Magnetosheath pressure pulses: Generation downstream of the bow shock from solar wind discontinuities

Abstract

We present multipoint Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations of transient dynamic pressure pulses in the magnetosheath 3–10 times the background in amplitude, due to enhancements in both the ion density and velocity. Their spatial dimensions are of the order of ∼1 RE parallel to the flow and ∼0.2–0.5 RE perpendicular to it, inferred from the difference in the amplitudes observed by the different spacecraft. For the first time, simultaneous observations of the solar wind and foreshock are also shown, proving no similar dynamic pressure enhancements exist upstream of the bow shock and that the majority of pulses are downstream of the quasi‐parallel shock. By considering previously suggested mechanisms for their generation, we show that the pressure pulses cannot be caused by reconnection, hot flow anomalies, or short, large‐amplitude magnetic structures and that at least some of the pressure pulses appear to be consistent with previous simulations of solar wind discontinuities interacting with the bow shock. These simulations predict large‐amplitude pulses when the local geometry of the shock changes from quasi‐perpendicular to quasi‐parallel, while the opposite case should also produce notable pulses but typically of lower amplitude. Therefore, in a given region of the magnetosheath, some of the discontinuities in the solar wind should generate pressure pulses, whereas others are expected not to. There is also evidence that the pulses can impinge upon the magnetopause, causing its motion.