Magnetosheath Jets Research Articles

Connection Between Foreshock Structures and the Generation of Magnetosheath Jets: Vlasiator Results

AbstractEarth’s magnetosheath consists of shocked solar wind plasma that has been compressed and slowed down at the Earth’s bow shock. Magnetosheath jets are pulses of enhanced dynamic pressure in the magnetosheath. Jets have been observed by numerous spacecraft missions, but their origin has remained unconfirmed, though several formation mechanisms have been suggested. In this study, we use a method for automatically identifying and tracking jets as well as foreshock compressive structures (FCSs) in four 2D runs of the global hybrid‐Vlasov simulation Vlasiator. We find that up to 75% of magnetosheath jets are caused by FCSs impacting the bow shock. These jets propagate deeper into the magnetosheath than the remaining 25% of jets that are not caused by FCSs. We conduct a visual case study of one jet that was not caused by FCSs and find that the bow shock was not rippled before the formation of the jet.

Solar Wind Control of Magnetosheath Jet Formation and Propagation to the Magnetopause

AbstractMagnetosheath jets are localized high‐dynamic pressure pulses originating at Earth's bow shock and propagating earthward through the magnetosheath. Jets can influence magnetospheric dynamics upon impacting the magnetopause; however, many jets dissipate before reaching it. In this study we present a database of 13,096 jets observed by the Time History of Events and Macroscale Interactions during Substorms spacecraft from 2008 to 2018, spanning a solar cycle. Each jet is associated with upstream solar wind conditions from OMNI. We statistically examine how solar wind conditions control the likelihood of jets forming at the shock, and the conditions favorable for jets to propagate through the magnetosheath and reach the magnetopause. We see that, for each solar wind quantity, these two effects are separate, but when combined, we find that jets are over 17 times more likely to reach and potentially impact the magnetopause when the interplanetary magnetic field (IMF) orientation is at a low cone angle, and approximately 8 times more likely during high speed solar wind. Low IMF magnitude, high Alfvén Mach number, and low density approximately double the number of jets at the magnetopause, while and dynamic pressure display no net effect. Due to the strong dependence on wind speed, we infer that jet impact rates may be solar cycle dependent as well as vary during solar wind transients. This is an important step towards forecasting the magnetospheric effects of magnetosheath jets, as it allows for predictions of jet impact rates based on measurements of the upstream solar wind.

On the Generation of Pi2 Pulsations due to Plasma Flow Patterns Around Magnetosheath Jets

AbstractWe report observations of a magnetosheath jet followed by a period of decelerated background plasma. During this period, THEMIS‐A magnetometer showed abrupt disturbances which, in the wavelet spectrum, appeared as prominent and irregular pulsations in two frequency bands (7.6–9.2 and 12–17 mHz) within the Pi2 range. The observations suggest—for the first time to our knowledge—that these pulsations were locally generated by the abrupt magnetic field changes driven by the jet’s interaction with the ambient magnetosheath plasma. Furthermore, similar pulsations, detected by THEMIS‐D inside the magnetosphere with a 140 s time‐lag (which corresponds to the propagation time of a disturbance traveling with Alfvénic speed), are shown to be directly associated with the ones in the magnetosheath, which raises the question of how exactly these pulsations are propagated through the magnetopause.

Ground‐Based Magnetometer Response to Impacting Magnetosheath Jets

AbstractLocalized dynamic pressure pulses in the magnetosheath, or jets, have been a popular topic for discussion in recent decades. Studies show that they can propagate through the magnetosheath and impact the magnetopause, possibly showing up as geoeffective elements at ground level. However, questions still remain on how geoeffective they can be. Previous studies have been limited to case studies during few days and with only a handful of events. In this study we have found 65 cases of impacting jets using observations from the Multiscale Magnetospheric mission during 2015–2017. We examine their geoeffectiveness using ground‐based magnetometers (GMAGs). From our statistics we find that GMAGs observe responses as fluctuations in the geomagnetic field with amplitudes of 34 nT, frequencies of 1.9 mHz, and damping times of 370 s. Further, the parallel length and the maximum dynamic pressure of the jet dictate the amplitude of the observed GMAG response. Longer and higher pressure jets inducing larger amplitude responses in GMAG horizontal components. The median time required for the signal to be detected by GMAGs is 190 s. We also examine if jets can be harmful for human infrastructure and cannot exclude that such events could exist.

