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A comparison of the Polar magnetic field data with the Tsyganenko
[1996] (T96) model that includes various current systems in the
magnetosphere shows that the residuals in the inner magnetosphere on
May 11 are those expected from a weak ring current. We do not expect
the magnetopause and tail currents to contribute to the residuals
because they are proportional to the square root of the solar wind
dynamic pressure and we see no such contribution at high altitudes.
On May 11, the Dst index ranged from -4 nT to +10 nT and the daily
average was 2 nT. At face value one might take this to mean that the ring
current was nearly absent at this time, but we find that we need to
put in a significant ring current in the T96 model in order to obtain
the best fit between the data and the model. Figure 4 shows the residuals
on May 11 from the T96 model that uses solar wind and IMF conditions
shown in Figure 1 and a ring current with Dst=-10 nT as inputs. By
comparing Figure 4 with Figure 3, it is clear that the residual in
the magnetic field strength around the Polar orbit perigees (0433 UT
and 2229 UT) is greatly reduced. Thus, the ring current did not
disappear on May 11 when the solar wind almost disappeared. Again
this is expected because although the ring current derives its energy
ultimately from the solar wind [Burton et al., 1975], the mass appears
to be from the ionosphere and once energized the ring current
persists for many hours.
Next: Polar cusp
Up: Polar Magnetic Field Observations
Previous: Magnetopause currents
Guan Le
12/23/1999