Spontaneous Hot Flow Anomalies in Earth's Foreshock

Abstract In recent years, we have learned that foreshock transients play an influential role in solar wind coupling with Earth's magnetosphere. These transients include spontaneous hot flow anomalies (SHFAs), which are characterized by dips in magnetic field magnitude and ion density, enhanced temperature, decelerated and deflected solar wind flow, and are surrounded by ultra low frequency (ULF) waves. SHFAs share some characteristics with hot flow anomalies, but their formation mechanism does not require a discontinuity in the solar wind. SHFAs evolve close to the shock from caviton interaction with backstreaming ions. We use MMS magnetic field and plasma data to study SHFAs' internal structure and their evolution. In addition we analyze the properties of velocity distribution functions inside the events and in their surroundings. Our findings show that the morphology of the SHFAs is diverse depending on where they are located. They can appear deep in the foreshock or close to the ULF wave foreshock boundary, where they separate regions with different types of waves. Those SHFAs observed closer to the bow shock can experience more compression at their rear edge. Overall, our results show that the internal structure of SHFAs is more complex and diverse than previously thought.