STEREO observations of solar wind transients in the inner heliosphere

This thesis investigates the implications of relaxing assumptions inherent in techniques that analyse solar wind transients observed by NASA's Solar TErrestrial RElations Observatory (STEREO). In the first research chapter, I relaxed the assumption that the STEREO spacecraft are stationary while observing a transient. For much of the parameter space investigated, this effect was minimal, however in some cases it resulted in differences in derived radial speeds of hundreds of km s-1, leading to large errors. Using real data examples, the difference this effect makes was shown. The second research chapter applies the previous analysis to Corotating Interactions Region (CIR) observations. CIR events were identifed in STEREO HI J-maps, analysed, and their predicted arrival times calculated at each of the STEREO and Advanced Composition Explorer (ACE) spacecraft. A superposed epoch analysis was conducted using the predicted arrival times as the zero epoch time. It was found that when the fixed STEREO spacecraft assumption was relaxed, the CIR related transients that I observed had their estimated propagation speed increase such that they were propagating at (or close to) the slow solar wind speed, a physically realistic change. Changes in the structure of a stream interface over 1-2 days were seen, calling into question some of the underlying assumptions, which assume constant propagation characteristics over longer time-scales. Finally, I consider acceleration of solar wind transients close to the Sun. I use the analysis from previous chapters to perform fits to transient trajectories close to the Sun and infers the size of the acceleration region required to achieve convincing fits at low elongation values. It was found that the behaviour of the transients is consistent with an acceleration region within which the transient accelerates and then adopts a constant propagation speed. The acceleration region does not appear to occur at a fixed radial distance, but rather is different for each event analysed.