Interplanetary magnetic field control of polar patch velocity

Polar patch drift speed and direction have been investigated using oxygen 630 nm images recorded by an all‐sky imager at Eureka (89° CGM), Canada over an extended period, January and December 1998. Statistical studies showed that (1) the interplanetary magnetic field (IMF, from the WIND satellite) Bz component linearly controls the patch speed when Bz is between −7.5 nT and 0 nT. The speed tends to saturate when Bz is less than −7.5nT due to nonlinear coupling between the solar wind and the magnetosphere. The average patch speed of 600 m/s is in agreement with results from earlier studies; (2) The IMF By or the IMF clock angle has a clear control of the patch drift direction as determined by the drift azimuth angle. When ∣By∣ is less than 7.5nT, the drift azimuth angle is linearly and positively correlated with By. For a large ∣By∣ (>7.5 nT), the positive correlation is replaced by a negative correlated linear relation and the azimuth angle tends to turn towards 180 degrees; that is, the patches drift in an antisunward direction. These IMF By effects can be qualitatively explained by the northern winter polar ionospheric convection models developed by Weimer [1995] and Hairston and Heelis [1990]. Results from our quantitative study on the IMF control of polar patch speed and drift direction provide constraints for the development of future polar ionospheric convection models.