Ideal MHD simulation of magnetic clouds in the solar wind

Diese Dissertation untersucht die Ausbreitung magnetischer Wolken durch das innere Sonnensystem auf der Basis der Gesetze der idealen Magnetohydrodynamik (MHD). Magnetische Wolken bilden eine Untergruppe der koronalen Massenausw\ufcrfe (CMEs) mit speziellen Plasma- und Magnetfeld-Eigenschaften. Das Eintreffen einer magnetischen Wolke bei der Erde ist von besonderer Bedeutung, weil dadurch St\uf6rungen im Erdmagnetfeld sowie in der Ionosph\ue4re der Erde ausgel\uf6st werden.Die Simulation besteht im Wesentlichen aus einer numerischen L\uf6sung des Systems der MHD Erhaltungsgleichungen (Erhaltung von Masse, Impuls, magnetischer Induktion und Gesamtenergie). Gel\uf6st werden diese Gleichungen auf einem zwei-dimensionalen Gitternetz in sph\ue4rischen Polarkoordinaten, zum einen in einer meridionalen Ebene, zum anderen in einer \ue4quatorialen Ebene. Das L\uf6sungsverfahren ist bekannt als "Roe-type approximate Riemann solver".Im ersten Schritt wird der Sonnenwind simuliert. Herausragende Merkmale dieser Simulation sind: eine Parker-Spirale f\ufcr das interplanetare Magnetfeld (IMF), eine heliosph\ue4rische Stromschicht, sowie breitenabh\ue4ngige Variationen von Dichte, Sonnenwindgeschwindigkeit, Magnetfeldst\ue4rke und Druck. Danach wird eine magnetische Wolke als kreisf\uf6rmige Querschnittsfl\ue4che am inneren Rand des Gitters platziert. Vier verschiedene Ausbreitungsszenarien werden simuliert. Spezielle Aufmerksamkeit wird dabei dem Wolken-Magnetfeld gewidmet. Je nach Umlaufsinn der Magnetfeldlinien um die Achse der Wolke entstehen v\uf6llig unterschiedliche Wechselwirkungen mit dem interplanetaren Magnetfeld als Folge magnetischer Rekonnexion. Die Ergebnisse dieser MHD Simulationen stimmen gut mit Beobachtungen durch Satelliten und Raumsonden \ufcberein. Diese Dissertation kann als erster Schritt in Richtung vergleichende MHD Studien angesehen werden, deren Ziel es ist, die Ausbreitung magnetischer Wolken unter dem Aspekt variierender Anfangsbedingungen besser zu verstehen.The propagation of Magnetic Clouds (MCs) through the inner heliosphere has been investigated on the basis of the laws of ideal magnetohydrodynamics (MHD). Magnetic clouds are a subset of coronal mass ejections exhibiting a special plasma and magnetic field configuration. The arrival of magnetic clouds at Earth is of special interest because they cause severe disturbances of the terrestrial magnetosphere and ionosphere system.The simulation solves numerically the set of ideal MHD governing equations which comprises the conservations of mass, momentum, magnetic induction, and total energy. A numerical solution is obtained on a two-dimensional spherical grid, either in a meridional plane or in an equatorial plane, by using a Roe-type approximate Riemann solver.In a first step, the solar wind is simulated exhibiting a Parker spiral interplanetary magnetic field, a heliospheric current sheet, and variations of density, velocity, magnetic field strength, and pressure with heliographic latitude. Then, a magnetic cloud cross section is placed near the inner boundary. Four different scenarios of initial MC configurations have been simulated in this thesis. The propagation of magnetic clouds and their interactions with the structured ambient solar wind have been investigated on a global scale. Special emphasis is put on the role of the initial magnetic helicity because this parameter severely influences the efficiency of magnetic reconnection between the MC's magnetic field and the interplanetary magnetic field. The principal features seen in these MHD simulations are in good agreement with in-situ measurements performed by satellites and spacecraft. This thesis may be considered as a first step towards a more extensive comparative study for investigating the complex interaction between magnetic clouds and a structured ambient solar wind.Ulrich TaubenschussAbweichender Titel laut cbersetzung der Verfasserin/des VerfassersGraz, Univ., Diss., 200

Shock deceleration in interplanetary coronal mass ejections (ICMEs) beyond Mercury’s orbit until one AU

The CDPP propagation tool is used to propagate interplanetary coronal mass ejections (ICMEs) observed at Mercury by MESSENGER to various targets in the inner solar system (VEX, ACE, STEREO-A and B). The deceleration of ICME shock fronts between the orbit of Mercury and 1 AU is studied on the basis of a large dataset. We focus on the interplanetary medium far from the solor corona, to avoid the region where ICME propagation modifications in velocity and direction are the most drastic. Starting with a catalog of 61 ICMEs recorded by MESSENGER, the propagation tool predicts 36 ICME impacts with targets. ICME in situ signatures are investigated close to predicted encounter times based on velocities estimated at MESSENGER and on the default propagation tool velocity (500 km s−1). ICMEs are observed at the targets in 26 cases and interplanetary shocks (not followed by magnetic ejecta) in two cases. Comparing transit velocities between the Sun and MESSENGER (${\bar{v}}_{\mathrm{SunMess}}$) and between MESSENGER and the targets (${\bar{v}}_{\mathrm{MessTar}}$), we find an average deceleration of 170 km s−1 (28 cases). Comparing ${\bar{v}}_{\mathrm{MessTar}}$ to the velocities at the targets (v Tar), average ICME deceleration is about 160 km s−1 (13 cases). Our results show that the ICME shock deceleration is significant beyond Mercury’s orbit. ICME shock arrival times are predicted with an average accuracy of about six hours with a standard deviation of eleven hours. Focusing on two ICMEs detected first at MESSENGER and later on by two targets illustrates our results and the variability in ICME propagations. The shock velocity of an ICME observed at MESSENGER, then at VEX and finally at STEREO-B decreases all the way. Predicting arrivals of potentially effective ICMEs is an important space weather issue. The CDPP propagation tool, in association with in situ measurements between the Sun and the Earth, can permit to update alert status of such arrivals

