Theory of Cosmic Ray Transport in the Heliosphere

Abstract

Modelling the transport of cosmic rays (CRs) in the heliosphere represents a global challenge in the field of heliophysics, in that such a study, if it were to be performed from first principles, requires the careful modelling of both large scale heliospheric plasma quantities (such as the global structure of the heliosphere, or the heliospheric magnetic field) and small scale plasma quantities (such as various turbulence-related quantities). Here, recent advances in our understanding of the transport of galactic cosmic rays are reviewed, with an emphasis on new developments pertaining to their transport coefficients, with a special emphasis on novel theoretical and numerical simulation results, as well as the CR transport studies that employ them. Furthermore, brief reviews are given of recent progress in CR focused transport modelling, as well as the modelling of non-diffusive CR transport.

Utilizing AI to unveil the nonlinear interplay of convection, drift, and diffusion on galactic cosmic ray modulation in the inner heliosphere

“…Our results drawn from the LightGBMs and PIs, which are better suitable for nonlinear processes, suggest that with the declining solar activity, especially after the maximum of SC-23, the modulation effect of diffusion became less important, while drift effects became more prominent. The physical explanation for this comes from the lack of emergence of stronger active regions and their decay products, such as coronal holes, leading to faster solar winds and CIRs and outward propagating diffusive barriers, whereas the drift of GCRs along the HCS due to gradients and curvature in the HMF 8 . Our results on the drift effects on GCR modulations in long and short timescales are in-line with those drawn from Empirical Mode Decomposition, showing that the contribution of the drift effects are much higher in the 11-and 22-year timescales compared with the 6-year timescale 51 .…”

“…In these equations, K || denotes parallel diffusion coefficient, while K ⊥r and K ⊥θ show a perpendicular diffusion coefficient in the radial and polar directions, respectively 5 . The parallel and perpendicular diffusion coefficients are related to the corresponding mean free path (MFP, ) length scales by ||,⊥ = 3K ||,⊥ /v , where v is the particle velocity 8 . The parallel and the perpendicular MFPs can be approximated via the relationships || ∝ P 2 /δB 2 N and ⊥ ∝ (δB 2 N ) 1/3 P 2/3 /B 4/3 0 9,10…”

Turbulence in the Outer Heliosphere

“…Coupling global heliospheric models and turbulence transport models provides not only mean-flow plasma and magnetic field parameters, but also the turbulence quantities, which makes them useful also for calculation of diffusion coefficients and modulation of galactic cosmic rays (GCRs) (see, e.g., Florinski et al 2013a;Engelbrecht and Burger 2013;Wiengarten et al 2016;Chhiber et al 2017;Engelbrecht 2017;Zhao et al 2018). This topic is reviewed by Engelbrecht et al (2022) in this journal.…”

On the fractal properties of cosmic rays and Sun dynamics cross-correlations

“…The transport of Galactic Cosmic Rays (GCR) in the heliosphere (understanding as the spatial region under the influence of the solar wind and its magnetic field) is one of the topics of greatest interest in space physics, given its close relationship with the radiation levels in the interplanetary space of the solar system, the study of which is part of what is known as space weather. This transport is modulated by different physical mechanisms, which in turn can be divided into large-scale processes (Boschini et al 2022;Engelbrecht et al 2022;Shlyk et al 2021): re-lated to the heliospheric magnetic field, transient phenomena: those generated by the ejection of solar matter, or with respect to their duration, which range from several solar cycles, such as processes related to the dynamics of the solar dynamo (Ossendrijver 2003;Charbonneau 2014), to a few hours, such as those produced by transients in small-scale activity that propagate along with the solar wind near the Earth. The 11-year variation is due to the solar activity cycle, and the 22-year variation is due to the magnetic solar cycle.…”