392 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
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
Arguments for the physical nature of the triggered ion-acoustic waves observed on the Parker Solar Probe
Triggered ion-acoustic waves are a pair of coupled waves observed in the
previously unexplored plasma regime near the Sun. They may be capable of
producing important effects on the solar wind. Because this wave mode has not
been observed or studied previously and it is not fully understood, the issue
of whether it has a natural origin or is an instrumental artifact can be
raised. This paper discusses this issue by examining 13 features of the data
such as whether the triggered ion-acoustic waves are electrostatic, whether
they are both narrow-band, whether they satisfy the requirement that the
electric field is parallel to the k-vector, whether the phase difference
between the electric field and the density fluctuations is 90 degrees, whether
the two waves have the same phase velocity as they must if they are coupled,
whether the phase velocity is that of an ion-acoustic wave, whether they are
associated with other parameters such as electron heating, whether the electric
field instrument otherwise performed as expected, etc. The conclusion reached
from these analyses is that triggered ion-acoustic waves are highly likely to
have a natural origin although the possibility that they are artifacts
unrelated to processes occurring in the natural plasma cannot be eliminated.
This inability to absolutely rule out artifacts as the source of a measured
result is a characteristic of all measurements.Comment: 21 pages, 9 figures, 1 table. arXiv admin note: substantial text
overlap with arXiv:2211.1441
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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