Solar wind magnetic holes can cross the bow shock and enter the magnetosheath

Solar wind magnetic holes are localized depressions of the magnetic field strength, on timescales of seconds to minutes. We use Cluster multipoint measurements to identify 26 magnetic holes which are observed just upstream of the bow shock and, a short time later, downstream in the magnetosheath, thus showing that they can penetrate the bow shock and enter the magnetosheath. For two magnetic holes, we show that the relation between upstream and downstream properties of the magnetic holes are well described by the MHD (magnetohydrodynamic) Rankine&ndash;Hugoniot (RH) jump conditions. We also present a small statistical investigation of the correlation between upstream and downstream observations of some properties of the magnetic holes. The temporal scale size and magnetic field rotation across the magnetic holes are very similar for the upstream and downstream observations, while the depth of the magnetic holes varies more. The results are consistent with the interpretation that magnetic holes in Earth's and Mercury's magnetosheath are of solar wind origin, as has previously been suggested. Since the solar wind magnetic holes can enter the magnetosheath, they may also interact with the magnetopause, representing a new type of localized solar wind&ndash;magnetosphere interaction. &nbsp;</p

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

Shocks are one of nature's most powerful particle accelerators and have been connected to relativistic electron acceleration and cosmic rays. Upstream shock observations include wave generation, wave-particle interactions and magnetic compressive structures, while at the shock and downstream, particle acceleration, magnetic reconnection and plasma jets can be observed. Here, using Magnetospheric Multiscale (MMS) we show in-situ evidence of high-speed downstream flows (jets) generated at the Earth's bow shock as a direct consequence of shock reformation. Jets are observed downstream due to a combined effect of upstream plasma wave evolution and an ongoing reformation cycle of the bow shock. This generation process can also be applicable to planetary and astrophysical plasmas where collisionless shocks are commonly found. Several mechanisms exist for formation of jets observed in Earth's magnetosheath. Here, the authors show evidence of high-speed downstream flows generated at the Earth's bow shock as a direct consequence of shock reformation, which is different than the proposed mechanisms.Peer reviewe

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

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 Alfven Mach number M-A 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 M-A.Peer reviewe

