Chapter 25 Cluster Hot Flow Anomaly Observations During Solar Cycle Minimum

Hot flow anomalies (HFAs) are studied using observations of the FGM magnetometer and the CIS plasma detector aboard the four Cluster spacecraft. Pre- viously we studied several specific features of tangential discontinuities on the basis of Cluster measurements in February-April 2003 and discovered a new condition for forming HFAs that is the solar wind speed is higher than the average. How- ever during the whole spring season of 2003, the solar wind speed was higher than average. In this study we analyse HFAs detected in 2007, the year of solar cycle minimum. Our earlier result was confirmed: the higher solar wind speed is a real condition for HFA formation; furthermore this constraint is independent of Schwartz et al.'s condition for HFA formation.