An ionospheric signature of possible enhanced magnetic field merging on the dayside magnetopause

Identifying the causative mechanisms at the magnetopause that produce a variety of transient plasma velocity signatures in the high latitude ionosphere is difficult. Correct identification is of fundamental importance in determining how solar wind energy is coupled to the magnetosphere. Observations in conjugate hemispheres offer the chance to distinguish between events triggered by merging and those initiated by solar wind pressure variations, if the direction of travel of the ionospheric signatures can be determined. Using data from two conjugate H F radars, high temporal resolution measurements of the F-region plasma convection in the vicinity of the cusp are presented for 22 April 1988. In a previous study of this dataset [Greenwaldet al., 1990. J. geophys. Res.95, 8057], the authors identified a particular region of the cusp ionosphere as being the footprint of the antiparallel merging line on the magnetopause. Following an enhancement in the Interplanetary Magnetic Field's (IMF) southward component the inferred ionospheric footprint of the magnetopause merging line moved equatorward for 20 min. In this period, two poleward-directed bursts of high plasma velocities (~2000 m/s) were observed in the southern hemisphere, occurring close to the ionospheric footprint of the merging line, with a weaker (~ 1000 m/s) response in the northern hemisphere. Poleward-directed flow at lower latitudes, crossing the inferred polar cap boundary, was also observed. We interpret the flow bursts as a signature of an event driven by magnetic merging at the magnetopause, primarily because the motion of the flow burst features in each hemisphere was conditioned by the prevailing east-west component of the IMF. The azimuthal variation of the radar's line-of-sight velocities may be interpreted as showing the presence of vortices which is consistent with theoretical models advanced for the ionospheric signature of patchy reconnection. However, the presence of vortices was not detected by the radar until at least 3 min after the onset of the poleward flow burst. The hemispherical differences in the plasma velocities of the flow burst events may be explained in terms of the differences in the mapping of the magnetopause merging lines to the conjugate hemispheres.

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