Magnetic switchbacks are large-amplitude fluctuations in the interplanetary magnetic field, and appear frequently in the near-Sun solar wind explored recently by Parker Solar Probe: these new observations have prompted many new studies into their properties and origins. Here, we first review what is known about how switchbacks evolve as they travel away from the Sun: both in terms of their expansion-driven growth and their decay due to various processes like turbulence, reconnection, dispersion, parametric instability, and interaction with interplanetary shocks. We then review the current state of knowledge on how switchbacks impact the physics of the solar wind as a whole: in terms of the turbulent cascade, acceleration and heating of the wind, modification of the open solar flux and scattering of energetic particles. Finally, we suggest future studies to further our understanding of switchback evolution and impacts on the heliosphere.

Magnetic geometry shapes how turbulence transports and dissipates energy in strongly magnetized plasmas. The solar corona, a maze of open and closed flux tubes with sharp transverse gradients, is a prominent example, yet most wave-turbulence models of coronal heating assume symmetric flux tubes or add geometric effects in ad hoc ways. Here we develop a geometry-complete multiscale transport theory for reduced-magnetohydrodynamic turbulence in an arbitrary background field, retaining squashing (magnetic shear), transverse gradients, curvature, and gravity at the same order as standard expansion-driven reflection, and coupling fast, anisotropic fluctuations to slow background evolution through conservation laws. Applied to the corona, it recovers the standard reflection-driven turbulent cascade in smooth regions such as coronal-hole interiors, but predicts that in structured regions geometry-driven channels can dominate: squashing drives reflection even when parallel Alfvén-speed gradients are weak; curvature and non-radial geometry drive compressive heating channels; and waves catalyze the relaxation of velocity shear into heat. The same dynamics drive cross-field transport of mass, composition, momentum, and heat across open-closed interfaces, at rates rivaling the field-parallel supply from the base. These effects bias heating to low altitudes in structured regions, giving a physical basis for the coronal-hole–boundary corrections used in empirical wind-speed predictors. Additionally, the framework's slow-timescale transport equations could be evolved in time, providing a route to a global, geometry-aware model of a structured wave-driven corona and wind. More broadly, the theory provides an energy-consistent account of turbulence, geometry, and transport effects relevant to various astrophysical and terrestrial settings, from magnetospheres and accretion flows to fusion experiments.

Magnetic switchbacks are large amplitude deflections of the magnetic field within the solar wind. They are Alfvénic in character and so are associated with a spike in velocity and a generally small variation in local plasma density. Early orbits of Parker Solar Probe revealed that the solar wind near the Sun is dominated by these structures, and therefore, they may be playing an important role in the energy budget and acceleration of the young solar wind. In this review, we present an overview of different mechanisms that have been proposed for how switchbacks could be formed. We group the mechanisms by whether they predominantly act in the low solar atmosphere or within the solar wind (in situ). We focus on mechanisms that can create reversals of the ambient magnetic field direction and, thus, account for the most extreme perturbations. The general consensus is that mechanisms in the lower solar atmosphere do not form such reversals on their own but provide the seed perturbations, flows, or particle beams necessary for in situ mechanisms to create switchbacks within the solar wind. Switchback observations thus likely contain an imprint of the coronal source of the seed perturbation or flow, which is evolved further locally by one of several plausible in situ mechanisms. We discuss the strengths and weaknesses of each mechanism and outline future observational and theoretical tests that could help differentiate between them.

Alfvénic turbulence is vital to powering the solar wind and corona, yet eludes a comprehensive understanding of the kinetic processes by which it dissipates. Minor ions are sensitive tracers of these processes, showing extreme perpendicular temperatures and mass-weighted temperature trends that can either correlate or anticorrelate with mass-to-charge ratio, A_i/Z_i. We use a combination of quasilinear theory and 3D hybrid-kinetic simulations to explain these features and their correlations with properties of turbulence in the fast solar wind. When Alfvénic turbulence is imbalanced, its cascade to ion-Larmor scales is throttled by the helicity barrier. This barrier ultimately leads to high-frequency proton-cyclotron waves (PCWs), both oblique and parallel, the latter of which produce very flat electric-energy spectra (ℰ_E_⊥∼ k_∥^-η with η<2) over the range of scales that are cyclotron resonant with minor ions. While steeper spectra lead to a positive correlation of heating with A_i/Z_i, the shallower spectra cause the dependence to invert, with Q_i∝ Q_pA_i(A_i/Z_i)^η-2. Six simulations of balanced and imbalanced turbulence spanning β_ p0={1,0.3,1/16} corroborate this prediction, showing minor-ion heating rates that follow (A_i/Z_i)^a. Minor-ion heating is strongest and most perpendicular in our lowest β_ p0=1/16 simulation of imbalanced turbulence, reaching T_⊥ O^5+/T_⊥ p≈40 and T_⊥ O^5+/T_∥ O^5+≈10, consistent with low-coronal observations. Future minor-ion measurements should test whether intervals in which minor-ion thermal speeds decrease with increasing mass-to-charge ratio are associated with a history of large cross helicity, enhanced power in parallel PCWs, and a steep transition-range spectrum.

