Modulation of nightside polar patches by substorm activity

Abstract. Results are presented from a multi-instrument study showing the influence of geomagnetic substorm activ-ity on the spatial distribution of the high-latitude ionospheric plasma. Incoherent scatter radar and radio tomography mea-surements on 12 December 2001 were used to directly ob-serve the remnants of polar patches in the nightside iono-sphere and to investigate their characteristics. The patches occurred under conditions of IMF Bz negative and IMF By negative. They were attributed to dayside photoionisation transported by the high-latitude convection pattern across the polar cap and into the nighttime European sector. The patches on the nightside were separated by some 5 ◦ latitude during substorm expansion, but this was reduced to some 2◦ when the activity had subsided. The different patch sepa-rations resulted from the expansion and contraction of the high-latitude plasma convection pattern on the nightside in response to the substorm activity. The patches of larger sep-aration occurred in the antisunward cross-polar flow as it en-tered the nightside sector. Those of smaller separation were also in antisunward flow, but close to the equatorward edge of the convection pattern, in the slower, diverging flow at the Harang discontinuity. A patch repetition time of some 10 to 30 min was estimated depending on the phase of the sub-storm