Dipolarization fronts in the near-Earth plasma sheet

Die Plasmaschicht des Erdmagnetosphärenschweifs beherbergt eine Vielzahl transienter Plasmaphänomene, die von Satelliten beobachtet werden können. Als Folge von rekonnektiven Prozessen in der mittleren und erdfernen Neutralschicht propagieren Plasmablasen und Fast Flows in Richtung Erde, vor denen sich sogenannte Dipolarisierungsfronten ausbilden. Sie kennzeichnen sich vor allem durch bipolare Signaturen in der Z-Komponente des Magnetfeldes. Diese Arbeit vereint zwei Handlungsstränge miteinander: Auf der einen Seite dient die Multi-Satellitenmission THEMIS, die seit 2007 in einem quasi­äquatorialen Orbit um die Erde ist, als Grundlage für die statistische Analyse einer Vielzahl von Beobachtungen von Dipolarisierungsfronten in der Plasmaschicht. Gleichzeitig dient ein eingeschränktes MHD-Modell einer Harris-Schicht im Simulationscode PLUTO dazu, die grundlegenden Eigenschaften von Dipolarisierungsfronten zu reproduzieren und zusätzliche, bisher nicht beobachtete Charakteristika abzuleiten. Nach der Validierung der Simulationen anhand der bekannten Eigenschaften werden neue Erkenntnisse bezüglich dieser Strukturen mit Hilfe der Satellitenbeobachtungen abgeglichen. Mit Hilfe einer Superpositionsanalyse werden mittlere Plasmaprofile von Dipolarisierungsfronten berechnet, die die Ergebnisse früherer Studien in ihrer Qualität übertreffen. Die Daten von Simulationen und Satellitenbeobachtungen zeigen, dass die Symmetrieebene von im Erdmagnetschweif propagierenden Dipolarisierungsfronten durch die Neutralschicht gebildet wird. Dieser Befund wird durch die statistische Analyse von Flächennormalen, Plasmasignaturen abseits der Symmetrieebene und durch die Modellierung der Deformation einer solchen Front gestützt. Gleichzeitig legen die Ergebnisse zur Asymmetrie der magnetischen Profile und die der Superpositionsanalyse eine enge Verwandtschaft zwischen Dipolarisierungsfronten und Flux Ropes nahe. Über die Formulierung eines simulationsempirischen Modells können die Satellitenbeobachtungen derart analysiert werden, dass Rückschlüsse auf das Alter der beobachteten Dipolarisierungsfronten und ihren Herkunftsort möglich sind. Als obere Abschätzung liegt die Lebenszeit dieser Strukturen zum Zeitpunkt der Beobachtung bei 1160 s. Ihr Ursprungsort liegt dadurch mit hoher Wahrscheinlichkeit in einer Distanz von bis zu 60 RE hinter der Erde.The plasma sheet of the Earth’s magnetotail is home to various transient plasma phenomena that are observed via spacecraft technologies. Initiated by reconnection processes, plasma bubbles and fast flows propagate towards Earth. Often, their plasma is separated from the background plasma by dipolarization fronts, characterized by sharp turnings of the z component of the magnetic field. In this study, both statistical analysis of multi-spacecraft mission THEMIS, as well as MHD simulations will be used to investigate dipolarization fronts in the magnetotail. THEMIS, being in orbit since 2007 on close-to-equatorial orbits, provides the basis for the statistical examination of the basic properties of dipolarization fronts. In the simulations, a simple Harris sheet model will serve as the most basic approach for the representation of the plasma sheet geometry. Both conistituents can be used to derive new findings from modeling and observations. Using THEMIS, superposed epoch analysis provide average plasma profiles that are more detailed than those of previous studies are. Combining simulations and satellite obervations it can be shown, that the fundamental plane of symmetry of dipolarization fronts is based on the neutral sheet geometry of the magnetotail. This result is based upon statistical analysis of front normal directions, plasma profiles slightly away from the symmetry line, and the deformation of dipolarization fronts during their passage through the neutral sheet. At the same time, the asymmetry of the magnetic profiles suggests a very close relationship between dipolarization fronts and flux ropes. By deriving an empirical model based upon the simulation results, the spacecraft observations can be used to estimate lifetime and origin of the dipolarizations fronts encountered by THEMIS. As an upper estimate, these structures have travelled for an average of 1160 s through the magnetotail, originating at a downtail distance of up to 60 RE

Spin axis offset calibration on THEMIS using mirror modes

A newly developed method for determining spin axis offsets of magnetic field instruments on spacecraft is applied to THEMIS. The formerly used determination method, relying on solar wind Alfvénic fluctuations, was rarely applicable due to the orbital restrictions of the mission. With the new procedure, based on magnetic field observation of mirror modes in the magnetosheath, updated spin axis offsets can be estimated approximately once per year. Retrospective calibration of all THEMIS magnetic field measurements is thereby made possible. Since, up to this point, spin axis offsets could hardly ever be calculated due to the mission's orbits, this update represents a substantial improvement to the data. The approximate offset stability is estimated to be < 0.75 nT year−1 for the complete course of the mission

Extracting planetary waves from geomagnetic time series using Empirical Mode Decomposition

Empirical Mode Decomposition is presented as an alternative to traditional analysis methods to decompose geomagnetic time series into spectral components. Important comments on the algorithm and its variations will be given. Using this technique, planetary wave modes of 5-, 10-, and 16-day mean periods can be extracted from magnetic field components of three different stations in Germany. In a second step, the amplitude modulation functions of these wave modes can be shown to contain significant contribution from solar cycle variation through correlation with smoothed sunspot numbers. Additionally, the data indicate connections with geomagnetic jerk occurrences, supported by a second set of data providing reconstructed near-Earth magnetic field for 150 years. Usually attributed to internal dynamo processes within the Earth's outer core, the question of who is impacting whom will be briefly discussed here

Average plasma sheet polytropic index as observed by THEMIS

Multi-spacecraft data from the years 2008 to 2015 of the THEMIS mission particularly in the near-Earth plasma sheet are used in order to empirically determine the polytropic index in the quiet and active time magnetotail. The results of a number of previous studies in the 1990s can be confirmed. An analysis of the total database, although showing poor correlation, results in an average polytropic index of γ = 1. 72. The active time plasma sheet is well correlated with an average γ = 1. 49. However, the data scattering suggests that the analysis of the data in total is not adequate. In order to reduce the timescales, individual spacecraft orbits are analyzed, giving a broad distribution of polytropic indices throughout the plasma sheet. The major part of the distribution falls in a range between γ = 0. 67 and γ = 2. Our results indicate a variety of thermodynamic processes in the magnetotail and an all-time presence of heat exchange of the plasma. A description of the plasma sheet using an equation of state with a single γ is probably inadequate. This necessitates the application of more sophisticated approaches, such as a parametrization of the heat flux vector in magnetohydrodynamic equations or a superposition of polytropic indices