Resonant drag instabilities (RDIs) are a novel type of dust/fluid instability relevant to a diverse range of astrophysical environments. They are driven by a resonant interaction between streaming dust and waves in a background medium, which results in dust density fluctuations and amplification of the waves. This broad class of instabilities includes recently-proposed modes incorporating acoustic and magnetohydrodynamic waves, as well as the well-studied disk streaming instability. As the study of RDIs is at an early stage, their evolution beyond the linear regime is not well understood. In order to make inroads into the nonlinear theory of RDIs, we performed simulations of the simplest case, the acoustic RDI, in which sound waves in a gas are amplified by interaction with supersonically streaming dust. This particular instability is of interest both due its potential relevance in various poorly ionized environments, and due to its resemblance to the fast magnetosonic RDI. We find that the nonlinear growth and saturation of the instability are characterized by a balance between time scales of instability growth and turbulent eddy turnover. The simulations demonstrate a saturated state possessing an anisotropic outer forcing range in which this balance is maintained, and suggest the presence of an isotropic turbulent inertial range below this scale. By presenting a model for the nonlinear growth and saturated state of the acoustic RDI, this work provides a framework for further study of the nonlinear behavior of this and other RDIs.

We present global magnetohydrodynamic (MHD) simulations of idealized accretion disks with a strong toroidal magnetic field using an equation of state that fixes the gas thermal scale height. The disk forms from the inflow of a rotating magnetized gas cloud with a toroidal magnetic field. We find that the system maintains a moderately strong mean azimuthal field in the midplane, with plasma- β ∼ 1 , trans-Alfv'enic fluctuations, and large accretion stresses α ∼ 0.1 . The azimuthal field in the disk is continuously escaping along the vertical direction but is also replenished via a local dynamo. The inflowing gas initially forms a strongly magnetized Keplerian disk with β ≪ 1 and α ≫ 1 . The disk gradually collapses from the inside out over ∼ 50 − 80 orbits to form a moderately magnetized disk with β ∼ 1 and α ∼ 0.1 . Radial advection of azimuthal magnetic field can maintain β ≲ 1 exterior to the circularization radius but not inside of it. Inclusion of a net initial vertical magnetic field can lead to an even more strongly magnetized disk midplane, consistent with previous work. When the gas thermal scale is not resolved ( ≲ 4 cells per thermal scale height), however, the disk remains highly magnetized with $ $. We discuss our results in the context of related shearing box simulations and other global disk simulations. The level of angular momentum transport found here is consistent with that inferred observationally in dwarf novae and X-ray transient outbursts, unlike simulations of weakly magnetized accretion disks.

In collisionless astrophysical plasmas, turbulence mediates the partitioning of free energy among cascade channels, and its dissipation into ion and electron heat. The resulting ion heating is often anisotropic, with ions observed to be preferentially heated perpendicular to the local magnetic field; understanding the mechanisms responsible for this heating is a key step in understanding the evolution of such plasmas. In this paper, we use the framework of quasi-linear theory to compute analytically the heating rates of ions interacting with turbulent, large-scale Alfv & eacute;nic fluctuations. We show how the imbalance of the turbulence (the difference in energies between Alfv & eacute;nic fluctuations travelling parallel and antiparallel to the magnetic field) modifies the spatiotemporal spectrum of these fluctuations, allowing the heating mechanism to smoothly transition between stochastic heating in balanced turbulence and cyclotron-resonant heating in imbalanced turbulence. The resultant heating rate is found to have a general form regardless of the level of imbalance, exhibiting a suppression related to the conservation of the ions' magnetic moment at small turbulent amplitudes and recovering previous empirical results in a formal calculation. The results of this work help to consolidate our qualitative understanding of ion heating within astrophysical plasmas, as well as yielding specific quantitative predictions to analyse simulations and observations.

