Based on global hybrid simulation results, we predict that foreshock turbulence can reach the magnetopause and lead to reconnection as well as Earth‐sized indents. Both the interplanetary magnetic ...field (IMF) and solar wind are constant in our simulation, and hence, all dynamics are generated by foreshock instabilities. The IMF in the simulation is mostly Sun‐Earth aligned with a weak northward and zero dawn‐dusk component, such that subsolar magnetopause reconnection is not expected without foreshock turbulence modifying the magnetosheath fields. We show a reconnection example to illustrate that the turbulence can create large magnetic shear angles across the magnetopause to induce local bursty reconnection. Magnetopause reconnection and indents developed from the impact of foreshock turbulence can potentially contribute to dayside loss of planetary plasmas.
Plain Language Summary
Turbulence structures are commonly generated as ions reflected by planetary bow shocks interact with the incoming solar wind. We use a large‐scale simulation treating ions as particles and electrons as a fluid to investigate the impact of these turbulence structures on the magnetopsheres. Based on the simulation results, we predict that the turbulence can open magnetic field lines on the dayside and lead to planet‐sized indents. Our work unfolds a potential pathway through which planetary plasmas can escape to the upstream solar wind.
Key Points
Foreshock turbulence can reach the magnetopause under a northward quasi‐radial interplanetary magnetic field with zero dawn‐dusk component
The turbulence can create large magnetic shear angles across the magnetopause, leading to local bursty reconnection
Bombardments of the turbulence cause Earth‐sized magnetopause indents under constant interplanetary magnetic field and solar wind
Results from particle-in-cell simulations of reconnection with asymmetric upstream conditions are reported to elucidate electron energization and structure of the electron diffusion region (EDR). ...Acceleration of unmagnetized electrons results in discrete structures in the distribution functions and supports the intense current and perpendicular heating in the EDR. The accelerated electrons are cyclotron turned by the reconnected magnetic field to produce the outflow jets, and as such, the acceleration by the reconnection electric field is limited, leading to resistivity without particle-particle or particle-wave collisions. A map of electron distributions is constructed, and its spatial evolution is compared with quantities previously proposed to be EDR identifiers to enable effective identifications of the EDR in terrestrial magnetopause reconnection.
Abstract Collisionless shocks are ubiquitous structures throughout the Universe. Shock waves in space and astrophysical plasmas convert the energy of a fast-flowing plasma to other forms of energy, ...including thermal and magnetic energies. Plasma turbulence and high-amplitude electric and magnetic fluctuations are necessary for effective energy conversion and particle acceleration. We survey and characterize in situ observations of reflected ions and magnetic field amplification rates at quasiperpendicular shocks under a wide range of upstream conditions. We report magnetic amplification rates as high as 25 in our current data set. Reflected ions interacting with the incoming plasma create magnetic perturbations that cause magnetic amplification in upstream and downstream regions of quasiperpendicular shocks. Our observations show that, in general, magnetic amplification increases with the fraction of reflected ions, which itself increases with Mach number. Both parameters plateau once full reflection is reached. Magnetic amplification continuously increases with the inverse of the magnetization parameter of the upstream plasma. We find that the extended foot region upstream of shocks and nonlinear processes within that region are key factors for intense magnetic amplification. Our observations at nonrelativistic shocks provide the first experimental evidence that below a certain magnetization threshold, the magnetic amplification efficiency at quasiperpendicular shocks becomes comparable to that at the quasiparallel shocks.
We investigate waves close to the lower-hybrid frequency in 12 magnetotail reconnection electron diffusion region (EDR) events with guide field levels of near-zero to 30%. In about half of the ...events, the wave vector has a small component along the current sheet normal, consistent with known lower-hybrid drift wave properties, but the perpendicular magnetic field fluctuations can be comparable or greater than the parallel component, a feature unique to the waves inside and adjacent to EDRs. Another new wave property is that the wave vector has a significant component along the current sheet normal in some events and completely along the normal for one event. In 1/4of the events, the ∇∙𝑷(sub 𝑒) term has a significant contribution to the wave electric field, possibly a feature of lower-hybrid waves more likely to exist in the diffusion region than further away from the X-line. Electron temperature variations are correlated with the wave potential, due to wave electric field acceleration and crossings at the corrugated separatrix region with different amounts of mixing between reconnection inflowing and outflowing populations. The latter also leads to the anti-correlation between parallel and perpendicular temperature components. Using four-spacecraft measurements, the magnetic field line twisting is demonstrated by the correlated fluctuations in (∇×𝑽(sub 𝐸×𝐵))||and (∇×𝐁)||. The lower-hybrid wave in the EDR of weak guide field reconnection may be generated near separatrices and penetrate to the mid-plane or locally generated, and the latter possibility is beyond the prediction of previous reconnection simulations.
