of the ring current is still not well understood and is the subject of ongoing research. However, the potential effect of wave-induced particle losses on the global energy content of the ring current is illustrated by the results of a recent computer simulation that included proton scattering by ion cyclotron waves. In this simulation, a region of strong wave activity developed just inside the plasmapause, inducing enhanced proton precipitation in the afternoon-dusk local time sector. The resulting energy loss produced a ~8 nT recovery of the Dst during the one-hour simulation period.
From the location and time scale of the decay of the ENA emissions, we will be able to identify the dominant loss process for a given ring current species at a given region in the inner magnetosphere. Ion pitch-angle distributions (PAD) deconvolved from the ENA images will also furnish clues about the process responsible for the loss of a particular species. For example, charge exchange tends to produce a PAD peaking at 90 degrees, while wave-particle interactions produce smooth and sometimes "head and shoulder" distributions.
The ring current is a dynamic system with a complex structure that varies with local time, radial distance, and storm phase. A pronounced noon-midnight asymmetry exists during injection, for example, and other asymmetries in the particle distributions become evident as the ring current grows and decays. The variable structure of the ring current results from differences in the drift velocities of the trapped particles and in their susceptibility to particular loss processes, both of which, in turn, depend upon the ion species involved and its energy and pitch-angle distributions.
ENA imaging will reveal the drift patterns of energetic particles under changing magnetic field configurations and will also provide information on how and where energetic plasmas are injected, accelerated, and lost during magnetic storms and substorms. For example, acceleration and injection regions will be identifiable in the ENA images through the relatively sudden increase in emission intensity.