Strong Preferential Ion Heating is Limited to within the Solar Alfven Surface

Abstract

The decay of the solar wind helium to hydrogen temperature ratio due to Coulomb thermalization can be used to measure how far from the Sun strong preferential ion heating occurs. Previous work has shown that a zone of preferential ion heating, resulting in mass-proportional temperatures, extends about $20-40R_{\odot }$ from the Sun on average. Here we look at the motion of the outer boundary of this zone with time and compare it to other physically meaningful distances. We report that the boundary moves in lockstep with the Alfvén point over the solar cycle, contracting and expanding with solar activity with a correlation coefficient of better than 0.95 and with an RMS difference of $4.23R_{\odot }$. Strong preferential ion heating apparently is predominatly active below the Alfvén point. To definitively identify the underlying preferential heating mechanisms, it will be necessary to make in situ measurements of the local plasma conditions below the Alfvén surface. We predict Parker Solar Probe (PSP) will be the first spacecraft to directly observe this heating in action, but only a couple of years after launch as activity increases, the zone expands, and PSP's perihelion drops.

5 pages, 5 figures, accepted in ApJL

References in corpus (3)

• Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence
• Fading Coronal Structure and the Onset of Turbulence in the Young Solar Wind
• A zone of preferential ion heating extends tens of solar radii from Sun

Cited by in corpus (8)

• Sub-Alfvenic Solar Wind observed by PSP: Characterization of Turbulence, Anisotropy, Intermittency, and Switchback
• The Enhancement of Proton Stochastic Heating in the near-Sun Solar Wind
• Alpha-proton Differential Flow of the Young Solar Wind: Parker Solar Probe Observations
• Modeling ion beams, kinetic instabilities, and waves observed by the Parker Solar Probe near perihelia
• Flux conservation, radial scalings, Mach numbers, and critical distances in the solar wind: magnetohydrodynamics and Ulysses observations
• Linear Stability in the Inner Heliosphere: Helios Reëvaluated
• Reconstructing the Sun's Alfvén surface and wind braking torque with Parker Solar Probe
• Perpendicular ion heating in turbulence and reconnection: magnetic moment breaking by coherent fluctuations