Context. The solar wind provides a natural laboratory for plasma turbulence. The core problem is the energy cascade process in the inertial range, which has been a fundamental long-standing question. Much effort has been put into theoretical models to explain the observational features in the solar wind. However, there are still many questions that remain unanswered. Aims. Here, we report the observational evidence for the existence of two subranges in the inertial regime of the solar wind turbulence and show the scaling features for each subranges. Methods. We performed multi-order structure function analyses for one high-latitude fast solar wind interval at 1.48 au measured by Ulysses and one slow but Alfvénic solar wind at 0.17 au measured by the Parker Solar Probe (PSP). We also conducted statistical analyses on 103 fast solar wind intervals observed by Wind. Results. We identify the existence of two subranges in the inertial range according to the distinct scaling features of the magnetic field. The multi-order scaling indices versus the order for the two subranges demonstrates a clear disparity, with the second-order scaling index being 1/2 in the larger-scale subrange 1 and 2/3 in the smaller-scale subrange 2. Both subranges display apparent but different anisotropies. The velocity exhibits similar features as the magnetic field. The PSP interval shows that subrange 1 follows Yaglom scaling law, while subrange 2 does not. The Ulysses interval shows that the intermittency abruptly grows to a maximum 5% of the interval from subrange 1 to subrange 2. Conclusions. Based on the observational features, we propose a new scenario that the inertial regime of the solar wind turbulence consists of two subranges. The observational evolution of the scaling as the solar wind expands may be a consequence of observing different subranges at different radial distances.

The solar wind is highly turbulent, and intermittency effects are observed for fluctuations within the inertial range. By analyzing magnetic field spectra and fourth-order moments, we perform a comparative study of turbulence and intermittency in different types of solar wind, measured during periods of solar minima and a maximum. Using eight fast-solar-wind intervals measured during solar minima between 0.3 and 3.16 au, we find a clear signature of two inertial subranges with f−3/2 and f−5/3 power laws in the magnetic power spectra. The intermittency, measured through the scaling law of the kurtosis of the magnetic field fluctuations, further confirms the existence of two different power laws separated by a clear break. A systematic study of the evolution of the said subranges as a function of heliospheric distance shows the correlation of the break scale with both the turbulence outer scale and the typical ion scales. During solar maximum, on the contrary, the two subranges are not omnipresent, thus showing more variability in the power spectra and intermittency scaling properties.

We investigate the radial evolution of solar wind turbulence during the radial alignment of Parker Solar Probe (PSP) and Solar Orbiter (SO) on 2022 December 10, with PSP located at approximately 0.11 au and SO near 0.88 au. To identify nearly the same plasma parcel crossing both spacecraft, we apply a ballistic propagation model with time-constant acceleration constrained by in situ solar wind velocity measurements at PSP and SO. We trace the magnetic footpoint of the plasma parcel back to the photosphere using a potential field source surface model based on a Global Oscillations Network Group synoptic magnetogram. Field and plasma measurements from PSP and SO are used to analyze power spectral density (PSD), spectral scaling, magnetic compressibility, and intermittency. Our results show that (1) the trace PSD of magnetic fluctuations steepens in the inertial range and flattens in the dissipation range with increasing radial distance; (2) the spectral break shifts to lower frequencies at SO; and (3) the Castaing model reveals multifractal intermittency in the inertial range, with slightly weaker intermittency at SO. These findings based on the same plasma parcel are consistent with the results of statistical studies on the radial evolution of turbulence and provide a reference for theoretical modeling of turbulence in the inner heliosphere.

