A Zone of Preferential Ion Heating Extends Tens of Solar Radii from the Sun

J. C. Kasper, K. G. Klein, T. Weber, M. Maksimovic, A. Zaslavsky, S. D. Bale, B. A. Maruca, M. L. Stevens, A. W. Case

Research output: Contribution to journalArticlepeer-review

46 Scopus citations


The extreme temperatures and nonthermal 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 α-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 the Parker Solar Probe will be the first spacecraft with a perihelion 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.

Original languageEnglish (US)
Article number126
JournalAstrophysical Journal
Issue number2
StatePublished - Nov 10 2017


  • Sun: corona
  • acceleration of particles
  • magnetic fields
  • plasmas
  • solar wind
  • turbulence

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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