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 < 1.1 R _ { \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 ) .