Magnetic Field Rotations in the Solar Wind at Kinetic Scales

The solar wind magnetic field contains rotations at a broad range of scales, which have been extensively studied in the MHD range. Here we present an extension of this analysis to the range between ion and electron kinetic scales. The distribution of rotation angles was found to be approximately log-normal, shifting to smaller angles at smaller scales almost self-similarly, but with small, statistically significant changes of shape. The fraction of energy in fluctuations with angles larger than $\alpha$ was found to drop approximately exponentially with $\alpha$, with e-folding angle $9.8^\circ$ at ion scales and $0.66^\circ$ at electron scales, showing that large angles ($\alpha > 30^\circ$) do not contain a significant amount of energy at kinetic scales. Implications for kinetic turbulence theory and the dissipation of solar wind turbulence are discussed

Ion kinetic energy conservation and magnetic field strength constancy in multi-fluid solar wind Alfv\'enic turbulence

We investigate properties of the plasma fluid motion in the large amplitude low frequency fluctuations of highly Alfv\'enic fast solar wind. We show that protons locally conserve total kinetic energy when observed from an effective frame of reference comoving with the fluctuations. For typical properties of the fast wind, this frame can be reasonably identified by alpha particles, which, owing to their drift with respect to protons at about the Alfv\'en speed along the magnetic field, do not partake in the fluid low frequency fluctuations. Using their velocity to transform proton velocity into the frame of Alfv\'enic turbulence, we demonstrate that the resulting plasma motion is characterized by a constant absolute value of the velocity, zero electric fields, and aligned velocity and magnetic field vectors as expected for unidirectional Alfv\'enic fluctuations in equilibrium. We propose that this constraint, via the correlation between velocity and magnetic field in Alfv\'enic turbulence, is at the origin of the observed constancy of the magnetic field: while the constant velocity corresponding to constant energy can be only observed in the frame of the fluctuations, the correspondingly constant total magnetic field, invariant for Galilean transformations, remains the observational signature, in the spacecraft frame, of the constant total energy in the Alfv\'en turbulence frame.Comment: 6 pages, 6 figures, Accepted for publication in The Astrophysical Journa

Experimental determination of whistler wave dispersion relation in the solar wind

The origins and properties of large-amplitude whistler wavepackets in the solar wind are still unclear. In this Letter, we utilize single spacecraft electric and magnetic field waveform measurements from the ARTEMIS mission to calculate the plasma frame frequency and wavevector of individual wavepackets over multiple intervals. This allows direct comparison of experimental measurements with theoretical dispersion relations to identify the observed waves as whistler waves. The whistlers are right-hand circularly polarized, travel anti-sunward, and are aligned with the background magnetic field. Their dispersion is strongly affected by the local electron parallel beta in agreement with linear theory. The properties measured are consistent with the electron heat flux instability acting in the solar wind to generate these waves

Radial evolution of the solar wind in pure high-speed streams: HELIOS revised observations

Spacecraft observations have shown that the proton temperature in the solar wind falls off with radial distance more slowly than expected for an adiabatic prediction. Usually, previous studies have been focused on the evolution of the solar-wind plasma by using the bulk speed as an order parameter to discriminate different regimes. In contrast, here, we study the radial evolution of pure and homogeneous fast streams (i.e. well-defined streams of coronal-hole plasma that maintain their identity during several solar rotations) by means of re-processed particle data, from the HELIOS satellites between 0.3 and 1 au. We have identified 16 intervals of unperturbed high-speed coronal-hole plasma, from three different sources and measured at different radial distances. The observations show that, for all three streams, (i) the proton density decreases as expected for a radially expanding plasma, unlike previous analysis that found a slower decrease; (ii) the magnetic field deviates from the Parker prediction, with the radial component decreasing more slowly and the tangential more quickly than expected; (iii) the double-adiabatic invariants are violated and an increase of entropy is observed; (iv) the collisional frequency is not constant, but decreases as the plasma travels away from the Sun. This work provides an insight into the heating problem in pure fast solar wind, fitting in the context of the next solar missions, and, especially for Parker Solar Probe, it enables us to predict the high-speed solar-wind environment much closer to the Sun

Scale dependence and cross-scale transfer of kinetic energy in compressible hydrodynamic turbulence at moderate Reynolds numbers

