How important are the alpha‐proton relative drift and the electron heat flux for the proton heating of the solar wind in the inner heliosphere?

Abstract

This report explores the feasibility of explaining the observed proton heating in the inner heliosphere (1) by tapping the field-aligned relative drift between alpha particles and protons in the solar wind plasma and (2) by tapping the strahl-electron heat flux from the Sun. The observed reduction of the alpha-proton drift kinetic energy from 0.3 to 1 AU and the observed reduction of electron heat flux from 0.3 to 1 AU are each about half of the energy needed to account for the observed heating of protons from 0.3 to 1 AU. A mechanism is identified to transfer the free energy of the alpha-proton relative drift into proton thermal energy: the alpha-proton magnetosonic instability. A mechanism is identified to transfer kinetic energy from the strahl-electron heat flux into proton thermal energy: weak double layers. At the current state of knowledge, the plausibility of heating the solar wind protons via the alpha-proton magnetosonic instability is high. The properties of the weak double layers that have been observed in the solar wind are not well known; more data analysis and plasma simulations are needed before the plausibility of heating the solar wind protons by the double-layer mechanism can be evaluated.

The plasma structure of coronal hole solar wind: Origins and evolution

“…Weaker correlation may have to do with cumulative in situ heating of the protons in the coronal hole origin solar wind [cf. Schwenn et al ., ; Freeman and Lopez , ; Hellinger et al ., ; Borovsky and Gary , ]. In Table the positive correlation between S p and v A is also seen but again apparently weaker than at 1 AU in Table .…”

“…The proton temperature at ~2 AU is cooler than the temperature at 0.3 AU owing to adiabatic cooling by the solar wind expansion being greater than nonadiabatic proton heating [cf. Borovsky and Gary , , Figure 2]. The relative amplitude of the temperature variations is greatly reduced from a factor of more than 4 at 0.3 AU to a factor of about 1.5 at ~2 AU.…”

“…In Table the Ulysses polar‐crossing data show a positive correlation between the alpha‐to‐proton density ratio and S p that is not seen at 1 AU in Table : this positive correlation further from the Sun may represent cumulative heating of the coronal hole origin solar wind protons by alpha particles with more proton heating (higher S p ) associated with higher fractional densities of alphas [cf. Schwartz et al ., ; Safrankova et al ., ; Borovsky and Gary , ].…”

“…It is well known that the protons of the fast wind are nonadiabatically heated with distance from the Sun, which produces a temperature decrease with distance that is slower than the decrease associated with pure adiabatic expansion [cf. Schwenn et al ., ; Freeman and Lopez , ; Hellinger et al ., ; Borovsky and Gary , ]. Figure shows the progression of the increase in proton specific entropy from 0.3 AU (black and gray curves), to 0.6 AU (light blue curve), to 1 AU (multiple blue, green, and purple curves), to ~2 AU (orange and red curves).…”

Time‐Integral Correlations of Multiple Variables With the Relativistic‐Electron Flux at Geosynchronous Orbit: The Strong Roles of Substorm‐Injected Electrons and the Ion Plasma Sheet

“…An example of this is the high correlation between the solar wind proton temperature T p and the AE index, which almost certainly is not physical because the information about the upstream solar wind proton temperature is lost as the solar wind crosses the bow shock (cf. Formisano et al, ; Tidman & Krall, ); this T p ‐ AE correlation is undoubtedly caused by the fact that the solar wind velocity v sw physically controls the driving of the magnetosphere (as measured by AE ) (e.g., Borovsky & Birn, ; Newell et al, ) and that proton temperature T p is correlated with the solar wind velocity v sw (e.g., Elliott et al, ; Lopez & Freeman, ) (although the physical reason for the T p ‐ v sw correlation is not known (Borovsky & Gary, ; Marsch & Richter, ). In examining the solar wind driving of the Earth's magnetosphere‐ionosphere system, one can find a web of correlations involving multiple solar and solar wind variables and multiple Earth‐based variables.…”