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We study anisotropic magnetohydrodynamic (MHD) turbulence in the slow solar wind measured by Parker Solar Probe (PSP) and Solar Orbiter (SolO) during its first orbit from the perspective of variance anisotropy and correlation anisotropy. We use the Belcher & Davis approach (M1) and a new method (M2) that decomposes a fluctuating vector into parallel and perpendicular fluctuating vectors. M1 and M2 calculate the transverse and parallel turbulence components relative to the mean magnetic field direction. The parallel turbulence component is regarded as compressible turbulence, and the transverse turbulence component as incompressible turbulence, which can be either Alfvénic or 2D. The transverse turbulence energy is calculated from M1 and M2, and the transverse correlation length from M2. We obtain the 2D and slab turbulence energy and the corresponding correlation lengths from those transverse turbulence components that satisfy an angle between the mean solar wind flow speed and mean magnetic field θ UB of either (i) 65° < θ UB < 115° or (ii) 0° < θ UB < 25° (155° < θ UB < 180°), respectively. We find that the 2D turbulence component is not typically observed by PSP near perihelion, but the 2D component dominates turbulence in the inner heliosphere. We compare the detailed theoretical results of a nearly incompressible MHD turbulence transport model with the observed results of PSP and SolO measurements, finding good agreement between them.

Using data collected by the Indian Mars Orbiter Mission (MOM) in 2021 October, we investigated coronal regions of the Sun by analyzing the Doppler spectral width of radio signals to estimate solar wind velocity. A simplified equation is introduced to directly relate these two parameters. The study focuses on observations conducted from 2021 October 2 to October 14, a relatively quiet phase of solar cycle 25. The analysis targeted the coronal region within heliocentric distances of 5–8 R⊙, near the ecliptic plane. In this region, solar wind velocities ranged from 100 to 150 km s−1, while electron densities were on the order of 1010 m−3. We also compared our results with electron density observations and models derived from previous studies. Though the decrease in the electron densities with respect to increasing heliocentric distance matches quite well with the theoretical models, MOM estimates fall at the lower edge of the distribution. This difference may be attributed to the prolonged weak solar activity during the MOM observations, in contrast to prior studies conducted during periods of comparatively higher solar activity in earlier solar cycles.

The interaction between the solar wind and the Martian upper atmosphere involves mass loading by newly created pickup ions (PUIs). Accordingly, these PUIs can modify the solar wind flow upstream of the Martian bow shock through momentum exchange. While the relevant solar wind deceleration and a deflection opposite to the solar wind convective electric field direction have been identified in previous studies, the present study reveals an extra solar wind deflection transverse to both the solar wind flow and the convective electric field by statistically analyzing the Solar Wind Ion Analyzer and magnetometer data of Mars Atmosphere and Volatile EvolutioN between 2015 and 2019. This transverse deflection demonstrates opposite shifts under two groups of interplanetary magnetic field orientations, with mode values of approximately +2.3 and −2.1 km s−1. Both Student’s t-test and the Mann–Whitney U-test confirm the statistical significance of the transverse deflection reversal. An estimate based on momentum conservation between the solar wind and PUIs indicates that the observed transverse deflection is consistent with the expectation from PUI mass loading. This study provides the first direct observational evidence of the transverse solar wind deflection upstream of the Martian bow shock and demonstrates that the impact of PUI mass loading on the solar wind velocity is inherently three-dimensional, involving simultaneous a deceleration along the solar wind flow direction, a deflection opposite to the convective electric field direction, as well as an extra deflection perpendicular to both the solar wind flow and the convective electric field.

An outstanding gap in our knowledge of the solar wind is the relationship between switchbacks and solar wind turbulence. Switchbacks are large fluctuations, even reversals, of the background magnetic field embedded in the solar wind flow. It has been proposed that switchbacks may form as a product of turbulence and decay via coupling with the turbulent cascade. In this work, we examine how properties of solar wind magnetic field turbulence vary in the presence or absence of switchbacks. Specifically, we use in situ particle and fields measurements from Parker Solar Probe to measure magnetic field turbulent wave power, separately in the inertial and kinetic ranges, as a function of switchback magnetic deflection angle. We demonstrate that the angle between the background magnetic field and the solar wind velocity in the spacecraft frame (θ vB ) strongly determines whether Parker Solar Probe samples wave power parallel or perpendicular to the background magnetic field. Further, we show that θ vB is strongly modulated by the switchback magnetic deflection angle. In this analysis, we demonstrate that switchback deflection angle does not correspond to any significant increase in wave power in either the inertial range or at kinetic scales. This result implies that switchbacks do not strongly couple to the turbulent cascade in the inertial or kinetic ranges via turbulent wave–particle interactions. Therefore, we do not expect switchbacks to contribute significantly to solar wind heating through this type of energy conversion pathway although contributions via other mechanisms, such as magnetic reconnection, may still be significant.