Causes of Jets in the Quasi‐Perpendicular Magnetosheath

AbstractMagnetosheath jets are currently an important topic in the field of magnetosheath physics. It is thought that 97% of the jets are produced by the shock rippling at quasi‐parallel shocks. Recently, large statistical studies of magnetosheath jets have been performed, however, it is not clear whether rippling also produces jets found downstream of quasi‐perpendicular shocks. We analyze four types of events in the quasi‐perpendicular magnetosheath with signatures characteristic of magnetosheath jets, namely increased density and/or dynamic pressure that were not produced by the shock rippling: (a) magnetic flux tubes connected to the quasi‐parallel bow‐shock, (b) nonreconnecting current sheets, (c) reconnection exhausts, and (d) mirror‐mode waves. The flux tubes are downstream equivalents of the upstream traveling foreshocks. Magnetosheath jets can impact the magnetopause, so knowing the conditions under which they form may enable us to understand their signatures in the magnetosphere.

Foreshock Cavities: Direct Transmission Through the Bow Shock

AbstractGlobal hybrid code simulations predict enhanced densities and magnetic field strengths not only in foreshock compressional boundaries on the flanks of the steady‐state dayside foreshock under near‐radial IMF conditions but also on the edges of the traveling foreshock cavities formed when slabs of interplanetary magnetic field (IMF) lines connect to the bow shock. The simulations predict only modest velocity or temperature perturbations attending the foreshock compressional/cavity boundaries. However, they predict pronounced flow enhancements and depressed temperatures accompanying the enhanced densities and magnetic field strengths on foreshock compressional/cavity boundaries transmitted into the magnetosheath. Many of these properties are similar to those of magnetosheath jets, previously attributed to processes occurring at locations where the solar wind flow makes a grazing encounter with a locally oblique bow shock. Simultaneous THEMIS observations upstream and downstream from the Earth’s bow shock from 1430 to 1630 UT on June 14, 2007 confirm simulation predictions for the direct transmission of foreshock cavities into the magnetosheath. Foreshock cavity density and magnetic field strength perturbations diminish very rapidly with distance upstream from the bow shock.

Magnetosheath jet evolution as a function of lifetime: global hybrid-Vlasov simulations compared to MMS observations

Abstract. Magnetosheath jets are regions of high dynamic pressure, which can traverse from the bow shock towards the magnetopause. Recent modelling efforts, limited to a single jet and a single set of upstream conditions, have provided the first estimations about how the jet parameters behave as a function of position within the magnetosheath. Here we expand the earlier results by doing the first statistical investigation of the jet dimensions and parameters as a function of their lifetime within the magnetosheath. To verify the simulation behaviour, we first identify jets from Magnetosphere Multiscale (MMS) spacecraft data (6142 in total) and confirm the Vlasiator jet general behaviour using statistics of 924 simulated individual jets. We find that the jets in the simulation are in quantitative agreement with the observations, confirming earlier findings related to jets using Vlasiator. The jet density, dynamic pressure, and magnetic field intensity show a sharp jump at the bow shock, which decreases towards the magnetopause. The jets appear compressive and cooler than the magnetosheath at the bow shock, while during their propagation towards the magnetopause they thermalise. Further, the shape of the jets flatten as they progress through the magnetosheath. They are able to maintain their flow velocity and direction within the magnetosheath flow, and they end up preferentially to the side of the magnetosheath behind the quasi-parallel shock. Finally, we find that Vlasiator jets during low solar wind Alfvén Mach number MA are shorter in duration, smaller in their extent, and weaker in terms of dynamic pressure and magnetic field intensity as compared to the jets during high MA.

