Observations and Simulations of a Double‐Core Hot Flow Anomaly.

Hot Flow anomalies (HFAs), one of the most well‐analyzed transient phenomena in the Earth's foreshock, are known as kinetic structures driven by tangential discontinuities (TDs). Recently, a 2‐dimensional (2D) magnetohydrodynamics (MHD) model reproduced HFAs with either a high‐ or low‐density core. Further investigation of an HFA with two cores observed by the Magnetospheric Multiscale (MMS) mission is reported. The observation via the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission suggests this MHD HFA is associated with a foreshock density hole‐like structure. The trailing flux tube in simulation may propagate with a TD in the foreshock. Our work suggests that HFAs with two low‐density cores can also be achieved in MHD process. Results show the total ram pressure can be an excellent diagnostic for the presence of transient structures, such as HFAs, at the bow shock. Plain Language Summary: The hot flow anomaly (HFA) is a typical foreshock transient on the upstream of Earth's bow shock. This phenomenon is characterized by heating and significant flow deflection inside its core region and is traditionally believed to be a kinetic structure associated with tangential discontinuities (TDs). More recently, HFA‐like structures have also been generated through magnetohydrodynamic (MHD) simulations. Here, we present an HFA with two low‐density cores captured by the Magnetospheric Multiscale (MMS) satellite. Observation from the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission suggests the observed double‐core HFA is driven by foreshock density hole‐like structure, which is propagating with the observed TD to the bow shock. The observed double‐core HFA is reproduced by a 2‐dimensional (2D) MHD model, in which two low‐density flux tubes are inputted simultaneously. Our observation and simulations show that the total ram pressure is an excellent indicator for the presence of transient structures at the bow shock. Key Points: Double‐core hot flow anomalies can be reproduced by 2‐dimensional magnetohydrodynamic model with two simultaneous low‐density flux tubesThe simulation results and the observation show general agreementThe total ram pressure is an excellent indicator for the formation of hot flow anomalies at the bow shock [ABSTRACT FROM AUTHOR]