Collisionless shock waves, found in supernova remnants, interstellar, stellar, and planetary environments, and laboratories, are one of nature's most powerful particle accelerators. This study combines in situ satellite measurements with recent theoretical developments to establish a reinforced shock acceleration model for relativistic electrons. Our model incorporates transient structures, wave-particle interactions, and variable stellar wind conditions, operating collectively in a multiscale set of processes. We show that the electron injection threshold is on the order of suprathermal range, obtainable through multiple different phenomena abundant in various plasma environments. Our analysis demonstrates that a typical shock can consistently accelerate electrons into very high (relativistic) energy ranges, refining our comprehension of shock acceleration while providing insight on the origin of electron cosmic rays.

We use the Magnetospheric Multiscale mission to observe a bi-directional electron acceleration event in the electron foreshock upstream of Earth's quasi-perpendicular collisionless bow shock. The acceleration region is associated with a decrease in wave activity, inconsistent with common electron acceleration mechanisms such as Diffusive Shock Acceleration and Stochastic Shock Drift Acceleration. We propose a two-step acceleration process where an electron field-aligned beam acts as a seed population further accelerated by a shrinking magnetic bottle process, with the shock acting as the magnetic mirror(s).

Shocklets and short large-amplitude magnetic structures (SLAMS) are steepened magnetic fluctuations commonly found in Earth's upstream foreshock. Here we present Venus Express observations from the 26th of February 2009 establishing their existence in the steady-state foreshock of Venus, building on a past study which found SLAMS during a substantial disturbance of the induced magnetosphere. The Venusian structures were comparable to those reported near Earth. The 2 Shocklets had magnetic compression ratios of 1.23 and 1.34 with linear polarization in the spacecraft frame. The 3 SLAMS had ratios between 3.22 and 4.03, two of which with elliptical polarization in the spacecraft frame. Statistical analysis suggests SLAMS coincide with unusually high solar wind Alfvén mach-number at Venus (12.5, this event). Thus, while we establish Shocklets and SLAMS can form in the stable Venusian foreshock, they may be rarer than at Earth. We estimate a lower limit of their occurrence rate of ≳14%.

We present the first statistical study on the velocity of magnetic holes (MHs) in the solar wind. Magnetic holes are localized depressions of the magnetic field, often divided into two classes: rotational and linear MHs. We have conducted a timing analysis of observations of MHs from the Cluster mission in the first quarter of 2005. In total, 69 events were used; out of these, there were 40 linear and 29 rotational MHs, where the limit of magnetic field rotation was set to 50 degrees. The resulting median velocity was 7.4 +/- 45 and 25 +/- 42 km s(-1) for linear and rotational MHs, respectively. For both classes, around 70% of the events had a velocity in the solar wind frame that was lower than the Alfven velocity. Therefore, we conclude that within the observational uncertainties, both linear and rotational MHs are convected with the solar wind.

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.

We investigate the dynamics of Earth's quasi-parallel terrestrial bow shock based on measurements from the Magnetospheric MultiScale (MMS) spacecraft constellation during a period of near-radial interplanetary magnetic conditions, when the interplanetary magnetic field and the solar wind (SW) velocity are nearly anti-parallel. High-speed earthward ion flows with properties that are similar to those of the pristine SW are observed to be embedded within the magnetosheath-like plasma. These flows are accompanied by Interplanetary Magnetic Field (IMF) intensity of less than about 10 nT, compared to nearby magnetosheath intensities of generally greater than 10 nT. The high-speed flow intervals are bounded at their leading and trailing edges by intense fluxes of more energetic ions and large amplitude quasi-sinusoidal magnetic oscillations, similar to ultra-low frequency waves known to steepen and pileup on approach toward Earth to form the quasi-parallel bow shock. The MMS string-of-pearls configuration is aligned with the outbound trajectory and provides inter-spacecraft separations of several hundred km along its near 10(3) length, allowing sequential observation of the plasma and magnetic field signatures during the event by the four spacecraft. The SW-like interval is most distinct at the outer-most MMS-2 and sequentially less distinct at each of the trailing MMS spacecraft. We discuss the interpretation of this event alternatively as MMS having observed a quasi-rigid bow shock contraction/expansion cycle, ripples or undulations propagating on the bow shock surface, or a more spatially local evolution in the context of either a deeply deformed shock surface or a porous shock surface, as in the three-dimensional patchwork concept of the quasi-parallel bow shock, under the extant near-radial IMF condition. Published under an exclusive license by AIP Publishing.

We use Magnetospheric Multiscale (MMS) data to study electron kinetic entropy per particle Se across Earth'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 S-e being positive or zero within their error margin. There is a strong dependence of Delta S-e on the change in electron temperature, Delta T-e, and the upstream electron plasma beta, beta(e). Shocks with large Delta T-e have large Delta S-e. Shocks with smaller beta(e) are associated with larger Delta S-e. We use the values of Delta S-e, Delta Te and density change Delta n(e) to determine the effective adiabatic index of electrons for each shock crossing. The average effective adiabatic index is <gamma(e)> = 1.64 +/- 0.07.

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).

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. 

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-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-magnetosphere interaction.