Electron acceleration in interplanetary space: Radio signatures and in-situ observations

Abstract

Coronal Mass Ejections (CMEs) are large-scale releases of hot plasma, to which the magnetic field is frozen-in. If the CMEs are faster than the local magnetosonic velocity in the solar wind, they create shock waves as they travel through the corona and Interplanetary (IP) space. Shock waves driven by CMEs can accelerate Solar Energetic Particles (SEPs). Both phenomena involve the acceleration of electrons, which can be observed as electromagnetic radiation and plasma radiation. This doctoral thesis presents analyses of the presence and propagation of accelerated electrons in the IP medium. By utilizing the observations of multiple science satellites, such as STEREO-A, STEREO-B, and Wind, we get a comprehensive picture of the accelerated particles and solar radio bursts, at various wavelengths. We can use this information to determine the origin of the eruptions, their directivity, and connections to other solar events. In particular, the role of shock waves in the acceleration of relativistic electrons is the subject of this research. Earlier studies have already confirmed the role of shock waves in the acceleration of electrons to keV energies using radio bursts, but for the higher energies, the research is still in progress.

As a result of observations by many space instruments we now have a 3D view of the Sun, particularly in the analysis of type IV radio bursts in multi-spacecraft radio dynamic spectra. The directivity of radio bursts, i.e., being seen only toward a certain direction, can be explained either by absorption in the surrounding medium or by obstruction of dense plasma region, even by the solar disk itself. The presence of dense plasma regions like solar streamers, in directions where no radiation is visible, strengthens this conclusion. Type II radio bursts can be associated with the interaction of streamers and shock waves. Our analysis of three separate type IV radio bursts revealed that their radiation was not visible toward directions where type II radio bursts were observed. The eruptions were generated by the same active region on three different days, and the location of the eruption region on the Sun changed from the disk center to the solar limb. The directivity of the type IV radio bursts could therefore be explained as absorption in the type II burst regions, as the shock fronts contain higher-density plasma.

In the study of isolated type II radio bursts, i.e., when separated from other bursts in time and also in frequency, we found that contrary to the previous studies, the majority of these radio bursts were associated with shocks that were created near the CME leading fronts. The analysis suggests the necessity of special coronal conditions, to form this subgroup of low-frequency type II radio bursts.

The creation of relativistic electrons in IP shocks led to the investigation of whether these shocks can continue to accelerate electrons up to one Astronomical Unit (AU). Using in-situ observations of the electron flux, SEP events, and associated Energetic Storm Particle (ESP) occurrences, we identified nine cases observed by High Energy Telescope (HET) onboard STEREO where MeV electrons showed a significant increase associated with shocks driven by fast speed Interplanetary CMEs (ICMEs). We also found that such events were rare at a distance of 1 AU. The research suggests the necessity to make observations with satellites orbiting closer to the Sun, such as the Parker Solar Probe and Solar Orbiter, so that we can find out how electrons are accelerated in IP shocks. Finally, in-situ observations show clear signatures of local acceleration of electrons during the passage of the shock wave or the sheath region of the ICME during the ESP event.