Magnetosheath high speed jets and foreshock transients observed by Cluster and MMS

Abstract

<p>Magnetosheath High Speed Jets (HSJs) are regularly observed downstream of the Earth&#8217;s bow shock. Determining their origin from spacecraft observations is however a challenge since (1) L1 solar wind monitors are usually used with their inherent inaccuracy when plasma and magnetic data are propagated to the bow shock, (2) the number of measurement points around the bow shock are always limited. Various mechanisms have been proposed to explain HSJs such as bow shock ripples, solar wind discontinuities, foreshock transients, pressure pulses or nano dust clouds and it is difficult to relate these to HSJs with the lack of simultaneous measurements near the bow shock and immediately upstream. &#160;We will use a special Cluster campaign, where one spacecraft was lagged 8 hours behind the three other spacecraft, to obtain near-Earth solar wind measurements upstream of the bow shock, together with simultaneous measurements in the magnetosheath. The event of interest is first observed by ACE on 13 January 2019 as a short 10 minutes period of IMF-Bx dominant (cone angle around 140 deg.). This IMF-Bx dominant period is also observed, one hour later, by THEMIS B and C (ARTEMIS) and Geotail, which were at 60 and 25 R_E from Earth on the dawnside. Cluster 1 and Cluster 2 just upstream of the bow shock, at 17 R_E from Earth, observed also such IMF-Bx dominant period together with energetic ions reflected from the bow shock and foreshock transients. Preliminary analysis indicate that these transients would be hot flow anomalies. Finally, Cluster 3 and 4 and MMS1-4, a few R_E from each other downstream of the shock, observed a turbulent magnetosheath with HSJs for 15 minutes. The HSJ characteristics are investigated with the constellation of 6 spacecraft, as well as their relation to hot flows anomalies observed upstream.</p>