Substorm Currents on the Equatorial Plane
W. Sun and S.-I. Akasofu
(1) Horizontal current vectors and field-aligned currents in the ionosphere
The upper and middle rows in Figure 1 show the distribution of ionospheric current vectors and field-aligned currents at 1130, 1140, 1200 and 1210 UT on March 19, 1978. These patterns are produced by using the KRM algorithm [Kamide et al., 1981] on the basis of magnetometer records at 71 stations in the polar region. As the insert in Figure 1 shows, the growth phase that began at about 1010 UT is reaching near its end at 1135 UT. The substorm onset is at 1135 UT, reaching the maximum epoch at 1210 UT. One can see a dramatic increase of the westward current (blue arrows) and the intense wedge-like field-aligned currents (red contours is upward, blue is downward) in the late evening sector at 11:40 UT, 5 minutes after the onset.
Fig 1
(2) Calculation of currents in the equatorial plane
Field-aligned currents must be closed in the equatorial plane to form current loops on the basis of the principle of the current continuity. However, it is not well understood how the current is closed in the equatorial plane. Bostrom [1964] suggested that the longitudinal (azimuthal) and latitudinal (radial) components of the ionospheric current are closed separately in the equatorial plane through the field-aligned currents to form longitudinal and latitudinal current loops. Bostrom's model provides a basic mode of current connection between the ionosphere and the magnetosphere. Sun et al. [1996] demonstrated the feasibility of inferring the distribution of substorm electric currents in the equatorial plane on the basis of ionospheric current vectors and field-aligned currents during substorms by using Bostrom's model. They found that the projected radial currents in the equatorial plane are in good agreement with the results observed by the satellite AMPTE/CCE [Iijima et al., 1990]. It was also shown that the azimuthal currents observed by Iijima et al. are mainly composed of the ring current and the cross-tail current. The ionospheric contribution to the azimuthal component is estimated to be about 20% of the total current.
The bottom row in Figure 1 shows the equatorial currents inferred from the ionospheric currents on the basis of Bostrom's model. One can see that the dipolarization at the onset may be resulted from the current that has a strong eastward component (red arrows) which counters the westward cross-tail current (blue arrows) that grew during the growth phase (1030 -1130 UT). In fact, the total current direction had an eastward component at 1140 UT. Such a current can cause a sort of "over dipolarization" that was sometimes observed at the geosynchronous satellites. This shows that the dipolarization is not just reducing or diverting the cross-tail current, which indicates that currents grow against the normal tail current.
(3) Reconfiguration of magnetic field lines during substorms
Fig 2 |
Figure 2 shows the configuration of magnetic field lines from 67°, 68° and 69° in latitude in 23 MLT sector between 1000 UT and 1210 UT on March 19, 1978, which was calculated by the currents in the plasma sheet from Tsyganenko's model (1987) plus mapped equatorial current from the ionosphere as shown in the bottom row of Figure 1 [Sun et al., 1991]. One can see that the stretched field lines prior to substorm onset at 1130 UT contract and become dipolar after the onset (1135 UT). This phenomenon is often referred to as "dipolarization" . |
