Propagation of coronal mass ejections in the inner heliosphere

Abstract

Solar Coronal mass ejections (CMEs) are large-scale ejections of plasma and magnetic field from the corona, which propagate through interplanetary space. CMEs are the most significant drivers of adverse space weather on Earth, but the physics governing their propagation through the Heliosphere is not well understood. This is mainly due to the limited fields-of-view and plane-of-sky projected nature of previous observations. The Solar Terrestrial Relations Observatory (STEREO) mission launched in October 2006, was designed to overcome these limitations. In this thesis, a method for the full three dimensional (3D) reconstruction of the trajectories of CMEs using STEREO was developed. Observations of CMEs close to the Sun (<15 R) were used to derive the CMEs trajectories in 3D. These reconstructions supported a pseudo-radial propagation model. Assuming pseudo-radial propagation, the CME trajectories were extrapolated to large distances from the Sun (15-240 R). It was found that CMEs slower than the solar wind were accelerated, while CMEs faster than the solar wind were decelerated, with both tending to the solar wind velocity. Using the 3D trajectories, the true kinematics were derived, which were free from projection effects. Evidence for solar wind (SW) drag forces acting in interplanetary space were found, with a fast CME decelerated and a slow CME accelerated toward typical SW velocities. It was also found that the fast CME showed a linear dependence on the velocity difference between the CME and the SW, while the slow CME showed a quadratic dependence. The differing forms of drag for the two CMEs indicated the forces responsible for their acceleration may have been different. Also, using a new elliptical tie-pointing technique the entire front of a CME was reconstructed in 3D. This enabled the quantification of its deflected trajectory, increasing angular width, and propagation from 2 to 46R_ (0.2 AU). Beyond 7R, its motion was shown to be determined by aerodynamic drag. Using the reconstruction as an input for a 3D magnetohydrodynamic simulation, an accurate arrival time at the L1 Lagrangian point near Earth was determined. CMEs are known to generate bow shocks as they propagate through the corona and SW. Although CME-driven shocks have previously been detected indirectly via their emission at radio frequencies, direct imaging has remained elusive due to their low contrast at optical wavelengths. Using STEREO observations, the first images of a CME-driven shock as it propagates through interplanetary space from 8R to 120R (0.5 AU) were captured. The CME was measured to have a velocity of ~1000kms-1 and a Mach number of 4:1+-1:2, while the shock front standoff distance was found to increase linearly to ~20R at 0.5 AU. The normalised standoff distance showed reasonable agreement with semi-empirical relations, where DO is the CME radius. However, when normalised using the radius of curvature, the standoff distance did not agree well with theory, implying that RO was underestimated by a factor of ~3 - 8. This is most likely due to the difficulty in estimating the larger radius of curvature along the CME axis from the observations, which provide only a cross-sectional view of the CME. The radius of curvature of the CME at 1AU was estimated to be ~0.95AU