High-speed jets and related phenomena at Earth's bow shock and magnetosheath

Magnetosheath high-speed jets are transient and localized dynamic pressure enhancements downstream of Earth’s bow shock. Their formation has been associated with several mechanisms, including solar transient events and the dynamical evolution of the bow shock. After their formation, jets interact with the background magnetosheath population, exciting various waves and accelerating particles. When they reach the magnetosphere, they can penetrate the magnetopause, drive surface waves, and cause magnetopause reconnection. Their effects to the inner geospace environment can be seen through substorm activity and ground magnetometer measurements. In this thesis, a series of papers on the formation, evolution and statistical properties of jets is presented. Most of the work is done using NASA’s Magnetosphere Multiscale (MMS) mission, while other missions like THEMIS and upstream solar wind monitors (e.g., ACE and Wind) are also used. For our analysis, we also make complementary use of neural networks and computer simulations. Our investigation initially showed the importance of classifying jets based on the shock orientation and interplanetary magnetic field (IMF), resulting in an open-access database of magnetosheath jets using MMS. This dataset was then used to derive statistical properties for each class of magnetosheath jets (Paper I). The jets were also classified using neural networks (Paper II), while a comparison between their statistical properties and computer simulated jets was performed (Paper III). Another aspect we investigated through multi-point measurements is the excitation of waves due to the interaction of jets with the magnetosheath (Paper IV). We then focused on the formation and evolution of jets close to the Earth’s bow shock. We showed direct in-situ evidence that shock reformation and the evolution of upstream waves can generate downstream high-speed jets (Paper V). By evaluating the properties of jets on a kinetic level, we demonstrated that jets exhibit complex velocity distribution functions (VDFs) throughout their lifetime. Deriving partial plasma moments to isolate the jet from the background population, we revealed the limitations of studying these phenomena from a single-fluid perspective and how the derived partial plasma moments are related to the upstream solar wind and its foreshock structures (Paper VI).Plasmajetar i magnetoskiktet är transienta och lokaliserade förhöjningar av det dynamiska trycket nedströms om jordens bogchock. Flera olika generationsmekanismer har föreslagits, t ex transienta strukturer i solvinden eller dynamisk omformning av bogchocken. Efter att de har genererats vid bogchocken växelverkar de med bakgrundsplasmat i magnetoskiktet, där de exciterar plasmavågor och accelererar partiklar. När de når magnetopausen kan de korsa den, driva ytvågor, eller initiera magnetisk omkoppling. Plasmajetars effekt på rymdmiljön nära Jorden manifesterar sig genom substormar och markbaserade mätningar av jordens magnetfält. Denna avhandling innehåller att antal artiklar om genereringen, utvecklingen och de statistiska egenskaperna hos plasmajetar. Huvuddelen av arbetet är baserad på mätningar från NASAs MMS-satelliter, tillsammans med kompletterande data från andra satellitmissioner, som THEMIS och solavindsmonitorer (t ex  ACE och Wind). För dataanalysen använder vi också neurala nätverk och plasmasimuleringar. Våra första resultat visade på vikten av att klassificera jetar baserat på relationen mellan bogchockens orientering och riktningen på det interplanetära magnetfältet. Denna klassificering resulterade i en offentligt tillgänglig databas, innehållande MMS-observationer av plasmajetar. Detta dataset användes för att bestämma jetarnas statistiska egenskaper för de olika klasserna (Artikel I), vilket följdes upp med en klassificering baserade på neurala nätverk (Artikel II), vilket jämfördes med plasmasimuleringar (Artikel III). En ytterligare egenskap hos plasmajetar, excitation av plasmavågor, undersöktes med flerpunktsmätningar (Artikel IV). Därefter fokuserade vi på genereringen och evolutionen av jetar nära jordens bogchock. Vi visar att direkta in situ-mätningar tyder på att dynamisk omformning av bogchocken och vågor uppströms om den kan generera plasmajetar i magnetoskiktet (Artikel V). Genom att studera jetars plasmakinetiska egenskaper visar vi också att deras distributionsfunktioner uppvisat ett komplext beteende under jetarnas livstid. Beräkningar av partiella plasmamoment för att isolera jetarna från bakgrundsplasmat visar på begränsningarna i att betrakta dessa fenomen som en enkel fluid, och hur momenten är relaterade till solvinden uppströms om bogchocken (Artikel VI).QC 20221031</p

