Study of shock waves in the solar corona and interplanetary space

In this thesis, I provide a detailed study of shock waves that are observed in the solar corona and interplanetary space. In particular, I have mainly focused on the study of some of the shock waves characteristics such as their regions of origin, the determination of their driver, the calculation of their compression ratio, their kinematics and spatial characteristics during their propagation in the solar corona and interplanetary (IP) space. Furthermore, for a shock wave that was observed in both the low corona and IP space, I studied the role of its spatial evolution in relation to the release of solar energetic particles (SEPs) in different - widely-distributed observing points in heliosphere. Additionally, I studied the relative contribution of shock waves and solar flares on the properties of SEP events.

For the implementation of this thesis, I exploited a big amount of solar and inner heliosphere observations that are provided by space-born and ground-based instruments. The above-mentioned studies are key elements in our understanding of the physical processes and phenomena that are involved in the development and evolution of coronal and IP shock waves in the heliosphere. Furthermore, they elucidate their nature and characteristics alongside with their capability to accelerate and release charged particles in open magnetic field lines in the heliosphere.

The combined observations of SDO/ΑΙΑ in extreme ultraviolet with the high-resolution dynamic spectra from the ARTEMIS-IV radiospectrograph, allowed me to study the nature and the driver of the coronal shock wave that was responsible for the band-splitted type-II radio emission. It was found that the CME’s propagation and the non-linear evolution of the EUV wave in the low corona are responsible for the observed coronal shock wave. Additionally, the shock wave was found to be piston-driven in the first stages of its evolution and afterwards, it results in a blast wave. In this study, I introduce a new method to calculate a compression-ratio proxy from the EUV images of AIA. From the comparison of the compression ratio from the type-II radio burst’s band split and the compression ratio proxy from the EUV images, I found that the type-II radio burst could originate in the sheath region between the bubble and the EUV shock front, in both radial and lateral directions.

Furthermore, I studied two homologous eruptive events that gave rise to a major SEP event observed at widely separated (120◦ in the ecliptic plane) points in the heliosphere. With my analysis, I mainly promoted the usefulness of the three-dimensional representation of a shock wave for the study of its spacial evolution in the inner heliosphere. Additionally, this study is a demonstration of the importance of shock waves for tracing the generation and distribution of energetic particles. From the three-dimensional reconstruction of the shock wave that is related to the first CME, I studied its lateral expansion higher in the corona and I connected its longitudinal extent with the energetic proton and electron release times. I found that the SEP event that was observed in STEREO-B (STB) and the first Lagrange point (L1) was associated with the shock wave driven by the first CME that was initiated in active region 11429.

Finally, I studied the relative role of CME-related shock waves and solar flares in the acceleration and release of high-energySEP events. For this study I studied selected properties of SEP events as inferred from their associated radio emissions, focusing in particular on the relation of the SEP release time with the transient radio emissions. From my study, it is clear that both flare- and shock-related particle release processes are observed in high-energy solar proton events and a clear-cut distinction between flare-related and CME/shock-related SEP events is difficult to establish. I conclude that typically, the protons are released after the start of the associated type III bursts and simultaneously or before the release of energetic electrons traced in-situ.