The solar wind is observed to undergo substantial heating as it expands through the heliosphere, with measured temperature profiles exceeding those expected from adiabatic cooling. A plausible source of this heating is reflection-driven turbulence (RDT), in which gradients in the background Alfv & eacute;n speed partially reflect outward-propagating Alfv & eacute;n waves, seeding counter-propagating fluctuations that interact and dissipate via turbulence. Previous RDT models assume a radial-background magnetic field, but at larger radii the interplanetary field is known to be twisted into the Parker spiral (PS). Here, we generalise RDT phenomenology to include a PS, using three-dimensional expanding-box magnetohydrodynamic simulations to test the ideas and compare the resulting turbulence with the radial-background-field case. We argue that the underlying RDT dynamics remains broadly similar with a PS, but the controlling scales change: as the azimuthal field grows it 'cuts across' perpendicularly stretched, pancake-like eddies, producing outer scales perpendicular to the magnetic field that are much smaller than in the radial-background case. Consequently, the outer-scale nonlinear turnover time increases more slowly with heliocentric distance in PS geometry, weakening the tendency (seen in radial-background models) for the cascade to 'freeze' into quasi-static, magnetically dominated structures. This allows the system to dissipate a larger fraction of the fluctuation energy as heat, also implying that the turbulence remains strongly imbalanced (with high normalised cross-helicity) out to larger heliocentric distances. We complement our heating results with a detailed characterisation of the turbulence (e.g. spectra, switchbacks and compressive fractions), providing a set of concrete predictions for comparison with spacecraft observations.

Minor ions in the solar corona are heated to extreme temperatures, far in excess of those of the electrons and protons that comprise the bulk of the plasma. These highly nonthermal distributions make minor ions sensitive probes of the collisionless processes that heat the corona and power the solar wind. The recent discovery of the “helicity barrier” offers a mechanism in which imbalanced Alfvénic turbulence in low- β plasmas preferentially heats protons over electrons, generating high-frequency, proton-cyclotron-resonant fluctuations. We use the hybrid-kinetic particle-in-cell code Pegasus++ to drive imbalanced Alfvénic turbulence in a 3D low- β plasma with additional passive ion species, He ^2+ and O ^5+ . A helicity barrier naturally develops, followed by clear phase-space signatures of oblique proton-cyclotron-wave heating and Landau-resonant heating from the imbalanced Alfvénic fluctuations. The former results in characteristically arced ion velocity distribution functions, whose non-bi-Maxwellian features are shown by linear ALPS calculations to be critical to the heating process. Additional features include a steep transition-range electromagnetic spectrum, proton-cyclotron waves propagating in the direction of the imbalance, significantly enhanced proton-to-electron heating ratios, ion temperatures that are considerably more perpendicular with respect to magnetic field, and extreme heating of heavier species in a manner consistent with mass scalings inferred from spacecraft measurements. None of these features are realized in an otherwise equivalent simulation of balanced turbulence. If seen simultaneously in the fast solar wind, these signatures of the helicity barrier would testify to the necessity of incorporating turbulence imbalance in a complete theory for the evolution of the solar wind.

We study the evolution of collisionless plasmas that, due to their macroscopic evolution, are susceptible to the firehose instability, using both analytic theory and hybrid-kinetic particle-in-cell simulations. We establish that, depending on the relative magnitude of the plasma beta, the characteristic time scale of macroscopic evolution and the ion-Larmor frequency, the saturation of the firehose instability in high-beta plasmas can result in three qualitatively distinct thermodynamic (and electromagnetic) states. By contrast with the previously identified 'ultra-high-beta' and 'Alfv & eacute;n-inhibiting' states, the newly identified 'Alfv & eacute;n-enabling' state, which is realised when the macroscopic evolution time tau exceeds the ion-Larmor frequency by a beta-dependent critical parameter, can support linear Alfv & eacute;n waves and Alfv & eacute;nic turbulence because the magnetic tension associated with the plasma's macroscopic magnetic field is never completely negated by anisotropic pressure forces. We characterise these states in detail, including their saturated magnetic-energy spectra. The effective collision operator associated with the firehose fluctuations is also described; we find it to be well approximated in the Alfv & eacute;n-enabling state by a simple quasi-linear pitch-angle scattering operator. The box-averaged collision frequency is nu eff similar to beta/tau, in agreement with previous results, but certain subpopulations of particles scatter at a much larger (or smaller) rate depending on their velocity in the direction parallel to the magnetic field. Our findings are essential for understanding low-collisionality astrophysical plasmas including the solar wind, the intracluster medium of galaxy clusters and black hole accretion flows. We show that all three of these plasmas are in the Alfv & eacute;n-enabling regime of firehose saturation and discuss the implications of this result.

