[P-EM16] Heliosphere and Interplanetary Space

[PEM16-09] Bayesian estimation of the galactic cosmic-ray anisotropy and its rigidity dependence

*Masayoshi Kozai¹, Kazuoki Munakata², Chihiro Kato², Yuki Hayashi², Ryuho Kataoka³, Shoko Miyake⁴, Akira Kadokura¹ (1.ROIS-DS, 2.Shinshu Univ., 3.NIPR, 4.KOSEN Gifu College)

Keywords:Galactic cosmic rays, Anisotropy, Muons, Solar modulation

Anisotropy of galactic cosmic-rays (GCRs) represents their momentum-space distribution in the interplanetary plasma, playing a key role in revealing the GCR propagation and space environment. The ground-based muon detector network has been a unique means to observe the anisotropy, thanks to its excellent angular resolution, angular acceptance, and statistics. However, quantitative analysis of rigidity dependence of the anisotropy has been difficult because of poor energy resolutions of ground-based muon or neutron detectors. To overcome this situation and derive the rigidity dependence from differences in geomagnetic and atmospheric effects in the network, we are developing a new analysis method utilizing the Bayesian estimation approach. The Gaussian process is introduced as a prior distribution of the Bayesian estimation. It allows us to confine the smoothness of the rigidity spectrum, which is required to derive the spectrum from broad rigidity responses of ground-based observations, while being tolerant of spectrum shapes without assuming any analytical function. A focused analysis of the North-South (NS) anisotropy demonstrates the usefulness of this new method (https://doi.org/10.3847/1538-4357/ad8577). It reveals solar cycle variations of the NS anisotropy and, moreover, the diffusion coefficient or mean-free-path length of GCRs (attached figure) based on the GCR propagation theory. We are attempting to generalize this Bayesian estimation approach to three-dimensional anisotropy and will report on its application to short-term events, such as interplanetary shocks.