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Magnetopause position is controlled mainly by the solar wind dynamic pressure and north‐south interplanetary magnetic field component and these quantities are included in different empirical magnetopause models. We have collected about 50,000 of dayside magnetopause crossings observed by THEMIS in course of 2007–2019 and compared the observed magnetopause position with model prediction. The difference between observed and predicted magnetopause radial distance, Robs − Rmod is used for quantifying the model‐observation agreement. Its median values are well predicted for cases up toRobs ≈ 12 RE for all models but higher positive deviations are found for larger magnetopause distances, mainly under a nearly radial field and low dynamic pressure. The analysis reveals their connection with transient magnetopause displacements caused by strong sunward flows in the magnetosheath. We discuss the possible origin of the observed magnetosheath flow switching in terms of the interaction of magnetosheath jets with the magnetopause. Plain Language Summary: Comparison of the observed magnetopause crossings with prediction of the magnetopause models reveals that under nearly radial interplanetary magnetic field can be the magnetopause observed several Earth radii farther from the Earth than the models predict. Comprehensive examination of several cases characterized by suitable spacecraft locations leads to the conclusion that the source of such magnetopause displacements is connected with the reformation of nearly parallel bow shock resulting in a strong antisunward jet in the magnetosheath. The jet creates a dip in the magnetopause surface that reverses its direction. The sunward flow in the magnetosheath pulls the magnetopause also sunward. Since these effects are transient in their nature, they cannot be captured by statistical magnetopause models. Key Points: The magnetopause is often observed several RE upstream its nominal position under nearly radial IMFExtreme magnetopause displacements are accompanied with strong antisunward magnetosheath jetsReversal of the jet direction is associated with the magnetopause outward displacement [ABSTRACT FROM AUTHOR]

Abstract Magnetopause position is controlled mainly by the solar wind dynamic pressure and north‐south interplanetary magnetic field component and these quantities are included in different empirical magnetopause models. We have collected about 50,000 of dayside magnetopause crossings observed by THEMIS in course of 2007–2019 and compared the observed magnetopause position with model prediction. The difference between observed and predicted magnetopause radial distance, Robs − Rmod is used for quantifying the model‐observation agreement. Its median values are well predicted for cases up to Robs ≈ 12 RE for all models but higher positive deviations are found for larger magnetopause distances, mainly under a nearly radial field and low dynamic pressure. The analysis reveals their connection with transient magnetopause displacements caused by strong sunward flows in the magnetosheath. We discuss the possible origin of the observed magnetosheath flow switching in terms of the interaction of magnetosheath jets with the magnetopause.

Magnetosheath jets are localized plasma structures with high dynamic pressure which are frequently observed downstream of the Earth's bow shock. In this work we analyze Magnetospheric MultiScale magnetic field and plasma data and show that jets can be found in the quasi‐perpendicular magnetosheath in regions permeated by Mirror mode waves (MMWs). We show that structures identified as jets by their enhanced dynamic pressure can have very different internal structure, with variable signatures in magnetic field magnitude and components, velocity, and density and can be associated to ion distribution functions of various types. This suggests that jets observed in the quasi‐perpendicular magnetosheath are generated by different mechanisms. We find that jets can be related to traveling foreshocks, flux transfer events, and some have MMWs inside them. Our results suggest that some jets have a local source and their formation does not depend on upstream structures. We find that different types of ion distributions can exist inside the jets, while in some cases anisotropic distributions are present, in others counterstreaming distributions exist. We also show that for jets with MMWs inside them, ion distributions can be modulated. This highlights the importance of using ion distributions to identify and classify different types of jets. Key Points: We study high speed magnetosheath jets coexisting with mirror mode wavesWe use Magnetospheric MultiScale data to show that high speed magnetosheath jets have a much more complex structure than previously thought. Velocity distribution functions can be a tool to describe the micro‐physics inside jetsHigh speed jets in the quasi‐perpendicular magnetosheath could have different generation mechanisms that those in the quasi‐parallel sheath [ABSTRACT FROM AUTHOR]

