Abstract
One of the most important early results from the Parker Solar Probe (PSP) is the ubiquitous presence of magnetic switchbacks, whose origin is under debate. Using a three-dimensional direct ...numerical simulation of the equations of compressible magnetohydrodynamics from the corona to 40 solar radii, we investigate whether magnetic switchbacks emerge from granulation-driven Alfvén waves and turbulence in the solar wind. The simulated solar wind is an Alfvénic slow-solar-wind stream with a radial profile consistent with various observations, including observations from PSP. As a natural consequence of Alfvén-wave turbulence, the simulation reproduced magnetic switchbacks with many of the same properties as observed switchbacks, including Alfvénic
v
–
b
correlation, spherical polarization (low magnetic compressibility), and a volume filling fraction that increases with radial distance. The analysis of propagation speed and scale length shows that the magnetic switchbacks are large-amplitude (nonlinear) Alfvén waves with discontinuities in the magnetic-field direction. We directly compare our simulation with observations using a virtual flyby of PSP in our simulation domain. We conclude that at least some of the switchbacks observed by PSP are a natural consequence of the growth in amplitude of spherically polarized Alfvén waves as they propagate away from the Sun.
We present three-dimensional, hybrid-kinetic numerical simulations of driven Alfvén-wave turbulence of relevance to the collisionless near-Earth solar wind. Special attention is paid to the spectral ...transition that occurs near the ion-Larmor scale and to the origins of preferential perpendicular ion heating and of nonthermal wings in the parallel distribution function. Several novel diagnostics are used to show that the ion heating rate increases as the kinetic-Alfvén-wave fluctuations, which comprise the majority of the sub-ion-Larmor turbulent cascade, attain near-ion-cyclotron frequencies. We find that 75%-80% of the cascade energy goes into heating the ions, broadly consistent with the near-Earth solar wind. This heating is accompanied by clear velocity-space signatures in the particle energization rates and the distribution functions, including a flattened core in the perpendicular-velocity distribution and non-Maxwellian wings in the parallel-velocity distribution. The latter are attributed to transit-time damping and the pitch-angle scattering of perpendicularly heated particles into the parallel direction. Accompanying these features is a steepening of the spectral index of sub-ion-Larmor magnetic-field fluctuations beyond the canonical −2.8, as field energy is transferred to thermal energy. These predictions may be tested by measurements in the near-Earth 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 βc regimes, where βc 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-βc conditions when , where U0s is the strahl speed and wc 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 βc ∼ 1. The comparison of our theoretical results with data from the Wind spacecraft confirms the relevance of the oblique FM/W instability for the solar wind. The whistler heat-flux, ion-acoustic heat-flux, kinetic-Alfvén-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 Parker Solar Probe and Solar Orbiter.
We study the propagation, reflection, and turbulent dissipation of Alfven waves in coronal holes and the solar wind. We start with the Heinemann-Olbert equations, which describe non-compressive ...magnetohydrodynamic fluctuations in an inhomogeneous medium with a background flow parallel to the background magnetic field. Following the approach of Dmitruk et al., we model the nonlinear terms in these equations using a simple phenomenology for the cascade and dissipation of wave energy and assume that there is much more energy in waves propagating away from the Sun than waves propagating toward the Sun. We then solve the equations analytically for waves with periods of hours and longer to obtain expressions for the wave amplitudes and turbulent heating rate as a function of heliocentric distance. We also develop a second approximate model that includes waves with periods of roughly one minute to one hour, which undergo less reflection than the longer-period waves, and compare our models to observations. Our models generalize the phenomenological model of Dmitruk et al. by accounting for the solar wind velocity, so that the turbulent heating rate can be evaluated from the coronal base out past the Alfven critical point-that is, throughout the region in which most of the heating and acceleration occurs. The simple analytical expressions that we obtain can be used to incorporate Alfven-wave reflection and turbulent heating into fluid models of the solar wind.
ABSTRACT Compressive fluctuations are a minor yet significant component of astrophysical plasma turbulence. In the solar wind, long-wavelength compressive slow-mode fluctuations lead to changes in ...and in , where and are the perpendicular and parallel temperatures of the protons, B is the magnetic field strength, and is the proton density. If the amplitude of the compressive fluctuations is large enough, crosses one or more instability thresholds for anisotropy-driven microinstabilities. The enhanced field fluctuations from these microinstabilities scatter the protons so as to reduce the anisotropy of the pressure tensor. We propose that this scattering drives the average value of away from the marginal stability boundary until the fluctuating value of stops crossing the boundary. We model this "fluctuating-anisotropy effect" using linear Vlasov-Maxwell theory to describe the large-scale compressive fluctuations. We argue that this effect can explain why, in the nearly collisionless solar wind, the average value of is close to unity.
