Ionospheric signature of the cusp as seen by incoherent scatter radar

Measurements with the Sondre Stromfjord incoherent scatter radar, co-ordinated with the observations by the Freja satellite, have been performed during three campaigns, April 1993, February 1994, and May-June 1994. Radar signatures of various types of magnetosheath particle injections in the cusp-cleft region are investigated. The measurement days represent very different geomagnetic conditions, from very quiet to a Kp index of 7+. On three occasions both Freja and the radar detected the cusp. A unique cusp signature is found for a relatively stable cusp, distinguishing it from the many other soft precipitation features seen around noon. The signature includes extremely high electron temperatures in a latitudinally well-defined region with a sharp equatorward border, some F region electron density enhancement, ion outflow, and mainly poleward plasma flow. Enhanced ion temperatures are also seen in the vicinity of, but not exactly coincident with, the electron temperature enhancements. Other dayside precipitation features observed with an intense soft component are narrow arcs, which usually have an accompanying accelerated electron component of several hundred eV to some keV energy. These are typically seen in, or bordering, convection regions where the plasma flow vorticity implies upward field-aligned currents.