Magnetosheath jets are localized dynamic pressure enhancements in the magnetosheath. We make use of the high time resolution burst mode data of the Magnetospheric Multiscale mission for an analysis of waves in plasmas associated with three magnetosheath jets. We find both electromagnetic and electrostatic waves over the frequency range from 0 to 4 kHz that can be probed by the instruments on board the MMS spacecraft. At high frequencies we find electrostatic solitary waves, electron acoustic waves, and whistler waves. Electron acoustic waves and whistler waves show the typical properties expected from theory assuming approximations of a homogeneous plasma and linearity. In addition, 0.2 Hz waves in the magnetic field, 1 Hz electromagnetic waves, and lower hybrid waves are observed. For these waves the approximation of a homogeneous plasma does not hold anymore and the observed waves show properties from several different basic wave modes. In addition, we investigate how the various types of waves are generated. We show evidence that, the 1 Hz waves are connected to gradients in the density and magnetic field. The whistler waves are generated by a butterfly‐shaped pitch‐angle distribution and the electron acoustic waves by a cold electron population. The lower hybrid waves are probably generated by currents at the boundary of the jets. As for the other waves we can only speculate about the generation mechanism due to limitations of the instruments. Studying waves in jets will help to address the microphysics in jets which can help to understand the evolution of jets better. Plain Language Summary: There is a constant plasma flow from the sun, the solar wind. The Earth's magnetic field deflects the solar wind as it flows toward Earth. As the solar wind plasma approaches Earth it gets decelerated and heated at the bow shock. Earthward of the bow shock, the magnetosheath is located where the flow diverges around Earth. In the magnetosheath plasma flows that are denser and faster than normal can sometimes be observed, so called magnetosheath jets. We investigate waves in plasmas in these magnetosheath jets and how they are generated. Studying these waves will help to understand the interaction of magnetosheath jets with their environment. Key Points: MMS burst mode data is used to investigate waves in, and in the vicinity of, magnetosheath jets0.2 Hz waves, 1 Hz waves, whistler waves, electron acoustic waves, lower hybrid waves, and solitary waves are observedWaves with low frequencies cannot be explained by "basic" wave modes that are derived in homogeneous plasmas [ABSTRACT FROM AUTHOR]

Magnetosheath jets constitute a significant coupling effect between the solar wind (SW) and the magnetosphere of the Earth. In order to investigate the effects and forecasting of these jets, we present the first‐ever statistical study of the jet production during large‐scale SW structures like coronal mass ejections (CMEs), stream interaction regions (SIRs) and high speed streams (HSSs). Magnetosheath data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft between January 2008 and December 2020 serve as measurement source for jet detection. Two different jet definitions were used to rule out statistical biases induced by our jet detection method. For the CME and SIR + HSS lists, we used lists provided by literature and expanded on incomplete lists using OMNI data to cover the time range of May 1996 to December 2020. We find that the number and total time of observed jets decrease when CME‐sheaths hit the Earth. The number of jets is lower throughout the passing of the CME‐magnetic ejecta (ME) and recovers quickly afterward. On the other hand, the number of jets increases during SIR and HSS phases. We discuss a few possibilities to explain these statistical results. Key Points: Occurrence rate of magnetosheath jets is found to vary due to the arriving CMEs and SIRsFewer jets are found when magnetic ejecta regions of CMEs hit the Earth, more jets are found when SIRs and high speed streams hit the EarthThe jet duration does not appear to vary much during individual SW structures [ABSTRACT FROM AUTHOR]

We perform 2D local hybrid simulations of collisionless shocks in order to study the properties of simulated magnetosheath jets as a function of shock properties, namely their Alfvénic Mach number (MA) and geometry (angle between the upstream magnetic field and the shock normal, θBN). In total we perform 15 simulations with inflow speeds of Vin = 3.3 CA (Alfvén velocity), 4.5 CA and 5.5 CA and θBN = 15°, 30°, 45°, 50°, and 65°. Under these conditions, the shock MA varied between 4.28 and 7.42. In order to identify magnetosheath jets in the simulation outputs, we use four different criteria, equivalent to those utilized to identify subsets of magnetosheath jets, called high-speed jets (Plaschke and Hietala and Angelopoulos, Ann. Geophys., 2013, 31, 1877–1889), transient flux enhancements (Němeček et al., Geophys. Res. Lett., 1998, 25, 1273–1276), density plasmoids (Karlsson et al., J. Geophys. Res., 2012, 117, a–n; Karlsson et al., J. Geophys. Res., 2015, 120, 7390–7403) and high-speed plasmoids (Gunell et al., Ann. Geophys., 2014, 32, 991–1009). In our simulations, the density plasmoids were produced only by shocks with MA ≥5.7, while the high-speed plasmoids only formed downstream of shocks with MA ≥6.97. We show that higher MA shocks tend to produce faster jets that tend to have larger surface area, mass, linear momentum and kinetic energy, while these quantities tend to be anticorrelated with θBN. In general, the increase of θBN to up to 45° results in increased jet formation rates. In the case of high-speed jets in runs with Vin = 3.3 CA and high-speed plasmoids, the jet formation anticorrelates with θBN. The jet production all but ceases for θBN = 65° regardless of the shock’s MA. The maximum distances of the magnetosheath jets from the shocks were ≲140 di (upstream ion intertial lengths), which, estimating 1 di ∼100–150 km at Earth, corresponds to 2.4–3.3 Earth radii. Thus, none of the simulated jets reached distances equivalent to the average extension of the Earth’s subsolar magnetosheath, which would make them the equivalents of geoeffective jets. Higher MA shocks are probably needed in order to produce such jets.

