Abstract
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 20–40 R ⊙ 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 4.23 R ⊙. Strong preferential ion heating is apparently predominately active below the Alfvén surface. 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 that the 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.
Highlights
Ions in the solar corona and solar wind are too hot, a puzzle since the start of the space age and the first in situ observations of solar wind plasma by spacecraft
We report that the boundary moves in lockstep with the Alfven 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
Remote observations provide some insight into the mechanisms injecting energy at the base of the corona (McIntosh et al 2011; Grant et al 2018) but these observations are not sufficient to distinguish between the various mechanisms that have been proposed to lead to preferential ion heating throughout the near-Sun environment, including wave damping, turbulent dissipation, shocks, reconnection, nano-flares, and velocity filtration (see reviews by Ofman (2010), Hansteen & Velli (2012)
Summary
Ions in the solar corona and solar wind are too hot, a puzzle since the start of the space age and the first in situ observations of solar wind plasma by spacecraft. The ratio of heavy ion species temperature to proton temperature Ts/Tp is observed to reach and even exceed the mass ratio ms/mp This suggests a kinetic heating process involving interactions with waves or fluctuations with a characteristic velocity, as ions have equal thermal speeds when they have mass proportional temperatures. Remote observations provide some insight into the mechanisms injecting energy at the base of the corona (McIntosh et al 2011; Grant et al 2018) but these observations are not sufficient to distinguish between the various mechanisms that have been proposed to lead to preferential ion heating throughout the near-Sun environment, including wave damping, turbulent dissipation, shocks, reconnection, nano-flares, and velocity filtration (see reviews by Ofman (2010), Hansteen & Velli (2012)
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