Arcadi V. Usmanov

An evolving turbulent flow like the solar wind can be meaningfully characterized by its “turbulence age”—an estimate of the number of nonlinear times that have elapsed during the plasma’s propagation from the Sun to the point of observation. Recent observations of the near-Sun solar wind by the Parker Solar Probe (PSP) indicate high correlation between velocity and magnetic fluctuations (i.e., cross helicity, σ _c ), which is known to impede development of magnetohydrodynamic (MHD) turbulence. Here we propose a new formulation of the solar wind’s turbulence age ( A _t ) that explicitly accounts for the Alfvénic nature of the fluctuations in the inner heliosphere. A _t is then evaluated for slow and fast wind streams using a variety of data sources—observations from the PSP, Advanced Composition Explorer, and Voyager missions, and a global solar wind simulation that includes turbulence transport. Compared to the formulation employed in previous work that neglected Alfvénicity, the present approach yields smaller values of A _t in medium-to-high σ _c solar wind; similar turbulence ages are then obtained for slow and fast wind in the ecliptic. The time derivative of A _t (while always positive) decreases between 0.2 and ∼5 au, indicating a gradual slowing of the in situ development of turbulence in the inner heliosphere. Beyond ∼5 au this rate begins to increase, likely due to turbulence driving by pick-up ions. This Letter highlights the important role of σ _c in modulating MHD turbulence, and the results will aid in better understanding the radial evolution of solar wind turbulence.

Close to the Sun, Parker Solar Probe (PSP) traverses the sub-Alfvénic solar wind, a magnetically controlled plasma environment. Farther from the Sun, the magnetic field amplitude and plasma density weaken enough to establish a super-Alfvénic environment where kinetic energy and turbulence become more prominent. Switchbacks (i.e. large directional deviations from the mean field) are shown to occur almost exclusively in the super-Alfvénic regime. In this study, we analyse magnetic fluctuations from PSP encounters 8 through 19 to study their relationship to Alfvén Mach number ($M_\mathrm{ A}$) and switchback parameter (Z). We find that the fluctuation of the magnetic field magnitude normalized to the mean magnetic field and of the radial velocity normalized to local Alfvén speed increase with both Alfvén Mach number and switchback parameter. However, there is distinct saturation in the increments of the normalized radial velocity fluctuations in the Alfvén Mach number range of $\sim$4–6. Results are interpreted in terms of the Chandrasekhar criterion for Kelvin–Helmholtz activity. Overall, these findings are in agreement with earlier studies suggesting switchback generation through non-linear shear flow dynamics.

Close to Earth, the solar wind is usually super-Alfvénic, i.e., the speed of the solar wind is much larger than the Alfvén speed. However, in the lower coronal regions, the solar wind is mostly sub-Alfvénic. With the Parker Solar Probe (PSP) crossing the boundary between the sub- and super-Alfvénic flow, R. Bandyopadhyay et al. performed a turbulence characterization of the sub-Alfvénic solar wind with initial data from encounters 8 and 9. In this study, we reexamine the turbulence properties such as turbulence amplitude, anisotropy of the magnetic field variance, intermittency, and switchback strength using PSP data from encounters 8–19. The later orbits probe lower altitudes and experience sub-Alfvénic conditions more frequently, providing a greater statistical coverage to contrast sub- and super-Alfvénic solar wind. These later orbits also extend the observations from near solar minimum at launch to near solar maximum conditions. Also, by isolating the intervals where the solar wind speed is approximately equal to the Alfvén speed, we explore the transition in more detail. We show that the amplitude of the normalized magnetic field fluctuation is smaller for the sub-Alfvénic samples. While solar wind turbulence in general is shown to be anisotropic, the sub-Alfvénic samples are more anisotropic than the super-Alfvénic samples, in general. Further, we show that the sub- and super-Alfvénic samples do not show much distinction in terms of intermittency strength. Finally, consistent with prior results, we find no evidence for polarity reversing >90° switchbacks in the sub-Alfvénic solar wind.

We present results from a unified three-dimensional magnetohydrodynamic model of the solar corona, solar wind, and global heliosphere that incorporates turbulence transport. The fully three-dimensional model is based on Reynolds-averaged mean-flow equations coupled with transport equations for turbulence energy, cross helicity, energy difference, and correlation length scale. The computational domain extends from the coronal base to the local interstellar medium and is divided into four regions: (1) the corona, 1–30 R _⊙ ; (2) the solar wind, 30 R _⊙ –5 au; (3) the distant solar wind, 5–40 au; and (4) the global heliosphere, 40–1200 au. Representative boundary conditions are imposed at the coronal base and in the interstellar medium. Steady-state solutions are obtained sequentially using time relaxation in each of the four regions. The model accounts for Reynolds stresses, eddy viscosity, turbulent heating, electron heat conduction, Coulomb collisions between thermal protons and electrons, and—beyond 5 au—the effects of pickup protons. In this study, we add an evolution equation for the energy difference and revise the correlation length scale equation to include additional terms associated with cross helicity and eddy viscosity. We show that a region of enhanced turbulence energy forms ahead of the hydrogen wall in the outer heliosheath, and that pickup protons have a negligible influence on the angular momentum flux carried by the solar wind. We compare our simulation results with in situ observations from the Parker Solar Probe, WIND, Ulysses, and New Horizons. The simulation results show overall agreement with the observed plasma, magnetic field, and turbulence quantities.

