Abstract
Using the auroral boundary index derived from DMSP electron precipitation data and the Dst index, changes in the size of the auroral belt during magnetic storms are studied. It is found that the equatorward boundary of the belt at midnight expands equatorward, reaching its lowest latitude about one hour before Dst peaks. This time lag depends very little on storm intensity. It is also shown that during magnetic storms, the energy of the ring current quantified with Dst increases in proportion to Leâ3, where Le is the L-value corresponding to the equatorward boundary of the auroral belt designated by the auroral boundary index. This means that the ring current energy is proportional to the ion energy obtained from the earthward shift of the plasma sheet under the conservation of the first adiabatic invariant. The ring current energy is also proportional to Emag, the total magnetic field energy contained in the spherical shell bounded by Le and Leq, where Leq corresponds to the quiet-time location of the auroral precipitation boundary. The ratio of the ring current energy ER to the dipole energy Emag is typically 10%. The ring current leads to magnetosphere inflation as a result of an increase in the equivalent dipole moment.Key words. Ionosphere (Auroral ionosphere) · Magnetospheric physics (Auroral phenomena; storms and substorms)
Highlights
During the last two decades, a series of extensive statistical studies have shown that the auroral belt expands/contracts in latitude systematically, respondingCorrespondence to: Y
Using a data set of geomagnetic storms for the period from 1983 to 1991, we have shown in the present study that changes in the auroral boundary index follow very closely those in the Dst index
The energy of the ring current calculated from the Dst index is found to correlate with vÀe 3, where ve is the v-value corresponding to the equatorward boundary of the auroral belt
Summary
During the last two decades, a series of extensive statistical studies have shown that the auroral belt expands/contracts in latitude systematically, respondingCorrespondence to: Y. The equatorward shift of the auroral belt has been closely associated with the development of geomagnetic storms, i.e., decreases in the Dst index, their study is limited to only a few individual storms. There have been, no quantitative, as well as statistical, studies regarding the relationship between the size of the auroral belt and the intensity of geomagnetic storms, because it is intrinsically dicult to dierentiate the substorm and storm eects in terms of the location of auroras. During the main phase of a magnetic storm, which is associated with southward IMF, intense substorms occur successively. It is under debate whether the storm-time ring current develops due to sustained, southward IMF or due to frequent occurrence of substorms (Kamide, 1992; Gonzalez et al, 1994)
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