We argue that stochastic and resonant ion heating, often viewed as distinct processes in low-β collisionless plasmas, are the far limits of a continuum controlled by nonlinear broadening of turbulent fluctuations, and thus by the normalized cross helicity. We propose a simple empirical formula that captures both regimes, generalizing the model commonly used to describe stochastic heating. Simulations of test ions interacting with turbulence confirm our scalings across a wide range of different ion and turbulence properties, including with a steep ion-kinetic transition range as seen in the solar wind. Our results provide a unified framework for understanding ion heating processes across diverse astrophysical environments from black-hole accretion disks to the solar corona, also providing a compact and versatile subgrid model for larger-scale simulations.

The magnetohydrodynamics (MHD) equations plus 'non-ideal' (Ohmic, Hall, ambipolar) resistivities are widely used to model weakly-ionized astrophysical systems. We show that if gradients in the magnetic field become too steep, the implied charge drift speeds become much faster than microphysical signal speeds, invalidating the assumptions used to derive both the resistivities and MHD equations themselves. Generically this situation will excite microscale instabilities that suppress the drift and current. We show this could be relevant at low ionization fractions especially if Hall terms appear significant, external forces induce supersonic motions, or dust grains become a dominant charge carrier. Considering well-established treatments of super-thermal drifts in laboratory, terrestrial, and Solar plasmas as well as conduction and viscosity models, we generalize a simple prescription to rectify these issues, where the resistivities are multiplied by a correction factor that depends only on already-known macroscopic quantities. This is generalized for multi-species and weakly-ionized systems, and leaves the equations unchanged in the drift limits for which they are derived, but restores physical behavior (driving the system back towards slow drift by diffusing away small-scale gradients in the magnetic field) if the limits are violated. This has important consequences: restoring intuitive behaviors such as the system becoming hydrodynamic in the limit of zero ionization; suppressing magnetic structure on scales below a critical length which can comparable to circumstellar disk sizes; limiting the maximum magnetic amplification; and suppressing the effects of the Hall term in particular. This likely implies that the Hall term does not become dynamically important under most conditions of interest in these systems.

Collisionless dissipation of turbulence is important for heating plasmas in astrophysical, space physics, and laboratory environments, controlling energy, momentum, and particle transport. We analyze Parker Solar Probe observations to understand the collisionless heating of the sub-Alfvénic solar wind, which is connected to the solar corona. Our results show that linear resonant heating through parallel-propagating cyclotron waves cannot account for turbulent dissipation in the sub-Alfvénic region, which observations suggest may dissipate turbulence at distances further from the Sun. Instead, we find that stochastic heating can account for the observed ion energization; however, because the dominant contributions arise from infrequent, large-amplitude events, turbulent intermittency must be explicitly incorporated. These observations directly connect stochastic heating via breaking of the proton magnetic moment with the intermittent and inhomogeneous heating of turbulence reported in many previous studies. Our identification of stochastic heating as a dynamic mechanism responsible for intermittent heating of the solar wind has significant implications for turbulent dissipation in the lower corona, other astrophysical environments, and laboratory plasmas.

In magnetized, stratified environments such as the Sun's corona and solar wind, Alfvénic fluctuations ‘reflect’ from background gradients, enabling nonlinear interactions that allow their energy to dissipate into heat. This process, termed ‘reflection-driven turbulence’, likely plays a key role in coronal heating and solar-wind acceleration, explaining a range of detailed observational correlations and constraints. Building on previous works focused on the inner heliosphere, here we study the basic physics of reflection-driven turbulence using reduced magnetohydrodynamics in an expanding box – the simplest model that can capture local turbulent plasma dynamics in the super-Alfvénic solar wind. Although idealized, our high-resolution simulations and simple theory reveal a rich phenomenology that is consistent with a diverse range of observations. Outwards-propagating fluctuations, which initially have high imbalance (high cross-helicity), decay nonlinearly to heat the plasma, becoming more balanced and magnetically dominated. Despite the high imbalance, the turbulence is strong because Elsässer collisions are suppressed by reflection, leading to ‘anomalous coherence’ between the two Elsässer fields. This coherence, together with linear effects, causes the growth of ‘anastrophy’ (squared magnetic potential) as the turbulence decays, forcing the energy to rush to larger scales and forming a ‘ $1/f$ -range’ energy spectrum in the process. Eventually, expansion overcomes the nonlinear and Alfvénic physics, forming isolated, magnetically dominated ‘Alfvén vortices’ with minimal nonlinear dissipation. These results can plausibly explain the observed radial and wind-speed dependence of turbulence imbalance (cross-helicity), residual energy, fluctuation amplitudes, plasma heating and fluctuation spectra, as well as making a variety of testable predictions for future observations.