The transfer of a star’s angular momentum to its atmosphere is a topic of considerable and wide-ranging interest in astrophysics. This Letter considers the effect of kinetic and magnetic turbulence on the solar wind’s angular momentum. The effects are quantified in a theoretical framework that employs Reynolds-averaged mean field magnetohydrodynamics, allowing for fluctuations of arbitrary amplitude. The model is restricted to the solar equatorial (r–ϕ) plane with axial symmetry, which permits the effect of turbulence to be expressed in analytical form as a modification to the classic E. J. Weber & L. Davis theory, dependent on the r, ϕ shear component of the Reynolds stress tensor. A solar wind simulation with turbulence transport modeling and Parker Solar Probe observations at the Alfvén surface are employed to quantify this turbulent modification to the solar wind’s angular momentum, which is found to be ∼3%–10% and tends to be negative. Implications for solar and stellar rotational evolution are discussed.

The existing Alfven Wave Turbulence Based Solar Atmosphere Model (AWSoM) as used in the SWMF framework of the University of Michigan to simulate the Solar Corona and Inner Heliosphere (i.e. to 1 AU heliocentric distance) meets an ideological problem while compared with the equations for turbulence usually employed in modeling the outer heliosphere (i.e. beyond 1 AU). While for the turbulence in the outer heliosphere the energy difference (the difference between the averaged kinetic and magnetic energy densities) is used as one of the Reynolds-averaged quantity describing the local state of turbulence, the present AWSoM model lacks the energy difference at all. Besides an evident inconsistency between the models of turbulence, employing the different sets of variables below and beyond 1 AU, to have a full description of turbulence is important by two reasons, both relating to the charged particle transport producing the radiation hazards in space. First, for simulation both solar energetic particle and galactic cosmic rays at 1 AU a computational domain should extend at least to 2-3 AU. Second, even if below 1 AU the energy difference effect on turbulence might be negligible and not necessary to be included into the turbulence model, it is still needed to calculate transport coefficients for the high energy charged particles in the turbulent magnetic field. To evaluate it from the averaged quantities characterizing the turbulence, both the total energy density and the said energy difference are needed. In the presented research, an extra equation for the energy difference in introduced and solved in such way that at small heliocentric distances the turbulence model reduces to that used in the AWSoM with no loss in generality. On the other hand at larger heliocentric distances it becomes very close (if not identical) to the typical outer heliosphere model. In addition to averaged energetic characteristics of turbulence we evaluate the correlation spatial scales for directions parallel and perpendicular to the averaged interplanetary magnetic field was well as the transport coefficients for high-energy charged particles

Using Parker Solar Probe data from orbits 8 through 17, we examine fluctuation amplitudes throughout the critical region where the solar wind flow speed approaches and then exceeds the Alfvén wave speed, taking account of various exigencies of the plasma data. In contrast to WKB theory for noninteracting Alfvén waves streaming away from the Sun, the magnetic and kinetic fluctuation energies per unit volume are not monotonically decreasing. Instead, there is clear violation of conservation of standard WKB wave action, which is consistent with previous indications of strong in situ fluctuation energy input in the solar wind near the Alfvén critical region. This points to strong violations of WKB theory due to nonlinearity (turbulence) and major energy input near the critical region, which we interpret as likely due to driving by large-scale coronal shear flows.

The transition from subAlfvénic to superAlfvénic flow in the solar atmosphere is examined by means of Parker Solar Probe (PSP) measurements during solar encounters 8 to 14. Around 220 subAlfvénic periods with a duration ≥10 min are identified. The distribution of their durations, heliocentric distances, and Alfvén Mach number are analysed and compared with a global magnetohydrodynamic model of the solar corona and wind which includes turbulence effects. The results are consistent with a patchy and fragmented morphology, and suggestive of a turbulent Alfvén zone within which the transition from subAlfvénic to superAlfvénic flow occurs over an extended range of helioradii. These results inform and establish context for detailed analyses of subAlfvénic coronal plasma that are expected to emerge from PSP’s final mission phase, as well as for NASA’s planned PUNCH mission.

The solar wind is the extension of the Sun’s hot and ionized corona, and it exists in a state of continuous expansion into interplanetary space. The radial distance at which the wind’s outflow speed exceeds the phase speed of Alfvénic and fast-mode magnetohydrodynamic (MHD) waves is called the Alfvén radius. In one-dimensional models, this is a singular point beyond which most fluctuations in the plasma and magnetic field cannot propagate back down to the Sun. In the multi-dimensional solar wind, this point can occur at different distances along an irregularly shaped “Alfvén surface.” In this article, we review the properties of this surface and discuss its importance in models of solar-wind acceleration, angular-momentum transport, MHD waves and turbulence, and the geometry of magnetically closed coronal loops. We also review the results of simulations and data-analysis techniques that aim to determine the location of the Alfvén surface. Combined with recent perihelia ofParker Solar Probe,these studies seem to indicate that the Alfvén surface spends most of its time at heliocentric distances between about 10 and 20 solar radii. It is becoming apparent that this region of the heliosphere is sufficiently turbulent that there often exist multiple (stochastic and time-dependent) crossings of the Alfvén surface along any radial ray. Thus, in many contexts, it is more appropriate to use the concept of a topologically complex “Alfvén zone” rather than one closed surface. This article also reviews how thePolarimeter to Unify the Corona and Heliosphere(PUNCH) mission will measure the properties of the Alfvén surface and provide key constraints on theories of solar-wind acceleration.