Magnetic holes (MH) are localized depressions of the magnetic field. They are found in various plasma environments, most commonly in the solar wind. The magnetic field direction sometimes varies across the hole, and it is common to divide MH into two categories, linear and rotational, where the first shows little to no rotation of the field. Linear MH are often in pressure balance with the surrounding plasma and can survive for long periods. It is unknown exactly where and how they are formed, but there is strong evidence that they are related to the mirror instability and waves associated with this instability. Rotational MH, on the other hand, appear as current sheet-like structures and are suggested to be related to magnetic reconnection or possibly in connection with wave activity. Both linear and rotational MH have similar properties, such as magnetic field magnitude and temporal scales. In this thesis, we use the Cluster mission, which has four spacecraft moving closely together providing multipoint measurements, to understand the phenomenon of magnetic holes better. Cluster provides over 20 years of data, with varying spacecraft spacing. Sometimes, the spacing of the satellites is such that one satellite is located in the solar wind while another is in the magnetosheath. In these instances, we have identified the same MH in the solar wind and later in the magnetosheath, suggesting they can cross the bowshock (Paper I). Using the Cluster database, we have identified times when the same MH was observed with all four spacecraft. Applying a timing analysis on a subset of these, we have derived their velocity, and conclude that they are convected with the solar wind speed (Paper II). When the same MH is observed by all four satellites in the solar wind, based on the spacecraft separation, one has four measuring points separated by distances comparable to the size of the MH. These observations can be combined with a local coordinate transformation and a model to estimate the scales of the MH and derive information about the three-dimensional morphology of MH (PaperIII). A large number of events were identified, and the results were combined to provide a statistical analysis of the morphology of solar wind magneticholes (Paper IV).

Magnetiska hål (MH) är lokaliserade reduktioner av magnetfältet. De finns i olika plasmamiljöer, oftast i solvinden. Det magnetiska fältets riktning varierar ibland över hålet, och det är vanligt att dela in MH i två kategorier, linjära och roterande, där den första visar liten eller ingen rotation av fältet. Linjära MH är ofta i tryckbalans med omgivande plasma och kan överleva under långa perioder. Det är okänt exakt var och hur de bildas, men det finns starka bevis för att de är relaterade till spegelinstabiliteten och vågor associerade med denna instabilitet. Roterande MH, å andra sidan, uppträder som strömskiktliknande strukturer och föreslås vara relaterade till magnetisk omkoppling eller möjligen till vågaktivitet. Både linjära och roterande MH har liknande egenskaper, såsom magnetfältets styrka och tidsskalor. I den här avhandlingen använder vi Cluster-missionen, som har fyra satelliter som rör sig tätt tillsammans och ger flerpunktsmätningar, för att förstå fenomenet magnetiska hål bättre. Cluster tillhandahåller över 20 års data, med varierande avstånd mellan satelliterna. Ibland är avståndet mellan satelliterna sådan att den ena satelliten befinner sig i solvinden medan en andra är i magnetoskiktet. I dessa fall har vi identifierat samma MH i solvinden och senare i magnetoskiktet, vilket tyder på att de kan korsa bogchocken (PaperI). Med hjälp av Cluster-databasen har vi identifierat tillfällen då samma MH observerades med alla fyra satelliter. Genom att tillämpa en tidsanalys på en delmängd av dessa har vi härlett deras hastighet och drar slutsatsen att de konvekteras med solvindens hastighet (Paper II). När samma MH observeras av alla fyra satelliterna i solvinden, baserat på rymdfarkostens separation, har man fyra mätpunkter åtskilda av avstånd jämförbara med storleken på MH. Dessa observationer kan kombineras med en lokal koordinattransformation och en modell för att uppskatta skalstorlekarna för MH och härleda information om MH tredimensionella morfologi (Paper III). Ett stort antal händelser identifierades, och resultaten kombinerades för att ge en statistisk analys av morfologin för solvindens magnetiska hål (Paper IV).