High-speed jets and related phenomena at Earth's bow shock and magnetosheath

Magnetosheath high-speed jets are transient and localized dynamic pressure enhancements downstream of Earth’s bow shock. Their formation has been associated with several mechanisms, including solar transient events and the dynamical evolution of the bow shock. After their formation, jets interact with the background magnetosheath population, exciting various waves and accelerating particles. When they reach the magnetosphere, they can penetrate the magnetopause, drive surface waves, and cause magnetopause reconnection. Their effects to the inner geospace environment can be seen through substorm activity and ground magnetometer measurements. In this thesis, a series of papers on the formation, evolution and statistical properties of jets is presented. Most of the work is done using NASA’s Magnetosphere Multiscale (MMS) mission, while other missions like THEMIS and upstream solar wind monitors (e.g., ACE and Wind) are also used. For our analysis, we also make complementary use of neural networks and computer simulations. Our investigation initially showed the importance of classifying jets based on the shock orientation and interplanetary magnetic field (IMF), resulting in an open-access database of magnetosheath jets using MMS. This dataset was then used to derive statistical properties for each class of magnetosheath jets (Paper I). The jets were also classified using neural networks (Paper II), while a comparison between their statistical properties and computer simulated jets was performed (Paper III). Another aspect we investigated through multi-point measurements is the excitation of waves due to the interaction of jets with the magnetosheath (Paper IV). We then focused on the formation and evolution of jets close to the Earth’s bow shock. We showed direct in-situ evidence that shock reformation and the evolution of upstream waves can generate downstream high-speed jets (Paper V). By evaluating the properties of jets on a kinetic level, we demonstrated that jets exhibit complex velocity distribution functions (VDFs) throughout their lifetime. Deriving partial plasma moments to isolate the jet from the background population, we revealed the limitations of studying these phenomena from a single-fluid perspective and how the derived partial plasma moments are related to the upstream solar wind and its foreshock structures (Paper VI).Plasmajetar i magnetoskiktet är transienta och lokaliserade förhöjningar av det dynamiska trycket nedströms om jordens bogchock. Flera olika generationsmekanismer har föreslagits, t ex transienta strukturer i solvinden eller dynamisk omformning av bogchocken. Efter att de har genererats vid bogchocken växelverkar de med bakgrundsplasmat i magnetoskiktet, där de exciterar plasmavågor och accelererar partiklar. När de når magnetopausen kan de korsa den, driva ytvågor, eller initiera magnetisk omkoppling. Plasmajetars effekt på rymdmiljön nära Jorden manifesterar sig genom substormar och markbaserade mätningar av jordens magnetfält. Denna avhandling innehåller att antal artiklar om genereringen, utvecklingen och de statistiska egenskaperna hos plasmajetar. Huvuddelen av arbetet är baserad på mätningar från NASAs MMS-satelliter, tillsammans med kompletterande data från andra satellitmissioner, som THEMIS och solavindsmonitorer (t ex  ACE och Wind). För dataanalysen använder vi också neurala nätverk och plasmasimuleringar. Våra första resultat visade på vikten av att klassificera jetar baserat på relationen mellan bogchockens orientering och riktningen på det interplanetära magnetfältet. Denna klassificering resulterade i en offentligt tillgänglig databas, innehållande MMS-observationer av plasmajetar. Detta dataset användes för att bestämma jetarnas statistiska egenskaper för de olika klasserna (Artikel I), vilket följdes upp med en klassificering baserade på neurala nätverk (Artikel II), vilket jämfördes med plasmasimuleringar (Artikel III). En ytterligare egenskap hos plasmajetar, excitation av plasmavågor, undersöktes med flerpunktsmätningar (Artikel IV). Därefter fokuserade vi på genereringen och evolutionen av jetar nära jordens bogchock. Vi visar att direkta in situ-mätningar tyder på att dynamisk omformning av bogchocken och vågor uppströms om den kan generera plasmajetar i magnetoskiktet (Artikel V). Genom att studera jetars plasmakinetiska egenskaper visar vi också att deras distributionsfunktioner uppvisat ett komplext beteende under jetarnas livstid. Beräkningar av partiella plasmamoment för att isolera jetarna från bakgrundsplasmat visar på begränsningarna i att betrakta dessa fenomen som en enkel fluid, och hur momenten är relaterade till solvinden uppströms om bogchocken (Artikel VI).QC 20221031</p

Current Sheet Statistics in the Magnetosheath

The magnetosheath (MSH) plasma turbulence depends on the structure and properties of the bow shock (BS). Under quasi-parallel (Q(||)) and quasi-perpendicular (Q(perpendicular to)) BS configurations the electromagnetic field and plasma quantities possess quite distinct behavior, e.g., being highly variable and structured in the Q(||) case. Previous studies have reported abundance of thin current sheets (with typical scales of the order of the plasma kinetic scales) in the Q(||) MSH, associated with magnetic reconnection, plasma heating, and acceleration. Here we use multipoint observations from Magnetospheric MultiScale (MMS) mission, where for the first time a comparative study of discontinuities and current sheets in both MSH geometries at very small spacecraft separation (of the order of the ion inertial length) is performed. In Q(||) MSH the current density distribution is characterized by a heavy tail, populated by strong currents. There is high correlation between these currents and the discontinuities associated with large magnetic shears. Whilst, this seems not to be the case in Q(perpendicular to) MSH, where current sheets are virtually absent. We also investigate the effect of the discontinuities on the scaling of electromagnetic fluctuations in the MHD range and in the beginning of the kinetic range. There are two (one) orders of magnitude higher power in the magnetic (electric) field fluctuations in the Q(||) MSH, as well as different spectral scaling, in comparison to the Q(perpendicular to) MSH configuration. This is an indication that the incoming solar wind turbulence is completely locally reorganized behind Q(perpendicular to) BS while even though modified by Q(||) BS geometry, the downstream turbulence properties are still reminiscent to the ones upstream, the latter confirming previous observations. We show also that the two geometries are associated with different temperature anisotropies, plasma beta, and compressibility, where the Q(perpendicular to) MSH is unstable to mostly mirror mode plasma instability, while the Q(||) MSH is unstable also to oblique and parallel fire-hose, and ion-cyclotron instabilities

