Midlatitude particle and electric field effects at the onset of the November 1993 geomagnetic storm

Millstone Hill incoherent scatter radar elevation scans across midlatitudes captured the ionospheric response to storm‐induced electric field and precipitation‐induced changes near the equatorward extent of the expanding auroral region during the early phases of the November 3–4, 1993, magnetic storm. Solar wind‐induced magnetospheric compression was observed at ∼2307 UT [ Borovsky et al. , this issue] and a prompt, short‐duration (10 min) increase in the upward plasma velocity to > 100 m/s at ∼2319 UT on November 3, 1993 signaled the onset of the storm‐induced enhancement of the eastward electric field over Millstone Hill in the premidnight sector at 54° invariant latitude (Λ). This resulted in an uplifting of the F region ionosphere above the site by ∼80 km by 2330 UT. Formation of a narrow ionospheric trough poleward of the Millstone site accompanied the auroral convection enhancement at somewhat later times while plasmasheet precipitation produced the ionization at altitudes between 200 and 300 km at Λ < 60° observed by the radar. Strong precipitation of energetic particles from the outer radiation belt was observed by SAMPEX at 58.5°Λ, near the equatorward limit of the plasmasheet precipitation observed by DMSP and Millstone Hill. Amplitude perturbations of VLF signals propagating in the Earth‐ionosphere waveguide serve to localize the energetic radiation belt precipitation to latitudes between 56°Λ and 58.5°Λ and provide accurate timing of storm‐induced energetic precipitation, whose onset was at ∼2332 UT in the premidnight sector. A later enhancement of the eastward electric field at latitudes equatorward of Millstone Hill and the storm‐induced trough led to a perturbation of the midlatitude ionosphere to A < 40° and is the subject of a companion paper [ Foster and Rich , this issue].