We have reported new observations of the magnetospheric responses to a typical CME incident upon the Earth.
The first response observed was the propagation of the MHD compression through
the polar cap, displacing the geopause inward past POLAR, immersing it in
mantle
plasma. The mantle flows consisted of a mixture of hot magnetosheath protons
(and He
) reflected from the cusp region, with relatively cold O
outflows of similar parallel velocity. The mantle was bursty, apparently in
response to gusty solar wind conditions.
The second response observed was the arrival at POLAR of the leading edge of
what we refer to as an ionospheric mass ejection. Relatively cold and slow
plasma, principally O, but including light and molecular ions at times, was
observed to flow from the ionosphere into the polar lobes, indicating energy
inputs that evidently originated with the incidence of the CME, and subsided
after it passed. We interpret the change from an H/O mixture to an
O-dominated outflow as resulting from the effects of CME incidence upon the
magnetosphere, and the dissipation of energy throughout the topside ionosphere
in the dayside auroral zone or cleft region, likely by field-aligned current
intensifications.
Topside ionospheric heating and outflow was observed in the dayside southern
hemisphere auroral zone after several hours of magnetospheric buffeting by
strong and variable solar wind dynamic pressure (and also after the development
of negative IMF 
.) Plasma upwelling was much stronger in flux (see below)
during this perigee pass than during the preceding pass. The POLAR apogee pass
following this one early on 25 Sept. exhibited O-dominated outflow
throughout the pass, but at significantly lower flux levels. Conditions similar
to those before the event reestablished themselves in the polar cap outflows
about two orbits (36hrs) after the leading edge of the CME arrived. Higher time
resolution observations of the low altitude heating and field-aligned currents
are available from the FAST spacecraft [R. Strangeway, personal communication].
To place these results in broader context, we plot in Figure 2 the
relationship observed during the DE-1 epoch, between peak upwelling flux and
solar wind dynamic pressure variability [Pollock et al., 1988]. A subset of the O
flux events used in the [Pollock et al., 1990] study were selected, for which solar
wind and magnetic field data sets from the ISEE-1 spacecraft could be obtained
from the NSSDC. The 1 min average peak upwelling flux was correlated with prior
hour-averaged interplanetary conditions, delayed to the magnetopause. The
Akasofu dynamo power parameter 
, the Reiff
polar cap potential proxy 
, the plasma
momentum flux 
, and its variability were studied. The peak
upwelling O flux was found to be best correlated with the variability of
the
dynamic pressure at the magnetopause (R=0.76). Correlations with the IMF were
poor (R 
±0.1-0.2), suggesting that the energy coupling to the
dayside
ionospheric outflows is controlled by the dynamic pressure and relatively
unaffected by dayside reconnection.
The perigee pass outflow fluxes from the present event have been plotted with
open symbols in Figure 2. The present observations lie below but otherwise
follow the trend established by the DE-1/ISEE-1 study. The 1981-2 DE-1 data
were
obtained in a period of average 10.7cm flux 208 [10
] while the present data were obtained at a time of lower solar activity
with
10.7cm flux = 136. The F10.7 effect on O outflow noted by
Yau
et al. [1985] suggests a factor of 2 difference between 1981 and
Sept.
1998. Other factors, such as the recent history of internal geomagnetic
activity, may have contributed to the somewhat larger ratio of the fluxes for
these two epochs.