Context. Large-amplitude inversions of the solar wind's interplanetary magnetic field have long been documented; however, observations from the Parker Solar Probe (PSP) mission have renewed interest in this phenomenon as such features, often termed switchbacks, may constrain both the sources of the solar wind as well as in situ nonlinear dynamics and turbulent heating. Aims. We aim to show that magnetic field fluctuations in the solar wind are consistent with Alfv & eacute;nic fluctuations that naturally form switchback inversions in the magnetic field through expansion effects. Methods. We examine PSP observations of the evolution of a single stream of solar wind in a radial scan from PSP's tenth perihelion encounter from approximate to 15 - 50 R-circle dot. We study the growth and radial scaling of normalized fluctuation amplitudes in the magnetic field, delta B/B, within the framework of spherical polarization. We compare heating rates computed via outer-scale decay from consideration of wave action to proton heating rates empirically observed through adiabatic expansion. Results. We find that the magnetic field fluctuations are largely spherically polarized and that the normalized amplitude of the magnetic field, delta B/B, increases with amplitude. The growth of the magnetic field amplitude leads to switchback inversions in the magnetic field. While the amplitude does not grow as fast as is predicted by the conservation of wave action, the deviation from the expected scaling yields an effective heating rate that is close to the empirically observed proton heating rate. Conclusions. The observed scaling of fluctuation amplitudes is largely consistent with a picture of expanding Alfv & eacute;n waves that seed turbulence, leading to dissipation. The expansion of the waves leads to the growth of wave amplitudes, resulting in the formation of switchbacks.

An understanding of how turbulent energy is partitioned between ions and electrons in weakly collisional plasmas is crucial for modelling many astrophysical systems. Using theory and simulations of a four-dimensional reduced model of low-beta gyrokinetics (the `Kinetic Reduced Electron Heating Model'), we investigate the dependence of collisionless heating processes on plasma beta and imbalance (normalised cross-helicity). These parameters are important because they control the helicity barrier, the formation of which divides the parameter space into two distinct regimes with remarkably different properties. In the first, at lower beta and/or imbalance, the absence of a helicity barrier allows the cascade of injected power to proceed to small (perpendicular) scales, but its slow cascade rate makes it susceptible to significant electron Landau damping, in some cases leading to a marked steepening of the magnetic spectra on scales above the ion Larmor radius. In the second, at higher beta and/or imbalance, the helicity barrier halts the cascade, confining electron Landau damping to scales above the steep `transition-range' spectral break, resulting in dominant ion heating. We formulate quantitative models of these processes that compare well to simulations in each regime, and combine them with results of previous studies to construct a simple formula for the electron-ion heating ratio as a function of beta and imbalance. This model predicts a `winner takes all' picture of low-beta plasma heating, where a small change in the fluctuations' properties at large scales (the imbalance) can cause a sudden switch between electron and ion heating.

We show that there exist two qualitatively distinct turbulent states of the zero-net-vertical-flux shearing box. The first, which has been studied in detail previously, is characterized by a weakly magnetized ($\beta\sim50$) midplane with slow periodic reversals of the mean azimuthal field (dynamo cycles). The second, the 'low-$\beta$ state,' which is the main subject of this paper, is characterized by a strongly magnetized $\beta\sim 1$ midplane dominated by a coherent azimuthal field with much stronger turbulence and much larger accretion stress ($\alpha \sim 1$). The low-$\beta$ state emerges in simulations initialized with sufficiently strong azimuthal magnetic fields. The mean azimuthal field in the low-$\beta$ state is quasi steady (no cycles) and is sustained by a dynamo mechanism that compensates for the continued loss of magnetic flux through the vertical boundaries; we attribute the dynamo to the combination of differential rotation and the Parker instability, although many of its details remain unclear. Vertical force balance in the low-$\beta$ state is dominated by the mean magnetic pressure except at the midplane, where thermal pressure support is always important (this holds true even when simulations are initialized at $\beta \ll 1$, provided the thermal scale height of the disk is well resolved). The efficient angular momentum transport in the low-$\beta$ state may resolve long-standing tension between predictions of magnetorotational turbulence (at high $\beta$) and observations; likewise, the bifurcation in accretion states we identify may be important for understanding the state transitions observed in dwarf novae, X-ray binaries, and changing-look AGN. We discuss directions for future work, including the implications of our results for global accretion disk models and simulations.

