Multipoint observations of ICMEs in the inner heliosphere: Forbush decreases and remote sensing


Coronal Mass Ejections (CMEs) are clouds of plasma and magnetic field regularly ejected from the Sun at high speeds that propagate out into interplanetary space. These events are one of the most important space weather phenomena. Their strong and turbulent magnetic field can cause disruptions of spacecraft electronics as well as terrestrial infrastructure, and they can be associated with the acceleration of energetic particles, which may cause increased radiation exposure, e.g. for astronauts. To be able to validate and consequently improve theoretical models predicting the arrival of interplanetary CMEs (ICMEs) at Earth and other locations in the heliosphere, it is important to employ many different data sources measuring the various signatures of ICMEs at different locations, so that their temporal and radial evolution can be studied. These investigations are also significantly aided by observations from remote sensing telescopes, which can directly observe the global structure of the ICMEs and track them out to large radial distances. The studies presented in this thesis introduce Mars into the framework of routinely available locations for the in situ observation of space weather. Here, ICMEs can be detected using Forbush decrease measurements by the Radiation Assessment Detector onboard the Mars Science Laboratory rover Curiosity. Forbush decreases are short-term decreases in the galactic cosmic ray flux caused by the magnetic structure of the ICME partly shielding away the cosmic rays. The measurements of these Forbush decreases are utilized in this thesis to determine ICME arrival times for statistical studies of events seen at two planets, Earth and Mars, or at one of the two STEREO spacecraft and Mars, during close longitudinal alignment. The measurements show for the first time that fast ICMEs can continue to decelerate beyond the orbit of Earth due to their interaction with the slower ambient solar wind. Using remote sensing observations from the STEREO heliospheric imagers, we study additional ICMEs that hit Mars and benchmark the accuracy of different approaches for the analysis of these heliospheric imager data. Subsequently, the Forbush decrease data for the thereby cataloged events are further investigated to infer not only the arrival time, but also more information about the radial evolution of the ICME properties by comparison with analytical modeling approaches. Finally, two case studies are performed: First, the major space weather events of September 2017 and their impact on Mars are examined, including the investigation of the solar energetic particle events and three associated CMEs that interacted and merged on their way towards Mars. Second, the first in situ observations of an ICME at the Solar Orbiter spacecraft, which launched in February 2020, are presented. In this study, we describe the capabilities of the Solar Orbiter’s High Energy Telescope for high-resolution observations of Forbush decreases and use its measurements in combination with a reverse modeling approach to show that the expansion of the ICME was non-uniform, possibly due to interaction with a following solar wind stream interaction region.