Journal
ASTROPHYSICAL JOURNAL
Volume 900, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/abab04
Keywords
Alfven waves; Solar coronal holes; Solar ultraviolet emission; Solar wind; Space plasmas; Spectroscopy
Categories
Funding
- National Aeronautics and Space Administration (NASA) [NNX15AW33G, NNX16AG87G]
- National Science Foundation (NSF) [1540094, 1613207]
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [1613207] Funding Source: National Science Foundation
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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 iLy 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 Ly 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.
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