The Role of Alfvén Wave Dynamics on the Large-scale Properties of the Solar Wind: Comparing an MHD Simulation with Parker Solar Probe E1 Data

Abstract

During Parker Solar Probe's first orbit, the solar wind plasma has been observed in situ closer than ever before, the perihelion on November 6th 2018 revealing a flow that is constantly permeated by large amplitude Alfvénic fluctuations. These include radial magnetic field reversals, or switchbacks, that seem to be a persistent feature of the young solar wind. The measurements also reveal a very strong, unexpected, azimuthal velocity component. In this work, we numerically model the solar corona during this first encounter, solving the MHD equations and accounting for Alfvén wave transport and dissipation. We find that the large scale plasma parameters are well reproduced, allowing the computation of the solar wind sources at Probe with confidence. We try to understand the dynamical nature of the solar wind to explain both the amplitude of the observed radial magnetic field and of the azimuthal velocities.

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

“…We also plan to apply such formulations to a more realistic wind model, because as we have explained it, the modeling of the turbulence presented here has limits; it is suited to have the turbulence evolving with the MHD model. Such work for a turbulence-based wind model with Alfvn wave heating is currently being undertaken, see Rville et al (2020). CRs propagation through the heliosphere, and especially in the lower corona, is bound to be an important subject in the years to come thanks to the combined efforts of Parker Solar Probe and Solar…”

“…Hence, choosing γ 5/3 is a simplified way of taking into account heating in the low corona, which is not modeled here. However a new model based on heating by Alfvn waves has been developed and has shown good agreement with the Parker Solar Probe data, see Rville et al (2020). The amplitude of the magnetic field is set by v A /v esc = 0.176, which corresponds to an amplitude of 0.5 G at the equator for a dipole.…”

“…For the Alfvn wave energy density , we needed an expression that could adapt to any amplitude and geometry of the magnetic field, as we wanted to vary these parameters. Instead of solving WKB computations for each case (similar to Usmanov et al 2000), we chose to perform a fit, using an Alfvn-wave turbulence model (Rville et al, 2020). This model propagates parallel and anti-parallel Alfvn waves following the WKB theory inside a MHD wind model similar to what has been described in the previous section.…”

“…It was first described using fluid dynamics (Parker, 1958), then magnetism was taken into account (Weber and Davis, 1967;Sakurai, 1985). Multiple models of the solar wind have been designed, from empirical models (Wang and Sheeley, 1990;Arge and Pizzo, 2000) to MHD numerical simulations in 1D (Lionello et al, 2001;Suzuki et al, 2013;Pinto and Rouillard, 2017), 2D (Keppens and Goedbloed, 1999;Matt and Pudritz, 2008;Rville et al, 2015) or 3D (Tth et al, 2012;Riley et al, 2015;Rville et al, 2020). The heating of the corona is modeled through a polytropic approximation (Rville and Brun, 2017) or via Alfvn waves perturbations (Usmanov et al, 2014).…”

“…The heating of the corona is modeled through a polytropic approximation (Rville and Brun, 2017) or via Alfvn waves perturbations (Usmanov et al, 2014). The complete list of phenomena leading to this heating still eludes our understanding and is a huge current numerical challenge (Rville et al, 2020).…”