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Register a new userMulti-spacecraft Study of the Solar Wind at Solar Minimum: Dependence on Latitude and Transient Outflows
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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.
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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.
We investigate the spatial correlation properties of the solar wind using simultaneous observations by the ACE and WIND spacecraft. We use mutual information as a nonlinear measure of correlation and compare this to linear correlation. We find that the correlation lengthscales of fluctuations in density and magnetic field magnitude vary strongly with the solar cycle, whereas correlation lengths of fluctuations in B field components do not. We find the correlation length of |B| ~ 120 Re at solar minimum and ~ 270 Re at maximum and the correlation length of density ~ 75 Re at minimum and ~ 170 Re at minimum. The components of the B field have correlation lengths ~ correlation length |B| at minimum.
The solar wind is a magnetized plasma and as such exhibits collective plasma behavior associated with its characteristic spatial and temporal scales. The characteristic length scales include the size of the heliosphere, the collisional mean free paths of all species, their inertial lengths, their gyration radii, and their Debye lengths. The characteristic timescales include the expansion time, the collision times, and the periods associated with gyration, waves, and oscillations. We review the past and present research into the multi-scale nature of the solar wind based on in-situ spacecraft measurements and plasma theory. We emphasize that couplings of processes across scales are important for the global dynamics and thermodynamics of the solar wind. We describe methods to measure in-situ properties of particles and fields. We then discuss the role of expansion effects, non-equilibrium distribution functions, collisions, waves, turbulence, and kinetic microinstabilities for the multi-scale plasma evolution.
According to emph{Wind} observations between June 2004 and May 2019, this Letter investigates the proton and alpha particle temperatures in the space of ($theta_d$, $V_d/V_A$) for the first time, where $theta_d$ and $V_d$ are the radial angle and magnitude of alpha$-$proton differential flow vector respectively, $V_A$ is the local Alfven speed. Results show that the temperatures significantly depend on $theta_d$ as well as $V_d/V_A$. In case of low proton parallel beta ($beta_{p{parallel}} < 1$), it is found that the proton perpendicular temperature is clearly enhanced when $theta_d$ is small ($lesssim 45^circ$) and $V_d/V_A gtrsim 0.5$. On the contrary, the perpendicular temperature of alpha particles is considerably enhanced when $theta_d$ is large ($gtrsim 90^circ$) or $V_d/V_A$ is sufficiently small. The maximum of proton parallel temperature takes place at $theta_d sim 90^circ$ accompanied by higher $beta_{p{parallel}}$ and by larger turbulence amplitude of magnetic fluctuations in inertial range. This study should present strong evidence for cyclotron resonance heating of protons and alpha particles in the solar wind. Other mechanisms including Landau resonance and stochastic heating are also proposed, which tend to have different ($theta_d$, $V_d/V_A$) spaces than cyclotron resonance heating.
The solar wind undergoes significant heating as it propagates away from the Sun; the exact mechanisms responsible for this heating are not yet fully understood. We present for the first time a statistical test for one of the proposed mechanisms, stochastic ion heating. We use the amplitude of magnetic field fluctuations near the proton gyroscale as a proxy for the ratio of gyroscale velocity fluctuations to perpendicular (with respect to the magnetic field) proton thermal speed, defined as $epsilon_p$. Enhanced proton temperatures are observed when $epsilon_p$ is larger than a critical value ($sim 0.019 - 0.025$). This enhancement strongly depends on the proton plasma beta ($beta_{||p}$); when $beta_{||p} ll 1$ only the perpendicular proton temperature $T_{perp}$ increases, while for $beta_{||p} sim 1$ increased parallel and perpendicular proton temperatures are both observed. For $epsilon_p$ smaller than the critical value and $beta_{||p} ll 1$ no enhancement of $T_p$ is observed while for $beta_{||p} sim 1$ minor increases in $T_{parallel}$ are measured. The observed change of proton temperatures across a critical threshold for velocity fluctuations is in agreement with the stochastic ion heating model of Chandran et al. (2010). We find that $epsilon_p > epsilon_{rm crit}$ in 76% of the studied periods implying that stochastic heating may operate most of the time in the solar wind at 1 AU.
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