N. C. Maynard, W. J. Burke, D. R. Weimer, F. S. Mozer, J. D.
Scudder, C. T. Russell, and W. K. Peterson
Using measurements from the EFI, MFE, HYDRA, TIMAS
instruments on POLAR and supplemented with solar wind
information from WIND, we have been able to observe the
existence of a counter-clockwise-rotating lobe convection cell
in the cusp/mantle post noon. This configuration is expected
in a four cell convection pattern and is necessary to fit
observed NBZ field aligned current patterns in the polar cap
for Bz north; however, it has been difficult to pinpoint in
statistical derivations of ionospheric convection patterns.
Its appearance is a consequence of transitioning from the
distorted two-cell convection pattern generally observed with
IMF By and Bz positive to a four cell convection pattern as
the IMF clock angle approaches zero. Two POLAR passes above
the northern dayside ionosphere with similar IMF
characteristics (as determined by the WIND satellite) but
slightly different orbit tracks were used for this study. In
one instance the downward moving ions displayed a ``reverse'',
velocity-dispersion feature, with the highest energies
detected near the poleward boundary of the cusp, indicating
that POLAR crossed magnetic field lines that map upward to a
merging region poleward of the cusp. This feature was driving
a counter-clockwise rotating (positive potential) cell, which
was adjacent to a negative potential afternoon cell
encompassing precipitation from CPS and boundary layer
sources. In the other case, which crossed the cusp-mantle
region at slightly later times, POLAR exited a wide boundary
layer directly into the polar cap, where it detected a
clockwise-rotating (negative potential) lobe cell, poleward of
its merging line. The wide boundary layer maps to the dusk
flank well down the tail. At different times during the
passes POLAR encountered particle fluxes with central plasma
sheet (CPS), boundary layer, cusp and polar cap spectral
characteristics. Electric fields measured during
cusp/boundary layer passages are marked by very large
variability. The short time required for low-altitude
satellites to cross these regions restricts observations into
the PC 1 frequency range. High-altitude variations extend
from the PC 1 through the PC 4 ranges. Comparing electric
potential distributions detected at altitudes near 5 Re
with predictions under prevailing solar wind conditions of the
Weimer [1996] model derived from ionospheric measurements, we
found that the model correctly predicted the low-latitude
boundaries of measurable convection in the CPS, the presence
of lobe convection cells, and a region of relative convective
stagnation. While it failed to predict the positive potential
cell in the cusp for the observed IMF conditions, the model
does indicate the appearance of such a cell at smaller IMF
clock angles. Analysis of northward IMF convection is
continuing with other cusp passes under different clock angle
conditions.