A rotational discontinuity can generate both a foreshock bubble and a hot flow anomaly

Abstract

Solar wind directional discontinuities can generate transient mesoscale structures upstream of Earth's bow shock, which can have a global impact on the near-Earth environment. Understanding the formation conditions of these transient structures is crucial to evaluate their contribution to solar wind-magnetosphere coupling. Hot flow anomalies (HFAs) are thought to be created only by tangential discontinuities, and develop where the discontinuities intersect the shock. Foreshock bubbles, on the other hand, are associated with both tangential and rotational discontinuities, and are generated before the discontinuities reach the bow shock, as they form due to foreshock suprathermal ions accumulation on the upstream side of the discontinuities. Here we present the results of a global 2D hybrid-Vlasov simulation of the interaction of a rotational discontinuity with near-Earth space performed with the Vlasiator model. As the discontinuity enters the simulation domain, a foreshock bubble forms duskward of the Sun-Earth line, where the foreshock is initially located. Shortly after the discontinuity makes first contact with the bow shock at the subsolar point, we find that a structure with enhanced temperature and strongly deflected flows develops at the intersection of the discontinuity with the bow shock. This structure displays typical features of an HFA. This suggests that both a foreshock bubble and an HFA can be generated concurrently by a single directional discontinuity, and that a rotational discontinuity can lead to HFA formation in some conditions. We compare the ion distribution functions inside the foreshock bubble and the HFA, and find significant solar wind core heating within the HFA, as expected from spacecraft observations. We discuss how the properties of the structures vary spatially and temporally, providing global context to localised spacecraft measurements.