  Hot flow anomalies (HFAs) are studied using observations of the RAPID
suprathermal charged particle detector, the FGM magnetometer, and the CIS
plasma detector aboard the four Cluster spacecraft. Previously, we studied
several specific features of tangential discontinuities on the basis of Cluster
measurements in February-April 2003. In this paper, we confirm the following
results: the angle between the Sun direction and the tangentional discontinuity
(TD) normal is larger than 45{\deg} during HFAs, the magnetic field directional
change is large. We then present evidence for a new necessary condition for the
formation of HFAs, that is, the solar wind speed is significantly ( about 200
km/s or dMf=2.3) higher than the long-term average. The existence of this
condition is also confirmed by simultaneous ACE MAG and SWEPAM solar wind
observations at the L1 point 1.4 million km upstream of the Earth. The results
are compared with recent hybrid simulations.
