Abstract
The CUTLASS Finland HF radar has been operational since February 1995. The radar frequently observes backscatter during the midnight sector from a latitude range 70â75° geographic, latitudes often associated with the polar cap. These intervals of backscatter occur during intervals of substorm activity, predominantly in periods of relatively quiet magnetospheric activity, with Kp during the interval under study being 2- and ΣKp for the day being only 8-. During August 1995 the radar ran in a high time resolution mode, allowing measurements of line-of-sight convection velocities along a single beam with a temporal resolution of 14 s, and measurement of a full spatial scan of line-of-sight convection velocities every four minutes. Data from such scans reveal the radar to be measuring return flow convection during the interval of substorm activity. For three intervals during the period under study, a reduction in the spatial extent of radar backscatter occurred. This is a consequence of D region HF absorption and its limited extent in the present study is probably a consequence of the high latitude of the substorm activity, with the electrojet centre lying between 67° and 71° geomagnetic latitude. The high time resolution beam of the radar additionally demonstrates that the convection is highly time dependent. Pulses of equatorward flow exceeding ~600 m sâ1 are observed with a duration of ~5 min and a repetition period of ~8 min. Their spatial extent in the CUTLASS field of view was 400â500 km in longitude, and 300â400 km in latitude. Each pulse of enhanced equatorward flow was preceded by an interval of suppressed flow and enhanced ionospheric Hall conductance. The transient features are interpreted as being due to ionospheric current vortices associated with field aligned current pairs. The relationship between these observations and substorm phenomena in the magnetotail is discussed.
Highlights
Substorm physics remains a very active ®eld and in spite of recent progress towards an understanding of the substorm process, that understanding is as yet incomplete
In the present study an analysis of HF radar measurements of ionospheric convection and the location and dynamics of the auroral electrojets from ground magnetometers is presented. This demonstrates that for intervals of quiet magnetic activity, when the main substorm electrojets are at high latitudes ($70° geomagnetic), HF absorption has only a limited eect on the radar observations of the substorm expansion phase
The limited eect of HF absorption in the present study is probably a consequence of the relatively high latitude of the substorm activity recorded here, with the electrojet centre lying between 67° and 71° geomagnetic latitude
Summary
Substorm physics remains a very active ®eld and in spite of recent progress towards an understanding of the substorm process, that understanding is as yet incomplete. In the present study an analysis of HF radar measurements of ionospheric convection and the location and dynamics of the auroral electrojets from ground magnetometers is presented This demonstrates that for intervals of quiet magnetic activity, when the main substorm electrojets are at high latitudes ($70° geomagnetic), HF absorption has only a limited eect on the radar observations of the substorm expansion phase. These observations provide a context for data from the high time resolution beam of the CUTLASS Finland radar, which has revealed new observations of transient convection vortices in the substorm expansion phase. These vortices are characterised with the radar and magnetometer data and their relationship with magnetotail phenomena is discussed
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