A zone of preferential ion heating extends tens of solar radii from Sun

Abstract

The extreme temperatures and non-thermal nature of the solar corona and solar wind arise from an unidentified physical mechanism that preferentially heats certain ion species relative to others. Spectroscopic indicators of unequal temperatures commence within a fraction of a solar radius above the surface of the Sun, but the outer reach of this mechanism has yet to be determined. Here we present an empirical procedure for combining interplanetary solar wind measurements and a modeled energy equation including Coulomb relaxation to solve for the typical outer boundary of this zone of preferential heating. Applied to two decades of observations by the Wind spacecraft, our results are consistent with preferential heating being active in a zone extending from the transition region in the lower corona to an outer boundary 20-40 solar radii from the Sun, producing a steady state super-mass-proportional $\alpha$-to-proton temperature ratio of $5.2-5.3$. Preferential ion heating continues far beyond the transition region and is important for the evolution of both the outer corona and the solar wind. The outer boundary of this zone is well below the orbits of spacecraft at 1 AU and even closer missions such as Helios and MESSENGER, meaning it is likely that no existing mission has directly observed intense preferential heating, just residual signatures. We predict that {Parker Solar Probe} will be the first spacecraft with a perihelia sufficiently close to the Sun to pass through the outer boundary, enter the zone of preferential heating, and directly observe the physical mechanism in action.

11 pages, 7 figures, accepted for publication in the Astrophysical Journal on 1 August 2017

References in corpus (2)

• Fading Coronal Structure and the Onset of Turbulence in the Young Solar Wind
• Measuring Collisionless Damping in Heliospheric Plasmas using Field-Particle Correlations

Cited by in corpus (7)

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• Nature of stochastic ion heating in the solar wind: testing the dependence on plasma beta and turbulence amplitude
• The Enhancement of Proton Stochastic Heating in the near-Sun Solar Wind
• Helium Variation Across Two Solar Cycles Reveals A Speed-Dependent Phase Lag
• Strong Preferential Ion Heating is Limited to within the Solar Alfven Surface
• Linear Stability in the Inner Heliosphere: Helios Reëvaluated
• Radial evolution of stochastic heating in low-$\beta$ solar wind