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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.

 

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.

 

Some of the most common processes in the solar wind, such as turbulence and wave generation by instabilities, are associated with spectral magnetic helicity. Therefore, the helicity is a convenient tool to investigate these processes. We use three-dimensional nonlinear kinetic simulations with particle ions and fluid electrons to analyze the magnetic helicity produced by proton temperature anisotropy instabilities coexisting with an ambient turbulence. The symmetry of the unstable system is violated by alpha-particle streaming with respect to protons along the mean magnetic field. At the same time, the turbulent fluctuations are also imbalanced by a nonzero cross-helicity. We show that in the nonlinear phase of the instability the resulting helicity structure is different from the prediction of the linear theory. In particular, it contains sign reversals and multiple domains of nonzero helicity. The turbulence generates its own magnetic helicity signature, which extends over a wide range of angles around the direction perpendicular to the mean magnetic field, and can have a sign the same as or opposite to that of the instability. These findings are consistent with the observed helicity spectra in the solar wind.

 

Abstract We perform a statistical analysis of observed magnetic spectra in the solar wind at 1 au with localized power elevations above the level of the ambient turbulent fluctuations. We show that the elevations are seen only when the intensity of the ambient fluctuations is sufficiently low. Assuming that the spectral elevations are caused by thermal-ion instabilities, this suggests that on average the effect of the solar wind background is strong enough to suppress the instability or obscure it or both. We then carry out nonlinear numerical simulations with particle ions and an electron fluid to model a thermal-ion instability coexisting with an ambient turbulence. The parameters of the simulation are taken from a known solar wind interval where an instability was assumed to exist based on the linear theory and a bi-Maxwellian fit of the observed distribution with core and secondary-beam protons. The numerical model closely matches the position of the observed spectral elevation in the wavenumber space. This confirms that the thermal-ion instability is responsible for the elevation. At the same time, the magnitude of the elevation turns out to be smaller than in the real solar wind. When higher intensity of the turbulence is used in the simulation, which is typical of solar wind in general, the power elevation is no longer seen. This is in agreement with the reduced observability of the elevations at higher intensities. However, the simulations show that the turbulence does not simply obscure the instability but also lowers its saturation level. 

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.

 

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.

 

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 consider the firehose instability coexisting with the omnipresent ambient solar wind turbulence. The characteristic temporal and spatial scales of the turbulence are comparable to those of the instability. Therefore, turbulence may violate the common assumption of a uniform and stationary background used to describe instabilities and make the properties of the instabilities different. To investigate this effect, we perform three-dimensional hybrid simulations with particle-in-cell ions and a quasi-neutralizing electron fluid. We find that the turbulence significantly reduces the growth rates and saturation levels of both instabilities. Comparing the cases with and without turbulence, the former results in a higher temperature anisotropy in the asymptotic marginally stable state at large times. In the former case, the distribution function averaged over the simulation box is also closer to the initial one. 

Abstract Three-dimensional hybrid kinetic simulations are conducted with particle protons and warm fluid electrons. Alfvénic fluctuations initialized at large scales and with wavevectors that are highly oblique with respect to the background magnetic field evolve into a turbulent energy cascade that dissipates at proton kinetic scales. Accompanying the proton scales is a spectral magnetic helicity signature with a peak in magnitude. A series of simulation runs are made with different large-scale cross helicity and different initial fluctuation phases and wavevector configurations. From the simulations a so-called total magnetic helicity peak is evaluated by summing contributions at a wavenumber perpendicular to the background magnetic field. The total is then compared with the reduced magnetic helicity calculated along spacecraft-like trajectories through the simulation box. The reduced combines the helicity from different perpendicular wavenumbers and depends on the sampling direction. The total is then the better physical quantity to characterize the turbulence. On average the ratio of reduced to total is 0.45. The total magnetic helicity and the reduced magnetic helicity show intrinsic variability based on initial fluctuation conditions. This variability can contribute to the scatter found in the observed distribution of solar wind reduced magnetic helicity as a function of cross helicity.