Radar and satellite observations of the storm time cleft

Abstract

During the magnetic storm of February 8–9, 1986, the region of strong ion convection in the vicinity of the dayside cusp expanded equatorward into the field of view of the Millstone Hill radar at lower mid‐latitudes. High‐speed (>1.5 km s−1) poleward ion flows were found at latitudes as low as 60° invariant latitude, at least 10° lower than the typical cleft/cusp position for moderately disturbed (Kp>4) magnetospheric conditions. The ion velocity pattern responded promptly to changes in the interplanetary magnetic field By direction. The large‐scale two‐dimensional convection pattern across the dayside was well resolved using radar azimuth scan data at Millstone Hill, thus enabling us to place the fine‐scale radar/satellite observations of the storm time cusp and cleft in the context of the large‐scale pattern. We present a detailed comparison of radar and DMSP F7 satellite observations in the prenoon sector during a period of Kp > 7, to examine the low‐altitude signatures of various plasma regions in the vicinity of the cusp. The combination of particle precipitation, magnetic field perturbation, radar measurements of ion heating, and convection consistently suggests the unusual low‐latitude position of cusp at 65° invariant latitude. Boundary plasma sheet particles were observed to coincide with a narrow region of magnetic‐field‐aligned currents, and with antisunward convection flows at the equatorward edge of the cleft. This may be an indication of the presence of a viscous cell. The radar and satellite observations indicate that the storm time cleft and cusp at ionospheric altitudes retain the general characteristics revealed in their average configuration. Particle and field signatures of the plasma sheet, plasma sheet boundary layer, low‐latitude boundary layer, cusp, and mantle are identified at unusually low magnetic latitude in the 9–10 magnetic local time sector during this event.