The Ionospheric Cusp's Response to a Sharp Southward Turning of the Interplanetary Magnetic Field

M. WATANABE (1), P.E. SANDHOLT (2), M. LESTER (3), and N. SATO (1).
1. National Institute of Polar Research, Japan
2. University of Oslo, Norway
3. University of Leicester, UK

We present preliminary results of a case study on a midday auroral breakup event associated with a sharp southward turning of the interplanetary magnetic field (IMF), using CUTLASS HF radar data and optical data over Svalbard. Around 0930 UT, January 21, 1999, a well-defined IMF transition form positive to negative impinged on the magnetosphere. The meridian scanning photometer at Ny Aalesund observed optical signatures of the southward turning in the midday sector. At 0936 UT, a small auroral activity started at 78.7 MLAT (magnetic latitude), and then moved to 75.9 MLAT by 0940 UT. After that, a spell of poleward moving auroral forms were found. Corresponding to these optical signatures, the Finland radar also observed typical southward turning signatures with a 20-s time resolution. The equatorward edge of the ionospheric scatter moved from 78.7 to 75.9 MLAT for 0936-0940 UT. At this stage, changes in lone-of-sight velocities are not so significant. At 0940 UT, poleward flow bursts (> 800 m/s) started as often interpreted as ionospheric signatures of flux transfer events. In this paper, we focus on the very early stage of the cusp's response (0936-0945 UT) and discuss its nature from the point of view of the magnetosphere-ionosphere coupling.

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