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Abstract We examine Ulysses magnetic field observations from 1993 to 1996 as the spacecraft made its first fast-latitude scan from the southern to the northern hemisphere. Most of the observations we use are representative of high-latitude solar minimum conditions. We examine magnetic field power spectra characteristics of interplanetary turbulence at high frequencies, where the spectrum breaks from an inertial range into the ion dissipation range. The onset and spectral index of the dissipation spectrum are consistent with low-latitude observations at 1 au. Both ranges have a ratio of power in perpendicular magnetic field components to parallel components near 3. The power spectrum ratio test developed by Bieber et al. for single-spacecraft analyses that determines the underlying anisotropy of the wave vectors yields only marginally more energy associated with field-aligned wave vectors than perpendicular wave vectors when comparing the inertial and dissipation-range spectra. The lack of significant change in the anisotropies between the inertial and dissipation ranges contrasts strongly with the turbulence found typically for 1 au near-ecliptic observations, where significant differences in both anisotropies are observed. 

Abstract Observational data at heliocentric distances of tens of solar radii suggest that fast magnetosonic modes make up a considerable fraction of the solar wind fluctuations. Furthermore, this fraction appears to increase closer to the Sun. We carry out three-dimensional kinetic simulations with particle ions and fluid electrons to evaluate the proton and alpha-particle heating produced by the damping of the fast waves in the solar corona. Realistic parameters at 5 solar radii, including the fluctuation amplitude, are used. We show that, due to the cyclotron resonance, the alphas are heated preferentially perpendicularly to the magnetic field and much more strongly than the protons. The presence of the alpha particles alters the energy partition by reducing the heating of the protons. Nevertheless, the proton heating is sufficient to account for the solar wind acceleration. 

Abstract We investigate a secondary proton beam instability coexisting with the ambient solar wind turbulence at 50R_☉. Three-dimensional hybrid numerical simulations (particle ions and a quasi-neutralizing electron fluid) are carried out with the plasma parameters in the observed range. In the turbulent background, the particle distribution function, in particular the slope of the “bump-on-tail” responsible for the instability, is time-dependent and inhomogeneous. The presence of the turbulence substantially reduces the growth rate and saturation level of the instability. We derive magnetic power spectra from the observational data and perform a statistical analysis to evaluate the average turbulence intensity at 50R_☉. This information is used to link the observed frequency spectrum to the wavenumber spectrum in the simulations. We verify that Taylor’s frozen-in hypothesis is valid for this purpose to a sufficient extent. To reproduce the typical magnetic power spectrum of the instability observed concurrently with the background turbulence, an artificial spacecraft probe is run through the simulation box. The thermal-ion instabilities are often seen as power elevations in the kinetic range of scales above an extrapolation of the turbulence spectrum from larger scales. We show that the elevated power in the simulations is much higher than the background level. Therefore, the turbulence at the average intensity does not obscure the secondary proton beam instability, as opposed to the solar wind at 1 au, in which the ambient turbulence typically obscures thermal-ion instabilities. 

Abstract We revisit the question of how the unstable scattering of interstellar pickup ions (PUIs) may drive turbulence in the outer solar wind and why the energy released into fluctuations by this scattering appears to be significantly less than the standard bispherical prediction. We suggest that energization of the newly picked-up ions by the ambient turbulence during the scattering process can result in a more spherical distribution of PUIs and reduce the generated fluctuation energy to a level consistent with the observations of turbulent intensities and core solar wind heating. This scenario implies the operation of a self-regulation mechanism that maintains the observed conditions of turbulence and heating in the PUI-dominated solar wind. 

Abstract Interstellar neutral atoms enter the heliosphere at a relatively slow speed corresponding to the motion of the Sun through the local interstellar medium, which is approximately 25 km s⁻¹. Neutral hydrogen atoms enter from the approximate location of the Voyager spacecraft and are eventually ionized primarily by collision with thermal solar wind ions. An earlier analysis by Hollick et al. examined low-frequency magnetic waves observed by the Voyager spacecraft from launch through 1990 that are thought to arise from the scattering of newborn interstellar pickup H⁺and He⁺. We report an analysis of Voyager 1 observations in 1991, which is the last year of high-resolution magnetic field data that are publicly available, and find 70 examples of low-frequency waves with the characteristics that suggest excitation by pickup H⁺and 10 examples of waves consistent with excitation by pickup He⁺. We find a particularly dense cluster of observations at the tail end of what is thought to be a Merged Interaction Region (MIR) that was previously studied by Burlaga & Ness using Voyager 2 observations. This is not unexpected if the MIR is followed by a large rarefaction region, as they tend to be regions of reduced turbulence levels that permit the growth of the waves over the long time periods that are generally required of this instability. 