Earth'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. Plain Language Summary: The space around Earth is filled with plasma, the fourth state of matter. Earth's magnetic field shields our planet from the stream of plasma coming from the Sun, the solar wind. The solar wind plasma is slowed down at the Earth's bow shock, before it flows against and around the Earth's magnetic field in the magnetosheath. Sometimes, pulses of high density or velocity can occur in the magnetosheath that have the potential to disturb the inner regions of near‐Earth space where many spacecraft orbit. We call these pulses magnetosheath jets. Magnetosheath jets have been observed by many spacecraft over the past few decades, but how they form has remained unclear. In this study, we use the Vlasiator model to simulate plasma in near‐Earth space and investigate the origins of magnetosheath jets. We find that the formation of up to 75% of these jets can be explained by compressive structures in the foreshock, a region populated by intense wave activity extending sunward of the quasi‐parallel bow shock, where interplanetary magnetic field lines allow shock‐reflected particles to travel back toward the Sun. Key Points: We investigated magnetosheath jets in four hybrid‐Vlasov simulation runs with different solar wind parametersForeshock compressive structures can explain the formation of up to 75% of magnetosheath jetsThe foreshock compressive structure‐caused jets travel deeper into the magnetosheath than the 25% of the jets whose origins remain unclear [ABSTRACT FROM AUTHOR]

Magnetosheath 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 β 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. Plain Language Summary: When the solar wind, a constant flow of plasma from the Sun, meets Earth's magnetic field, a shock wave forms in space. Like a rock in a stream, the plasma is diverted around the obstacle and a dense, turbulent layer—the magnetosheath—forms in front of it. We study instances of fast plasma jets bursting through the shock and traveling towards Earth. However, it appears that only a small proportion of jets hit the edge of our magnetic field—the magnetopause. To forecast their effects, we therefore need to know when jets will make it through. We use a database of 13,096 jets observed by spacecraft in the magnetosheath, alongside measurements of the solar wind, to determine when jets are most likely to hit the magnetopause. We find the highest probability is when the solar wind magnetic field is aligned with its flow direction and when it has a higher speed. We hope that, with this information, we may eventually be able to forecast space weather effects of jets based solely on measurements of the upstream solar wind. Key Points: Time History of Events and Macroscale Interactions during Substorms and OMNI data from 2008 to 2018 are used to constrain conditions for jet formation and propagation to the magnetopauseJets reach the magnetopause 17x more often during low IMF cone angles (<33°) and 8x more often during high solar wind speeds (>539 km/s)Low interplanetary magnetic field magnitude, high Alfvén Mach no., and low density double expected magnetopause impacts, dynamic pressure and β have no net effect [ABSTRACT FROM AUTHOR]

Magnetosheath 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. Plain Language Summary: Magnetosheath jets have been identified as strong local enhancements of dynamic pressure or similar quantity in the magnetosheath associated to velocity and/or density enhancements. It is currently thought that ∼97% of magnetosheath jets form due to rippling of the quasi‐parallel bow‐shock. However shock rippling at the quasi‐perpendicular shock occurs on much smaller spatial scales (∼5 di, upstream ion inertial scales) than at the quasi‐parallel shock (several tens of di). It is thus not clear whether the rippling produces magnetosheath jets at the quasi‐perpendicular shock. Here, we show for the first time four different phenomena, not associated to shock rippling, that can produce magnetosheath jets in the quasi‐perpendicular magnetosheath. Three of them are already known types of events: current sheets, reconnection exhausts, and mirror‐mode waves. The fourth phenomena are magnetic flux tubes that are embedded in the quasi‐perpendicular magnetosheath but are connected to the quasi‐parallel bow‐shock. Key Points: Jets in the quasi‐perpendicular (Qper) magnetosheath may be of different origin than those in the quasi‐parallel magnetosheathSome causes of jets in the Qper magnetosheath are known phenomena, such as mirror‐mode waves, current sheets, and reconnection exhaustsSome jets in the Qper magnetosheath are due to passing magnetic flux tubes, which are downstream equivalents of traveling foreshocks [ABSTRACT FROM AUTHOR]

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 (θBn45o). 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. As a result, even in the absence of certain upstream properties, such as the IMF direction, they are capable of accurately determining the jet class.

