The Magnetospheric Multiscale (MMS) mission's extended phase introduced an "unbiased campaign" that provided the first high-resolution data set of the terrestrial magnetosheath obtained without human selection, across complete crossings from the bow shock to the magnetopause. This is achieved by collecting 3 min of burst-mode data every 9 min. Leveraging this novel data set, we report average plasma parameters and observe the emergence of a range in the magnetic field power spectrum. A distinct ordering of both bulk and turbulence quantities is demonstrated when the Alfv & eacute;nic Mach number is used as a proxy for the distance from the bow shock. This ordering enables us to obtain clear insights into the occurrence of small-scale current sheets throughout the magnetosheath by evaluating magnetic field kurtosis. Furthermore, the evolution of the correlation time is opposite to that of kurtosis and aligns with the principle of turbulence relaxation. This information is dispersed when quantities are sorted by geocentric distance. Crucially, striking differences emerge in turbulence-related quantities when comparing unsupervised and supervised data sets. These findings provide valuable insights into magnetospheric turbulence, establishing the campaign as a critical resource for unbiased statistical analysis of this environment.

The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission is a NASA Small Explorer to determine the cross-scale processes that unify the solar corona and heliosphere. PUNCH has two science objectives: (1) understand how coronal structures become the ambient solar wind, and (2) understand the dynamic evolution of transient structures, such as coronal mass ejections, in the young solar wind. To address these objectives, PUNCH uses a constellation of four small spacecraft in Sun-synchronous low Earth orbit, to collect linearly polarized images of the K corona and young solar wind. The four spacecraft each carry one visible-light imager in a 1 + 3 configuration: a single Narrow Field Imager solar coronagraph captures images of the outer corona at all position angles, and at solar elongations from 1.5° (6 R⊙) to 8° (32 R⊙); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3° (12 R⊙) to 45° (180 R⊙) from the Sun. PUNCH images include linear-polarization data, to enable inferring the three-dimensional structure of visible features without stereoscopy. The instruments are matched in wavelength passband, support overlapping instantaneous fields of view, and are operated synchronously, to act as a single “virtual instrument” with a 90∘ wide field of view, centered on the Sun. PUNCH launched in March of 2025 and began science operations in June of 2025. PUNCH has an open data policy with no proprietary period, and PUNCH Science Team Meetings are open to all.

We present magnetohydrodynamic simulations of laser-driven plasma outflows propagating along an externally applied poloidal magnetic field, designed to mimic coronal open-field plasma jets. Using the FLASH code with non-ideal terms (resistivity, Biermann battery, and Nernst advection) included, we model a CH target driven by a 3 omega (351 nm) beam delivering 5 kJ over 10 ns and a uniform background field B-0 = 0-50 T. Under these conditions, the expanding plume develops a central low-density diamagnetic cavity bounded by a high-magnetic-pressure shell. Magnetic flux is advected from the plume center to its edge, and azimuthal diamagnetic currents form that decrease fields inside the cavity and amplify fields outside, producing a radial magnetic-pressure gradient that exerts an inward J x B force and radially confines the flow. We show that the collimation strengthens with increasing applied magnetic field, as stronger fields reduce the plasma a and correspondingly enhance the confining J x B force. We discuss scaling to solar coronal jets and argue that low-a, magnetic-pressure-dominated ambient conditions promote similar diamagnetic cavity formation and J x B collimation in coronal outflows.

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.

The invariants of the velocity gradient tensor in turbulence offer a compact description of local kinematics and flow topology. For incompressible magnetohydrodynamic turbulence, analysis of the second and third invariants (Q, R) of the velocity gradient tensor clarifies how coherent structures are organized and evolve. Extending the same analysis to the magnetic field gradient tensor provides additional information on the dynamics. In this study, pseudo-spectral simulation is used to obtain the velocity and magnetic field of the turbulent flow, and analysis of the flow field is conducted through joint probability density functions (PDFs) of the invariants. Furthermore, we explore the influence of the external mean magnetic field strength, B-0 . The results show that when an external magnetic field is present, the Q-R joint PDF no longer maintains the familiar teardrop distribution for the velocity field, and the flow field structure tends to be two-dimensional with increasing B-0 . For the fluctuation magnetic field, the Q-R joint PDF takes on a "cigar" shape that becomes more elongated as B(0 )increases. Moreover, as the strength of the external mean magnetic field increases, the turbulence exhibits enhanced small-scale dissipation and localization, accompanied by a reduction in the effective dimensionality of the system toward a quasi-two-dimensional regime.

