Origin of the SuperDARN broad Doppler spectra: simultaneous observation with Oersted satellite magnetometer

We perform a case study of a favorable conjunc- tion of an overpass of the Oersted satellite with the field-of- view of the SuperDARN Syowa East radar during an interval of the southward IMF Bz. At the time, the radar observed an L-shell aligned boundary in the spectral width around the dayside ionosphere. Simultaneously, high-frequency (0.2- 5 Hz) magnetic field fluctuations were observed by the Oer- sted satellite's high-time resolution magnetometer. These magnetic field fluctuations are considered to be Alfvwaves possibly associated with the particle which precipitates into the dayside high-latitude ionosphere when magnetic recon- nection occurs. It has been theoretically predicted that the time-varying electric field is the dominant physical process to expand the broad HF radar Doppler spectra. Our obser- vation clearly demonstrates that the boundary between nar- row and broad spectral widths is corresponding well to the boundary in the level of the fluctuations, which supports the previous theoretical prediction. A close relationship between electric and magnetic field fluctuations and particle precipi- tations during southward IMF conditions has been confirmed by many authors. The present observation allows us to sug- gest that the boundary between narrow and broad Doppler spectral widths observed in the dayside ionosphere is con- nected with the signature of the open/closed field line bound- ary, such as the cusp particle precipitations via electric and magnetic field fluctuations for the case of the negative IMF Bz conditions.