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 Alfvén speed partially reflect outward-propagating Alfvén 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 generalise RDT phenomenology to include a PS, using three-dimensional expanding-box magnetohydrodynamic simulations to test the ideas and compare the resulting turbulence with the radial-background-field case. We argue that the underlying RDT dynamics remains 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 normalised cross-helicity) out to larger heliocentric distances. We complement our heating results with a detailed characterisation of the turbulence (e.g. spectra, switchbacks and compressive fractions), providing a set of concrete predictions for comparison with spacecraft observations.