Heating Rates for Protons and Electrons in Polar Coronal Holes: Empirical Constraints from the Ultraviolet Coronagraph Spectrometer

TL;DR

This study uses ultraviolet spectroscopy data from the UVCS instrument to empirically constrain the heating rates and kinetic properties of protons and electrons in polar coronal holes, shedding light on coronal heating mechanisms.

Contribution

It introduces a novel empirical modeling approach combining Monte Carlo simulations with UVCS data to constrain plasma parameters and wave amplitudes in the solar corona.

Findings

Proton and electron temperatures are around 1.2-1.9 MK with little radial variation.

Weak Alfvén wave dissipation is indicated by the data.

The measured wave amplitudes align with the observed heating rates.

Abstract

Ultraviolet spectroscopy of the extended solar corona is a powerful tool for measuring the properties of protons, electrons, and heavy ions in the accelerating solar wind. The large coronal holes that expand up from the north and south poles at solar minimum are low-density collisionless regions in which it is possible to detect departures from one-fluid thermal equilibrium. An accurate characterization of these departures is helpful in identifying the kinetic processes ultimately responsible for coronal heating. In this paper, Ultraviolet Coronagraph Spectrometer (UVCS) measurements of the H I Lyman alpha line are analyzed to constrain values for the solar wind speed, electron density, electron temperature, proton temperature (parallel and perpendicular to the magnetic field) and Alfven-wave amplitude. The analysis procedure involves creating a large randomized ensemble of empirical models, simulating their Lyman alpha profiles, and building posterior probability distributions for only the models that agree with the UVCS data. The resulting temperatures do not exhibit a great deal of radial variation between heliocentric distances of 1.4 and 4 solar radii. Typical values for the electron, parallel proton, and perpendicular proton temperatures are 1.2, 1.8, and 1.9 MK, respectively. Resulting values for the "nonthermal" Alfven wave amplitude show evidence for weak dissipation, with a total energy-loss rate that agrees well with an independently derived total heating rate for the protons and electrons. The moderate Alfven-wave amplitudes appear to resolve some tension in the literature between competing claims of both higher (undamped) and lower (heavily damped) values.