Abstract. On the basis of geomagnetic activity indices and solar wind parameters, a superposed epoch analysis has been conducted for more than 300 geomagnetic storms. The intensity of magnetic storms is found to depend on the duration of the main phase; larger storms have longer timescales. For intense storms, however, not only the duration of energy injection into the ring current but also the strength of injection is important in determining their size. It is confirmed that the southward component of the interplanetary magnetic field plays a crucial role both in triggering the storm main phase and in determining the magnitude of magnetic storms. It is also found that the time profile of the energy injection rate during the main phase tends to have two peaks. This is particularly the case for intense magnetic storms, where the second peak is more intense than the first. Implications of our findings are discussed in terms of the existing concept of geomagnetic storms and recent observations of ring current particles and of interplanetary disturbances.

There have been numerous reports showing that space weather affects power grids through a geomagnetically induced current (GIC). Generally, power grids consist of power lines connected to transformers, of which neutral points are directly grounded. The GIC flows into those transformers through the neutral points if geomagnetic variations cause a ground level potential. These currents can damage power grids, especially transformers. It has been tacitly assumed, however, that the effect of the GIC is minor in Japan because of the country's location at geomagnetically lower latitudes. To examine the GIC effect in Japan, we conducted approximately 2 years of GIC measurements in Hokkaido, Japan. It is found that GICs associated with substorms can be detected in Japan even at the solar minimum although intense GICs do occur mostly during geomagnetic storms. Temporal variations of GICs show high correlation with geomagnetic field variations, rather than time derivatives of the geomagnetic field.

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Abstract. The occurrence frequency of precipitation bursts of > 1 MeV electrons in the outer radiation belt is examined using data from the SAMPEX satellite. Electron burst characteristics shown in this paper include the dependence of the precipitation on magnetic local time, radial distance and geomagnetic activity. Precipitation bursts with timescales < 1 s, i.e., microbursts, are studied in detail, including their dependence on the phases of geomagnetic storms. It is found that precipitation bursts occur typically in the region between L = 4 and L = 6. Microbursts tend to occur at L lower than the bursts with timescales of several tens of seconds. The number of observed microbursts significantly increases during storms, appearing mainly in the morning sector early in the recovery phase of storms. These findings suggest that the microbursts may be due to interactions with electron whistler waves, which take place near the dawnside plasmapause in the density irregularities that are perhaps created in the "recovering" plasmasphere. The prevalence of bursty precipitation indicates that this enhanced loss component of the relativistic electron flux should be taken into account in any quantitative model of relativistic electron acceleration processes.

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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 quanti®ed with Dst increases in proportion to v À3 e , where v e is the vvalue 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 ®rst adiabatic invariant. The ring current energy is also proportional to i mg , the total magnetic ®eld energy contained in the spherical shell bounded by v e and v eq , where v eq corresponds to the quiet-time location of the auroral precipitation boundary. The ratio of the ring current energy i to the dipole energy i mg is typically 10%. The ring current leads to magnetosphere in¯ation as a result of an increase in the equivalent dipole moment.

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It is shown that "having substorms" is not a necessary condition for a magnetic storm. The main phase of magnetic storms develops because of sustained, southward interplanetary magnetic field (IMF), not because of frequent occurrence of intense substorms.