The period of several days preceding this day was
mildly disturbed. There was a minor magnetic storm
during 28–29 December, with a minimum SYM-H
value of 46 nT (not shown here). In Fig. 1a, the
vertical dotted line indicates a southward turning of
the IMF at 1457 UT on 30 December, which
occurred in the recovery phase of the second minor
storm with a minimum SYM-H of 36 nT (not
shown here). Correspondingly, the direction of the
IEF Ey changes from dawnward to duskward. The
horizontal dashed lines are used to indicate the
maximum and minimum values of the IMF/IEF.
The IEF Ey changes from 1:97mVm1 (minimum)
at 1457 UT to 3:12mVm1 (maximum) at
1536 UT, and the net increase of the IEF Ey is
5:09mVm1. Similarly, the ionospheric electric field
changes from 0:17mVm1 at 1457 UT to 0:46
mVm1 at 1542 UT, and the net increase of the ionospheric electric field Ey is 0:63mVm1. We
assume that near local noon, the variation of the
background ionospheric electric field is relatively
small within a short interval. Therefore, the increase
of the ionospheric electric field is assumed to be
caused solely by the increase of the IEF. In other
words, the increase of the ionospheric electric field is
the penetration electric field.
The period of several days preceding this day was
mildly disturbed. There was a minor magnetic storm
during 28–29 December, with a minimum SYM-H
value of 46 nT (not shown here). In Fig. 1a, the
vertical dotted line indicates a southward turning of
the IMF at 1457 UT on 30 December, which
occurred in the recovery phase of the second minor
storm with a minimum SYM-H of 36 nT (not
shown here). Correspondingly, the direction of the
IEF Ey changes from dawnward to duskward. The
horizontal dashed lines are used to indicate the
maximum and minimum values of the IMF/IEF.
The IEF Ey changes from 1:97mVm1 (minimum)
at 1457 UT to 3:12mVm1 (maximum) at
1536 UT, and the net increase of the IEF Ey is
5:09mVm1. Similarly, the ionospheric electric field
changes from 0:17mVm1 at 1457 UT to 0:46
mVm1 at 1542 UT, and the net increase of the ionospheric electric field Ey is 0:63mVm1. We
assume that near local noon, the variation of the
background ionospheric electric field is relatively
small within a short interval. Therefore, the increase
of the ionospheric electric field is assumed to be
caused solely by the increase of the IEF. In other
words, the increase of the ionospheric electric field is
the penetration electric field.
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