Plasma Flow in the Magnetotail and its Implications for Substorm Theories
The neutral line theory of substorms has evolved from early thinking about magnetic energy conversion in solar flares and in the outer magnetosphere and has now attained a relatively high level of sophistication. Some large-scale phenomena that are predicted by this theory are (a) plasma will flow rapidly earthward and tailward along the tail’s midplane away from the merging region; (b) the component of the magnetic field normal to the midplane will be oppositely directed on the two sides of the merging region; (c) in large substorms magnetic merging can reach the field lines bounding the plasma sheet, possibly causing loss of the tail ward portion of the plasma sheet to the solar wind. Observations with satellites located near the center of the tail at distances beyond 20 RE have revealed combinations of these phenomena occurring in more than 25 substorms and often agreeing in remarkable detail with predictions of the theory. This paper deals primarily with plasma flow measurements during substorms and reports on a recent study that shows tailward flow to be much more prevalent than earthward flow during the expansive phase of substorms. This finding tends to contradict an earlier study that reported instances of earthward flow to far outnumber instances of tailward flow during (substorm-related) plasma sheet thinnings. It calls into question a “rarefaction wave model” of substorms that was partially based on that earlier report.
KeywordsSolar Wind Field Line Plasma Flow Plasma Sheet Neutral Line
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