The theme of my PhD has been to investigate the global shape and size of coronal mass ejections, or CMEs, as they propagate from the Sun towards the Earth. CMEs are large eruptive events originating from previously magnetically confined structures in the solar atmosphere. These phenomena are the single biggest drivers for geomagnetic disturbances at Earth. My research is focused on analysing spacecraft data obtained both by imaging observations and in situ instrumentation. The three pieces of work presented in this thesis are summarised below:
Using the NASA STEREO mission, launched in 2006, I have analysed data from the Heliospheric Imager (HI) instruments. This new instrument is uniquely positioned to observe CMEs as they propagate away from the Sun into the inner heliosphere between 0.1 and 1 AU. Using this data I have been able to estimate the radial expansion of a single CME as it propagates in the inner heliosphere.
Investigating another case study event seen by STEREO-B in November 2007, I have been able to show that the distortion of a CME can be directly attributed to a structured solar wind. By using a 3D MHD simulation of the solar wind in the vicinity of the CME, it has been shown that a bimodal velocity structure within this solar wind was driving the CME from behind and distorting it from a circular to a concave morphology.
Using in situ data, I have also attempted to deduce the shape of CMEs in the inner heliosphere. To do this I analysed the shock wave driven ahead of the propagating CME, applying a technique previously used to predict the distance of the shock upstream of Earth’s magnetosphere - this distance can be predicted when the object’s shape (Earth) is known. I have carried out a statistical survey of many CMEs over a range of distances from the Sun, and compared them to theoretical predictions of their shape based on geometry
