京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18084号 / 理博第3962号 / 新制||理||1571(附属図書館) / 30942 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)准教授 齊藤 昭則, 教授 余田 成男, 准教授 藤 浩明 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Observations from the Earth's geomagnetic tail have established that the current sheet is often bifurcated with two peaks in the current density. These so-called bifurcated current sheets have also been reported in a variety of simulations and often occur in conjunction with significant temperature anisotropy. In this work, a new self-consistent Vlasov equilibrium is developed that permits both the current profile and temperature anisotropy to be independently adjusted. This new equilibrium has a sufficient flexibility to model a wide variety of bifurcated sheets observed in both kinetic simulations and space observations, and transforms continuously back to the standard Harris sheet model with a single peak in the current density. The linear stability of these layers with respect to the tearing mode is examined in the framework of resistive MHD and full Vlasov theory. From the simplified fluid theory, it is demonstrated that a bifurcated current profile has a stabilizing influence on the resistive tearing instability. However, the resistive MHD model is not really appropriate to model the highly collisionless plasma conditions in the magnetosphere. To obtain reliable predictions, Vlasov theory is required and the approach in this thesis employs both standard analytic techniques and a formally exact treatment in which the full orbit integral is numerically evaluated. The resulting linear growth rate for the collisionless tearing instability and the mode structure are verified with 2D full kinetic particle-in-cell simulations. The simplified analytic theory is reasonably accurate in capturing these dependencies for long wavelength modes, but the short wavelength regime generally requires the full numerical treatment to accurately compute the growth rate. The results from these different approaches consistently demonstrate that a bifurcated current profile has a strong stabilizing influence on the collisionless tearing mode in comparison to centrally peaked layers with a similar thickness. In collisionless tearing, electron temperature anisotropy is strongly destabilizing in the limit $T_{e \perp} > T_{e \parallel}$ and strongly stabilizing when $T_{e \perp} < T_{e \parallel}$. Thus, the collisionless tearing instability is determined by the competition between these two influences.

The whole universe is filled with plasma. There are different kinds of plasmas filling large volumes, separated by distinct boundaries. Many important energy conversion, particle acceleration and plasma transport processes occur at these boundaries, and therefore it is important to study the plasma processes there. It will for example help us to better understand the interaction and energy exchange between the Sun and the Earth. The lower hybrid drift waves (LHDW) are strong plasma waves that are often excited within boundaries, but their role in different plasma processes are still unclear. Many studies of the LHDW have been done, both in space and in laboratory. However, the LHDW are electron scale waves, and due to their small wavelength it has been difficult to study them in detail experimentally. For the first time we are able to make very detailed studies of the LHDW using observations by the Cluster spacecraft in the plasma surrounding Earth. By making cross spacecraft correlations of the electric field and examining existence conditions, we were able to determine the velocity of propagation and wavelength of the waves and thereby identify them as LHDW. We also calculate the electrostatic potential and find that it corresponds to about a third of the electron temperature. This indicate that they might be able to affect the electrons and thus take part in the processes within the boundary layer. By deriving a linear relation between the electrostatic potential, and the wave magnetic field, we compare them both and find that they correspond very well. We can use this to estimate the electrostatic potential in cases when cross spacecraft correlation is not possible.

In this thesis, a simple model of the stretched magnetic field lines in the nightside magnetotail was created. The nightside magnetosphere model contains four main regions: plasmasphere, plasma sheet, magnetic lobes, and low latitude boundary layers. The plasma sheet is split into three regions based on the shape of the closed field lines present: dipole plasma sheet, transition plasma sheet, and stretched plasma sheet (SPS). The SPS, the focus of this thesis, is split into two regions: disruption zones (DZs) and a central neutral sheet (NS). The shape of the stretched field lines contain four inflection points. The convex curvature regions form the DZs and the central concave curvature region forms the NS. The NS is split into two regions: outer neutral sheet (ONS) and inner neutral sheet (INS). Due to the reversal of the x-component of the magnetic field at the center line of the NS, the protons are magnetized in the ONS and "unmagnetized" in the INS. There are two main current systems in the SPS. The first is a double vortex current system consisting of eastward current in the DZs that closes westward in the NS. The second system is the NS field-aligned current (FAC) system. It is generated in the INS mainly by the earthward convective drift of the electrons while the "unmagnetized" protons have little convective drift and remain tailward of the electrons. This FAC system produces the pre-onset electron auroral arc during the growth phase of the substorm. A simple model of the stretched magnetic field lines was created in order to calculate the current systems present in the SPS. The simple model was based entirely upon the shape of the stretched field lines. It passed two physical tests, divergence of the magnetic field and limits at infinity, so it was used to calculate currents. The total current using Ampere's law and the curvature current was found. Both results agreed with the double vortex current system.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

The contemporary terrestrial atmosphere loses matter at a rate of around 100,000 tons per year. A major fraction of the net mass loss is constituted by ions, mainly H+ and O+, which escape from the Earth’s ionosphere in the polar regions. Previously, the outflow has only been measured at low altitudes, but to understand what fraction actually escapes and does not return, the measurements should be conducted far from the Earth. However, at large geocentric distances the outflowing ions are difficult to detect with conventional ion instruments on spacecraft, since the spacecraft electrostatic potential normally exceeds the equivalent energy of the ions. This also means that little is known about the ion outflow properties and distribution in space far from the Earth. In this thesis, we present a new method to measure the outflowing low-energy ions in those regions where they previously have been invisible. The method is based on the detection by electric field instruments of the large wake created behind a spacecraft in a flowing, low-energy plasma. Since ions with low energy will create a larger wake, the method is more sensitive to light ions, and our measured outflow is essentially the proton outflow. Applying this new method on data from the Cluster spacecraft, we have been able to make an extensive statistical study of ion outflows from 5 to 19 Earth radii in the magnetotail lobes. We show that cold proton outflows dominate in these large regions of the magnetosphere in both flux and density. Our outflow values of low-energy protons are close to those measured at low altitudes, which confirms that the ionospheric outflows continue far back in the tail and contribute significantly to the magnetospheric content. We also conclude that most of the ions are escaping and not returning, which improves previous estimates of the global outflow. The total loss of protons due to high-latitude escape is found to be on the order of 1026 protons/s.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Title: Structure, Location, and Dynamics of the Magnetopause Author: Kostiantyn Grygorov Department: Department of Surface and Plasma Science Supervisor: Doc. RNDr. Lubomír Přech, Dr., Department of Surface and Plasma Science e-mail address: Prech@mbox.troja.mff.cuni.cz Abstract: The magnetopause is the key region where the collisionless plasma from the Sun penetrates into the Earth's magnetosphere. A transfer of the plasma, momentum, and energy across the magnetopause is highly variable and depends on solar wind conditions, similarly as the magnetopause location. This thesis addresses the solar wind-magnetosphere interaction in two different regions: the dayside equatorial magnetopause and the far magnetotail. In the first part of the thesis, we present a detailed analysis of the interplanetary (IP) shock propa- gation through the solar wind, its impact on the substorm development and its influence on the far magnetotail. We demonstrate that the change of the solar wind VZ component across the shock creates a huge kink that propagates along the tail. The second part of the thesis is devoted to the influence of prolonged intervals of a radial interplanetary magnetic field (IMF) on the dayside magnetopause. The statistical analysis of the equatorial magnetopause shape and location based on THEMIS observations...