Abstract
In this Letter, we report observations of magnetic switchback (SB) features near 1 au using data from the Wind spacecraft. These features appear to be strikingly similar to the ones observed ...by the Parker Solar Probe mission closer to the Sun: namely, one-sided spikes (or enhancements) in the solar-wind bulk speed
V
that correlate/anticorrelate with the spikes seen in the radial-field component
B
R
. In the solar-wind streams that we analyzed, these specific SB features near 1 au are associated with large-amplitude Alfvénic oscillations that propagate outward from the Sun along a local background (prevalent) magnetic field
B
0
that is nearly radial. We also show that, when
B
0
is nearly perpendicular to the radial direction, the large-amplitude Alfvénic oscillations display variations in
V
that are two sided (i.e.,
V
alternately increases and decreases depending on the vector Δ
B
=
B
−
B
0
). As a consequence, SBs may not always appear as one-sided spikes in
V
, especially at larger heliocentric distances where the local background field statistically departs from the radial direction. We suggest that SBs can be well described by large-amplitude Alfvénic fluctuations if the field rotation is computed with respect to a well-determined local background field that, in some cases, may deviate from the large-scale Parker field.
The deceleration of alpha particle observed in the fast solar wind can contribute to the plasma heating between 0.3 and 1 au. The observational data suggest that the energy released from the ...deceleration has to be channeled to perpendicular heating of the protons. A possible mechanism of the energy conversion is a proton-alpha drift instability. We present hybrid numerical simulations of this instability in a warm plasma with particle-in-cell ions and a neutralizing electron fluid. The parallel temperature of the alpha particles is assumed to be larger than the perpendicular temperature. This sense of the anisotropy makes parallel-propagating fast magnetosonic waves the most easily excited modes. For typical ion beta values at 0.3 to 1 au, we find that the instability does not produce evident perpendicular heating of the protons if the initial background plasma is uniform. The lack of the heating is related to inefficient cyclotron interaction of the protons with the parallel-propagating fast modes. However, the background plasma in the solar wind is unlikely to be uniform. We consider the background variations across the mean magnetic field in the form of single or multiple equilibrium structures. The inhomogeneity modifies the unstable waves by making them oblique. Furthermore, their wavenumber spectrum extends to perpendicular wavenumbers of the order of the inverse proton gyroradius. Such waves can interact with the protons more efficiently. We show that significant and preferentially perpendicular heating of the protons is present in the nonuniform plasma.
We investigate the conditions under which parallel-propagating Alfven/ion-cyclotron waves are driven unstable by an isotropic (T sub(perpendicular alpha ) = T sub(|| alpha )) population of alpha ...particles drifting parallel to the magnetic field at an average speed U alpha with respect to the protons. We derive an approximate analytic condition for the minimum value of U alpha needed to excite this instability and refine this result using numerical solutions to the hot-plasma dispersion relation. When the alpha-particle number density is Asymptotically = to5% of the proton number density and the two species have similar thermal speeds, the instability requires that beta sub(p) > ~ 1, where beta sub(p) is the ratio of the proton pressure to the magnetic pressure. For 1 <, ~ beta sub(p) <, ~ 12, the minimum U sub( alpha ) needed to excite this instability ranges from 0.7v sub(A) to 0.9v sub(A), where v sub(A) is the Alfven speed. This threshold is smaller than the threshold of 1.2v sub(A) for the parallel magnetosonic instability, which was previously thought to have the lowest threshold of the alpha-particle beam instabilities at beta sub(p) > ~ 0.5. We discuss the role of the parallel Alfvenic drift instability for the evolution of the alpha-particle drift speed in the solar wind. We also analyze measurements from the Wind spacecraft's Faraday cups and show that the U sub( alpha ) values measured in solar-wind streams with T sub(perpendicular alpha ) approximately T sub(|| alpha ) are approximately bounded from above by the threshold of the parallel Alfvenic instability.
ABSTRACT Protons and alpha particles in the fast solar wind are only weakly collisional and exhibit a number of non-equilibrium features, including relative drifts between particle species. Two ...non-collisional mechanisms have been proposed for limiting differential flow between alpha particles and protons: plasma instabilities and the rotational force. Both mechanisms decelerate the alpha particles. In this paper, we derive an analytic expression for the rate at which energy is released by alpha-particle deceleration, accounting for azimuthal flow and conservation of total momentum. We show that instabilities control the deceleration of alpha particles at , and the rotational force controls the deceleration of alpha particles at , where in the fast solar wind in the ecliptic plane. We find that is positive at and at , consistent with the previous finding that the rotational force does not lead to a release of energy. We compare the value of at with empirical heating rates for protons and alpha particles, denoted and , deduced from in situ measurements of fast-wind streams from the Helios and Ulysses spacecraft. We find that exceeds at , and that decreases with increasing distance from the Sun from a value of about one at r = 0.29-0.42 AU to about 1/4 at 1 AU. We conclude that the continuous energy input from alpha-particle deceleration at makes an important contribution to the heating of the fast solar wind. We also discuss the implications of the alpha-particle drift for the azimuthal flow velocities of the ions and for the Parker spiral magnetic field.
