CUTLASS IMAGE OBSERVATIONS OF HIGH-LATITUDE CONVECTION FEATURES DURING SUBSTORMS/

The CUTLASS Finland HF radar has been operational since February 1995. The radar frequently observes backscatter during the midnight sector from a latitude range 70-75 degrees geographic, latitudes often associ ated with the polar cap. These intervals of backscatter occur during i ntervals of substorm activity, predominantly in periods of relatively quiet magnetospheric activity, with K-p during the interval under stud y being 2- and Sigma K-p for the day being only 8-. During August 1995 the radar ran in a high time resolution mode, allowing measurements o f line-of-sight convection velocities along a single beam with a tempo ral resolution of 14 s, and measurement of a full spatial scan of line -of-sight convection velocities every four minutes. Data from such sca ns reveal the radar to be measuring return flow convection during the interval of substorm activity. For three intervals during the period u nder study, a reduction in the spatial extent of radar backscatter occ urred. This is a consequence of D region HF absorption and its limited extent in the present study is probably a consequence of the high lat itude of the substorm activity, with the electrojet centre lying betwe en 67 degrees and 71 degrees geomagnetic latitude. The high time resol ution beam of the radar additionally demonstrates that the convection is highly time dependent. Pulses of equatorward flow exceeding similar to 600 m s(-1) are observed with a duration of similar to 5 min and a repetition period of similar to 8 min. Their spatial extent in the CU TLASS field of view was 400-500 km in longitude, and 300-400 km in lat itude. Each pulse of enhanced equatorward flow was preceded by an inte rval of suppressed flow and enhanced ionospheric Hall conductance. The transient features are interpreted as being due to ionospheric curren t vortices associated with field aligned current pairs. The relationsh ip between these observations and substorm phenomena in the magnetotai l is discussed.