SPRG Seminars
February 23, 2010:
" Optical and ground based magnetic observations of substorms - Could they fit a simple qualitative model? "
Stephen Mende, University of California, Berkeley, Space Sciences Laboratory
It is widely accepted that the largest magnetospheric instabilty, the Magnetospheric Substorm, energizes particles by converting magnetic field energy into particle energy. During the substorm expansive phase highly stretched field lines attain more dipolar field geometry. This “dipolarization” is topologically equivalent to a reduction of the near earth cross tail current responsible for stretching the field. From 40 plus years of ground and spacecraft based observations there is overwhelming evidence that substorm onset occurs at relatively low latitudes which are likely to map to the inner magnetosphere. Observation of substorm current system and associated E field measurements show that at onset the equatorward ionospheric E field and the corresponding westward currents intensify and that the current is primarily a Hall current in the ionosphere.
At each end of the electrojet large field aligned currents (FAC) are driven, upward on the west and downward on the east ends of the electrojet. The appearance of the equatorward electric field in the ionosphere at onset is probably due to ion inertia during dipolarization. The Alfvenic auroras that appear near onset and are located poleward of the “breakup arc” signify the arrival of large amplitude Alfven waves which appear to be the foot print of dipolarization. The above phenomena appear qualitatively consistent with a model instability in which a slight perturbation reducing the cross tail current causes intense dipolarization through ionospheric positive feed back. There are several mechanisms that can provide the initial perturbation.