Starting at 14:57:06, MMS2 observes a localized structure of increased magnetic field and density (red shaded region, 1). 6 provides jet-associated measurements for the innermost (MMS4, MMS3) satellites. 5 shows 2D reduced velocity distribution functions (VDFs) for MMS2 and MMS3, while Fig. Figure 4 provides detailed measurements, during the jet observations, for the outermost (MMS2, MMS1) spacecraft. 3a, b provides ion and magnetic field measurements for MMS2 and MMS1. Figure 2a, b provides ion and magnetic field measurements for MMS2 and MMS1, during the corresponding period while Fig. Figure 1 shows the satellite separation in the xy and xy plane, which are effectively identical (string-of-pearls configuration). We use data from the MMS spacecraft 28 on from 14:56:50 UTC to 14:58:20 UTC. The string-of-pearls configuration and the relatively stable shock conditions allow us to observe the development of both phenomena, originating at the upstream region, evolving and ending up downstream in the magnetosheath. Furthermore, we observe localized downstream density enhancements (embedded plasmoids 23, 25) generated by the same process. In contrast to earlier suggested mechanisms, we show that high-speed jets downstream of the quasi-parallel bow shock can be generated as a direct consequence of the upstream wave evolution and the bow shock reformation cycle. In this work, we use data from recently available unique string-of-pearls configuration of the four Magnetosphere Multiscale MMS spacecraft 28 that allow to follow the jet formation at the shock. However, no direct observations have been made so far and the exact causal link has yet to be revealed. Some studies have speculated on the connection of jets to upstream magnetic compressive structures (e.g., SLAMS) 25, 26, 27. Although several mechanisms have been proposed to explain how jets are generated, their origin is still not understood. Some proposed generation mechanisms connect jets to the solar wind interaction with the local inclination of bow shock ripples 9, 18, 22, 23 or to solar wind discontinuities 24. They have been suggested to trigger magnetopause reconnection 19, excite surface eigenmodes on the magnetopause 20 and accelerate electrons 21. One important property of quasi-parallel shocks is the formation of downstream jets with high dynamic pressure, well above the solar wind dynamic pressure 9, 17, 18. Their interaction with the shock environment gives rise to a new shock front, while the previous one convects into the magnetosheath region (reformation) 5, 10, 12, 13, 14, 15, 16. These waves evolve, and get steepened to a larger amplitude as the solar wind brings them back to the shock. Furthermore, the shock is dynamically evolving through its interaction with the foreshock waves upstream of it. It has been shown that the quasi-parallel shock contains local curvature variations (ripples) 8, 9, 10, 11. The quasi-parallel shock itself is a place that is dynamically evolving, giving rise to several phenomena embedded in its structure. The shock and its upstream and downstream region create a complex environment in which several magnetospheric phenomena of diverse nature have been observed, like Short Large Amplitude Magnetic Structures (SLAMS), reconnecting current sheets, and fast plasma flows 4, 6, 7. Downstream of it, the shocked solar wind forms a highly variable environment named the magnetosheath. The type of bow shock that is most challenging to study is the so called quasi-parallel shock, where the upstream magnetic field is approximately parallel to the shock’s surface normal 4, 5. Earth’s bow shock, resulting from the interaction of the super-magnetosonic solar wind and Earth’s magnetic field, has been studied for over 50 years and due to the availability of in-situ measurements, serves as an ideal astrophysical laboratory to study collisionless shocks 1, 2, 3.
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