Abstract
Abstract 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.
Highlights
Observations of space over the last half century, including spectroscopic diagnostics of UV emission from coronal plasma and direct in situ sampling of solar wind by spacecraft, have shed light on the nonthermal nature of heating in the corona and solar wind
Are unequal temperatures in the solar wind maintained by ongoing local preferential heating, or are they a leftover of heating that happened close to the Sun? Is faster solar wind further from local thermodynamic equilibrium than slow wind because only fast wind experiences preferential heating in the corona, resulting in nonthermal structure? How far from the Sun does preferential ion heating continue? The purpose of this paper is to develop a technique for measuring how much time has elapsed since solar wind ions experienced preferential heating that was sufficiently strong to generate super-mass-proportional temperatures
We have examined the temperature ratio of fully ionized He2+ and H+ in the solar wind and its dependence on Coulomb collisional age in order to solve for the location of an outer boundary of an apparent zone of preferential ion heating in the inner heliosphere
Summary
Observations of space over the last half century, including spectroscopic diagnostics of UV emission from coronal plasma and direct in situ sampling of solar wind by spacecraft, have shed light on the nonthermal nature of heating in the corona and solar wind. Throughout the heliosphere, plasma is typically found in states other than local thermodynamic equilibrium, with relative drifts and unequal temperatures between species and anisotropic and otherwise non-Maxwellian velocity distribution functions (VDFs) commonly observed. Such nonthermal structure is indicative of mechanisms that selectively couple to particles with particular velocities, charges, or masses and preferentially heat different plasma species.
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