We consider ion heating by turbulent Alfven waves (AWs) and kinetic Alfven waves (KAWs) with wavelengths (measured perpendicular to the magnetic field) that are comparable to the ion gyroradius and ...frequencies {omega} smaller than the ion cyclotron frequency {Omega}. We focus on plasmas in which {beta} {approx}< 1, where {beta} is the ratio of plasma pressure to magnetic pressure. As in previous studies, we find that when the turbulence amplitude exceeds a certain threshold, an ion's orbit becomes chaotic. The ion then interacts stochastically with the time-varying electrostatic potential, and the ion's energy undergoes a random walk. Using phenomenological arguments, we derive an analytic expression for the rates at which different ion species are heated, which we test by simulating test particles interacting with a spectrum of randomly phased AWs and KAWs. We find that the stochastic heating rate depends sensitively on the quantity {epsilon} = {delta}v {sub {rho}/}v{sub perpendicular}, where v{sub perpendicular} (v {sub ||}) is the component of the ion velocity perpendicular (parallel) to the background magnetic field B {sub 0}, and {delta}v {sub {rho}} ({delta}B {sub {rho}}) is the rms amplitude of the velocity (magnetic-field) fluctuations at the gyroradius scale. In the case of thermal protons, when {epsilon} << {epsilon}{sub crit}, where {epsilon}{sub crit} is a constant, a proton's magnetic moment is nearly conserved and stochastic heating is extremely weak. However, when {epsilon}>{epsilon}{sub crit}, the proton heating rate exceeds half the cascade power that would be present in strong balanced KAW turbulence with the same value of {delta}v {sub {rho}}, and magnetic-moment conservation is violated even when {omega} << {Omega}. For the random-phase waves in our test-particle simulations, {epsilon}{sub crit} = 0.19. For protons in low-{beta} plasmas, {epsilon} {approx_equal} {beta}{sup -1/2{delta}}B{sub {rho}/}B {sub 0}, and {epsilon} can exceed {epsilon}{sub crit} even when {delta}B{sub {rho}/}B {sub 0} << {epsilon}{sub crit}. The heating is anisotropic, increasing v {sup 2}{sub perpendicular} much more than v {sup 2}{sub ||} when {beta} << 1. (In contrast, at {beta} {approx}> 1 Landau damping and transit-time damping of KAWs lead to strong parallel heating of protons.) At comparable temperatures, alpha particles and minor ions have larger values of {epsilon} than protons and are heated more efficiently as a result. We discuss the implications of our results for ion heating in coronal holes and the solar wind.
This paper proposes a new phenomenology for strong incompressible MHD turbulence with nonzero cross helicity. This phenomenology is then developed into a quantitative Fokker-Planck model that ...describes the time evolution of the anisotropic power spectra of the fluctuations propagating parallel and antiparallel to the background magnetic field image. It is found that in steady state the power spectra of the magnetic field and total energy are steeper than image and become increasingly steep as image increases, where image is the cross helicity, image is the fluctuation energy, and image is the wavevector component perpendicular to image. Increasing C with fixed image increases the time required for energy to cascade to smaller scales, reduces the cascade power, and increases the anisotropy of the small-scale fluctuations. The implications of these results for the solar wind and solar corona are discussed in some detail.
The dissipation of magnetized turbulence is an important paradigm for describing heating and energy transfer in astrophysical environments such as the solar corona and wind; however, the specific ...collisionless processes behind dissipation and heating remain relatively unconstrained by measurements. Remote sensing observations have suggested the presence of strong temperature anisotropy in the solar corona consistent with cyclotron resonant heating. In the solar wind, in situ magnetic field measurements reveal the presence of cyclotron waves, while measured ion velocity distribution functions have hinted at the active presence of cyclotron resonance. Here, we present Parker Solar Probe observations that connect the presence of ion-cyclotron waves directly to signatures of resonant damping in observed proton-velocity distributions using the framework of quasilinear theory. We show that the quasilinear evolution of the observed distribution functions should absorb the observed cyclotron wave population with a heating rate of 10^{-14} W/m^{3}, indicating significant heating of the solar wind.
Abstract
In this work, we investigate how the complex structure found in solar wind proton velocity distribution functions (VDFs), rather than the commonly assumed two-component bi-Maxwellian ...structure, affects the onset and evolution of parallel-propagating microinstabilities. We use the
Arbitrary Linear Plasma Solver
, a numerical dispersion solver, to find the real frequencies and growth/damping rates of the Alfvén modes calculated for proton VDFs extracted from Wind spacecraft observations of the solar wind. We compare this wave behavior to that obtained by applying the same procedure to core-and-beam bi-Maxwellian fits of the Wind proton VDFs. We find several significant differences in the plasma waves obtained for the extracted data and bi-Maxwellian fits, including a strong dependence of the growth/damping rate on the shape of the VDF. By applying the quasilinear diffusion operator to these VDFs, we pinpoint resonantly interacting regions in velocity space where differences in VDF structure significantly affect the wave growth and damping rates. This demonstration of the sensitive dependence of Alfvén mode behavior on VDF structure may explain why the Alfvén ion-cyclotron instability thresholds predicted by linear theory for bi-Maxwellian models of solar wind proton background VDFs do not entirely constrain spacecraft observations of solar wind proton VDFs, such as those made by the Wind spacecraft.
Low-frequency Alfven-wave turbulence causes ion trajectories to become chaotic, or 'stochastic', when the turbulence amplitude is sufficiently large. Stochastic orbits enable ions to absorb energy ...from the turbulence, increasing the perpendicular ion temperature T{sub perpendiculari} even when the fluctuation frequencies are too small for a cyclotron resonance to occur. In this paper, an analytic expression for the stochastic heating rate is used in conjunction with an observationally constrained turbulence model to obtain an analytic formula for T{sub perpendiculari} as a function of heliocentric distance r, ion mass, and ion charge in coronal holes at 2 R{sub sun} {approx}< r {approx}< 15 R{sub sun}. The resulting temperature profiles provide a good fit to observations of protons and O{sup +5} ions at 2 R {sub sun} {approx}< r {approx}< 3 R{sub sun} from the Ultraviolet Coronagraph Spectrometer (UVCS). Stochastic heating also offers a natural explanation for several detailed features of the UVCS observations, including the preferential and anisotropic heating of minor ions, the rapid radial increase in the O{sup +5} temperature between 1.6 R{sub sun} and 1.9 R{sub sun}, and the abrupt flattening of the O{sup +5} temperature profile as r increases above 1.9 R{sub sun}.
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