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

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 Ly α 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 Alfvén-wave amplitude. The analysis procedure involves creating a large, randomized ensemble of empirical models, simulating their Ly α 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” Alfvén 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 Alfvén-wave amplitudes appear to resolve some tension in the literature between competing claims of both higher (undamped) and lower (heavily damped) values.