Abstract Using high-resolution data from Solar Orbiter, we investigate the plasma conditions necessary for the proton temperature-anisotropy-driven mirror-mode and oblique firehose instabilities to occur in the solar wind. We find that the unstable plasma exhibits dependencies on the angle between the direction of the magnetic field and the bulk solar wind velocity which cannot be explained by the double-adiabatic expansion of the solar wind alone. The angle dependencies suggest that perpendicular heating in Alfvénic wind may be responsible. We quantify the occurrence rate of the two instabilities as a function of the length of unstable intervals as they are convected over the spacecraft. This analysis indicates that mirror-mode and oblique firehose instabilities require a spatial interval of length greater than 2–3 unstable wavelengths in order to relax the plasma into a marginally stable state and thus closer to thermodynamic equilibrium in the solar wind. Our analysis suggests that the conditions for these instabilities to act effectively vary locally on scales much shorter than the correlation length of solar wind turbulence. 

Abstract The proton–alpha drift instability is a possible mechanism of the alpha-particle deceleration and the resulting proton heating in the solar wind. We present hybrid numerical simulations of this instability with particle-in-cell ions and a quasi-neutralizing electron fluid for typical conditions at 1 au. For the parameters used in this paper, we find that fast magnetosonic unstable modes propagate only in the direction opposite to the alpha-particle drift and do not produce the perpendicular proton heating necessary to accelerate the solar wind. Alfvén modes propagate in both directions and heat the protons perpendicularly to the mean magnetic field. Despite being driven by the alpha temperature anisotropy, the Alfvén instability also extracts the energy from the bulk motion of the alpha particles. In the solar wind, the instabilities operate in a turbulent ambient medium. We show that the turbulence suppresses the Alfvén instability but the perpendicular proton heating persists. Unlike a static nonuniform background, the turbulence does not invert the sense of the proton heating associated with the fast magnetosonic instability and it remains preferentially parallel. 

Abstract We have surveyed magnetic field data from the Ulysses spacecraft and found examples of magnetic waves with the expected characteristics that point to excitation by newborn pickup He⁺. With interstellar neutrals as the likely source for the pickup ions, we have modeled the ion production rates and used them to produce wave excitation rates that we compare to the background turbulence rates. The source ions are thought to be always present, but the waves are seen when growth rates are comparable to or exceed the turbulence rates. With the exception of the fast latitude scans, and unlike the waves excited by newborn interstellar pickup H⁺, the waves are seen throughout the Ulysses orbit. 

Where and under what conditions the transfer of energy between electromagnetic fields and particles takes place in the solar wind remains an open question. We investigate the conditions that promote the growth of kinetic instabilities predicted by linear theory to infer how turbulence and temperature-anisotropy-driven instabilities are interrelated. Using a large dataset from Solar Orbiter, we introduce the radial rate of strain, a novel measure computed from single-spacecraft data, which we interpret as a proxy for the double-adiabatic strain rate. The solar wind exhibits high absolute values of the radial rate of strain at locations with large temperature anisotropy. We measure the kurtosis and skewness of the radial rate of strain from the statistical moments to show that it is non-Gaussian for unstable intervals and increasingly intermittent at smaller scales with a power-law scaling. We conclude that the velocity field fluctuations in the solar wind contribute to the presence of temperature anisotropy sufficient to create potentially unstable conditions. 

At kinetic scales in the solar wind, instabilities transfer energy from particles to fluctuations in the electromagnetic fields while restoring plasma conditions towards thermodynamic equilibrium. We investigate the interplay between background turbulent fluctuations at the small-scale end of the inertial range and kinetic instabilities acting to reduce proton temperature anisotropy. We analyse in situ solar wind observations from the Solar Orbiter mission to develop a measure for variability in the magnetic field direction. We find that non-equilibrium conditions sufficient to cause micro-instabilities in the plasma coincide with elevated levels of variability. We show that our measure for the fluctuations in the magnetic field is non-ergodic in regions unstable to the growth of temperature anisotropy-driven instabilities. We conclude that the competition between the action of the turbulence and the instabilities plays a significant role in the regulation of the proton-scale energetics of the solar wind. This competition depends not only on the variability of the magnetic field but also on the spatial persistence of the plasma in non-equilibrium conditions.