Storm time heavy ion outflow at mid‐latitude

Abstract

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.