A CME at the sun produced a magnetic cloud led by an interplanetary shock that reached the earth just before midnight on 24 Sept. 1998. The shock arrived at the Wind spacecraft at 22:30 UT as illustrated in Figure 1. Prior to shock arrival, the dynamic pressure was around 2 nPa, and it was smooth with a variability of only 10%. At shock passage, the solar wind speed increased abruptly from about 450 km/s to 650 km/s, and continued to ramp up to over 800 km/s. The dynamic pressure spiked to 12-15 nPa behind the shock, and fluctuated with a similar amplitude over the several hours after the shock. The Wind spacecraft was located 185 RE upstream of the Earth.
The magnetic field enhancement associated with the shock was largely in the and directions (toward the sun), but rotated over the next 2-3 hours to an away (from the sun) orientation. Over the same period after shock passage, the interplanetary field was initially variable, but turned and remained southward for about 12 hours beginning at about 02:00 UT on 25 Sept. (at the magnetopause), with peak component of -20 nT. More information concerning the magnetic field is given in Russell et al. .
POLAR descended from apogee over the north polar cap of the earth, headed sunward toward the dayside auroral zone region, as shown in Plate 1. At 23:45:30 UT, the magnetic field at POLAR abruptly tilted 20 toward the sun and increased in magnitude smoothly and exponentially (), from 125 to 185 nT, with a time constant of 0.95 min. The plasma source instrument (PSI) was operating during this period, so the spacecraft potential was steady at +2.5V, making low energy outflow observations possible in the high polar cap. The polar wind parallel flow prior to the event was relatively steady for several hours and contained fluxes of O, He, and H, the flow velocity being about 30-40 km/s for the protons.
During the abrupt field change, the net plasma flow was nearly perpendicular to the local magnetic field, at proton flow velocity of 100 km/s, after which the perpendicular flow subsided and field-aligned flow increased to proton bulk flow velocity of 100 km/s. The magnetopause displacement (integrated transverse proton flow) was . After the change, a large increase in the temperature of the H flow was observed, as evidenced by the large increase in angular extent of the H angular distribution around the local magnetic field direction (see TIDE data panels, Plate 1). The O flow energy rose abruptly to 1.2 keV (see TIMAS data panels, Plate 1), but the O remained relatively cold and beam-like.
We interpret these changes as resulting from the displacement of the mantle and geopause, such that POLAR left the cold polar wind outflow region and entered the mantle outflows of magnetosheath plasma from the cusp region. The mantle was mixed with relatively energetic ionospheric outflows originating from the cusp region ionosphere and having parallel velocities similar to the mantle protons, owing to the velocity filter effect. The four bursts containing an isotropic H component seen in the TIMAS (panel 4) and TIDE (Panel 2) data in Plate 1 at 00:20, 00:55, 01:20, and 01:50 were coincident with bursts of upflowing energetic He (not shown). These impulsive events may result from brief displacements of the magnetopause even closer to POLAR, but this is beyond the scope of the present study.
Beginning at about 02:00 UT on 25 Sept., POLAR flew across the geopause again in the opposite direction, and reentered exclusively polar wind outflows. The H content of the polar wind had by then fallen near the detection threshold while the O flux had increased and become dominant. From 03:00 to 03:30, the outflow was joined by a significant component of molecular ions, perhaps representing the peak of the mass ejection rate (well after negative IMF began). The O flow speed continued to decline as the flux increased until a minimum flow energy of 30-40 eV was reached at about 04:00-04:30 UT, near local noon, an altitude of 4.6 , and invariant latitude of 78. The flow accelerated and continued to increase in flux as the spacecraft continued equatorward toward the cusp, which was encountered at an unusually low invariant latitude of less than 70, reflecting the negative IMF at that time. The velocity changes of the O outflow were again consistent with the velocity filter dispersion of the cusp outflows.
POLAR flew through the inner magnetosphere and across the southern auroral zone and polar cap at an altitude of about 0.8 , observing plasma closer to the source region. In Plate 2 the ionospheric plasma at and immediately poleward of the cusp, dominantly O in this case, is typical of an upwelling ion event [Pollock et al., 1990]. At the equatorward edge, the flow exhibits strong bulk transverse heating and upward flow. At higher latitudes, the temperature and flow velocity drop rapidly. The integral upward flux at the peak observed by TIDE at 0710 UT, was 2x10, covering all relevant energies, normalized to 1000 km altitude. In the same location, TIMAS registered a total ion flux of about 2x10 above 15eV. In the more poleward part of the event, the temperature dropped to as low as 0.5eV, with similar total flux, but the outflow was by then entirely below the TIMAS energy range. Even further into the polar cap, a downward fountain flow was present.