3-D Modeling of Coronal Mass Ejections with STEREO/SECCHI Data

Abstract

This thesis reports on the three-dimensional analysis of coronal mass ejections (CMEs) in order to answer questions about their morphology and to derive their three-dimensional geometry. The questions are about the detailed 3-D structure, orientation and position of CMEs which are observed near the Sun (10 - 20 solar radii) with coronagraphs of the STEREO mission. The Sun as main actor for space weather and in particular as source of CMEs raises many exciting unanswered questions. Some of these about the CME's geometry are treated in this thesis. Getting a better understanding of CMEs is very helpful to improve forecasts of Earth directed CMEs for shielding humans and their infrastructure from the sometimes harmful space weather effects which are caused by CMEs. Within the scope of this work a study was started for the 3-D analysis of CMEs based on the coronagraph observations of the STEREO twin satellites. Both spacecraft observe the Sun and inner heliosphere from Earth-like orbits, with STEREO-A moving ahead and STEREO-B trailing behind Earth. The coronagraph observations are analysed for the time period starting at the beginning of the STEREO observations in January 2007 until December 2011 when the solar activity increased. 1071 CMEs were identified in the STEREO/SECCHI/COR2 coronagraph observations and summarised in an overall CME list. From this list a 'Best-of' CME list with 264 events was extracted based on the visual appearance of the CME's white-light structure in the coronagraph images. These CME events are analysed in this thesis in detail. During the inspection of the COR2 coronagraph images it was noticeable that the CMEs occur with different morphologies in their two dimensional white-light appearance. Because several shapes of CME appearances with certain patterns occurred often, ten CME classes are defined and introduced. Subsequently, the 'Best-of' CMEs are categorised according to these CME class definitions. The different CME morphologies which were found during the first inspections of the coronagraph data are investigated in order to find out if CMEs of different white-light appearances can be described as flux ropes with the Graduated Cylindrical Shell (GCS) model. In the second step it is examined how these CMEs look like in terms of this model. It is found that the different appearances and morphologies of CMEs observed with coronagraphs can indeed be fitted with the GCS modelling technique resulting in a 3-D flux rope geometry. The synthetic coronagraph images which are generated from Thomson scattering calculations confirm their 3-D flux rope geometry. Since the CMEs can be described with the Graduated Cylindrical Shell (GCS) model, their 3-D geometry, position and orientation is analysed and discussed in the second part of this thesis. For this purpose the 3-D GCS modeling technique is used to compute the 3-D geometry with STEREO/SECCHI/COR2 coronagraph data for 241 CMEs. The results from the 3-D GCS CME analysis show that during the phase of low solar activity (January 2007 - January 2010) small-scale CMEs at low latitudes dominate. This is proved by low values of the GCS model parameters aspect ratio ( < 0.4) and half angle (< 20°) which describe the spatial expansion of a CME. In contrast, during the second time period with increasing solar activity (February 2010 - December 2011) also large-scale CMEs are observed and CMEs occur also at higher latitudes up to +/- 60°. The spatial expansion of those CMEs is characterised by a GCS half angle > 20° and an aspect ratio > 0.4. The analysis of the diameter for GCS modelled CMEs reveals that CMEs observed between 10 and 20 solar radii exhibit a flux rope diameter ranging from 2 to 8 solar radii. It is found from GCS modeling that the CMEs are best suitable for fitting within a solar distance of 11 to 17 solar radii when they appear largely expanded and most bright and clear in structure in the COR2 coronagraph's field of view. In comparison to other previous CME studies this one provides a detailed 3-D parametrisation and analysis of a large set of 241 CMEs instead of plane-of-sky measurements which are affected by projection effects.