On the entry and transit of high‐density plasma across the polar cap

Observations are presented from SuperDARN and the EISCAT Svalbard Radar of two intense polar cap patch events on 6 February 2001. The interplanetary magnetic field (IMF) was dominated by a large positive By component, and for both events the electron density exceeded 10 12 m −3 in the F region. With SuperDARN we tracked the events all the way across the polar cap, from the dayside Svalbard sector to the nightside Alaska sector. The flow speed was highly dynamic and pulsed, and both patches underwent substantial rotation in the polar cap. On the nightside the leading edge had become the trailing edge. It suggests that the first patch to enter the polar cap on the dayside may not always be the first patch to reach the nightside; plasma might be stagnant in the polar cap or even overtaken. The study also provides evidence that momentum transfer in the dayside polar cap can last significantly longer than 10 min after reconnection, especially for extremely long field lines where IMF By is dominating, i.e., on “old open field lines.” Knowledge of the solar wind driver and the coupling processes is therefore extremely important for predicting the motion of a polar cap patch event across the polar cap. Gradients in the plasma flow associated with the rotation of the extreme density may in itself lead to a stronger growth of ionospheric irregularities. These irregularities may continue to grow all the way across the polar cap. The result is more efficient creation of ionospheric irregularities.