Observations of isolated polar cap patches by the EISCAT Svalbard and SuperDARN Finland radars

In this paper we present new observations of the F-region cusp ionosphere from the SuperDARN Finland and EISCAT Svalbard radars in a new joint experiment. On 16 December 2002 the EISCAT Svalbard radar was operated in a fast elevation scan along beam 9 of the SuperDARN Finland radar giving altitude/latitude profiles of the SuperDARN Finland backscatter volume every two minutes. Combining the two independent data sets we monitor the formation of an isolated polar cap patch of high electron density (1011.5 m-3) that slowly formed in the dark cusp ionosphere during an interval of northward interplanetary magnetic field. The patch formed on a stirred lobe cell, excluding the possibility for solar EUV ionized plasma to drift in from the south. This data set represents an unparalleled example of patch formation within the polar cap, locally by particle impact ionization. After the interplanetary magnetic field turned southward and By changed polarity the patch started to move poleward into the polar cap. Enhanced velocity gave rise to enhanced HF backscatter power and enhanced spectral widths, consistent with gradient drift instability as the formation mechanism of HF backscatter targets.