Studies in multi-scale turbulent dynamics of the solar wind
Date
2018
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
The solar wind is the continuous outflow of plasma from the Sun, driven by the
pressure difference between the hot corona and the interstellar medium. This complex,
dynamically evolving flow permeates all of interplanetary space, and its behavior and
properties have significant implications for near-Earth space weather, the health of
space travelers, and the proper functioning of both terrestrial and space-faring elec-
tronic systems. As the only astrophysical plasma where direct spacecraft measurements
are possible, the solar wind provides an archetype of a strongly turbulent magnetized
system. ☐ This dissertation examines several problems within the context of multi-scale
turbulent dynamics of the solar wind, employing a “model heliosphere” produced
by a well-tested global two-fluid magnetohydrodynamic (MHD) code. The three-
dimensional simulations are based on a Reynolds-averaging approach, in which resolved
large-scale flow is self-consistently coupled to smaller-scale fluctuations by means of a
dynamical turbulence transport model. The focus is on the inner heliosphere (coronal
surface to 3 astronomical units); effects of solar variability are incorporated through
changing source magnetic dipole tilts and magnetogram-derived boundary data from
different solar-activity epochs. ☐ The simulations are used to study the collisional history of the solar wind;
full integral calculations of the collisional age are compared with simpler one-point
estimates commonly employed in observational work, the relationship between the
collisional age and the Knudsen number is clarified, and the collisional age is contrasted
with the turbulent age of the solar wind. ☐ The diffusion tensor that describes scattering of energetic particles by magnetic
fluctuations is evaluated throughout the inner-heliosphere, with the heliospheric current sheet emerging as a region of strong diffusion perpendicular to the magnetic field.
The rigidity-dependence of the parallel diffusion coefficient is shown to evolve with
heliocentric distance. ☐ Critical (sonic, Alfvénic, and plasma-beta unity) surfaces that mark the transi-
tion of the magnetically-structured corona into the predominantly hydrodynamic solar
wind are localized. The flow in regions propinquitous to these surfaces is investigated,
and simulation results are compared with a variety of remote sensing observations.
The often-overlooked concept of a “range of influence” that limits the length scales at
which fluctuations may interact in the expanding solar wind is discussed. ☐ With the importance of the critical surfaces established, contextual predictions
for the soon-to-be-launched Parker Solar Probe (PSP ) mission are provided by com-
bining the simulations with the spacecraft’s planned trajectory. PSP crossings of the
critical surfaces are simulated, and the turbulence environment likely to be observed
during early orbits is discussed. ☐ In an ancillary observational study, comparative statistical analyses of multi-
scale intermittent turbulence in the Earth’s magnetosheath and the solar wind are
performed, employing high-resolution multi-spacecraft data provided by the Magneto-
spheric Multiscale mission. Strong signatures of intermittent turbulent structures at
electron and ion scales in the magnetosheath are observed. These signatures appear to
be absent at sub-ion scales in the solar wind, which does, nevertheless, exhibit inter-
mittency in the inertial range. The findings also include different power-law spectral
behavior in the two regions. Comparisons of a multi-spacecraft technique with single-
spacecraft estimates permits a verification of the accuracy of the Taylor “frozen-in”
hypothesis.
Description
Keywords
Pure sciences, Solar wind, Space physics, Turbulence