  Hot flow anomalies (HFAs) were first discovered in the early 1980s at the bow
shock of the Earth. In the 1990s these features were studied, observed and
simulated very intensively and many new missions focused the attention to this
phenomenon again. Many basic features and the HFA formation mechanism were
clarified observationally and using hybrid simulation techniques. We described
previous observational, theoretical and simulation results in the research
field of HFAs. We introduced HFA observations performed at the Earth, Mars,
Venus and Saturn in this paper. We share different observation results of space
mission to give an overview to the reader. Cluster multi-spacecraft
measurements gave us more observed HFA events and finer, more sophisticated
methods to understand them better. In this study, HFAs were studied using
observations of the Cluster magnetometer and the Cluster plasma detector aboard
the four Cluster spacecraft. Energetic particle measurements (28.2-68.9 keV)
were also used to detect and select HFAs. We studied several specific features
of tangential discontinuities generating HFAs on the basis of Cluster
measurements in the period February-April 2003, December 2005-April 2006 and
January-April, 2007, when the separation of spacecraft was large and the
Cluster fleet reached the bow shock. We have confirmed the condition for
forming HFAs, that the solar wind speed is higher than the average. This
condition was also confirmed by simultaneous ACE magnetic field and solar wind
plasma observations at the L1 point 1.4 million km upstream of the Earth. The
measured and calculated features of HFA events were compared with the results
of different previous hybrid simulations. During the whole spring season of
2003, the solar wind speed was higher than the average. Here we checked whether
the higher solar wind speed is a real condition of HFA formation also in 2006
and 2007.
