Modeling the Time-Dependent Cosmic-Ray Sun Shadow and its related Gamma-Ray and Neutrino Signatures

• Applicants: Privatdozent Dr. Horst Fichtner; Professorin Dr. Julia Tjus
• Subject Area: Astrophysics and Astronomy
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 437789084

For several decades, both the Sun and the Moon have been known to geometrically block Earth-bound cosmic rays, thus creating areas of reduced intensity in spatially resolved cosmic ray observations known as the "cosmic-ray shadow" of these celestial bodies. Charged cosmic rays up to and even above an energy of 1 TeV additionally experience a deflection in the solar coronal magnetic field. Cosmic ray ions, predominantly protons, initiate particle showers via their interaction with the solar atmospheric matter. Among the final decay products in these cascades are gamma-rays and neutrinos that can be detected by instruments like Fermi and IceCube, respectively. Furthermore, cosmic-ray electrons generate, via inverse-Compton scattering on the solar radiation field, additional gamma-rays. All these processes are directly or indirectly sensitive to the solar magnetic field that, therefore, must be expected to leave an imprint in such measurements. Besides quantifying the significance of an accurate modeling of the magnetic field for the Sun's cosmic-ray shadow, the solar gamma-ray as well as neutrino fluxes, the study will allow to derive constraints for the field itself in regions that are, as yet, not explored in situ. The central goal of the project is to quantitatively explore the signatures of the solar magnetic field in the measurements of high-energy cosmic rays, gamma-rays, and neutrinos, and to resolve the presently existing discrepancies between theory and measurements. The project can only be performed successfully by applying knowledge from both high-energy astroparticle physics (research area of J. Tjus) and heliophysics (research area of H. Fichtner). The study is very timely because it will, by combining expertise from the two research fields, contribute in a two-fold way to the contemporary research. First, it will deliver cosmic-ray particle trajectories in realistic, data-based, time-varying magnetic field configurations. Second, it will significantly improve the computation of gamma-ray and neutrino fluxes due to cosmic ray-initiated cascades. The already existing collaboration between the two research groups at the Ruhr-Universität Bochum offers a unique opportunity to develop a unified description of cosmic-ray transport and its interaction and radiation processes that will open the door for a detailed investigation of how signatures of the solar magnetic field can be used in order to better understand both the field itself as well as the transport of high-energy cosmic rays through it and the innermost heliosphere.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Paolo Desiati, Ph.D.