We investigate properties of the scale dependence and cross-scale transfer of kinetic energy in compressible three-dimensional hydrodynamic turbulence, by means of two direct numerical simulations of decaying turbulence with initial Mach numbers M = 1/3 and M = 1, and with moderate Reynolds numbers, R_lambda ~ 100. The turbulent dynamics is analyzed using compressible and incompressible versions of the dynamic spectral transfer (ST) and the Karman-Howarth-Monin (KHM) equations. We find that the nonlinear coupling leads to a flux of the kinetic energy to small scales where it is dissipated; at the same time, the reversible pressure-dilatation mechanism causes oscillatory exchanges between the kinetic and internal energies with an average zero net energy transfer. While the incompressible KHM and ST equations are not generally valid in the simulations, their compressible counterparts are well satisfied and describe, in a quantitatively similar way, the decay of the kinetic energy on large scales, the cross-scale energy transfer/cascade, the pressure dilatation, and the dissipation. There exists a simple relationship between the KHM and ST results through the inverse proportionality between the wave vector k and the spatial separation length l as k l ~ 3^1/2. For a given time the dissipation and pressure-dilatation terms are strong on large scales in the KHM approach whereas the ST terms become dominant on small scales; this is owing to the complementary cumulative behavior of the two methods. The effect of pressure dilatation is weak when averaged over a period of its oscillations and may lead to a transfer of the kinetic energy from large to small scales without a net exchange between the kinetic and internal energies. Our results suggest that for large-enough systems there exists an inertial range for the kinetic energy cascade ...Comment: 14 pages, 10 figure

Alpha particle thermodynamics in the inner heliosphere fast solar wind

Context. Plasma processes occurring in the corona and solar wind can be probed by studying the thermodynamic properties of different ion species. However, most in-situ observations of positive ions in the solar wind are taken at 1 AU, where information on their solar source properties may have been irreversibly erased. Aims. In this study we aimed to use the properties of alpha particles at heliocentric distances between 0.3 and 1 AU to study plasma processes occurring at the points of observation, and to infer processes occurring inside 0.3 AU by comparing our results to previous remote sensing observations of the plasma closer to the Sun. Methods. We reprocessed the original Helios positive ion distribution functions, isolated the alpha particle population, and computed the alpha particle number density, velocity, and magnetic field perpendicular and parallel temperatures. We then investigated the radial variation of alpha particle temperatures in fast solar wind observed between 0.3 and 1 AU. Results. Between 0.3 and 1 AU alpha particles are heated in the magnetic field perpendicular direction, and cooled in the magnetic field parallel direction. Alpha particle evolution is bounded by the alpha firehose instability threshold, which provides one possible mechanism to explain the observed parallel cooling and perpendicular heating. Closer to the Sun our observations suggest that the alpha particles undergo heating in the perpendicular direction, whilst the large magnetic field parallel temperatures observed at 0.3 AU may be due to the combined effect of double adiabatic expansion and alpha particle deceleration inside 0.3 AU

Plasma turbulence and kinetic instabilities at ion scales in the expanding solar wind

The relationship between a decaying strong turbulence and kinetic instabilities in a slowly expanding plasma is investigated using two-dimensional (2D) hybrid expanding box simulations. We impose an initial ambient magnetic field perpendicular to the simulation box, and we start with a spectrum of large-scale, linearly polarized, random-phase Alfvénic fluctuations that have energy equipartition between kinetic and magnetic fluctuations and vanishing correlation between the two fields. A turbulent cascade rapidly develops; magnetic field fluctuations exhibit a power-law spectrum at large scales and a steeper spectrum at ion scales. The turbulent cascade leads to an overall anisotropic proton heating, protons are heated in the perpendicular direction, and, initially, also in the parallel direction. The imposed expansion leads to generation of a large parallel proton temperature anisotropy which is at later stages partly reduced by turbulence. The turbulent heating is not sufficient to overcome the expansion-driven perpendicular cooling and the system eventually drives the oblique firehose instability in a form of localized nonlinear wave packets which efficiently reduce the parallel temperature anisotropy. This work demonstrates that kinetic instabilities may coexist with strong plasma turbulence even in a constrained 2D regime

Coherent deflection pattern and associated temperature enhancements in the near-Sun solar wind

Measurements of transverse magnetic field and velocity components from Parker Solar Probe have revealed a coherent quasi-periodic pattern in the near-Sun solar wind. As well as being Alfv\'enic and arc-polarised, these deflections were characterised by a consistent orientation and an increased proton core temperature, which was greater parallel to the magnetic field. We show that switchbacks represent the largest deflections within this underlying structure, which is itself consistent with the expected outflow from interchange reconnection simulations. Additionally, the spatial scale of the deflections was estimated to be around $1$\,Mm on the Sun, comparable to the jetting activity observed at coronal bright points within the base of coronal plumes. Therefore, our results could represent the in situ signature of interchange reconnection from coronal bright points within plumes, complementing recent numerical and observational studies. We also found a consistent relationship between the proton core temperature and magnetic field angle across the Parker Solar Probe encounters and discussed how such a persistent signature could be more indicative of an in situ mechanism creating a local increase in temperature. In future, observations of minor ions, radio bursts and remote sensing images could help further establish the connection between reconnection events on the Sun and signatures in the solar wind