Scale Sizes of Magnetosheath Jets

AbstractMagnetosheath jets are plasma entities that feature a significantly enhanced dynamic pressure with respect to the ambient plasma. They occur more often downstream of the quasi‐parallel bow shock. Jets can propagate through the entire magnetosheath and impact on the magnetopause. We reanalyze multi‐spacecraft data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission to obtain the first unbiased distributions of scale sizes of the jets, in the directions parallel and perpendicular to their propagation direction. These distributions are log‐normal; they fit well to the observations. We argue that jet scales should be log‐normally distributed as they should result from multiplicative processes in the foreshock and in the magnetosheath. We find that typical jet scales are on the order of 0.1 Earth radii (RE), one order of magnitude smaller than previously reported. Median scale sizes of 0.12 RE and 0.15 RE in the perpendicular and parallel directions are obtained. The small scales may be related to the substructure of Short Large Amplitude Magnetic Structures (SLAMS) in the foreshock, or to the break up of larger jets within the magnetosheath. Use of the log‐normal distributions also allows for an analysis of impact rates of small‐scale jets: While previous results on large jets hitting the magnetopause several times per hour remain largely unchanged, we now find that hundreds to thousands of mostly small‐scale jets could potentially impact the dayside magnetopause every hour.

Magnetosheath Microstructure: Mirror Mode Waves and Jets during Southward IP Magnetic Field

AbstractIn the Earth's magnetosheath plasma waves, nonlinear structures associated with characteristic ion populations can occur. Understanding the interaction of the solar wind with the magnetosphere requires a deeper knowledge of the underlying magnetosheath kinetic microstructure. We study a 45 min interval when the MMS spacecraft observed a southward magnetic field (Bz < 0 nT) in the dayside magnetosheath. Using magnetic field and plasma data, we analyze three transient dynamic pressure enhancements identified as magnetosheath jets. The characteristics of these jets are different, suggesting different origins. While two of them, called J1 and J2, exhibit large increment in velocity and almost no density increment, the third jet (J3) shows large density enhancements with almost no velocity increment. The duration of J3 is ∼7 times longer than those of J1 and J2. J3 occurs at the region where the negative Bz becomes positive. Ion distributions inside the jets are different. J1 and J2 show a secondary field‐aligned beam, which is not present in J3, suggesting that magnetic reconnection at the magnetopause is responsible for their formation. Distributions inside J3 are more isotropic. B field and plasma signatures inside J3 correspond to the crossing of a sector boundary, similar to the heliospheric plasma sheet, suggesting that J3 forms by the evolution and interaction of a solar wind structure with the bow shock and magnetosheath. Fluctuations inside J3 have larger transverse components, although they propagate at large angles to B. In contrast, waves in regions between the jets are compressive and are identified as mirror mode waves.

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.

Statistical Study of Magnetosheath Jet‐Driven Bow Waves

AbstractWhen a magnetosheath jet (localized dynamic pressure enhancements) compresses ambient magnetosheath at a (relative) speed faster than the local magnetosonic speed, a bow wave or shock can form ahead of the jet. Such bow waves or shocks were recently observed to accelerate particles, thus contributing to magnetosheath heating and particle acceleration in the extended environment of Earth's bow shock. To further understand the characteristics of jet‐driven bow waves, we perform a statistical study to examine which solar wind conditions favor their formation and whether it is common for them to accelerate particles. We identified 364 out of 2,859 (~13%) magnetosheath jets to have a bow wave or shock ahead of them with Mach number typically larger than 1.1. We show that large solar wind plasma beta, weak interplanetary magnetic field (IMF) strength, large solar wind Alfvén Mach number, and strong solar wind dynamic pressure present favorable conditions for their formation. We also show that magnetosheath jets with bow waves or shocks are more frequently associated with higher maximum ion and electron energies than those without them, confirming that it is common for these structures to accelerate particles. In particular, magnetosheath jets with bow waves have electron energy flux enhanced on average by a factor of 2 compared to both those without bow waves and the ambient magnetosheath. Our study implies that magnetosheath jets can contribute to shock acceleration of particles especially for high Mach number shocks. Therefore, shock models should be generalized to include magnetosheath jets and concomitant particle acceleration.

