STORM TIME ELECTRIC-FIELD PENETRATION OBSERVED AT MIDLATITUDE

Under disturbed geomagnetic conditions the latitudinal profile of the westward ion convection (equivalent to poleward electric field) observed with the Millstone Hill incoherent scatter radar at dusk, often exhibits a double peak (dual maxima). During the height of the February 8-9, 1986, magnetic storm the Millstone Hill radar was in the evening local time sector (1600-2200 MLT). Radar observations indicate that high speed (> 1000 m s-1) westward ion flow penetrated deeply below 50-degrees invariant latitude (LAMBDA) and persisted for 6 hours between 2100 UT on February 8 and 0300 UT on February 9. The double-peaked ion convection feature was pronounced throughout the period, and the separation in the dual maxima ranged from 4-degrees to 10-degrees. The latitude positions of the high-latitude ion drift peak and the convection reversal varied in unison. The low-latitude ion drift peak (approximately 49-degrees-LAMBDA or L = 2.3) did not show significant universal time/magnetic local time (UT/MLT) variation in its latitude location but showed a decrease in magnitude during the initial recovery phase of the storm. Using simultaneous particle (30 eV - 30 keV) precipitation data from the DMSP F6 and F7 satellites, we find the high-latitude ion drift peak to coincide with the boundary plasma sheet/central plasma sheet transition in the high ionospheric conductivity (> 15 mho) region. The low-latitude ion drift peak lay between the equatorward edges of the electron and soft (< 1 keV) ion precipitation in the low conductivity region (approximately 1 mho). A comparison between the low-altitude observations and simultaneous ring current observations from the high-altitude AMPTE satellite further suggests that the low-latitude ion drift peak is closely related to the maximum of the O+ dominated ring current energy density in magnetic latitude. The low-latitude ion drift peak is the low-altitude signature of the electric field shielding effect associated with ring current penetration into the outer layer of the storm time plasmasphere. Unlike the transient and localized subauroral ion drifts under moderately disturbed conditions, the intense westward ion drifts developed in response to heavy ion ring current shielding during a great magnetic storm can decouple from the high-latitude electric field and penetrate to very low latitudes and persist for long durations in the dusk and early afternoon MLT sectors. These features confirm the active role of storm time ring current dynamics in generating the low-latitude extension of the magnetospheric electric field.