The longitudinal distribution of energetic particles in the inner heliosphere - multi-point observations with STEREO-
The datasets of the STEREO spacecraft, utilized in this thesis, built an unprecedented platform to investigate longitudinal particle distributions in the inner heliosphere. In contrast to previous space missions these two nearly identical spacecraft fly on Earth-like orbits around the Sun with successively increasing longitudinal separation to Earth. One spacecraft, therefore, runs ahead of the Earth, STEREO A, while the other one, STEREO B, trails behind. Beside plenty of up to date instruments carried by the STEREO spacecraft, one of the main advantages of the mission is the radial distance to the Sun which is nearly the same as that of the Earth. This is of great use when investigating energetic particles because radial effects can almost be neglected. Another important advance is that both spacecraft do not orbit the Sun ’blindly’: In addition to a number of in-situ experiments they are also equipped with several remote sensing instruments. These provide optical observations of the Sun and the corona at different wavelengths which were only available from the Earth’s viewpoint previous to the STEREO mission. In the region of STEREO’s orbit, two energetic particle populations lend themselves for longitudinal investigations: Energetic particles associated with corotating interaction regions (CIRs), and solar energetic particles (SEPs). The first part of this thesis deals with the former and the different effects which can cause variable CIR observations and associated particle increases. For this purpose, the special configuration of the STEREO spacecraft enables us to disentangle temporal and spatial effects. Furthermore, local interactions with transient structures were observed which obviously favor the local particle acceleration. The second part of this thesis presents investigations of solar energetic particle events which show remarkably wide particle spreads of up to 360 degrees in longitude in the inner heliosphere. These so-called wide-spread events are of special interest because the processes, yielding to these unexpectedly wide distributions, are not completely understood yet. On the basis of a detailed study of such an event and comparison with a 3D propagation model, we conclude that particle transport perpendicular to the mean magnetic field in the interplanetary medium cannot be neglected for wide spread events. In a second study we identified 21 of such wide-spread events and investigate these in a statistical manner. As a key characteristic the longitudinal anisotropy distribution is also investigated. By means of this information different types of events could be distinguished. In contrast to the first study where strong perpendicular diffusion was supposed to play the main role, several observations require a pre-distribution of the particles over large angular ranges close to the Sun. The second study, therefore, concludes that it is likely that both processes, perpendicular diffusion in the interplanetary medium as well as a lateral distribution in the corona (due to coronal transport, a shock or another so far unknown process) must be present at the same time to explain the majority of our observations.