Radial evolution of the solar wind in pure high-speed streams: HELIOS revised observations
File(s)
Accepted version
Author(s)
Perrone, Denise
Stansby, David
Horbury, Timothy
Matteini, Lorenzo
Type
Journal Article
Abstract
Spacecraft observations have shown that the proton temperature in the solar wind falls off with radial distance more slowly than expected for an adiabatic prediction. Usually, previous studies have been focused on the evolution of the solar-wind plasma by using the bulk speed as an order parameter to discriminate different regimes. In contrast, here, we study the radial evolution of pure and homogeneous fast streams (i.e. well-defined streams of coronal-hole plasma that maintain their identity during several solar rotations) by means of re-processed particle data, from the HELIOS satellites between 0.3 and 1 au. We have identified 16 intervals of unperturbed high-speed coronal-hole plasma, from three different sources and measured at different radial distances. The observations show that, for all three streams, (i) the proton density decreases as expected for a radially expanding plasma, unlike previous analysis that found a slower decrease; (ii) the magnetic field deviates from the Parker prediction, with the radial component decreasing more slowly and the tangential more quickly than expected; (iii) the double-adiabatic invariants are violated and an increase of entropy is observed; (iv) the collisional frequency is not constant, but decreases as the plasma travels away from the Sun. This work provides an insight into the heating problem in pure fast solar wind, fitting in the context of the next solar missions, and, especially for Parker Solar Probe, it enables us to predict the high-speed solar-wind environment much closer to the Sun.
Date Issued
2019-03-01
Date Acceptance
2018-12-06
Citation
Monthly Notices of the Royal Astronomical Society, 2019, 483 (3), pp.3730-3737
ISSN
0035-8711
Publisher
Oxford University Press (OUP)
Start Page
3730
End Page
3737
Journal / Book Title
Monthly Notices of the Royal Astronomical Society
Volume
483
Issue
3
Copyright Statement
©2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record is available online at:https://academic.oup.com/mnras/article/483/3/3730/5237719
Sponsor
Science and Technology Facilities Council (STFC)
Grant Number
ST/N000692/1
Subjects
physics.space-ph
physics.plasm-ph
0201 Astronomical And Space Sciences
Astronomy & Astrophysics
Publication Status
Published
Date Publish Online
2018-12-10