Close to the Sun, Parker Solar Probe (PSP) traverses the sub-Alfv & eacute;nic solar wind, a magnetically controlled plasma environment. Farther from the Sun, the magnetic field amplitude and plasma density weaken enough to establish a super-Alfv & eacute;nic 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-Alfv & eacute;nic regime. In this study, we analyse magnetic fluctuations from PSP encounters 8 through 19 to study their relationship to Alfv & eacute;n 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 Alfv & eacute;n speed increase with both Alfv & eacute;n Mach number and switchback parameter. However, there is distinct saturation in the increments of the normalized radial velocity fluctuations in the Alfv & eacute;n 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.

Fluctuations and structure across a wide range of spatial and temporal scales are frequently studied in the solar wind. The properties of the low-frequency fluctuations are of relevance to turbulent energy injection into the plasma and the transport of high-energy cosmic rays. Correlation analysis of decade-long intervals of interplanetary data permits study of fluctuations at time scales much longer than suitably defined correlation times, and therefore at frequencies well below those associated with the Kolmogorov inertial range of in situ turbulence. At the frequencies of interest, we study the familiar occurrence of the [Formula: see text] spectral signature. We also study point spectral features due to solar rotation and their relation with the [Formula: see text] signal. We report properties at timescales ranging from minutes up to years, using data selected by wind speed, phase of solar cycle, and cartesian components of the magnetic field. A surprising finding is that the power in solar rotation harmonics is consistent with an extension of the [Formula: see text] spectrum, down to frequencies as low as around [Formula: see text]. The presence of a broadband [Formula: see text] spectrum across different wind types supports the interpretation that [Formula: see text] signals may be related to or even originate from the solar dynamo.

Context. Spectral anisotropy is a ubiquitous characteristic of turbulence in magnetized space plasma environments, as it occurs spontaneously due to the effect of a background magnetic field. This anisotropy significantly affects the turbulent energy cascade and modifies the scale-to-scale balance between different terms in the von K & aacute;rm & aacute;n-Howarth equation.Aims. We investigate the effect of the induced lag-space anisotropy on the determination of the scale-dependent energy transfer rate.Methods. We compared traditional single-spacecraft and novel multispacecraft approaches using hybrid particle-in-cell simulations and in situ observations from the Magnetospheric Multiscale (MMS) mission in the Earth's magnetosheath.Results. We show that the isotropy assumption, which is required for single-spacecraft measurements, leads to inaccuracies in the estimation of the turbulent cascade rate. On the other hand, the novel lag-polyhedra derivative ensemble method, which is specifically designed for the next generation of multipoint multiscale missions such as HelioSwarm and Plasma Observatory, can capture the anisotropy of the turbulent cascade.Conclusions. These findings are used to interpret observations from the MMS mission in the Earth's magnetosheath. They highlight the importance of accounting for anisotropy in space plasma diagnostics.

The interplanetary magnetic field exhibits a distinctive 1/f spectral density from frequencies of around [10^-6]Hz to around [10^-4]Hz, ranging from harmonics of the solar rotation to the reciprocal of the turbulence correlation time in the spacecraft frame. Various theories have been proposed to explain its origin, typically invoking either processes in the lower corona or in the solar interior, or local interplanetary dynamics. Here, we investigate the superposition principle that underlies explanations of the solar/coronal types, which in principle can generate the full observed range of 1/f noise. Using synthetic time series with scale-invariant or log-normal distributions of correlation times, we examine the efficacy of several superposition approaches in generating a 1/f regime. The persistence of 1/f spectrum is further illustrated with decade-long in situ magnetic field measurements from the ACE spacecraft. Together, these results help explain the ubiquity of 1/f noise under the unavoidable superposition inherent in long-duration heliospheric data.