Electron Acceleration by Magnetosheath Jet‐Driven Bow Waves

AbstractMagnetosheath jets are localized fast flows with enhanced dynamic pressure. When they supermagnetosonically compress the ambient magnetosheath plasma, a bow wave or shock can form ahead of them. Such a bow wave was recently observed to accelerate ions and possibly electrons. The ion acceleration process was previously analyzed, but the electron acceleration process remains largely unexplored. Here we use multipoint observations by Time History of Events and Macroscale during Substorms from three events to determine whether and how magnetosheath jet‐driven bow waves can accelerate electrons. We show that when suprathermal electrons in the ambient magnetosheath convect toward a bow wave, some electrons are shock‐drift accelerated and reflected toward the ambient magnetosheath and others continue moving downstream of the bow wave resulting in bidirectional motion. Our study indicates that magnetosheath jet‐driven bow waves can result in additional energization of suprathermal electrons in the magnetosheath. It implies that magnetosheath jets can increase the efficiency of electron acceleration at planetary bow shocks or other similar astrophysical environments.

Classification of Magnetosheath Jets Using Neural Networks and High Resolution OMNI (HRO) Data

Magnetosheath jets are transient, localized dynamic pressure enhancements found downstream of the Earth's bow shock in the magnetosheath region. Using a pre-existing database of magnetosheath jets we train a neural network to distinguish between jets found downstream of a quasi-parallel bow shock θBn 45). The initial database was compiled using MMS measurements in the magnetosheath (downstream) to identify and classify them as "quasi-parallel" or "quasi-perpendicular", while the neural network uses only solar wind (upstream) measurements from the OMNIweb database. To evaluate the results, a comparison with three physics-based modeling approaches is done. It is shown that neural networks are systematically outperforming the other methods by achieving a ~93 agreement with the initial dataset, while the rest of the methods achieve around 80%. The better performance of the neural networks likely is due to the fact that they use information from more solar wind quantities than the physics-based models, even in the absence of certain upstream properties, such as IMF direction.

On the alignment of velocity and magnetic fields within magnetosheath jets

Abstract. Jets in the subsolar magnetosheath are localized enhancements in dynamic pressure that are able to propagate all the way from the bow shock to the magnetopause. Due to their excess velocity with respect to their environment, they push slower ambient plasma out of their way, creating a vortical plasma motion in and around them. Simulations and case study results suggest that jets also modify the magnetic field in the magnetosheath on their passage, aligning it more with their velocity. Based on Magnetospheric Multiscale (MMS) jet observations and corresponding superposed epoch analyses of the angles ϕ between the velocity and magnetic fields, we can confirm that this suggestion is correct. However, while the alignment is more significant for faster than for slower jets, and for jets observed close to the bow shock, the overall effect is small: typically, reductions in ϕ of around 10∘ are observed at jet core regions, where the jets' velocities are largest. Furthermore, time series of ϕ pertaining to individual jets significantly deviate from the superposed epoch analysis results. They usually exhibit large variations over the entire range of ϕ: 0 to 90∘. This variability is commonly somewhat larger within jets than outside them, masking the systematic decrease in ϕ at core regions of individual jets.