Turbulent magnetic field fluctuations observed in the solar wind often maintain a constant-magnitude magnetic field accompanied by spherically polarized velocity fluctuations; these signatures are characteristic of large-amplitude Alfvén waves. Nonlinear energy transfer in Alfvénic turbulence is typically considered in the small-amplitude limit where the constant-magnitude condition may be neglected; in contrast, nonlinear energy transfer of large-amplitude fluctuations remains relatively unstudied. We develop a method to analyze large-amplitude turbulence through studying fluctuations as constant-magnitude rotations in the de Hoffmann–Teller frame, in which the convected electric field of the fluctuations vanishes such that the frame and fluctuations are copropagating. Our analysis reveals signatures of large-amplitude effects deep into the inertial range. While the dominant fluctuations are consistent with spherically polarized large-amplitude Alfvén waves, the subdominant fluctuations are relatively compressible. Signatures of nonlinear interaction between the large-amplitude spherically polarized mode with the subdominant population reveal highly aligned transverse components. In many theoretical models of Alfvénic turbulence, alignment is thought to reduce nonlinearity; our observations suggest that the observed alignment is sufficient to either reduce shear nonlinearity such that non-Alfvénic interactions may be responsible for energy transfer in spherically polarized states, or alternatively that counterpropagating fluctuations maintain anomalous coherence, a predicted signature of reflection-driven turbulence.

We propose a novel dust battery mechanism for generating seed magnetic fields in the early Universe, in which charged dust grains are radiatively accelerated, inducing strong electric currents that subsequently generate magnetic fields. Our analysis demonstrates that this process is effective even at very low metallicities (approximately ∼10 ^−5 Z _⊙ ), and capable of producing seed fields with significant amplitudes of B ∼ μ G around luminous sources over timescales of years to Myr and across spatial scales ranging from au to kpc. Crucially, we find that this mechanism is generically ∼10 ^8 times more effective than the radiatively driven electron battery or Biermann battery in relatively cool gas (≪10 ^5 K), including both neutral and ionized gas. Furthermore, our results suggest that, to first order, dissipation effects do not appear to significantly impede this process, and that it can feasibly generate coherent seed fields on macroscopically large interstellar medium (ISM) scales (much larger than turbulent dissipation scales or electron mean-free-paths in the ISM). These seed fields could then be amplified by subsequent dynamo actions to the observed magnetic fields in galaxies. Additionally, we propose a subgrid model for integration into cosmological simulations, and the required electric-field expressions for magnetohydrodynamic-particle-in-a-cell simulations that explicitly model dust dynamics. Finally, we explore the broad applicability of this mechanism across different scales and conditions, emphasizing its robustness compared to other known battery mechanisms.

Many hot, diffuse astrophysical plasmas are collisionless, with collision times that exceed dynamical timescales of interest. Without a standard viscosity, the mechanisms by which mechanical energy dissipates into heat via a turbulent cascade are complex and nonuniversal, with important consequences for the partition of heat between different species and directions. Recent theory has argued that in some parameter regimes a barrier to the turbulent cascade of energy forms at small scales, profoundly altering the nature of the heating of the plasma by changing how fluctuations dissipate. The solar wind is an environment, when in this parameter regime, where such a barrier would be expected to form. Here, using data from NASA’s , we find that the shape of the magnetic energy spectrum around ion scales varies with solar wind parameters in a manner consistent with the presence of the barrier. This allows us to identify critical values of parameters necessary for the barrier to form; we show that the barrier appears fully developed for ion plasma beta of below ≃0.5 and becomes increasingly prominent with increasing normalized cross helicity (the difference in energy in wave packets propagating parallel and antiparallel to the background magnetic field) for values greater than ≃0.4. As these conditions are frequently met in the solar wind, particularly close to the Sun, our results suggest that the barrier is likely playing a significant role in turbulent dissipation in the solar wind and so is an important mechanism in explaining its heating and acceleration.