The influence of Parker spiral on the reflection-driven turbulence

TL;DR

This study extends reflection-driven turbulence models to include the Parker Spiral magnetic field, revealing how the spiral's geometry influences turbulence scales, energy dissipation, and heating in the solar wind.

Contribution

It generalizes RDT phenomenology to the Parker Spiral, showing how the magnetic field's twist alters turbulence scales and energy dissipation compared to radial models.

Findings

Outer scales perpendicular to the magnetic field are smaller in PS geometry.

The nonlinear turnover time increases more slowly with distance in PS, enhancing heating.

Turbulence remains highly imbalanced at larger heliocentric distances.

Abstract

The solar wind is observed to undergo substantial heating as it expands through the heliosphere, with measured temperature profiles exceeding those expected from adiabatic cooling. A plausible source of this heating is reflection-driven turbulence (RDT), in which gradients in the background Alfv\'en speed partially reflect outward-propagating Alfv\'en waves, seeding counter-propagating fluctuations that interact and dissipate via turbulence. Previous RDT models assume a radial background magnetic field, but at larger radii the interplanetary field is known to be twisted into the Parker Spiral (PS). Here, we generalize RDT phenomenology to include a PS, using three-dimensional expanding-box magnetohydrodynamic (MHD) simulations to test the ideas and compare the resulting turbulence to the radial-background-field case. We argue that the underlying RDT dynamics remain broadly similar with a PS, but the controlling scales change: as the azimuthal field grows it "cuts across" perpendicularly stretched, pancake-like eddies, producing outer scales perpendicular to the magnetic field that are much smaller than in the radial-background case. Consequently, the outer-scale nonlinear turnover time increases more slowly with heliocentric distance in PS geometry, weakening the tendency (seen in radial-background models) for the cascade to 'freeze' into quasi-static, magnetically dominated structures. This allows the system to dissipate a larger fraction of the fluctuation energy as heat, also implying that the turbulence remains strongly imbalanced (with high normalized cross-helicity) out to larger heliocentric distances. We complement our heating results with a detailed characterization of the turbulence (e.g., spectra, switchbacks, and compressive fractions) providing a set of concrete predictions for comparison with spacecraft observations.