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Register a new userPrecision Electron Measurements in the Solar Wind at 1 au from NASAs Wind Spacecraft
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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.
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The recent launches of Parker Solar Probe (PSP), Solar Orbiter (SO) and BepiColombo, along with several older spacecraft, have provided the opportunity to study the solar wind at multiple latitudes and distances from the Sun simultaneously. We take advantage of this unique spacecraft constellation, along with low solar activity across two solar rotations between May and July 2020, to investigate how the solar wind structure, including the Heliospheric Current Sheet (HCS), varies with latitude. We visualise the sector structure of the inner heliosphere by ballistically mapping the polarity and solar wind speed from several spacecraft onto the Suns source surface. We then assess the HCS morphology and orientation with the in situ data and compare with a predicted HCS shape. We resolve ripples in the HCS on scales of a few degrees in longitude and latitude, finding that the local orientation of sector boundaries were broadly consistent with the shape of the HCS but were steepened with respect to a modelled HCS at the Sun. We investigate how several CIRs varied with latitude, finding evidence for the compression region affecting slow solar wind outside the latitude extent of the faster stream. We also identified several transient structures associated with HCS crossings, and speculate that one such transient may have disrupted the local HCS orientation up to five days after its passage. We have shown that the solar wind structure varies significantly with latitude, with this constellation providing context for solar wind measurements that would not be possible with a single spacecraft. These measurements provide an accurate representation of the solar wind within $pm 10^{circ}$ latitude, which could be used as a more rigorous constraint on solar wind models and space weather predictions. In the future, this range of latitudes will increase as SOs orbit becomes more inclined.
Electron velocity distribution functions in the solar wind according to standard models consist of 4 components, of which 3 are symmetric - the core, the halo, and the superhalo, and one is magnetic field-aligned, beam-like population, referred to as the strahl. We analysed in-situ measurements provided by the two Helios spacecrafts to study the behaviour of the last, the strahl electron population, in the inner Solar system between 0.3 and 1 au. The strahl is characterised with a pitch-angle width (PAW) depending on electron energy and evolving with radial distance. We find different behaviour of the strahl electrons for solar wind separated into types by the core electron beta parallel value ($beta_{ecparallel}$). For the low-$beta_{ecparallel}$ solar wind the strahl component is more pronounced, and the variation of PAW is electron energy dependent. At low energies a slight focusing over distance is observed, and the strahl PAW measured at 0.34 au agrees with the width predicted by a collisionless focusing model. The broadening observed for higher-energy strahl electrons during expansion can be described by an exponential relation, which points toward an energy dependent scattering mechanism. In the high-$beta_{ecparallel}$ solar wind the strahl appears broader in consistence with the high-$beta_{ecparallel}$ plasma being more unstable with respect to kinetic instabilities. Finally we extrapolate our observations to the distance of 0.16 au, predicting the strahl PAWs in the low-$beta_{ecparallel}$ solar wind to be $sim$ 29$^o$ for all energies, and in the high-$beta_{ecparallel}$ solar wind a bit broader, ranging between 37$^o$ and 65$^o$.
A canonical description of a corotating solar wind high speed stream, in terms of velocity profile, would indicate three main regions:a stream interface or corotating interaction region characterized by a rapid flow speed increase and by compressive phenomena due to dynamical interaction between the fast wind flow and the slower ambient plasma;a fast wind plateau characterized by weak compressive phenomena and large amplitude fluctuations with a dominant Alfvenic character;a rarefaction region characterized by a decreasing trend of the flow speed and wind fluctuations dramatically reduced in amplitude and Alfvenic character, followed by the slow ambient wind. Interesting enough, in some cases the region where the severe reduction of these fluctuations takes place is remarkably short in time, of the order of minutes, and located at the flow velocity knee separating the fast wind plateau from the rarefaction region. The aim of this work is to investigate which are the physical mechanisms that might be at the origin of this phenomenon. We firstly looked for the presence of any tangential discontinuity which might inhibit the propagation of Alfvenic fluctuations from fast wind region to rarefaction region. The absence of a clear evidence for the presence of this discontinuity between these two regions led us to proceed with ion composition analysis for the corresponding solar wind, looking for any abrupt variation in minor ions parameters (as tracers of the source region) which might be linked to the phenomenon observed in the wind fluctuations. In the lack of a positive feedback from this analysis, we finally propose a mechanism based on interchange reconnection experienced by the field lines at the base of the corona, within the region separating the open field lines of the coronal hole, source of the fast wind, from the surrounding regions mainly characterized by closed field lines.
Motivated by the upcoming Solar Orbiter and Solar Probe Plus missions, qualitative and quantitative predictions are made for the effects of the violation of the Taylor hypothesis on the magnetic energy frequency spectrum measured in the near-Sun environment. The synthetic spacecraft data method is used to predict observational signatures of the violation for critically balanced Alfvenic turbulence or parallel fast/whistler turbulence. The violation of the Taylor hypothesis can occur in the slow flow regime, leading to a shift of the entire spectrum to higher frequencies, or in the dispersive regime, in which the dissipation range spectrum flattens at high frequencies. It is found that Alfvenic turbulence will not significantly violate the Taylor hypothesis, but whistler turbulence will. The flattening of the frequency spectrum is therefore a key observational signature for fast/whistler turbulence.
One of the discoveries made by Parker Solar Probe during first encounters with the Sun is the ubiquitous presence of relatively small-scale structures standing out as sudden deflections of the magnetic field. They were called switchbacks as some of them show up the full reversal of the radial component of the magnetic field and then return to regular conditions. Analyzing the magnetic field and plasma perturbations associated with switchbacks we identify three types of structures with slightly different characteristics: 1. Alfvenic structures, where the variations of the magnetic field components take place while the magnitude of the field remains constant; 2. Compressional, the field magnitude varies together with changes of the components; 3. Structures manifesting full reversal of the magnetic field (extremal class of Alfvenic structures). Processing of structures boundaries and plasma bulk velocity perturbations lead to the conclusion that they represent localized magnetic field tubes with enhanced parallel plasma velocity and ion beta moving together with respect to surrounding plasma. The magnetic field deflections before and after the switchbacks reveal the existence of total axial current. The electric currents are concentrated on the relatively narrow boundary layers on the surface of the tubes and determine the magnetic field perturbations inside the tube. These currents are closed on the structure surface, and typically have comparable azimuthal and the axial components. The surface of the structure may also accommodate an electromagnetic wave, that assists to particles in carrying currents. We suggest that the two types of structures we analyzed here may represent the local manifestations of the tube deformations corresponding to a saturated stage of the Firehose instability development.
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