Impact of solar magnetic field amplitude and geometry on cosmic rays diffusion coefficients in the inner heliosphere

Abstract

p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.0px Helvetica} span.s1 {font: 5.5px Helvetica} Cosmic rays are remarkable tracers of solar events when they are associated with solar flares, but also galactic events such as supernova remnants when they come from outside our solar system. Solar Energetic Particles (SEPs) are correlated with the 11-year solar cycle while Galactic Cosmic Rays (GCRs) are anti correlated due to their interaction with the heliospheric magnetic field and the solar wind. Our aim is to quantify separately the impact of the amplitude and the geometry of the magnetic field, both evolving during the solar cycle, on the propagation of cosmic rays of various energies in the inner heliosphere (within Earth orbit). We focus especially on the diffusion caused by the magnetic field along and across the field lines. To do so, we use the results of 3D magnetohydrodynamics (MHD) wind simulations running from the lower corona up to 1 AU. This gives us the structure of the wind and the corresponding magnetic field. The wind is modeled using a polytropic approximation, and fits and power laws are used to account for the turbulence. Using these results, we compute the parallel and perpendicular diffusion coefficients of the Parker cosmic ray transport equation, yielding 3D maps of the diffusion of cosmic rays in the inner heliosphere. By varying the amplitude of the magnetic field, we change the amplitude of the diffusion by the same factor, and the radial gradients by changing the spread of the current sheet. By varying the geometry of the magnetic field, we change the latitudinal gradients of diffusion by changing the position of the current sheets. By varying the energy, we show that the distribution of values for SEPs is more peaked than GCRs. For realistic solar configurations, we show that diffusion is highly non-axisymmetric due to the configuration of the current sheets, and that the distribution varies a lot with the distance to the Sun with a drift of the peak value. This study shows that numerical simulations, combined with theory, can help quantify better the influence of the various magnetic field parameters on the propagation of cosmic rays. This study is a first step towards the resolution of the complete Parker transport equation to generate synthetic cosmic rays rates from numerical simulations.

An Analytical Model of Turbulence in Parker Spiral Geometry and Associated Magnetic Field Line Lengths

“…The energy-containing range spectrum affects the relation of the correlation scale and the spectral break scale and, through that connection, the field line and particle diffusion coefficients (e.g., Matthaeus et al 2007;Shalchi et al 2010;Engelbrecht & Burger 2015). For correlation length, one option is to use the Hollweg (1986) assumption of λ corr ∝ 1/B 1/2 , giving λ corr ∝ 1/r close to the Sun (e.g., Perri et al 2020), whereas some studies have used constant correlation lengths (e.g., Laitinen et al 2016;Strauss et al 2017), and some use the results of turbulence transport simulations (e.g Chhiber et al 2017;van den Berg et al 2021). Recent results from the PSP (Fox et al 2016) suggest a radial dependence of λ corr ∝ r 0.746 at heliocentric heights above 30r e , close to our model assumption.…”

Effects of mesh topology on MHD solution features in coronal simulations

“…For example, the domain can be split into two segments with additional boundary conditions at the poles (see, e.g., Perri et al. 2020), such that the degenerated regions are omitted. Whether such a solution leads to physical results is, however, debatable.…”

“…The basic mesh (1) was derived from the mesh commonly used by the Wind-Predict code (Perri et al. 2020) for MHD simulations with real magnetograms (Leitner et al, submitted). This is a good reference of what resolution is generally required to sufficiently resolve coronal features and what type of sphere is used for it.…”

Long-term forcing of Sun's coronal field, open flux and cosmic ray modulation potential during grand minima, maxima and regular activity phases by the solar dynamo mechanism

“…Different solar activity phases result in variable surface magnetic field strength which indicates a variation in solar wind speed as well. Solar wind models are often initialized with coupled solar dynamo and coronal magnetic field simulations to understand the variations in solar wind outflow and cosmic ray flux due to solar internal magnetic field dynamics (Perri et al 2018(Perri et al , 2020(Perri et al , 2021. The long-term solar dynamo simulations encompassing solar maximum, minimum and regular activity phases describe solar magnetic activity evolution that impact the state of the heliosphere.…”