Jets in the magnetosheath: IMF control of where they occur

Abstract. Magnetosheath jets are localized regions of plasma that move faster towards the Earth than the surrounding magnetosheath plasma. Due to their high velocities, they can cause indentations when colliding into the magnetopause and trigger processes such as magnetic reconnection and magnetopause surface waves. We statistically study the occurrence of these jets in the subsolar magnetosheath using measurements from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and OMNI solar wind data from 2008 to 2011. We present the observations in the BIMF–vSW plane and study the spatial distribution of jets during different interplanetary magnetic field (IMF) orientations. Jets occur downstream of the quasi-parallel bow shock approximately 9 times as often as downstream of the quasi-perpendicular shock, suggesting that foreshock processes are responsible for most jets. For an oblique IMF, with 30–60∘ cone angle, the occurrence increases monotonically from the quasi-perpendicular side to the quasi-parallel side. This study offers predictability for the numbers, locations, and magnetopause impact rates of jets observed during different IMF orientations, allowing us to better forecast the formation of these jets and their impact on the magnetosphere.

THEMIS Observations of Particle Acceleration by a Magnetosheath Jet‐Driven Bow Wave

AbstractLocalized magnetosheath jets with high dynamic pressure are frequently observed downstream of Earth's bow shock. When such a fast magnetosheath jet compresses the ambient magnetosheath plasma, an earthward compressional bow wave could form. Such bow waves have been predicted by simulations but have never been observed. Using multipoint Time History of Events and Macroscale Interactions during Substroms (THEMIS) observations, we report the first observation where such a bow wave driven by an intrinsically formed magnetosheath jet can reflect and accelerate particles up to tens of kiloelectron volt for ions and 100 keV for electrons. By analyzing the ion distributions, we infer how particles reach the spacecraft from the bow wave demonstrating good agreement with our model of single particle motion. Our study implies that particle acceleration at magnetosheath jets could contribute significantly to particle acceleration at shocks in general.

Can Enhanced Flux Loading by High‐Speed Jets Lead to a Substorm? Multipoint Detection of the Christmas Day Substorm Onset at 08:17 UT, 2015

AbstractThis paper describes highly interesting observations of the 25 December 2015 substorm onset at 08:17 UT during northward interplanetary magnetic field (IMF) while magnetosheath contained several intervals of negative Bz. A unique alignment of several spacecraft near the Earth‐Sun line together with magnetohydrodynamic simulations, ground‐based magnetometer, and auroral observations allow a comprehensive timing analysis of the events leading to substorm onset. Perplexingly, prior to substorm onset Geotail measured for 26 min positive IMF Bz just upstream of the bow shock, while simultaneously MMS spacecraft measured several intervals of strong negative Bz in the dayside magnetosheath. These strong pulses of negative Bz in the magnetosheath were associated with high dynamic pressure magnetosheath jets, likely created by foreshock transients during strongly radial IMF interval. Multipoint plasma and magnetic field measurements from ARTEMIS and THEMIS spacecraft were used to determine tail reconnection time at 8:14 and location at x=−33RE. Ground‐based observations of Pi2 pulsations and auroral brightening, with observations of a dipolarization front by THEMIS spacecraft, allowed determination of substorm onset to be at ≈08:17. All MMS spacecraft detected the same magnetosheath jet structure with Bz=−25 nT at ∼08:00 while IMF was northward. Based on DMSP observations and timing analysis we propose that these jets produced magnetopause reconnection leading to final, critical flux enhancement in the midtail region, which may have triggered reconnection 12–14 min later after jet observations.

Magnetosheath jet properties and evolution as determined by a global hybrid-Vlasov simulation