Shock deceleration in interplanetary coronal mass ejections (ICMEs) beyond Mercury’s orbit until one AU

The CDPP propagation tool is used to propagate interplanetary coronal mass ejections (ICMEs) observed at Mercury by MESSENGER to various targets in the inner solar system (VEX, ACE, STEREO-A and B). The deceleration of ICME shock fronts between the orbit of Mercury and 1 AU is studied on the basis of a large dataset. We focus on the interplanetary medium far from the solor corona, to avoid the region where ICME propagation modifications in velocity and direction are the most drastic. Starting with a catalog of 61 ICMEs recorded by MESSENGER, the propagation tool predicts 36 ICME impacts with targets. ICME in situ signatures are investigated close to predicted encounter times based on velocities estimated at MESSENGER and on the default propagation tool velocity (500 km s−1). ICMEs are observed at the targets in 26 cases and interplanetary shocks (not followed by magnetic ejecta) in two cases. Comparing transit velocities between the Sun and MESSENGER ( v ̅ SunMess ${\bar{v}}_{\mathrm{SunMess}}$ ) and between MESSENGER and the targets ( v ̅ MessTar ${\bar{v}}_{\mathrm{MessTar}}$ ), we find an average deceleration of 170 km s−1 (28 cases). Comparing v ̅ MessTar ${\bar{v}}_{\mathrm{MessTar}}$ to the velocities at the targets (v Tar), average ICME deceleration is about 160 km s−1 (13 cases). Our results show that the ICME shock deceleration is significant beyond Mercury’s orbit. ICME shock arrival times are predicted with an average accuracy of about six hours with a standard deviation of eleven hours. Focusing on two ICMEs detected first at MESSENGER and later on by two targets illustrates our results and the variability in ICME propagations. The shock velocity of an ICME observed at MESSENGER, then at VEX and finally at STEREO-B decreases all the way. Predicting arrivals of potentially effective ICMEs is an important space weather issue. The CDPP propagation tool, in association with in situ measurements between the Sun and the Earth, can permit to update alert status of such arrivals

A slow mode transition region adjoining the front boundary of a magnetic cloud as a relic of a convected solar wind feature: Observations and MHD simulation

We identify a planar, pressure-balanced structure bounded by sharp changes in the dynamic pressure plastered against the front boundary of the magnetic cloud which passed Earth on 20 November 2003. The front boundary of the magnetic cloud (MC) is particularly well-defined in this case, being located where the He++/H+ number density ratio jumps from 4 to 10% for the first time and the proton plasma beta decreases sharply from ∼1 to ∼0.001. The feature, estimated to have a length scale ∼50 RE in the Sun-Earth direction, bears close resemblance to a slow mode transition region in that the magnetic pressure decreases, the plasma pressure increases, and their temporal variations are anticorrelated. Using a 2-D MHD simulation, we hypothesize that a pressure-balanced structure was encountered by the MC en route to Earth. Our calculations reproduce qualitatively the major features of the observations. Using a simplified geometry suggested by the observations, we find that the lateral deflection speed of the plasma is less than the lateral expansion speed of the MC. We infer that the structure traversed the MC sheath in ∼20 h, consistent with its crossing of the MC\u27s shock at 0.6–0.7 AU. The finding is consistent with the recent paradigm according to which solar wind plasma and magnetic field tend to pile up in front of interplanetary ejecta because the expansion of the ejecta hinders the shocked solar wind plasma from deflecting effectively around the object. Also, the inferred “age” of the layer contiguous to the surface of the MC, the earliest relic of its passage through the inner heliosphere, is in agreement with general estimates

Observations of the First Harmonic of Saturn Kilometric Radiation During Cassini's Grand Finale

International audienceClear first harmonic emissions of Saturn Kilometric Radiation are discovered during the CassiniGrand Finale orbits. Both ordinary (O) and extraordinary (X) mode fundamental emissions accompanied by Xmode harmonics are observed. Analysis shows that the frequency ratio between the fundamental and harmonicemissions is 2.01 ± 0.08, and the harmonic emissions display weaker intensities than the fundamental, by30–40 dB for the X-X (fundamental-harmonic) type harmonic and 10–30 dB for the O-X type harmonic.The intensity relations between the two types of harmonics, that is, O-X and X-X show different patternsthat we attribute to different conditions of emission at the source. Direction-finding results shows that thefundamental and harmonic emissions are plausibly generated in the same source region. In agreement withprevious studies at Earth, the generation of the two types of harmonics can be attributed to the cyclotron maserinstability operating with different plasma density and electron energy distributions in the source region

New high sensitive investigation of the modulation effects in the sporadic Jovian DAM emission

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Linear filtering of Saturn kilometric radiation and the solar wind

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High Sensitive Investigations of the Sporadic Jovian Radio Emission

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New high sensitive investigation of the modulation effects in the sporadic Jovian DAM emission

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High Sensitive Investigations of the Sporadic Jovian Radio Emission

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