Is there local energization of ions to the suprathermal reservoir in the inner heliosphere?

• Applicant: Dr. Verena Heidrich-Meisner
• Subject Area: Astrophysics and Astronomy
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 502050603

Suprathermal particles are found throughout the interplanetary space, which have higher energies than the bulk thermal population and are more abundant than a thermal (Maxwellian) velocity distribution function would predict and thus form a suprathermal tail.These particles are the seed population for further acceleration at coronal and interplanetary shocks. Therefore, it is important to understand the properties of the suprathermal particle reservoir and especially how it is maintained. Here, we propose to study the role and relative importance of local, small-scale processes such as turbulence, wave damping, velocity fluctuations, etc., in energizing suprathermal ions. We focus on suprathermal ions in the energy range of ~2-100 keV/nucleon. Several sources and acceleration processes have been proposed to explain the heliospheric population of suprathermal ions. These acceleration mechanisms fall into two categories: heliospheric (local) and solar (remote) acceleration sites. Both local and remote sources contribute to the reservoir of suprathermal particles but our focus lies on the local continuous processes that maintain the suprathermal particle reservoir. Thus, we pose the research question which of the local acceleration processes from the literature, namely bulk velocity fluctuations (BVF), compressional turbulence (CT), waves and turbulence in the interplanetary magnetic field (WTI), and reconnection between magnetic islands (RMI), dominate under which conditions.Each of these proposed mechanisms provides predictions that are testable against observations and thereby we can verify and evaluate the relative importance of these mechanisms. Suitable observations are now available with ESA’s Solar Orbiter which was launched in February 2020 [Müller et al., 2020]. We will use observations by Solar Wind Analyser (SWA, Owen et al.[2020]) and Energetic Particle Detector (EPD, Rodríguez-Pacheco et al.[2020], Wimmer-Schweingruber et al.[2021]) on Solar Orbiter to investigate the above mentioned mechanisms.We will address our research question by applying three tasks to Solar Orbiter's observations. We first derive velocity distribution functions of suprathermal ions under different solar wind conditions and magnetic field properties, which provides us with the tools needed to test each theoretical prediction. Then, we select a suitable collection of time periods when we can study the local acceleration processes and the resulting suprathermal particle population in the form of both case studies and statistical studies. Finally, we apply the tools developed in the first task to the time periods of interest and then pair observations with theoretical predictions as well as study the radial evolution of suprathermal particles, in order to investigate each proposed acceleration mechanism. With this approach, we can address our research question and investigate the influence of local acceleration on the reservoir of suprathermal particles.

• DFG Programme: Research Grants