On magnetosheath jet kinetic structure and plasma properties

High-speed plasma jets downstream of Earth's bow shock are high velocity streams associated with a variety of shock and magnetospheric phenomena. In this work, using the Magnetosphere Multiscale mission, we study the properties of a jet found downstream of the Quasi-parallel bow shock using high-resolution (burst) data. By doing so, we demonstrate how the jet is an inherently kinetic structure described by highly variable velocity distributions. The observed distributions show the presence of two plasma population, a cold/fast jet and a hotter/slower background population. We derive partial moments for the jet population to isolate its properties. The resulting partial moments appear different from the full ones which are typically used in similar studies. These discrepancies show how jets are more similar to upstream solar wind beams compared to what was previously believed. Finally, we explore the consequences of our results and methodology regarding the characterization, origin, and evolution of jets

Electron Kinetic Entropy across Quasi-Perpendicular Shocks

We use Magnetospheric Multiscale (MMS) data to study electron kinetic entropy per particle Se across Earth&rsquo;s quasi-perpendicular bow shock. We have selected 22 shock crossings covering a wide range of shock conditions. Measured distribution functions are calibrated and corrected for spacecraft potential, secondary electron contamination, lack of measurements at the lowest energies and electron density measurements based on plasma frequency measurements. All crossings display an increase in electron kinetic entropy across the shock &Delta;Se being positive or zero within their error margin. There is a strong dependence of &Delta;Se on the change in electron temperature, &Delta;Te, and the upstream electron plasma beta, &beta;e. Shocks with large &Delta;Te have large &Delta;Se. Shocks with smaller &beta;e are associated with larger &Delta;Se. We use the values of &Delta;Se, &Delta;Te and density change &Delta;ne to determine the effective adiabatic index of electrons for each shock crossing. The average effective adiabatic index is &#10216;&gamma;e&#10217;=1.64&plusmn;0.07

High-speed Downstream Plasma Jet Generated due to Shock Reformation

Magnetosheath jets are transient, localized dynamic pressure enhancements found behind Earth’s bow shock. They have been associated to a variety of phenomena and effects, including, magnetopause reconnection, excitation of ULF waves and direct plasma penetration in the magnetosphere. While the have been observed for several decades, their origin is not yet fully understood. In this work, we use Magnetosphere Mutliscale (MMS) measurement to show the generation of a high-speed downstream jet resulting from the shock reformation process. The jet appears to be associated to the evolution of the upstream waves found upstream of a Short Large Amplitude Magnetic Structure (SLAMS). As the initial SLAMS eventually continues to form the magnetosheath region, a newly formed foreshock magnetic structure appears, acting as the local bow shock front. This process allows the solar wind to be effectively found downstream of the new local shock front, forming a magnetosheath jet. The limited interaction of the solar wind with the old shock (initial SLAMS) allow the slightly compressed solar wind to retain its initial high velocity, which correspond to a plasma jet relatively to the background. The formation mechanism, we show, does not require any external solar wind related transient phenomena to occur and could provide an answer on how jets could form in situations where strong rippling is not observed in the quasi-parallel bow shock