We examine the applicability of homogeneous, negligible cross-helicity magnetohydrodynamic (MHD) turbulence models incorporating "critical balance" (CB) and "scale-dependent dynamic alignment" (SDDA) to Parker Solar Probe observations of a highly Alfv & eacute;nic, high cross-helicity solar wind stream. At energy-injection scales, both Alfv & eacute;nic modes satisfy chi lambda +/-equivalent to tau A +/-/tau nl +/-<< 1 , where tau A +/- and tau nl +/- denote the linear and nonlinear timescales. The outward cascade remains weak, chi lambda+approximate to 0.2 , across all scales, while the inward cascade reaches CB at the onset of the inertial range ( chi lambda-similar to 1 ), yet with spectral scalings that depart from the canonical weak-to-strong transition. Within the domain conventionally designated as the inertial range, we identify two statistically distinct subranges. At larger scales (R2; 200-6000, di), the average eddy displays a field-aligned, tube-like morphology. An inverse correlation is observed between alignment angle and gradient intensity, and the conditional structure-function exponents zeta n-measured perpendicular to both the local mean field and fluctuation direction-agree with the predictions of B. D. G. Chandran et al. and A. Mallet & A. A. Schekochihin, although the parallel and displacement components show more concave scaling than anticipated. At smaller scales (R1; 10-100, di), spectra steepen, eddies become ribbon-like, and intermittency weakens. Within a narrow interval near the ion characteristic scales, eddies approach isotropy before the trend of increasing anisotropy resumes at smaller scales. Analysis using five-point increments further demonstrates a stronger multifractal character at kinetic scales than is resolved with conventional two-point methods. Finally, we discuss the influence of solar wind expansion, finite cross helicity, "anomalous coherence," and the emergence of a "helicity barrier" in modifying CB/SDDA phenomenology and shaping the statistical properties of solar wind turbulence.

Context. It has been recently accepted that the standard classification of the solar wind solely according to flow speed is outdated, and particular interest has been devoted to the study of the origin and evolution of so-called Alfvénic slow solar wind streams and to what extent such streams resemble or differ from fast wind. Aims. In March 2022, Solar Orbiter completed its first nominal phase perihelion passage. During this interval, it observed several Alfvénic streams, allowing for characterization of fluctuations in three slow wind intervals (AS1-AS3) and comparison with a fast wind stream (F) at almost the same heliocentric distance. Methods. This work makes use of Solar Orbiter plasma parameters from the Solar Wind Analyzer (SWA) and magnetic field measurements from the magnetometer (MAG). The magnetic connectivity to the solar sources of selected solar wind intervals was reconstructed using a ballistic extrapolation based on measured solar wind speed down to the (spherical) source surface at 2.5 Rs below which a potential field extrapolation was used to map back to the Sun. The source regions were identified using SDO/AIA observations. A spectral analysis of in situ measured magnetic field and velocity fluctuations was performed to characterize correlations, Alfvénicity, normalized cross-helicity, and residual energy in the frequency domain as well as intermittency of the fluctuations and spectral energy transfer rate estimated via mixed third-order moments. A machine learning technique was used to separate proton core, proton beam, and alpha particles and to study v-b correlations for the different ion populations in order to evaluate the role played by each population in determining the Alfvénic content of solar wind fluctuations. Results. The comparison between fast wind and Alfvénic slow wind intervals highlights the differences between the two solar wind regimes: The fast wind is characterized by larger amplitude fluctuations, and magnetic and velocity fluctuations are closer to equipartition of energy. In fact the Alfvénic slow wind streams appear to be on a spectrum of wind types, with AS1, originating from open field lines neighboring active regions and displaying similarities with the fast wind in terms of fluctuation amplitude and turbulence characteristics, but not with respect to the alpha particles and proton beams. The other two slow streams differed both in their sources as well as plasma characteristics, with AS2 coming from the expansion of a narrow coronal hole corridor and AS3 from a region straddling a pseudostreamer. The latter displayed the coldest and highest density but the slowest stream with the smallest fluctuation amplitude and greatest magnetic energy excess. It also showed the largest scatter in proton beam speeds and the greatest difference in speed between proton beam and alpha particles. Conclusions. This study shows how the old fast- slow solar wind dichotomy, already called into question by the observations of slower Alfvénic solar wind streams, should further be refined, as the Alfvénic slow wind, originating in different solar wind regions, show significant differences in density, temperature, and proton and alpha-particle properties in the inner heliosphere. The observations presented here provide the starting point for a better understanding of the origin and evolution of different solar wind streams as well as the evolving turbulence contained within.