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, sigma c), which is known to impede development of magnetohydrodynamic (MHD) turbulence. Here we propose a new formulation of the solar wind's turbulence age (At) that explicitly accounts for the Alfv & eacute;nic nature of the fluctuations in the inner heliosphere. At 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 Alfv & eacute;nicity, the present approach yields smaller values of At in medium-to-high sigma c solar wind; similar turbulence ages are then obtained for slow and fast wind in the ecliptic. The time derivative of At (while always positive) decreases between 0.2 and similar to 5 au, indicating a gradual slowing of the in situ development of turbulence in the inner heliosphere. Beyond similar to 5 au this rate begins to increase, likely due to turbulence driving by pick-up ions. This Letter highlights the important role of sigma c in modulating MHD turbulence, and the results will aid in better understanding the radial evolution of solar wind turbulence.

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.

In solar wind turbulence, the energy transfer/dissipation rate is typically estimated using MHD third-order structure functions calculated using spacecraft observations. However, the inherent anisotropy of solar wind turbulence leads to significant variations in structure functions along different observational directions, thereby affecting the accuracy of energy dissipation rate estimation. An unresolved issue is how to optimise the selection of observation angles under limited directional sampling to improve estimation precision. We conduct a series of MHD turbulence simulations with different mean magnetic field strengths, B0. Our analysis of the third-order structure functions reveals that the global energy dissipation rate estimated around a polar angle of theta = 60 degrees agrees reasonably with the exact one for 0 <= B0/brms <= 5, where brms denotes the rms magnetic field fluctuation. The speciality of 60 degrees polar angle can be understood by the mean value theorem of integrals, since the spherical integral of the polar-angle component ( T theta ) of the divergence of Yaglom flux is zero, and T theta changes sign around 60 degrees. Existing theory on the energy flux vector as a function of the polar angle is assessed, and supports the speciality of the 60 degrees polar angle. The angular dependence of the third-order structure functions is further assessed with virtual spacecraft data analysis. The present results can be applied to measure the turbulent dissipation rates of energy in the solar wind, which are of potential importance to other areas in which turbulence takes place, such as laboratory plasmas and astrophysics.

While dissipation in collisional plasma is defined in terms of viscosity and resistivity, the exact functional form of dissipation, i.e., the so-called dissipation function in nearly collisionless plasma, is unknown. Nevertheless, previous studies have suggested that there exists viscous-like energy conversion in collisionless plasma with scaling characteristics analogous to collisional plasma, and in particular that the average dissipation is proportional to the square of the rate of strain as in hydrodynamics. In this study, using 2.5D kinetic particle-in-cell (PIC) simulation of collisionless plasma turbulence, we provide an estimate of effective viscosity at each scale, obtained via a scale-filtering approach. We then compare the turbulent dynamics of the PIC simulation with that from MHD and two-fluid simulations in which the viscosity is equal to the effective viscosity estimate obtained from the PIC simulation. We find that the global behavior in these MHD and two-fluid simulations has a striking similarity to that in their kinetic/PIC counterpart. In addition, we explore the scale dependence of the effective viscosity and discuss implications of this approach for space plasmas.

Low-frequency signals having a band-limited scale-invariant power, or “1/ f ” noise, have been detected in the interplanetary medium and inferred from coronal and photospheric observations. Their origin remains under debate, with the principal issue being whether the signal originates from local processes or has a solar origin, possibly emerging from a superposition of scale-invariant structures. Detection of such interplanetary signals at frequencies as low as a few times 10 ^−6 Hz at distances well within 1 au is important in developing an understanding of this phenomenon, but this is challenging due to a lack of the requisite long-duration data records. Here, we employ magnetic field data from the MESSENGER spacecraft, processed to remove Mercury’s wake, to analyze spectra from a year of data. The 1/ f signal is detected down to the lowest expected frequencies, consistent with the observed lognormal distribution of correlation times and the Machlup–Montroll–Shlesinger superposition principle. This supports the hypothesis that the observed 1/ f signals have an origin in solar processes, possibly the dynamo, and disfavors local in situ generation, as described in W. H. Matthaeus & M. L. Goldstein and W. H. Matthaeus et al.