Using Magnetospheric Multiscale (MMS) data, we find, classify, and analyze transient dynamic pressure enhancements in the magnetosheath (jets) from May 2015 to May 2019. A classification algorithm is presented, using in situ MMS data to classify jets (N=8,499) into different categories according to their associated angle between interplanetary magnetic field (IMF) and the bow shock normal vector (θBn). Jets appearing for θBn < 45 are referred to as quasi‐parallel, while jets appearing for θBn > 45 as quasi‐perpendicular jets. Furthermore, we define those jets that occur at the boundaries between quasi‐parallel and quasi‐perpendicular magnetosheath as boundary jets. Finally, encapsulated jets are jet‐like structures with similar characteristics to quasi‐parallel jets while the surrounding plasma is of quasi‐perpendicular nature. We present the first statistical results of such a classification and provide comparative statistics for each class. Furthermore, we investigate correlations between jet quantities. Quasi‐parallel jets have the highest dynamic pressure while occurring more often than quasi‐perpendicular jets. The infrequent quasi‐perpendicular jets have a much smaller duration, velocity, and density and are therefore relatively weaker. We conclude that quasi‐parallel and boundary jets have similar properties and are unlikely to originate from different generation mechanisms. Regarding the encapsulated jets, we suggest that they are a special subset of quasi‐parallel jets originating from the flanks of the bow shock, for large IMF cone angles although a relation to flux transfer events (FTEs) and magnetospheric plasma is also possible. Our results support existing generation theories, such as the bow shock ripple and SLAMS‐associated mechanisms while indicating that other factors may contribute as well. Key Points: Classification of jet database based on θBn, using MMS data is presentedAll classes show different properties with some classes being compatible with existent generation mechanismsBow shock ripple mechanism and SLAMS are generally supported by statistical properties [ABSTRACT FROM AUTHOR]

Magnetosheath jets are dynamic pressure enhancements observed in the terrestrial magnetosheath. Their generation mechanisms are currently debated but the majority of jets can be linked to foreshock processes. Recent results showed that jets are less numerous when coronal mass ejections (CMEs) cross the magnetosheath and more numerous when stream interaction regions (SIRs) cross it. Here, we show for the first time how the pronounced substructures of CMEs and SIRs are related to jet production. We distinguish between compression and magnetic ejecta (ME) regions for the CME as well as compression region associated with the stream interface and high-speed streams (HSSs) for the SIR. Based on THEMIS and OMNI data covering 2008-2021, we show the 2D probability distribution of jet occurrence using the cone angle and Alfvén Mach number. We compare this distribution with the values within each solar wind (SW) structure. We find that both high cone angles and low Alfvén Mach numbers within CME-MEs are unfavorable for jet production as they may inhibit a well-defined foreshock region. 1D histograms of all parameters show, which SW parameters govern jet occurrence in each SW structure. In terms of the considered parameters the most favorable conditions for jet generation are found for HSSs due to their associated low cone angles, low densities, and low magnetic field strengths., (© 2023. The Authors.)

We have investigated MESSENGER magnetic field data from the Mercury magnetosheath and near solar wind, to identify isolated magnetic field structures (defined as clear, isolated changes in the field magnitude). Their properties are studied in order to determine if they may be considered as analogues to plasmoids and jets known to exist in Earth's magnetosheath. Both isolated decreases of the magnetic field absolute value (‘negative magnetic field structures’) and increases (‘positive structures’) are found in the magnetosheath, whereas only negative structures are found in the solar wind. The similar properties of the solar wind and magnetosheath negative magnetic field structures suggests that they are analogous to diamagnetic plasmoids found in Earth's magnetosheath and near solar wind. The latter have earlier been identified with solar wind magnetic holes. Positive magnetic field structures are only found in the magnetosheath, concentrated to a region relatively close to the magnetopause. Their proximity to the magnetopause, their scale sizes, and the association of a majority of the structures with bipolar magnetic field signatures identify them as flux transfer events (which generally are associated with a decrease of plasma density in the magnetosheath). The positive magnetic field structures are therefore not likely to be analogous to terrestrial paramagnetic plasmoids but possibly to a sub-population of magnetosheath jets. At Earth, a majority of magnetosheath jets are associated with the quasi-parallel bow shock. We discuss some consequences of the findings of the present investigation pertaining to the different nature of the quasi-parallel bow shock at Mercury and Earth. [ABSTRACT FROM AUTHOR]