310 research outputs found
Precision Electron Measurements in the Solar Wind at 1 au from NASA's Wind Spacecraft
This work aims to characterize precisely and systematically the non-thermal
characteristics of the electron Velocity Distribution Function (eVDF) in the
solar wind at 1 au using data from the Wind spacecraft. We present a
comprehensive statistical analysis of solar wind electrons at 1 au using the
electron analyzers of the 3D-Plasma instrument on board Wind. This work uses a
sophisticated algorithm developed to analyze and characterize separately the
three populations - core, halo and strahl - of the eVDF up to 2 keV. The eVDF
data are calibrated using independent electron parameters obtained from the
quasi-thermal noise around the electron plasma frequency measured by the
Thermal Noise Receiver. The code determines the respective set of total
electron, core, halo and strahl parameters through non-linear least-square fits
to the measured eVDF, taking properly into account spacecraft charging and
other instrumental effects. We use four years, ~ 280000 independent
measurements of core, halo and strahl parameters to investigate the statistical
properties of these different populations in the solar wind. We discuss the
distributions of their respective densities, drift velocities, temperature, and
temperature anisotropies as functions of solar wind speed. We also show
distributions with solar wind speed of the total density, temperature,
temperature anisotropy and heat flux, as well as those of the proton
temperature, proton-to-electron temperature ratio, proton and electron beta.
Intercorrelations between some of these parameters are also discussed. The
present dataset represents the largest, high-precision, collection of electron
measurements in the pristine solar wind at 1~AU. It provides a new wealth of
information on electron microphysics. Its large volume will enable future
statistical studies of parameter combinations and their dependencies under
different plasma conditions.Comment: total of 21 pages, 17 figures, 1 appendix and 7 table
Evolution of large-amplitude Alfv\'en waves and generation of switchbacks in the expanding solar wind
Motivated by recent Parker Solar Probe (PSP) observations of "switchbacks"
(abrupt, large-amplitude reversals in the radial magnetic field, which exhibit
Alfv\'enic correlations) we examine the dynamics of large-amplitude Alfv\'en
waves in the expanding solar wind. We develop an analytic model which makes
several predictions: switchbacks should preferentially occur in regions where
the solar wind plasma has undergone a greater expansion, the switchback
fraction at radii comparable to PSP should be an increasing function of radius,
and switchbacks should have their gradients preferentially perpendicular to the
mean magnetic field direction. The expansion of the plasma generates small
compressive components as part of the wave's nonlinear evolution: these are
maximized when the normalized fluctuation amplitude is comparable to
, where is the angle between the propagation direction and
the mean magnetic field. These compressive components steepen the primary
Alfv\'enic waveform, keeping the solution in a state of nearly constant
magnetic field strength as its normalized amplitude grows due to
expansion. The small fluctuations in the magnetic-field-strength are minimized
at a particular -dependent value of , usually of order unity,
and the density and magnetic-field-strength fluctuations can be correlated or
anticorrelated depending on and . Example solutions of our
dynamical equation are presented; some do indeed form magnetic-field reversals.
Our predictions appear to match some previously unexplained phenomena in
observations and numerical simulations, providing evidence that the observed
switchbacks result from the nonlinear evolution of the initially
small-amplitude Alfv\'en waves already known to be present at the coronal base.Comment: 20 pages, 1 table, 9 figures. Submitted to Ap
Proton Heating in Solar Wind Compressible Turbulence with Collisions between Counter-propagating Waves
Magnetohydronamic turbulence is believed to play a crucial role in heating
the laboratorial, space, and astrophysical plasmas. However, the precise
connection between the turbulent fluctuations and the particle kinetics has not
yet been established. Here we present clear evidence of plasma turbulence
heating based on diagnosed wave features and proton velocity distributions from
solar wind measurements by the Wind spacecraft. For the first time, we can
report the simultaneous observation of counter-propagating magnetohydrodynamic
waves in the solar wind turbulence. Different from the traditional paradigm
with counter-propagating Alfv\'en waves, anti-sunward Alfv\'en waves (AWs) are
encountered by sunward slow magnetosonic waves (SMWs) in this new type of solar
wind compressible turbulence. The counter-propagating AWs and SWs correspond
respectively to the dominant and sub-dominant populations of the imbalanced
Els\"asser variables. Nonlinear interactions between the AWs and SMWs are
inferred from the non-orthogonality between the possible oscillation direction
of one wave and the possible propagation direction of the other. The associated
protons are revealed to exhibit bi-directional asymmetric beams in their
velocity distributions: sunward beams appearing in short and narrow patterns
and anti-sunward broad extended tails. It is suggested that multiple types of
wave-particle interactions, i.e., cyclotron and Landau resonances with AWs and
SMWs at kinetic scales, are taking place to jointly heat the protons
perpendicularly and parallel
A zone of preferential ion heating extends tens of solar radii from Sun
The extreme temperatures and non-thermal nature of the solar corona and solar
wind arise from an unidentified physical mechanism that preferentially heats
certain ion species relative to others. Spectroscopic indicators of unequal
temperatures commence within a fraction of a solar radius above the surface of
the Sun, but the outer reach of this mechanism has yet to be determined. Here
we present an empirical procedure for combining interplanetary solar wind
measurements and a modeled energy equation including Coulomb relaxation to
solve for the typical outer boundary of this zone of preferential heating.
Applied to two decades of observations by the Wind spacecraft, our results are
consistent with preferential heating being active in a zone extending from the
transition region in the lower corona to an outer boundary 20-40 solar radii
from the Sun, producing a steady state super-mass-proportional
-to-proton temperature ratio of . Preferential ion heating
continues far beyond the transition region and is important for the evolution
of both the outer corona and the solar wind. The outer boundary of this zone is
well below the orbits of spacecraft at 1 AU and even closer missions such as
Helios and MESSENGER, meaning it is likely that no existing mission has
directly observed intense preferential heating, just residual signatures. We
predict that {Parker Solar Probe} will be the first spacecraft with a perihelia
sufficiently close to the Sun to pass through the outer boundary, enter the
zone of preferential heating, and directly observe the physical mechanism in
action.Comment: 11 pages, 7 figures, accepted for publication in the Astrophysical
Journal on 1 August 201
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