Alpha particle thermodynamics in the inner heliosphere fast solar wind
File(s)
Published version
Author(s)
Stansby, David
Perrone, Denise
Matteini, Lorenzo
Horbury, Timothy
Salem, Chadi
Type
Journal Article
Abstract
Context. Plasma processes occurring in the corona and solar wind can be probed by studying the thermodynamic properties of
different ion species. However, most in-situ observations of positive ions in the solar wind are taken at 1 AU, where information on
their solar source properties may have been irreversibly erased.
Aims. In this study we aimed to use the properties of alpha particles at heliocentric distances between 0.3 and 1 AU to study plasma
processes occurring at the points of observation, and to infer processes occurring inside 0.3 AU by comparing our results to previous
remote sensing observations of the plasma closer to the Sun.
Methods. We reprocessed the original Helios positive ion distribution functions, isolated the alpha particle population, and computed
the alpha particle number density, velocity, and magnetic field perpendicular and parallel temperatures. We then investigated the radial
variation of alpha particle temperatures in fast solar wind observed between 0.3 and 1 AU.
Results. Between 0.3 and 1 AU alpha particles are heated in the magnetic field perpendicular direction, and cooled in the magnetic
field parallel direction. Alpha particle evolution is bounded by the alpha firehose instability threshold, which provides one possible
mechanism to explain the observed parallel cooling and perpendicular heating. Closer to the Sun our observations suggest that the
alpha particles undergo heating in the perpendicular direction, whilst the large magnetic field parallel temperatures observed at 0.3 AU
may be due to the combined effect of double adiabatic expansion and alpha particle deceleration inside 0.3 AU.
different ion species. However, most in-situ observations of positive ions in the solar wind are taken at 1 AU, where information on
their solar source properties may have been irreversibly erased.
Aims. In this study we aimed to use the properties of alpha particles at heliocentric distances between 0.3 and 1 AU to study plasma
processes occurring at the points of observation, and to infer processes occurring inside 0.3 AU by comparing our results to previous
remote sensing observations of the plasma closer to the Sun.
Methods. We reprocessed the original Helios positive ion distribution functions, isolated the alpha particle population, and computed
the alpha particle number density, velocity, and magnetic field perpendicular and parallel temperatures. We then investigated the radial
variation of alpha particle temperatures in fast solar wind observed between 0.3 and 1 AU.
Results. Between 0.3 and 1 AU alpha particles are heated in the magnetic field perpendicular direction, and cooled in the magnetic
field parallel direction. Alpha particle evolution is bounded by the alpha firehose instability threshold, which provides one possible
mechanism to explain the observed parallel cooling and perpendicular heating. Closer to the Sun our observations suggest that the
alpha particles undergo heating in the perpendicular direction, whilst the large magnetic field parallel temperatures observed at 0.3 AU
may be due to the combined effect of double adiabatic expansion and alpha particle deceleration inside 0.3 AU.
Date Issued
2019-03
Date Acceptance
2019-02-19
Citation
Astronomy and Astrophysics, 2019, 623
ISSN
0004-6361
Publisher
EDP Sciences
Journal / Book Title
Astronomy and Astrophysics
Volume
623
Copyright Statement
© ESO 2019
Subjects
Science & Technology
Physical Sciences
Astronomy & Astrophysics
solar wind
Sun: heliosphere
VELOCITY DISTRIBUTIONS
TEMPERATURE ANISOTROPY
ION TEMPERATURES
HELIUM
INSTABILITIES
ACCELERATION
EQUATION
PROTONS
0.3-AU
CORONA
physics.space-ph
physics.space-ph
astro-ph.SR
Astronomy & Astrophysics
0201 Astronomical and Space Sciences
Publication Status
Published
Article Number
ARTN L2
Date Publish Online
2019-02-28