Polar cap patches are regions of enhanced plasma density, seen to drift fromthe sunlit ionosphereon the day side, over the polar cap, and into the night side Auroral oval. The patches are relatedto substorm activity and reconnection processes in Earth’s magnetic tail.
The fact that the patches crosses the open-closed field line boundary (OCB) and goes intothe poleward edge of the aurora, can be used as an indicator of the night side merging gap, andthe ongoing reconnection processes in the tail.
A statistics of the occurrence of polar cap patch OCB crossings at the night side and itsdependence on IMF By are made using data from Meridian scanning photometer (MSP) andAll-sky camera (ASI) in Ny- Ålesund, Svalbard. This statistics can give indications of the positioningof the night side merging gap and it’s IMF By dependence.
Polar cap patches was found to cross the night side OCB over Svalbard in the interval 18:30– 04:50 MLT with an average of 23:25 MLT, more than half an our before magnetic midnight.The changes in IMF By seems to be able to move the area where patches crosses the OCB by∼ 1 hour MLT.