101 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
Constraining a Model of the Radio Sky Below 6 MHz Using the Parker Solar Probe/FIELDS Instrument in Preparation for Upcoming Lunar-based Experiments
We present a Bayesian analysis of data from the FIELDS instrument on board
the Parker Solar Probe (PSP) spacecraft with the aim of constraining low
frequency ( 6 MHz) sky in preparation for several upcoming
lunar-based experiments. We utilize data recorded during PSP's ``coning roll''
maneuvers, in which the axis of the spacecraft is pointed 45 off of
the Sun. The spacecraft then rotates about a line between the Sun and the
spacecraft with a period of 24 minutes. We reduce the data into two formats:
roll-averaged, in which the spectra are averaged over the roll, and
phase-binned, in which the spectra are binned according to the phase of the
roll. We construct a forward model of the FIELDS observations that includes
numerical simulations of the antenna beam, an analytic emissivity function of
the galaxy, and estimates of the absorption due to free electrons. Fitting 5
parameters, we find that the roll-averaged data can be fit well by this model
and we obtain posterior parameter constraints that are in general agreement
with previous estimates. The model is not, however, able to fit the
phase-binned data well, likely due to limitations such as the lack of
non-smooth emission structure at both small and large scales, enforced symmetry
between the northern and southern galactic hemispheres, and large uncertainties
in the free electron density. This suggests that significant improvement in the
low frequency sky model is needed in order to fully and accurately represent
the sky at frequencies below 6 MHz.Comment: 18 pages, 10 figures, 5 tables. Under review in the Astrophysical
Journa
Self-induced Scattering of Strahl Electrons in the Solar Wind
We investigate the scattering of strahl electrons by microinstabilities as a
mechanism for creating the electron halo in the solar wind. We develop a
mathematical framework for the description of electron-driven
microinstabilities and discuss the associated physical mechanisms. We find that
an instability of the oblique fast-magnetosonic/whistler (FM/W) mode is the
best candidate for a microinstability that scatters strahl electrons into the
halo. We derive approximate analytic expressions for the FM/W instability
threshold in two different regimes, where is the ratio of the core electrons' thermal pressure to the magnetic
pressure, and confirm the accuracy of these thresholds through comparison with
numerical solutions to the hot-plasma dispersion relation. We find that the
strahl-driven oblique FM/W instability creates copious FM/W waves under
low- conditions when ,
where is the strahl speed and is the thermal
speed of the core electrons. These waves have a frequency of about half the
local electron gyrofrequency. We also derive an analytic expression for the
oblique FM/W instability for . The comparison of our
theoretical results with data from the \emph{Wind} spacecraft confirms the
relevance of the oblique FM/W instability for the solar wind. The whistler
heat-flux, ion-acoustic heat-flux, kinetic-Alfv\'en-wave heat-flux, and
electrostatic electron-beam instabilities cannot fulfill the requirements for
self-induced scattering of strahl electrons into the halo. We make predictions
for the electron strahl close to the Sun, which will be tested by measurements
from \emph{Parker Solar Probe} and \emph{Solar Orbiter}.Comment: 11 pages, 11 figure
Weak Solar Radio Bursts from the Solar Wind Acceleration Region Observed by Parker Solar Probe and Its Probable Emission Mechanism
The Parker Solar Probe (PSP) provides us the unprecedentedly close approach
observation to the Sun, and hence the possibility of directly understanding the
"elementary process" which occurs in the kinetic scale of particles collective
interactioin in solar coronal plasmas. We reported a kind of weak solar radio
bursts (SRBs), which are detected by PSP when it passed a low-density magnetic
channel during its second encounter phase. These weak SRBs have low starting
frequecny MHz and narrow frequency range from a few tens MHz to a few
hundres kHz. Their dynamic spectra display a strongly evolving feature of the
intermediate relative drift rate decreasing rapidly from above 0.01/s to below
0.01/s. Analyses based on common empirical models of solar coronal plasmas
indicate that these weak SRBs originate from the heliocentric distance (the solar radius), a typical solar wind acceleration region with
a low- plasma, and indicate that their soruces have a typic motion
velociy (Alfv\'en velocity) obviously lower than that of fast
electrons required by effectively exciting SRBs. We propose that solitary
kinetic Alfv\'en waves with kinetic scales can be responsible for the
generation of these small-scalevweak SRBs, called solitary wave radiation
(SWR)
Periodicities in an active region correlated with Type III radio bursts observed by Parker Solar Probe
Context. Periodicities have frequently been reported across many wavelengths
in the solar corona. Correlated periods of ~5 minutes, comparable to solar
p-modes, are suggestive of coupling between the photosphere and the corona.
