Reconstructing ICMEs with the toroidal Grad-Shafranov method

Abstract

The main objective of this thesis is to model the magnetic structure of interplanetary coronal mass ejections (ICMEs) measured in-situ by the WIND spacecraft positioned at L1. The modeling is done by a magnetohydrodynamic reconstruction technique based on the Grad-Shafranov equation with a toroidal geometry. The purpose has been to extend the application of the reconstruction program to real data and to test its performance when various input parameters are changed. Two events are presented; the 16-17 May 2012 and 15-16 May 2005 ICMEs have been successfully reconstructed with this model.

The main achievements of the study are that a) the code now works for real data b) the important parameters that can be changed for different reconstructions in the code are the number of iterations used to find the optimal Z-axis, the plasma pressure and the order of the polynomial fitting of the flux functional, c) if all cross section reconstructions for different variations of these parameters strongly resembles each other then this is an indication that the model approximation is good and that the fluxrope exists. The results have been compared and verified with previously published studies of these events.

Using a toroidal geometry for the GS reconstruction method we obtain very similar results to the one obtained with other reconstruction techniques. This implies that at L1, the ICMEs have expanded so much that a cylindrical geometry is sufficient to describe the flux rope geometry. The toroidal Grad-Shafranov reconstruction technique is best suited for circular, or slightly elongated, flux rope cross section profiles but have been proven to work for one complex ejecta consisting of two merged fluxropes. The toroidal model might become an important asset in the future when data from spacecraft closer to the Sun, such as Parker Solar Probe and Solar Orbiter, become public. When the major radius of the flux rope is smaller the choice of geometry will most likely have a larger role than for measurements at L1 and so, the toroidal Grad-Shafranov reconstruction technique will probably be the better alternative of the models that exists today.

Results

Example of an ICME cross section as reconstructed from single-point s/c data using the G-S technique.