Magnetotail Assimilation Model

S. Wing and P. T. Newell
The Johns Hopkins University Applied Physics Lab.
Simon.wing@jhuapl.edu/Fax:240-228-6670

2D/3D global distributions of the plasma sheet pressure, temperature, and
density are crucial to understand many physical processes and dynamics in the
magnetotail.  Providing these global quantities is a goal of several future NASA
multi-spacecraft missions.  However, assimilating the unprecedentedly large
multi-point data sets in a coherent and unified manner poses a challenge.  A
technique that can serve as a foundation for assimilating low-, mid-, and
high-altitude observations from these multispacecraft missions to create 2D/3D
images of the plasma sheet ion pressure, temperature, and density is presented.
This method relies on the plasma sheet isotropy, which has a strong theoretical
foundation and overwhelming observational support.  Previously developed
algorithms are used to detect the plasma isotropy boundary and to detect and
exclude acceleration events.  Global equatorial plasma sheet pressure, density,
and temperature inferred from DMSP particle observations are presented.  A
statistical and a case study show a favorable comparison with in situ
measurements.  The power this method for magnetotail investigations is
illustrated in two examples:  (1) the dynamics of the 2D equatorial plasma sheet
under northward and southward IMF orientations and (2) the quiet time plasma
sheet ion pressure contribution to the field-aligned currents.  During periods
of northward IMF, cold dense ions can be found plentifully along the dawn and
dusk flanks of the plasma sheet, suggesting massive LLBL ion entry into the
magnetosphere along the flanks.  However, during periods of southward IMF, the
presence of the cold dense ions is noticeably diminished, especially along the
dusk flank where the density peak is barely discernible.  The nightside region-1
Birkeland currents exhibit a dawn-dusk asymmetry that can be explained by the
dawn-dusk asymmetry in the plasma pressure arising from the E x B and
curvature/gradient drifts of ions originating from the deep tail and LLBL.  In
the region tailward/poleward of region-1 currents, the pressure gradient
generates additional currents, that have been called region-0 and that flow in
the opposite sense as region-1.