In the diffusion region of magnetotail reconnection, particle distributions are highly structured, exhibiting triangular shapes and multiple striations that deviate dramatically from the Maxwellian ...distribution. Fully kinetic simulations have been demonstrated to be capable of producing the essential structures of the observed distribution functions, yet are computationally not feasible for 3D global simulations. The fluid models used for large‐scale simulations, on the other hand, do not have the kinetic physics necessary for describing reconnection accurately. Our study aims to bridge fully kinetic and fluid simulations by quantifying the information required to capture the non‐Maxwellian features in the distributions underlying the closures used in the fluid code. We compare the results of fully kinetic simulations with observed electron velocity distributions in a magnetotail reconnection diffusion region and use the maximum entropy model to reconstruct electron and ion distributions using various numbers of moments obtained from the simulation. Our results indicate that using only local moments, the maximum entropy model can reproduce many of the features of the distributions: (1) the electron outflow distribution with a tilted triangular structure is reproduced with 21 or more moments in agreement with Ng et al. (2018, https://doi.org/10.1063/1.5041758) and (2) counterstreaming distributions can be captured with the 35‐moment model when the separation in velocity space between the populations is large.
Plain Language Summary
Fluid modeling of plasmas in collisionless environments such as the magnetosphere is challenging because the approximation that collisions are important does not hold. It is important to understand the motion of individual particles as collisions do not bring them back to equilibrium, However, this is computationally intensive and cannot be used to model the Earth at the present time. We use a recent fluid model which is based on maximizing the entropy of an underlying distribution of particles and attempt to reconstruct particle distributions during reconnection in the Earth's magnetotail. We show that most features of the distribution function can be reproduced using an increasing number of fluid moments.
Key Points
Particle distribution functions in a reconnection diffusion region are reconstructed using a maximum entropy closure
Kinetic features such as electron anisotropy and acceleration are reproduced
Inclusion of fourth‐order velocity moments allows the modeling of counterstreaming populations
During a storm-time interval around winter solstice, observations by the Magnetospheric Multi-Scale (MMS) Mission show multiple distinct magnetopause boundary layers (BLs) in the vicinity of the ...southern cusp. The microphysics of the solar wind-magnetosphere interaction during storm times are not well understood, because the observations are relatively lacking. This event enables the opportunity to probe the storm-time magnetopause, and observations support that MMS was near a reconnection site equatorward of the southern cusp, suggesting active reconnection in close proximity to closed magnetic flux regions in the BL. The Grid Agnostic magnetohydrodynamics (MHD) for Extended Research Applications global MHD simulation shows evidence for transient secondary reconnection sites near the southern cusp, demonstrating mechanisms to form closed field line regions of the BL.
We report evidence of magnetic reconnection in the transition region of the Earth's bow shock when the angle between the shock normal and the immediate upstream magnetic field is 65°. An ...ion‐skin‐depth‐scale current sheet exhibits the Hall current and field pattern, electron outflow jet, and enhanced energy conversion rate through the nonideal electric field, all consistent with a reconnection diffusion region close to the X‐line. In the diffusion region, electrons are modulated by electromagnetic waves. An ion exhaust with energized field‐aligned ions and electron parallel heating are observed in the same shock transition region. The energized ions are more separated from the inflowing ions in velocity above the current sheet than below, possibly due to the shear flow between the two inflow regions. The observation suggests that magnetic reconnection may contribute to shock energy dissipation.
Plain Language Summary
Collisionless shock and magnetic reconnection are two fundamental plasma processes where significant energy conversion between electromagnetic fields and particles occurs. Knowledge is still lacking for whether reconnection occurs at the shock transition region and whether the reconnection property at the shock is different from that which occurs elsewhere. In this letter, we report the existence of reconnection at the Earth's bow shock. Many features are consistent with those in standard reconnection, such as the electron flow and field structures in the diffusion region, and the feature of ion and electron heating in the exhaust. The results suggest reconnection to be one energy dissipation mechanism at the shock, which encourages further investigation.