In this study, we revisit the pressure-strain interaction in kinetic turbulence, and in particular we re-examine the decomposition of pressure-strain interaction into compressive and incompressive parts. The pressure dilatation ingredient is clearly due to plasma compressions, but here using kinetic particle-in-cell (PIC) simulations of plasma turbulence, it is demonstrated that the remaining anisotropic part, often called Pi-D, also contains contributions due to compressive, non solenoidal velocities of the particle species. The compressive Pi-D can play a significant role in systems with low plasma β even if the system starts with small density variations. The compressive ingredient of Pi-D is found to be anticorrelated with both incompressive Pi-D and pressure dilatation.

Fluctuations in the Sun's photospheric magnetic field are the primary source of the turbulence that can heat and accelerate the solar atmosphere, and thus play an important role in the production and evolution of the solar wind that permeates the heliosphere. A key parameter that characterizes this turbulence is the correlation scale of fluctuations, which determines the injection of turbulent energy into the plasma and the diffusive transport of solar energetic particles. This study employs magnetogram data from the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory to characterize an ensemble of spatial autocorrelation functions (ACFs) of turbulence in the photosphere. It is shown that the two-point ACFs satisfy the similarity-decay hypothesis of von Karman and Howarth, a fundamental property of turbulent systems: rescaling the ACFs by their respective energies and correlation lengths yields a quasi-universal exponential form. The probability distribution function of transverse correlation lengths ( lambda) is shown to be approximately log-normal, which is consistent with observations of turbulence in the solar wind. A 'mosaic' of the spatial distribution of A over the photosphere is presented; the 'quiet Sun' tends to have lambda similar to 1500 km (albeit with a wide distribution), which is close to the scale of solar granulation; systematically longer lengths are associated with active regions. A positive correlation is observed between mean magnetic field magnitude and A, and empirical fits quantify this relationship. These results improve our understanding of solar turbulence while providing observational constraints for models that describe turbulence transport from solar and stellar photospheres into their atmospheres.

In recent years, pressure-strain interaction, which describes a pathway of internal energy evolution due to a spatially varying bulk flow, has received significant attention in collisionless magnetic reconnection. However, the influence of an out-of-plane (guide) magnetic field on the structure and amplitude of the pressure-strain interactions during reconnection has yet to be systematically explored. In this study, we use 2.5-dimensional kinetic particle-in-cell simulations of antiparallel and guide field reconnection and explore the changes due to the guide field of the electron pressure-strain interaction and its decompositions close to the X-line. We find that the structure and amplitude of the electron pressure-strain interaction for larger guide field cases differ strongly from the antiparallel case; it is non-zero over a larger region of space, and its amplitude is considerably greater when there is a guide field. It is mostly dominated by Pi-D, especially the contribution due to velocity shear. Physically, this is because in guide field reconnection, the electrons form thin current sheets along one branch of the separatrices in the exhaust region, leading to strong velocity shear. We perform a scaling analysis near the electron diffusion region in the large guide field limit to estimate the value of electron pressure-strain interaction as a function of upstream parameters for symmetric guide field reconnection and confirm the result with the simulations. We anticipate that this scaling and structure will be useful for spacecraft observations of a wide range of guide field reconnection in Earth's magnetosphere and the solar wind.

We present a new scale decomposition method to investigate turbulence in wavenumber-frequency space. Using 3D magnetohydrodynamic turbulence simulations, we show that magnetic fluctuations with time scales longer than the nonlinear time exhibit an inverse cascade toward even smaller frequencies. Low frequency magnetic fluctuations support turbulence, acting as an energy reservoir that is converted into plasma kinetic energy, the latter cascading toward large wavenumbers and frequencies, where it is dissipated. Our results shed new light on the spatio-temporal properties of turbulence, potentially explaining the origin and role of low frequency turbulent fluctuations in the solar wind.

In this study, we revisit the pressure-strain interaction in kinetic plasma turbulence. We reexamine the decomposition of pressure-strain interaction into compressive and incompressive parts using Helmholtz theorem. The pressure dilatation ingredient is clearly due to plasma compressions, but here, using 2.5 dimensional kinetic particle-in-cell (PIC) simulations of plasma turbulence, it is demonstrated that the remaining anisotropic part, often called Pi-D, also contains contributions due to compressive, non-solenoidal velocities of the particle species. The compressive Pi-D can play a significant role in systems with low plasma beta even if the system starts with small density variations. In addition, the compressive ingredient of Pi-D is found to be strongly anticorrelated with both incompressive Pi-D and pressure dilatation along the current sheets.

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.