Abstract. We use a global hybrid-Vlasov simulation for the magnetosphere, Vlasiator, to investigate magnetosheath high-speed jets. Unlike many other hybrid-kinetic simulations, Vlasiator includes an unscaled geomagnetic dipole, indicating that the simulation spatial and temporal dimensions can be given in SI units without scaling. Thus, for the first time, this allows investigating the magnetosheath jet properties and comparing them directly with the observed jets within the Earth's magnetosheath. In the run shown in this paper, the interplanetary magnetic field (IMF) cone angle is 30∘, and a foreshock develops upstream of the quasi-parallel magnetosheath. We visually detect a structure with high dynamic pressure propagating from the bow shock through the magnetosheath. The structure is confirmed as a jet using three different criteria, which have been adopted in previous observational studies. We compare these criteria against the simulation results. We find that the magnetosheath jet is an elongated structure extending earthward from the bow shock by ∼2.6 RE, while its size perpendicular to the direction of propagation is ∼0.5 RE. We also investigate the jet evolution and find that the jet originates due to the interaction of the bow shock with a high-dynamic-pressure structure that reproduces observational features associated with a short, large-amplitude magnetic structure (SLAMS). The simulation shows that magnetosheath jets can develop also under steady IMF, as inferred by observational studies. To our knowledge, this paper therefore shows the first global kinetic simulation of a magnetosheath jet, which is in accordance with three observational jet criteria and is caused by a SLAMS advecting towards the bow shock.

Plasma flow patterns in and around magnetosheath jets

Abstract. The magnetosheath is commonly permeated by localized high-speed jets downstream of the quasi-parallel bow shock. These jets are much faster than the ambient magnetosheath plasma, thus raising the question of how that latter plasma reacts to incoming jets. We have performed a statistical analysis based on 662 cases of one THEMIS spacecraft observing a jet and another (second) THEMIS spacecraft providing context observations of nearby plasma to uncover the flow patterns in and around jets. The following results are found: along the jet's path, slower plasma is accelerated and pushed aside ahead of the fastest core jet plasma. Behind the jet core, plasma flows into the path to fill the wake. This evasive plasma motion affects the ambient magnetosheath, close to the jet's path. Diverging and converging plasma flows ahead and behind the jet are complemented by plasma flows opposite to the jet's propagation direction, in the vicinity of the jet. This vortical plasma motion results in a deceleration of ambient plasma when a jet passes nearby. Keywords. Magnetospheric physics (magnetosheath; MHD waves and instabilities; solar wind–magnetosphere interactions)

Investigating the anatomy of magnetosheath jets – MMS observations

Abstract. We use Magnetosphere Multiscale (MMS) mission data to investigate a small number of magnetosheath jets, which are localized and transient increases in dynamic pressure, typically due to a combined increase in plasma velocity and density. For two approximately hour-long intervals in November, 2015 we found six jets, which are of two distinct types. (a) Two of the jets are associated with the magnetic field discontinuities at the boundary between the quasi-parallel and quasi-perpendicular magnetosheath. Straddling the boundary, the leading part of these jets contains an ion population similar to the quasi-parallel magnetosheath, while the trailing part contains ion populations similar to the quasi-perpendicular magnetosheath. Both populations are, however, cooler than the surrounding ion populations. These two jets also have clear increases in plasma density and magnetic field strength, correlated with a velocity increase. (b) Three of the jets are found embedded within the quasi-parallel magnetosheath. They contain ion populations similar to the surrounding quasi-parallel magnetosheath, but with a lower temperature. Out of these three jets, two have a simple structure. For these two jets, the increases in density and magnetic field strength are correlated with the dynamic pressure increases. The other jet has a more complicated structure, and no clear correlations between density, magnetic field strength and dynamic pressure. This jet has likely interacted with the magnetosphere, and contains ions similar to the jets inside the quasi-parallel magnetosheath, but shows signs of adiabatic heating. All jets are associated with emissions of whistler, lower hybrid, and broadband electrostatic waves, as well as approximately 10 s period electromagnetic waves with a compressional component. The latter have a Poynting flux of up to 40 µW m−2 and may be energetically important for the evolution of the jets, depending on the wave excitation mechanism. Only one of the jets is likely to have modified the surrounding magnetic field into a stretched configuration, as has recently been reported in other studies. None of the jets are associated with clear signatures of either magnetic or thermal pressure gradient forces acting on them. The different properties of the two types also point to different generation mechanisms, which are discussed here. Their different properties and origins suggest that the two types of jets need to be separated in future statistical and simulation studies. Keywords. Magnetospheric physics (magnetosheath; plasma waves and instabilities; solar wind–magnetosphere interactions)