Short Large-Amplitude Magnetic Structures (SLAMS) are non-linear isolated magnetic field structures commonly observed in the foreshock region of quasi-parallel collisionless shocks. In this work, we use an automated algorithm to create a database of SLAMS detections made in the foreshock of Earth by the Cluster mission between the years 2002-2012. We define SLAMS to have amplitudes of at least two times the background magnetic field, leading to a detection of 1736 SLAMS during the studied period. Subsequently, the statistical properties of the SLAMS in the database are studied, such as their amplitude and temporal scale size. Correlations with the upstream environment are also studied, together with the conditions required for SLAMS formation and solar cycle dependencies. We find a mean temporal scale size of 3.3 s and an amplitude normalized by the background field, Delta B/Bbg ${\Delta }B/{B}_{bg}$, varying between 2 and 9, with a mean value of 2.9. 81% of the SLAMS are right-hand polarized in the spacecraft frame. We find that the magnetosonic and Alfv & eacute;n Mach numbers are important for SLAMS formation, with an increasing observation rate with increasing Mach numbers. Higher Mach numbers also tend to increase Delta B/Bbg ${\Delta }B/{B}_{bg}$ and decrease the temporal scale size of the structures. SLAMS are often associated with peaks in the plasma density, and we find a positive correlation between the amplitude of the magnetic field peaks and the amplitude of the density peaks, confirming the fast magnetosonic nature of SLAMS.

We investigate the collection of dust particles in the mesosphere with the MESS (MEteoric Smoke Sampler) instrument that is designed to fly on a sounding rocket. We assume that the ice particles that form in the polar mesosphere between 80 and 85 km altitude in summer contain meteoric smoke particles; and these should be collected with MESS. The instrument consists of a collection device with an opening and closure mechanism, as well as an attached conic funnel which increases the sampling area in comparison to the collection area. Dust particles are collected either directly after passing through the instrument or indirectly after colliding with and fragmenting on the funnel wall. We calculate the dust and fragment trajectories in the detector to determine the collection efficiency for different particle sizes, rocket velocities, and heights, and we find the final velocities and the temperatures of the particles. The considered design has a sampling area of 62.78 mm diameter and a collection area of 20 mm diameter. For the conditions at the rocket launch site in And & oslash;ya, Norway, we estimate the collection of meteoric smoke particles contained in the ice particles to be similar to 1012-1014 amumm-2. The estimated temperatures suggest that the composition of these smoke particles is not affected by the collection. Our calculations also show that keeping the instrument open above 85 km altitude increases the amount of small smoke particles that are directly collected. The directly collected smoke particles are heated as they decelerate, which can affect their composition.

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.

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.

We investigate and evaluate the possibility of using local magnetosheath measurements to classify the plasma according to upstream conditions. In order to do this, we use simultaneous measurements from the Cluster spacecraft from time intervals when one of them is located in the solar wind, and the other in the magnetosheath. In particular, we study the classification of the magnetosheath plasma into the classes quasi-parallel versus quasi-perpendicular and foreshock/no foreshock (referring to the geometry of the upstream bow shock). We evaluate this method based on the magnetosheath measurements of the high-energy ion energy flux, magnetic field standard deviation, and ion temperature anisotropy. We find that the method is promising and useful, in that it eliminates the uncertainties associated with propagating upstream measurements made far from the bow shock. Finally, we discuss some possible extensions of the methodology to be investigated in the future.

Magnetic holes have been studied for decades, but their three-dimensional structure has not been thoroughly investigated until now. We have identified solar wind magnetic holes observed simultaneously by the four Cluster spacecraft. By transforming the observations into a local coordinate system and identifying a principal axis, we fit the results to a three-dimensional Gaussian model to estimate their scales. We present four events that highlight the various aspects of this method, emphasizing the critical role of coordinate system selection in the analysis. The principal axis of the holes varied, and in three out of four cases, were not aligned with the magnetic field, instead forming angles between 50 and 80 degrees, suggesting that magnetic holes are not necessarily oriented along the magnetic field. Finally, our analysis reveals that the scale of the holes along one direction is significantly longer than along the other two, suggesting an elongated ellipsoid as a typical shape for their morphology.