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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° invariant latitude (Λ) 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° to 10°. The latitude positions of the high‐latitude ion drift peak and the convection reversal varied in unison. The low‐latitude ion drift peak (∼49°Λ 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 (∼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.

Abstract. In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22-23, 1999, with the hourly Dst index being -167 nT at -2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of -231 nT at -0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS) -

Local ionospheric observations with the Millstone Hill incoherent scatter radar reveal an upward ion bulk velocity in excess of 3 km s−1 at 1000 km altitude during the very large magnetic storm on February 8, 1986. The upward flux of O+ ions exceeded 3×109 cm−2s−1 at 42° geodetic latitude (55°Λ) for a 3‐hour period around 18 MLT during the event. Frictional ion heating with ion temperatures in excess of 4000°K at 500 km altitude was observed by the radar in the vicinity of the ion outflow event. Satellite observations place the ion outflow event within a region of intense ion and electron precipitation on field lines associated with the storm‐perturbed ring current. For a one‐dimensional analysis of the observed plasma profiles, continuity considerations indicate a region of intense O+ production (200 cm−3 s−1) as well as significant upward acceleration (5–10 m s−2) in the region between 600 km and 800 km altitude where the outflow approaches supersonic speed. Ionizing collisions involving fast backsplash neutral O atoms (Torr et al., 1974) produced by ring current heavy ion precipitation can provide sufficient upward momentum to account for the acceleration in the observed outflowing thermal O+ fluxes. Alternatively, the outflow event can be explained in terms of a time‐dependent diffusion process triggered by a sudden change in the frictional heating rate in the collision‐dominated F region (St.‐Maurice, 1989). The concurrence of rapid ion convection and energetic ring current precipitation is unique at mid‐latitudes during intense magnetic storms. Under these conditions, our observations indicate that the mid‐latitude ionosphere constitutes a significant source of upflowing thermal O+ fluxes to the overlying magnetosphere.

Regions of intense keV electron precipitation, such as inverted‐V structures, at times colocate with ionospheric plasma depletion regions in the high‐latitude polar ionosphere. When Defense Meteorological Satellite Program (DMSP) F6 and F7 satellites, at 840 km, enter these regions in darkness, ion signatures of high spacecraft‐to‐ambient plasma potential differences (several hundred volts negative) are observed with the new SSJ/4 ion detectors. A systematic survey of charging events and the environment in which they occur was made using the DMSP F6 and F7 precipitating ion and electron detectors, the SSIE thermal plasma probes, and the SSM (F7 only) vector magnetometer. The charging events of November 26, 1983, are analyzed in detail since they occurred on both satellites. Critical levels of number flux and average energy for the precipitating electrons, and the threshold density of the thermal ionospheric ions are defined for different levels of spacecraft charging.