Spectroscopic Measurements of the Ion Velocity Distribution at the Base of the Fast Solar Wind

Abstract

In situ measurements of the fast solar wind reveal non-thermal distributions of electrons, protons and, minor ions extending from $0.3$ AU to the heliopause. The physical mechanisms responsible for these non-thermal properties and the location where these properties originate remain open questions. Here we present spectroscopic evidence, from extreme ultraviolet spectroscopy, that the velocity distribution functions (VDFs) of minor ions are already non-Gaussian at the base of the fast solar wind in a coronal hole, at altitudes of $<1.1R_{\odot }$. Analysis of Fe, Si, and Mg spectral lines reveal a peaked line-shape core and broad wings that can be characteristed by a kappa VDF. A kappa distribution fit gives very small kappa indices off-limb of $\kappa \approx 1.9-2.5$, indicating either (a) ion populations far from thermal equilibrium, (b) fluid motions such as non-Gaussian turbulent fluctuations or non-uniform wave motions, or (c) some combination of both. These observations provide important empirical constraints for the source region of the fast solar wind and for the theoretical models of the different acceleration, heating, and energy deposition processes therein. To the best of our knowledge, this is the first time that the ion VDF in the fast solar wind has been probed so close to its source region. The findings are also a timely precursor to the upcoming 2018 launch of the Parker Solar Probe, which will provide the closest in situ measurements of the solar wind at approximately $0.04$ AU ($8.5$ solar radii).

arXiv comment9 pages, 4 figures, accepted for publication in the Astrophysical Journal Letters

References in corpus (14)

• The Interface Region Imaging Spectrograph (IRIS)
• Coronal Holes
• Solar Wind Turbulence and the Role of Ion Instabilities
• Acceleration and propagation of Solar Energetic Particles
• Turbulent kinetic energy in the energy balance of a solar flare
• Evidence of Wave Damping at Low Heights in a Polar Coronal Hole
• Observational Quantification of the Energy Dissipated by Alfvén Waves in a Polar Coronal Hole: Evidence that Waves Drive the Fast Solar Wind
• Ion Temperatures in the Low Solar Corona: Polar Coronal Holes at Solar Minimum
• Non-Maxwellian analysis of the transition-region line profiles observed by the Interface Region Imaging Spectrograph
• First evidence of non-Gaussian solar flare EUV spectral line profiles and accelerated non-thermal ion motion
• Non-Gaussian Velocity Distributions in Solar Flares from Extreme Ultraviolet Lines: A Possible Diagnostic of Ion Acceleration
• Measurements of Anisotropic Ion Temperatures, Non-Thermal Velocities, and Doppler Shifts in a Coronal Hole
• Spectroscopic indication of suprathermal ions in the solar corona
• Spectral line shape modeling and ion temperature fluctuations in tokamak edge plasmas

Cited by in corpus (4)

• Broad Non-Gaussian Fe XXIV Line Profiles in the Impulsive Phase of the 2017 September 10 X8.3 class Flare Observed by Hinode/EIS
• Modeling ion beams, kinetic instabilities, and waves observed by the Parker Solar Probe near perihelia
• Signatures of the non-Maxwellian $\kappa$-distributions in optically thin line spectra. II. Synthetic Fe XVII--XVIII X-ray coronal spectra and predictions for the Marshall Grazing-Incidence X-ray Spectrometer (MaGIXS)
• Estimating Ion Temperatures at the Polar Coronal Hole Boundary