Morphology of the ring current derived from magnetic field observations

Abstract

Abstract. Our examination of the 20 years of magnetospheric magnetic field data from ISEE, AMPTE/CCE and Polar missions has allowed us to quantify how the ring current flows and closes in the magnetosphere at a variety of disturbance levels. Using intercalibrated magnetic field data from the three spacecraft, we are able to construct the statistical magnetic field maps and derive 3-dimensional current density by the simple device of taking the curl of the statistically determined magnetic field. The results show that there are two ring currents, an inner one that flows eastward at ∼3 R E and a main westward ring current at ∼4-7 R E for all levels of geomagnetic disturbances. In general, the insitu observations show that the ring current varies as the D st index decreases, as we would expect it to change. An unexpected result is how asymmetric it is in local time. Some current clearly circles the magnetosphere but much of the energetic plasma stays in the night hemisphere. These energetic particles appear not to be able to readily convect into the dayside magnetosphere. During quiet times, the symmetric and partial ring currents are similar in strength (∼0.5 MA) and the peak of the westward ring current is close to local midnight. It is the partial ring current that exhibits most drastic intensification as the level of disturbances increases. Under the condition of moderate magnetic storms, the total partial ring current reaches ∼3 MA, whereas the total symmetric ring current is ∼1 MA. Thus, the partial ring current contributes dominantly to the decrease in the D st index. As the ring current strengthens the peak of the partial ring current shifts duskward to the pre-midnight sector. The partial ring current is closed by a meridional current system through the ionosphere, mainly the field-aligned current, which maximizes at local times near the dawn and dusk. The closure currents flow in the sense of region-2 field-aligned currents, downward into the ionosphere near the dusk and upward out of the ionosphere near the dawn.

Energetic ion injection and formation of the storm-time symmetric ring current

“…There has been a symmetric ring current paradigm for years in which the ring current quickly becomes symmetric after the storm main phase, when ions drifting on open drift trajectories are trapped in closed drift paths, in response to the decrease in the convection electric field (Le et al, 2004). However, both the present work and Chen et al (1994Chen et al ( , 1997 challenge this paradigm that trapping of injected ions in closed paths may appear earlier in the main phase.…”

“…In the conventional ring current pictiure, the "symmetric" ring current consists of trapped particles with closed orbit, while the particles with open convection paths create an "asymmetric" or partial ring current (Le et al, 2004). The D st index and SYM-H index are reliable indicators of the symmetric ring current strength, while the ASYM-H index measures the asymmetric ring current.…”

Statistical study on the occurrence of ASAID electric fields

“…Le et al (2004) showed that the center position of the ring current also moves to larger distances from Earth with decreasing magnetic activity: it is located at about 5 Earth radii during storm-time periods and it extends to about 6 Earth radii far from the Earth during magnetic quiet times.…”

The distribution of the ring current: Cluster observations

“…These values show the expected westward ring current around the equator. The morphology of the ring current system suggested by Iijima et al (1990) and Le et al (2004) partially closes in the ionosphere via up and down Region 2 FACs, and the ring current can extend from −65 to 65 • in latitude all over the evening and post-midnight sectors (Vallat et al, 2005). Considering the effect of the FACs on the accuracy of the current calculations, we focus here on the results in the equatorial plane (−30 to 30 • ).…”

“…It is understood in terms of the gradient and curvature drifts of energetic particles (∼1 keV to a few hundreds of keV), trapped in the geomagnetic field. Le et al (2004) examined the 20 years of magnetospheric magnetic field data from the ISEE, AMPTE/CCE, and Polar missions, and showed that there are two ring currents: an inner one flowing eastward at ∼3 R E , and the main westward ring current at ∼4-7 R E for all levels of geomagnetic disturbances. The ring current evolution is dependent on particle injections during geomagnetic activity and on loss mechanisms .…”