Cluster and MMS Simultaneous Observations of Magnetosheath High Speed Jets and Their Impact on the Magnetopause

C P Escoubet ,B L Giles,G Paschmann,Owen Roberts,R C Fear,G Lapenta,C T Russell,D G Sibeck,H Laakso,R Nakamura,Daniel B Graham,I Dandouras,Y V Bogdanova,A Varsani,J Berchem,A P Dimmock,N Aunai,K J Genestreti,R B Torbert,C Carr,Stein Haaland ,Kyoung‐Joo Hwang ,A Masson ,A N Fazakerley ,Primož Kajdič ,Sergio Toledo‐Redondo ,J P Eastwood ,Lucile Turc ,Jérémy Dargent ,M W Dunlop ,J L Burch ,Yu V Khotyaintsev ,C J Pollock ,Matt Taylor ,B Lavraud ,Cecilia Norgren ,H S Fu

doi:10.3389/fspas.2019.00078

Abstract

Discontinuities in the solar wind, bow shock ripples or ionized dust clouds carried by the solar wind, high speed jets (HSJs) are observed in the magnetosheath. These HSJs have typically a Vx component larger than 200 km s-1 and their dynamic pressure can be a few times the solar wind dynamic pressure. We use a conjunction of Cluster and MMS, crossing simultaneously the magnetopause, to study the characteristics of these HSJs and their impact on the magnetopause. Over one hour-fifteen minutes interval in the magnetosheath, Cluster observed 21 HSJs. During the same period, MMS observed 12 HSJs and entered the magnetosphere several times. A jet was observed simultaneously by both MMS and Cluster and it is very likely that they were two distinct HSJs. TDuring this period, two and six magnetopause crossings were observed respectively on Cluster and MMS with a significant angle between the observation and the expected normal deduced from models. The angles observed range between from 11° up to 114°. One inbound magnetopause crossing observed by Cluster (magnetopause moving out at 142 km s-1) was observed simultaneous to an outbound magnetopause crossing observed by MMS (magnetopause moving in at -83 km s-1), showing that the magnetopause can have multiple local indentation places, most likely independent from each other. Under the continuous impacts of HSJs, the magnetopause is deformed significantly and can even move in opposite directions at different places. It can therefore not be considered as a smooth surface anymore but more as surface full of local indents. Four dust impacts were observed on MMS, although not at the time when HSJs are observed, showing that dust clouds would have been present during the observations. No dust cloud in the form of Interplanetary Field Enhancements was however observed in the solar wind which may exclude large clouds of dust as a cause of HSJs. Radial IMF and Alfven Mach number above 10 would fulfil the criteria for the creation of bow shock ripples and the subsequent crossing of HSJs in the magnetosheath.

Highlights

The coupling between the solar wind and the Earth’s magnetosphere is one of the most studied phenomena since the first spacecraft measurements of the magnetopause at the beginning of the 1960s (Cahill and Amazeen, 1963)
We have studied high speed jets (HSJs) characteristics and their impact on the magnetopause at two widely separated points (10 RE) across the dayside magnetosheath, using the Cluster and MMS constellations
- interplanetary magnetic field (IMF) was radial with a low cone angle at the center of the event; - HSJs were observed at Cluster 25 min before MMS; - HSJs were characterized by a dominant Vx component with strong Vy at –Y position (MMS) and strong Vz components at –Z position (Cluster); - 21 and 12 HJSs were observed by Cluster and MMS, respectively; - Two HJSs were observed simultaneously at Cluster and MMS and given their characteristics and size, they would most likely be two separated HSJs; - The largest HSJs observed, respectively, by Cluster and MMS had a computed size along the flow of 4.3 and 9.8 RE

Summary

INTRODUCTION

The coupling between the solar wind and the Earth’s magnetosphere is one of the most studied phenomena since the first spacecraft measurements of the magnetopause at the beginning of the 1960s (Cahill and Amazeen, 1963). The maximum flow observed by C1 in Vx was −350 km s−1 and the maximum of Pvx was 4.37 nPa. The two magnetopause crossings can be clearly seen on MMS1 data (Figures 8E–H) with the sharp change of energy in the ions going from sheath like plasma with energy around 1 keV to magnetospheric plasma with energy around 10. The first magnetopause crossing shows a short negative Vz flow of −245 km s−1 at 00:39:46 (Figure 8F, blue line), which was larger in absolute terms than the velocity components (Vx = −94 km s−1 and Vy = −110 km s−1) This may be an indication of reconnection taking place at the magnetopause between the southward magnetic field in the magnetosheath and the northward magnetic field in the magnetosphere. The normal to the magnetopause model at 01:06:24

Method

DISCUSSION AND CONCLUSION

Findings

DATA AVAILABILITY STATEMENT