Aims. Our study investigates whether there are correlations in the periodic
behavior of Type III radio bursts, indicative of non-thermal electron
acceleration processes, and coronal EUV emission, assessing heating and
cooling, in an active region when there are no large flares. Methods. We use
coordinated observations of Type III radio bursts from the FIELDS instrument on
Parker Solar Probe (PSP), of extreme ultraviolet emissions by the Solar
Dynamics Observatory (SDO)/AIA and white light observations by SDO/HMI, and of
solar flare x-rays by Nuclear Spectroscopic Telescope Array (NuSTAR) on April
12, 2019. Several methods for assessing periodicities are utilized and compared
to validate periods obtained. Results. Periodicities of about 5 minutes in the
EUV in several areas of an active region are well correlated with the
repetition rate of the Type III radio bursts observed on both PSP and Wind.
Detrended 211A and 171A light curves show periodic profiles in multiple
locations, with 171A peaks lagging those seen in 211A. This is suggestive of
impulsive events that result in heating and then cooling in the lower corona.
NuSTAR x-rays provide evidence for at least one microflare during the interval
of Type III bursts, but there is not a one-to-one correspondence between the
x-rays and the Type-III bursts. Our study provides evidence for periodic
acceleration of non-thermal electrons (required to generate Type III radio
bursts) when there were no observable flares either in the x-ray data or the
EUV. The acceleration process, therefore, must be associated with small
impulsive events, perhaps nanoflares
Wind Observations of Wave Heating and/or Particle Energization at Supercritical Interplanetary Shocks
We present the first observations at supercritical interplanetary shocks of large amplitude (> 100 mV/m pk-pk) solitary waves, approx.30 mV/m pk-pk waves exhibiting characteristics consistent with electron Bernstein waves, and > 20 nT pk-pk electromagnetic lower hybrid-like waves, with simultaneous evidence for wave heating and particle energization. The solitary waves and the Bernstein-like waves were likely due to instabilities driven by the free energy provided by reflected ions [Wilson III et al., 2010]. They were associated with strong particle heating in both the electrons and ions. We also show a case example of parallel electron energization and perpendicular ion heating due to a electromagnetic lower hybrid-like wave. Both studies provide the first experimental evidence of wave heating and/or particle energization at interplanetary shocks. Our experimental results, together with the results of recent Vlasov [Petkaki and Freeman, 2008] and PIC [Matsukyo and Scholer, 2006] simulations using realistic mass ratios provide new evidence to suggest that the importance of wave-particle dissipation at shocks may be greater than previously thought
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Closed magnetic topology in the Venusian magnetotail and ion escape at Venus.
Venus, lacking an intrinsic global dipole magnetic field, serves as a textbook example of an induced magnetosphere, formed by interplanetary magnetic fields (IMF) enveloping the planet. Yet, various aspects of its magnetospheric dynamics and planetary ion outflows are complex and not well understood. Here we analyze plasma and magnetic field data acquired during the fourth Venus flyby of the Parker Solar Probe (PSP) mission and show evidence for closed topology in the nightside and downstream portion of the Venus magnetosphere (i.e., the magnetotail). The formation of the closed topology involves magnetic reconnection-a process rarely observed at non-magnetized planets. In addition, our study provides an evidence linking the cold Venusian ion flow in the magnetotail directly to magnetic connectivity to the ionosphere, akin to observations at Mars. These findings not only help the understanding of the complex ion flow patterns at Venus but also suggest that magnetic topology is one piece of key information for resolving ion escape mechanisms and thus the atmospheric evolution across various planetary environments and exoplanets
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