Magnetosheath Jets and Plasmoids: Characteristics and Formation Mechanisms from Hybrid Simulations

Abstract

Magnetosheath jets and plasmoids are very common phenomena downstream of Earth’s quasi-parallel bow shock. As the increase of the dynamic pressure is one of the principal characteristics of magnetosheath jets, the embedded paramagnetic plasmoids have been considered as an special case of the former. Although the properties of both types of structures have been widely studied during the last 20 years, their formation mechanisms have not been examined thoroughly. In this work we perform a 2D local hybrid simulation (kinetic ions – fluid electrons) of a quasi-parallel (θ Bn = 15°), supercritical (M A = 7) collisionless shock in order to study these mechanisms. Specifically, we analyze the formation of one jet and one plasmoid, showing for the first time that they can be produced by different mechanisms related to the same shock. In our simulation, the magnetosheath jet is formed according to the mechanism proposed by Hietala, where at the shock ripples the upstream solar wind suffers locally less deceleration and the flow is focused in the downstream side, producing a compressed and high-velocity region that leads to an increase of dynamic pressure downstream of the shock. The formation of the plasmoid, however, follows a completely new scenario being generated by magnetic reconnection between two plasma layers with opposite B-field orientation in the region just behind the shock.

Downstream high-speed plasma jet generation as a direct consequence of shock reformation

“…The structure now, as viewed in the magnetosheath, includes a pile-up region (patterned red region) of the waves associated to the shock’s ramp evolution. Similar events have been discussed in recent studies 23 , 35

Fig. 7 Time-shifted magnetic field measurements for Magnetosphere Multiscale (MMS) 1–4.

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“…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. Furthermore, we observe localized downstream density enhancements (embedded plasmoids 23 , 25 ) generated by the same process. 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.…”

“…They have been suggested to trigger magnetopause reconnection 19 , excite surface eigenmodes on the magnetopause 20 and accelerate electrons 21 . 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 . Although several mechanisms have been proposed to explain how jets are generated, their origin is still not understood.…”

Downstream Super-magnetosonic Plasma Jet Generation as a Direct Consequence of Shock Reformation

“…The structure now includes a pile-up region (patterned red region) of the waves associated to the shock's ramp evolution. Similar events have been discussed in recent studies [17,25] At approximately 14:57:35 -14:57:45 (as viewed in FIG. 3), MMS4 observes a new compressed plasma region ("2"), spatially detached from the initial shock ("1"), forming upstream of the first, becoming the new local shock front, and thus completing a bow shock reformation process/cycle.…”

“…During this time, region "1" interacts with the faster upstream waves, forming a pile-up region. Thus, while travelling downstream, the region 1 gets extended, forming a region of increased magnetic field and density that corresponds to the equivalent downstream phenomenon of "embedded plasmoid" or "shock remnant" at the magnetosheath [17,20]. Note that, this structure, due to the pile-up process, is not anymore polarized, and its origin would have been unknown in the absence of upstream measurements.…”