ABSTRACT The Solar Probe Plus (SPP) spacecraft will explore the near-Sun environment, reaching heliocentric distances less than . Near Earth, spacecraft measurements of fluctuating velocities and ...magnetic fields taken in the time domain are translated into information about the spatial structure of the solar wind via Taylor's "frozen turbulence" hypothesis. Near the perihelion of SPP, however, the solar-wind speed is comparable to the Alfvén speed, and Taylor's hypothesis in its usual form does not apply. In this paper, we show that under certain assumptions, a modified version of Taylor's hypothesis can be recovered in the near-Sun region. We consider only the transverse, non-compressive component of the fluctuations at length scales exceeding the proton gyroradius, and we describe these fluctuations using an approximate theoretical framework developed by Heinemann and Olbert. We show that fluctuations propagating away from the Sun in the plasma frame obey a relation analogous to Taylor's hypothesis when and , where is the component of the spacecraft velocity perpendicular to the mean magnetic field and ( ) is the Elsasser variable corresponding to transverse, non-compressive fluctuations propagating away from (toward) the Sun in the plasma frame. Observations and simulations suggest that, in the near-Sun solar wind, the above inequalities are satisfied and fluctuations account for most of the fluctuation energy. The modified form of Taylor's hypothesis that we derive may thus make it possible to characterize the spatial structure of the energetically dominant component of the turbulence encountered by SPP.
The heating, acceleration, and pitch-angle scattering of charged particles by magnetohydrodynamic (MHD) turbulence are important in a wide range of astrophysical environments, including the solar ...wind, accreting black holes, and galaxy clusters. We simulate the interaction of high-gyrofrequency test particles with fully dynamical simulations of subsonic MHD turbulence, focusing on the parameter regime with beta ~ 1, where beta is the ratio of gas to magnetic pressure. We use the simulation results to calibrate analytical expressions for test particle velocity-space diffusion coefficients and provide simple fits that can be used in other work. The test particle velocity diffusion in our simulations is due to a combination of two processes: interactions between particles and magnetic compressions in the turbulence (as in linear transit-time damping; TTD) and what we refer to as Fermi Type-B (FTB) interactions, in which charged particles moving on field lines may be thought of as beads sliding along moving wires. We show that test particle heating rates are consistent with a TTD resonance that is broadened according to a decorrelation prescription that is Gaussian in time (but inconsistent with Lorentzian broadening due to an exponential decorrelation function, a prescription widely used in the literature). TTD dominates the heating for upsilonS >> upsilonA (e.g., electrons), where upsilonS is the thermal speed of species S and upsilonA is the Alfven speed, while FTB dominates for upsilonS << upsilonA (e.g., minor ions). Proton heating rates for beta ~ 1 are comparable to the turbulent cascade rate. Finally, we show that velocity diffusion of collisionless, large gyrofrequency particles due to large-scale MHD turbulence does not produce a power-law distribution function.
Three-dimensional numerical hybrid simulations with particle protons and quasi-neutralizing fluid electrons are conducted for a freely decaying turbulence that is anisotropic with respect to the ...background magnetic field. The turbulence evolution is determined by both the combined root-mean-square (rms) amplitude for fluctuating proton bulk velocity and magnetic field and by the ratio of perpendicular to parallel wavenumbers. This kind of relationship had been considered in the past with regard to interplanetary turbulence. The fluctuations nonlinearly evolve to a turbulent phase whose net wave vector anisotropy is usually more perpendicular than the initial one, irrespective of the initial ratio of perpendicular to parallel wavenumbers. Self-similar anisotropy evolution is found as a function of the rms amplitude and parallel wavenumber. Proton heating rates in the turbulent phase vary strongly with the rms amplitude but only weakly with the initial wave vector anisotropy. Even in the limit where wave vectors are confined to the plane perpendicular to the background magnetic field, the heating rate remains close to the corresponding case with finite parallel wave vector components. Simulation results obtained as a function of proton plasma to background magnetic pressure ratio beta sub(p) in the range 0.1-0.5 show that the wave vector anisotropy also weakly depends on beta sub(p).
Previous studies have shown that the observed temperature anisotropies of protons and alpha particles in the solar wind are constrained by theoretical thresholds for pressure and anisotropy driven ...instabilities such as the Alfven/ion-cyclotron (A/IC) and fast-magnetosonic/whistler (FM/W) instabilities. In this Letter, we use a long period of in situ measurements provided by the Wind spacecraft's Faraday cups to investigate the combined constraint on the alpha proton differential flow velocity and the alpha particle temperature anisotropy due to A/IC and FM/W instabilities. We show that the majority of the data are constrained to lie within the region of parameter space in which A/IC and FM/W waves are either stable or have extremely low growth rates. In the minority of observed cases in which the growth rate of the A/IC (FM/W) instability is comparatively large, we find relatively higher values of T sub(perpendicular alpha )/T sub(perpendicularp) (T sub(|| alpha )/T sub(||p)) when the alpha proton differential flow velocity is small, where T sub(perpendicular alpha ) and T sub(perpendicularp) (T sub(|| alpha )a and T sub(||p)) are the perpendicular (parallel) temperatures of alpha particles and protons. We conjecture that this observed feature might arise from preferential alpha particle heating which can drive the alpha particles beyond the instability thresholds.
We present two-dimensional hybrid simulations of proton-cyclotron and mirror instabilities in a proton-alpha plasma with particle-in-cell ions and a neutralizing electron fluid. The instabilities are ...driven by the protons with temperature perpendicular to the background magnetic field larger than the parallel temperature. The alpha particles with initially isotropic temperature have a nonzero drift speed with respect to the protons. The minor ions are known to influence the relative effect of the proton-cyclotron and mirror instabilities. In this paper, we show that the mirror mode can dominate the power spectrum at the nonlinear stage even if its linear growth rate is significantly lower than that of the proton-cyclotron mode. The proton-cyclotron instability combined with the alpha-proton drift is a possible cause of the nonzero magnetic helicity observed in the solar wind for fluctuations propagating nearly parallel to the magnetic field. Our simulations generally confirm this concept but reveal a complex helicity spectrum that is not anticipated from the linear theory of the instability.
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