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Ann. Geophys., 21, 869-879, 2003
www.ann-geophys.net/21/869/2003/
© European Geosciences Union 2003
Substorm-associated large-scale magnetic field changes in the magnetotail: a prerequisite for "magnetotail deflation" events
H. Nakai1 and Y. Kamide2
1Ibaraki High School, Ibaraki, Osaka, Japan
2Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa, Japan
Abstract. An attempt is made to
search for a critical condition in the lobe magnetic field to initiate
large-scale magnetic field changes associated with substorm expansions. Using
data from ISEE-1 for 1978, sudden decreases in the lobe magnetic field
accompanied by magnetic field dipolarizations are identified. In this study,
such events are designated as the magnetotail deflation. The magnetic field
component parallel to the equatorial plane, BE , is normalized to a
fixed geocentric distance, BEN , and is corrected for the
compression effect of the solar wind dynamic pres-sure, BENC . It is
shown that the BENC value just prior to a magnetotail deflation
correlates well with the Dst index; BENC = 37.5 - 0.217 Dst0,
where Dst0 denotes the Dst value corrected for the
solar wind dynamic pressure. This regression function appears to delineate the
upper limit of BENC values, when they are sorted by the Dst0
index. On the basis of this finding it is suggested that a prerequisite
condition for magnetotail deflations must exist in the magnetosphere.
Key words. Magnetospheric physics (magnetotail;
current systems; storms and substorms)
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