The problems of heating and acceleration of solar wind particles are of significant and enduring interest in astrophysics. The interactions between waves and particles are crucial in determining the distributions of proton and alpha particles, resulting in non-Maxwellian characteristics, including temperature anisotropies and particle beams. These processes can be better understood as long as the beam can be separated from the core for the two major components of the solar wind. We utilized an alternative numerical approach that leverages the clustering technique employed in machine learning to differentiate the primary populations within the velocity distribution rather than employing the conventional bi-Maxwellian fitting method. Separation of the core and beam revealed new features for protons and alphas. We estimated that the total temperature of the two beams was slightly higher than that of their respective cores, and the temperature anisotropy for the cores and beams was larger than 1. We concluded that the temperature ratio between alphas and protons largely over 4 is due to the presence of a massive alpha beam, which is approximately 50% of the alpha core. We provided evidence that the alpha core and beam populations are sensitive to Alfvénic fluctuations and the surfing effect found in the literature can be recovered only when considering the core and beam as a single population. Several similarities between proton and alpha beams would suggest a common and local generation mechanism not shared with the alpha core, which may not have necessarily been accelerated and heated locally.

The Parker Solar Probe's discovery that magnetic switchbacks and velocity spikes in the young solar wind are abundant has prompted intensive research into their origin(s) and formation mechanism(s) in the solar atmosphere. Recent studies, based on in situ measurements and numerical simulations, argue that velocity spikes are produced through interchange magnetic reconnection. Our work studies the relationship between interplanetary velocity spikes and coronal brightenings induced by changes in the photospheric magnetic field. Our analysis focuses on the characteristic periodicities of velocity spikes detected by the Proton Alpha Sensor on the Solar Orbiter during its fifth perihelion pass. Throughout the time period analyzed here, we estimate their origin along the boundary of a coronal hole. Around the boundary region, we identify periodic variations in coronal brightening activity observed by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. The spectral characteristics of the time series of in situ velocity spikes, remote coronal brightenings, and remote photospheric magnetic flux exhibit correspondence in their periodicities. Therefore, we suggest that the localized small-scale magnetic flux within coronal holes fuels a magnetic reconnection process that can be observed as slight brightness augmentations and outward fluctuations or jets. These dynamic elements may act as mediators, bonding magnetic reconnection with the genesis of velocity spikes and magnetic switchbacks.

Context. The nature and evolution of the solar wind magnetic field rotations is studied in data from the Parker Solar Probe. Aims. We investigated the magnetic field deflections in the inner heliosphere below 0.5 au in a distance-and scale-dependent manner to shed some light on the mechanism behind their evolution. Methods. We used the magnetic field data from the FIELDS instrument suite to study the evolution of the magnetic field vector increment and rotation distributions that contain switchbacks. Results. We find that the rotation distributions evolve in a scale-dependent fashion. They have the same shape at small scales regardless of the radial distance, in contrast to larger scales, where the shape evolves with distance. The increments are shown to evolve towards a log-normal shape with increasing radial distance, even though the log-normal fit works quite well at all distances, especially at small scales. The rotation distributions are shown to evolve towards a previously developed rotation model moving away from the Sun. Conclusions. Our results suggest a scenario in which the evolution of the rotation distributions is primarily the result of the expansion-driven growth of the fluctuations, which are reshaped into a log-normal distribution by the solar wind turbulence.

We analyze data from multiple flybys by the Solar Orbiter, BepiColombo, and Parker Solar Probe (PSP) missions to study the interaction between Venus' plasma environment and the solar wind forming the induced magnetosphere. Through examination of magnetic field and plasma density signatures we characterize the spatial extent and dynamics of Venus' magnetotail, focusing mainly on boundary crossings. Notably, we observe significant differences in boundary crossing location and appearance between flybys, highlighting the dynamic nature of Venus' magnetotail. In particular, during Solar Orbiter's third flyby, extreme solar wind conditions led to significant variations in the magnetosheath plasma density and magnetic field properties, but the increased dynamic pressure did not compress the magnetotail. Instead, it is possible that the increased EUV flux at this time rather caused it to expand in size. Key findings also include the identification of several far downstream bow shock (BS), or bow wave, crossings to at least 60 (Formula presented.) (1 (Formula presented.) = 6,052 km is the radius of Venus), and the induced magnetospheric boundary to at least (Formula presented.) 20 (Formula presented.). These crossings provide insight into the extent of the induced magnetosphere. Pre-existing models from Venus Express were only constrained to within (Formula presented.) 5 (Formula presented.) of the planet, and we provide modifications to better fit the far-downstream crossings. The new model BS is now significantly closer to the central tail than previously suggested, by about 10 (Formula presented.) at 60 (Formula presented.) downstream.

A rare alignment of two spacecraft near the Sun captures energetics in the heliosphere.