Magnetospheric access of solar particles and the configuration of the distant geomagnetic field
<p>The access of 1.2-40 MeV protons and 0.4-1.0 MeV electrons from interplanetary space to the polar cap regions has been investigated with an experiment on board a low altitude, polar orbiting satellite (OG0-4).</p> <p>A total of 333 quiet time observations of the electron pola...
| Summary: | <p>The access of 1.2-40 MeV protons and 0.4-1.0 MeV electrons from
interplanetary space to the polar cap regions has been investigated with
an experiment on board a low altitude, polar orbiting satellite (OG0-4).</p>
<p>A total of 333 quiet time observations of the electron polar cap
boundary give a mapping of the boundary between open and closed geomagnetic
field lines which is an order of magnitude more comprehensive than
previously available.</p>
<p>Persistent features (north/south asymmetries) in the polar cap proton
flux, which are established as normal during solar proton events,
are shown to be associated with different flux levels on open geomagnetic
field lines than on closed field lines. The pole in which these persistent
features are observed is strongly correlated to the sector structure
of the interplanetary magnetic field and uncorrelated to the north/south
component of this field. The features were observed in the north (south)
pole during a negative (positive) sector 91% of the time, while the
solar field had a southward component only 54% of the time. In addition,
changes in the north/south component have no observable effect on the
persistent features.</p>
<p>Observations of events associated with co-rotating regions of enhanced
proton flux in interplanetary space are used to establish the
characteristics of the 1.2 - 40 MeV proton access windows: the access window
for low polar latitudes is near the earth, that for one high polar
latitude region is ~250 R<sub>⊕</sub> behind the earth, while that for the other
high polar latitude region is ~1750 R<sub>⊕</sub> behind the earth. All of the
access windows are of approximately the same extent (~120 R<sub>⊕</sub>). The following
phenomena contribute to persistent polar cap features: limited
interplanetary regions of enhanced flux propagating past the earth,
radial gradients in the interplanetary flux, and anisotropies in the
interplanetary flux.</p>
<p>These results are compared to the particle access predictions of
the distant geomagnetic tail configurations proposed by Michel and Dessler,
Dungey, and Frank. The data are consistent with neither the model of
Michel and Dessler nor that of Dungey. The model of Frank can yield a
consistent access window configuration provided the following constraints
are satisfied: the merging rate for open field lines at one polar neutral
point must be ~5 times that at the other polar neutral point, related
to the solar magnetic field configuration in a consistent fashion,
the migration time for open field lines to move across the polar cap
region must be the same in both poles, and the open field line merging
rate at one of the polar neutral points must be at least as large as
that required for almost all the open field lines to have merged in
0 (one hour). The possibility of satisfying these constraints is investigated
in some detail.</p>
<p>The role played by interplanetary anisotropies in the observation
of persistent polar cap features is discussed. Special emphasis is given
to the problem of non-adiabatic particle entry through regions where the
magnetic field is changing direction. The degree to which such particle
entry can be assumed to be nearly adiabatic is related to the particle
rigidity, the angle through which the field turns, and the rate at which
the field changes direction; this relationship is established for the
case of polar cap observations.</p>
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