Recent theoretical studies argue that the rate of stochastic ion heating in low-frequency Alfven-wave turbulence is given by Q sub(perpendicular) = c sub(1)(( delta u) super(3)/rho) exp(-c ...sub(2)/subset or is implied by), where delta u is the rms turbulent velocity at the scale of the ion gyroradius rho, subset or is implied by = delta u/v sub(perpendiculari), v sub(perpendiculari) is the perpendicular ion thermal speed, and c sub(1) and c sub(2) are dimensionless constants. We test this theoretical result by numerically simulating test particles interacting with strong reduced magnetohydrodynamic (RMHD) turbulence. The heating rates in our simulations are well fit by this formula. The best-fit values of c sub(1) are ~ 1. The best-fit values of c sub(2) decrease (i.e., stochastic heating becomes more effective) as the Reynolds number and the number of grid points in the RMHD simulations increase. As an example, in a 1024 super(2) x 256 RMHD simulation with a dissipation wavenumber of the order of the inverse ion gyroradius, we find c sub(2) = 0.21. We show that stochastic heating is significantly stronger in strong RMHD turbulence than in a field of randomly phased Alfven waves with the same power spectrum, because coherent structures in strong RMHD turbulence increase orbit stochasticity in the regions where ions are heated most strongly. We find that c sub(1) increases by a factor of ~3 while c sub(2) changes very little as the ion thermal speed increases from values << v sub(A) to values ~v sub(A), where v sub(A) is the Alfven speed. We discuss the importance of these results for perpendicular ion heating in the solar wind.
In this Letter, weak-turbulence theory is used to investigate interactions among Alfvén waves and fast and slow magnetosonic waves in collisionless low-beta plasmas. The wave kinetic equations are ...derived from the equations of magnetohydrodynamics, and extra terms are then added to model collisionless damping. These equations are used to provide a quantitative description of a variety of nonlinear processes, including parallel and perpendicular energy cascade, energy transfer between wave types, "phase mixing," and the generation of backscattered Alfvén waves.
We investigate the effects of pitch-angle scattering on the efficiency of particle heating and acceleration by MHD turbulence using phenomenological estimates and simulations of non-relativistic test ...particles interacting with strong, subsonic MHD turbulence. We include an imposed pitch-angle scattering rate, which is meant to approximate the effects of high-frequency plasma waves and/or velocity space instabilities. We focus on plasma parameters similar to those found in the near-Earth solar wind, though most of our results are more broadly applicable. An important control parameter is the size of the particle mean free path lambda sub(mfp) relative to the scale of the turbulent fluctuations L. For small scattering rates, particles interact quasi-resonantly with turbulent fluctuations in magnetic field strength. Scattering increases the long-term efficiency of this resonant heating by factors of a few times 10, but the distribution function does not develop a significant non-thermal power-law tail. For higher scattering rates, the interaction between particles and turbulent fluctuations becomes non-resonant, governed by particles heating and cooling adiabatically as they encounter turbulent density fluctuations. Rapid pitch-angle scattering can produce a power-law tail in the proton distribution function, but this requires fine-tuning of parameters. Moreover, in the near-Earth solar wind, a significant power-law tail cannot develop by this mechanism because the particle acceleration timescales are longer than the adiabatic cooling timescale set by the expansion of the solar wind. Our results thus imply that MHD-scale turbulent fluctuations are unlikely to be the origin of the v super(-5) tail in the proton distribution function observed in the solar wind.
Understanding the physical processes in the solar wind and corona that actively contribute to heating, acceleration, and dissipation is a primary objective of NASA's Parker Solar Probe (PSP) mission. ...Observations of circularly polarized electromagnetic waves at ion scales suggest that cyclotron resonance and wave-particle interactions are dynamically relevant in the inner heliosphere. A wavelet-based statistical study of circularly polarized events in the first perihelion encounter of PSP demonstrates that transverse electromagnetic waves at ion resonant scales are observed in 30-50% of radial field intervals. Average wave amplitudes of approximately 4 nT are measured, while the mean duration of wave events is on the order of 20 s; however, long-duration wave events can exist without interruption on hour-long timescales. Determination of wave vectors suggests propagation parallel/antiparallel to the mean magnetic field. Though ion-scale waves are preferentially observed during intervals with a radial mean magnetic field, we show that measurement constraints, associated with single spacecraft sampling of quasi-parallel waves superposed with anisotropic turbulence, render the measured coherent ion-wave spectrum unobservable when the mean magnetic field is oblique to the solar wind flow; these results imply that the occurrence of coherent ion-scale waves is not limited to a radial field configuration. The lack of radial scaling of characteristic wave amplitudes and duration suggests that the waves are generated in situ through plasma instabilities. Additionally, observations of proton distribution functions indicate that temperature anisotropy may drive the observed ion-scale waves.
We develop a model for stochastic acceleration of electrons in solar flares. As in several previous models, the electrons are accelerated by turbulent fast magnetosonic waves ("fast waves") via ...transit-time-damping (TTD) interactions. (In TTD interactions, fast waves act like moving magnetic mirrors that push the electrons parallel or anti-parallel to the magnetic field). We also include the effects of Coulomb collisions and the waves' parallel electric fields. Unlike previous models, our model is two-dimensional in both momentum space and wavenumber space and takes into account the anisotropy of the wave power spectrum F sub(k) and electron distribution function f sub(e). We use weak turbulence theory and quasilinear theory to obtain a set of equations that describes the coupled evolution of F sub(k) and f sub(e). We solve these equations numerically and find that the electron distribution function develops a power-law-like non-thermal tail within a restricted range of energies E isin (E sub(nt), E sub(max)). We obtain approximate analytic expressions for E sub(nt) and E sub(max), which describe how these minimum and maximum energies depend upon parameters such as the electron number density and the rate at which fast-wave energy is injected into the acceleration region at large scales. We contrast our results with previous studies that assume that F sub(k) and f sub(e) are isotropic, and we compare one of our numerical calculations with the time-dependent hard-X-ray spectrum observed during the 1980 June 27 flare. In our numerical calculations, the electron energy spectra are softer (steeper) than in models with isotropic F sub(k) and f sub(e) and closer to the values inferred from observations of solar flares.
Cross Helicity Reversals in Magnetic Switchbacks McManus, Michael D.; Bowen, Trevor A.; Mallet, Alfred ...
The Astrophysical journal. Supplement series,
02/2020, Letnik:
246, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We consider 2D joint distributions of normalized residual energy, r(s, t), and cross helicity, c(s, t), during one day of Parker Solar Probe's (PSP's) first encounter as a function of wavelet scale ...s. The broad features of the distributions are similar to previous observations made by Helios in slow solar wind, namely well-correlated and fairly Alfvénic wind, except for a population with negative cross helicity that is seen at shorter wavelet scales. We show that this population is due to the presence of magnetic switchbacks, or brief periods where the magnetic field polarity reverses. Such switchbacks have been observed before, both in Helios data and in Ulysses data in the polar solar wind. Their abundance and short timescales as seen by PSP in its first encounter is a new observation, and their precise origin is still unknown. By analyzing these MHD invariants as a function of the wavelet scale, we show that magnetohydrodynamic (MHD) waves do indeed follow the local mean magnetic field through switchbacks, with a net Elsässer flux propagating inward during the field reversal and that they, therefore, must be local kinks in the magnetic field and not due to small regions of opposite polarity on the surface of the Sun. Such observations are important to keep in mind as computing cross helicity without taking into account the effect of switchbacks may result in spurious underestimation of c as PSP gets closer to the Sun in later orbits.
PDF
