The hot flow anomaly (HFA) is a region of the solar wind where the plasma density and temperature are significantly higher than in the surrounding area. The HFA was first observed in the early 1970s by spacecraft such as NASA’s Pioneer 10 and 11, and has been studied extensively since then by many different spacecraft.
HFAs are typically found at high latitudes, away from the sunward direction, and tend to be associated with regions of low magnetic field strength. They are believed to be caused by a combination of factors, including the interaction of the solar wind with the interstellar medium, turbulence within the solar wind itself, and/or Alfvén waves propagating through the plasma.
HFAs can have a significant impact on planetary atmospheres, particularly if they interact with a planet’s magnetosphere. For example, HFAs have been linked to auroral activity on Earth and Jupiter. Additionally, HFAs can accelerate particles to very high energies, making them potentially hazardous to both spacecraft and astronauts.
Despite their importance, HFAs remain one of the least understood aspects of solar wind-planetary interactions. continued research is needed to better understand how these anomalies form and evolve over time.