Solar Corona

Introduction Observations of the time-dependent cosmic-ray Sun shadow have proven to be a valuable diagnostic tool for assessing solar magnetic field models. This project compares several years of IceCube data with solar activity and solar magnetic field models. For the first time, a quantitative comparison of solar magnetic field models with IceCube data at the event rate level is conducted. Additionally, we present an initial energy-dependent analysis compared to recent predictions. We utilize seven years of IceCube data for the moon and the Sun, comparing them with simulations at the data rate level. The simulations are performed under the geometrical shadow hypothesis for both the moon and the Sun and under a cosmic-ray propagation model influenced by the solar magnetic field for the Sun case. Our findings indicate a linearly decreasing relationship between Sun shadow strength and solar activity, which is preferred over a constant relationship at the 6.4σ level. We evaluate two commonly used coronal magnetic field models, both in conjunction with a Parker spiral, by modeling cosmic-ray propagation in the solar magnetic field. Both models predict a weakening of the shadow during periods of high solar activity, which is also observable in the data. We observe tensions with the data on the order of 3σ for both models, assuming only statistical uncertainties. The magnetic field model CSSS fits the data slightly better than the PFSS model. This finding generally aligns with earlier results from the Tibet AS-γ Experiment; however, the deviation of the data from the two models is not significant at this time. Regarding the energy dependence of the sun shadow, we find indications that the shadowing effect increases with energy during periods of high solar activity, which is consistent with theoretical predictions. In contrast, the Sun’s shadow varies seasonally, with IceCube detecting a significant deviation from the mean Sun shadow (χ²/ndof = 22.47/4, 3.8σ). This variation is likely linked to the Sun’s magnetic field changes, consistent with past Tibet observations at lower energies. A Spearman’s rank test suggests a 96% likelihood of correlation between sunspot number and the Sun shadow’s amplitude. However, further data are needed to confirm this relationship, given the limited observation periods and a weak solar cycle. Future studies should refine models of cosmic ray deflection by solar magnetic fields and improve point spread function treatments to enhance comparisons between observational data and simulations. Publication of IceCube research The investigations on the Sun’s cosmic-ray shadow’s time variabilities were published in The Astrophysical Journal. A more detailed study was published in Physical Review D where the Sun shadow’s time variability is associated with solar cycles, as opposed to the constant Moon cosmic-ray shadow, whose slow variability with time is only associated to the change in average Moon-Earth’s distance. Publication of numerical calculations The latest observational result was paired with a dedicated study of cosmic-ray particle trajectories propagating near the Sun, assuming different models of the solar corona’s magnetic field and its variability across the solar cycles. The report was published in Astronomy & Astrophysics.

Feb 1, 2025