Key Points
Magnetic reconnection is observed in the shock transition region
Features of a reconnection diffusion region are observed: electron outflow jet, Hall flow and fields, and enhanced energy conversion rates
Ion energization to suprathermal energies and parallel electron heating are found to be consistent with a reconnection exhaust
Whistler wave generation near the magnetospheric separatrix during reconnection at the dayside magnetopause is studied with data from the Magnetospheric Multiscale mission. The dispersion relation of ...the whistler mode is measured for the first time near the reconnection region in space, which shows that whistler waves propagate nearly parallel to the magnetic field line. A linear analysis indicates that the whistler waves are generated by temperature anisotropy in the electron tail population. This is caused by loss of electrons with a high velocity parallel to the magnetic field to the exhaust region. There is a positive correlation between activities of whistler waves and the lower hybrid drift instability both in laboratory and space, indicating the enhanced transport by lower hybrid drift instability may be responsible for the loss of electrons with a high parallel velocity.
Plain Language Summary
Magnetic reconnection is a fundamental process in magnetized plasma, during which magnetic energy is converted to particle energy. Due to this nature of magnetic reconnection, there are many free energy sources that can excite plasma waves such as lower hybrid and whistler waves. Whistler waves near the boundary between the magnetosphere and the exhaust region of magnetic reconnection have been observed over many decades. However, the propagation characteristic and the exact excitation mechanism associated with magnetic reconnection have not been well understood. Here the dispersion relation of the whistler wave is clearly measured for the first time by using correlations between four satellites of the Magnetospheric Multiscale mission. The measured dispersion shows that the whistler wave propagates mostly parallel to the background magnetic field toward the central reconnection region, which agrees well with a linear theory. A linear calculation with the measured electron distribution function verifies that the whistler wave is excited by temperature anisotropy in energetic electrons whose energy is much larger than that of bulk electrons. Observations both in space and laboratory suggest that lower hybrid drift instabilities may cause the anisotropy in energetic electrons, which is an interesting wave‐wave‐particle phenomenon.
Key Points
The whistler wave dispersion relation is measured for the first time by using correlation between multiple spacecraft
The whistler wave propagating toward the X line is generated by temperature anisotropy in the electron tail
Positive correlations between LHDI and whistler activities suggest that the anisotropy is generated by transport due to LHDI
Shock waves are common in the heliosphere and beyond. The collisionless nature of most astrophysical plasmas allows for the energy processed by shocks to be partitioned amongst particle ...sub‐populations and electromagnetic fields via physical mechanisms that are not well understood. The electrostatic potential across such shocks is frame dependent. In a frame where the incident bulk velocity is parallel to the magnetic field, the deHoffmann‐Teller frame, the potential is linked directly to the ambipolar electric field established by the electron pressure gradient. Thus measuring and understanding this potential solves the electron partition problem, and gives insight into other competing shock processes. Integrating measured electric fields in space is problematic since the measurements can have offsets that change with plasma conditions. The offsets, once integrated, can be as large or larger than the shock potential. Here we exploit the high‐quality field and plasma measurements from NASA's Magnetospheric Multiscale mission to attempt this calculation. We investigate recent adaptations of the deHoffmann‐Teller frame transformation to include time variability, and conclude that in practice these face difficulties inherent in the 3D time‐dependent nature of real shocks by comparison to 1D simulations. Potential estimates based on electron fluid and kinetic analyses provide the most robust measures of the deHoffmann‐Teller potential, but with some care direct integration of the electric fields can be made to agree. These results suggest that it will be difficult to independently assess the role of other processes, such as scattering by shock turbulence, in accounting for the electron heating.
Plain Language Summary
Shock waves form when a supersonic flow encounters an immovable object. Thus, ahead of the magnetic bubble formed by the Earth's extended magnetic field, the flow of charged particles emanating from the Sun known as the solar wind is shocked, slowed, and deflected around the Earth. In dense fluids, the conversion of the incident bulk flow energy into heat is accomplished by collisions between particles or molecules. However, the solar wind is so rarefied that such collisions are negligible, and the energy conversion involves more than one kinetic process that couples the different particles to the electromagnetic fields. In particular, electric potentials are believed to control the energy split between positive and negative particles. Measuring electric potentials in space is challenging because there is no available zero “earth” potential. In this work, we explore alternative measurements of the potential associated with the electron physics. Some methods can be made to agree with direct determinations using the measured electrons, but we conclude that despite the unprecedented data quality, they are not sufficient to provide an independent determination of the potential. This poses challenges in assessing other, non‐potential physics that also influences the electron energization.
Key Points
Measuring directly the cross‐shock deHoffmann‐Teller potential in space is challenging
Proposed adaptive frame transformation techniques have limited utility for shocks with 2D or 3D time‐varying structure
Electron inferences of the potential are robust but assume scattering is negligible
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