The Martian bow shock stands as the first defense against the solar wind and shapes the Martian magnetosphere. Previous studies showed the correlation between the Martian bow shock location and solar wind parameters. Here we present direct evidence of solar wind effects on the Martian bow shock by analyzing Tianwen‐1 and MAVEN data. We examined three cases where Tianwen‐1 data show rapid oscillations of the bow shock, while MAVEN data record changes in solar wind plasma and magnetic field. The results indicate that the bow shock is rapidly compressed and then expanded during the dynamic pressure pulse in the solar wind, and is also oscillated during the IMF rotation. The superposition of variations in multiple solar wind parameters leads to more intensive bow shock oscillation. This study emphasizes the importance of joint observations by Tianwen‐1 and MAVEN for studying the real‐time response of the Martian magnetosphere to the solar wind. Plain Language Summary The Martian bow shock is a standing shock wave that forms ahead of Mars due to the interaction with the solar wind, where the supersonic solar wind flow drops sharply to subsonic. The bow shock plays a crucial role in shaping the Martian magnetosphere and controlling the energy, mass, and momentum exchange between the solar wind and the Martian atmosphere. Previous research has shown that the position of Mars' bow shock is related to the solar wind. This research presents two‐spacecraft observations of how the solar wind affects the Martian bow shock. By analyzing data obtained by two orbiters, Tianwen‐1 and MAVEN, we find that the bow shock quickly contracts when the solar wind dynamic pressure rises or when the interplanetary magnetic field direction turns radial. When there are multiple changes in the solar wind at the same time, the bow shock moves around even more. This study shows how important it is to look at data from Tianwen‐1 and MAVEN at the same time to understand how Mars' magnetosphere reacts to the solar wind. Key Points First observations of the real‐time response of the Martian bow shock to changes in the upstream solar wind Direct evidence of the compression of the Martian bow shock under increased solar wind dynamic pressure Direct evidence of motion of the Martian bow shock caused by the rotation of the interplanetary magnetic field

We investigate the origin of mesoscale structures in the solar wind called microstreams, defined as enhancements in the solar wind speed and temperature that last several hours. They were first clearly detected in Helios and Ulysses solar wind data and are now omnipresent in the “young” solar wind measured by the Parker Solar Probe and Solar Orbiter. These recent data reveal that microstreams transport a profusion of Alfvénic perturbations in the form of velocity spikes and magnetic switchbacks. In this study, we use a very-high-resolution 2.5D MHD model of the corona and the solar wind to simulate the emergence of magnetic bipoles interacting with the preexisting ambient corona and the creation of jets that become microstreams propagating in the solar wind. Our high-resolution simulations reach sufficiently high Lundquist numbers that capture the tearing mode instability that develops in the reconnection region and produces plasmoids released with the jet into the solar wind. Our domain runs from the lower corona to 20 R ⊙, which allows us to track the formation process of plasmoids and their evolution into Alfvénic velocity spikes. We obtain perturbed solar wind flows lasting several hours with velocity spikes occurring at characteristic periodicities of about 19 minutes. We retrieve several properties of the microstreams measured in the pristine solar wind by the Parker Solar Probe, namely an increase in wind velocity of about 100 km s−1 during a stream's passage together with superposed velocity spikes of also about 100 km s−1 released into the solar wind.

Magnetic field fluctuations measured in the heliosheath by the Voyager spacecraft are often characterized as compressible, as indicated by a strong fluctuating component parallel to the mean magnetic field. However, the interpretation of the turbulence data faces the caveat that the standard Taylor’s hypothesis is invalid because the solar wind flow velocity in the heliosheath becomes subsonic and slower than the fast magnetosonic speed, given the contributions from hot pickup ions (PUIs) in the heliosheath. We attempt to overcome this caveat by introducing a 4D frequency-wavenumber spectral modeling of turbulence, which is essentially a decomposition of different wave modes following their respective dispersion relations. Isotropic Alfvén and fast mode turbulence are considered to represent the heliosheath fluctuations. We also include two dispersive fast wave modes derived from a three-fluid theory. We find that (1) magnetic fluctuations in the inner heliosheath are less compressible than previously thought, an isotropic turbulence spectral model with about 25% in compressible fluctuation power is consistent with the observed magnetic compressibility in the heliosheath; (2) the hot PUI component and the relatively cold solar wind ions induce two dispersive fast magnetosonic wave branches in the perpendicular propagation limit, PUI fast wave may account for the spectral bump near the proton gyrofrequency in the observable spectrum; (3) it is possible that the turbulence wavenumber spectrum is not Kolmogorov-like although the observed frequency spectrum has a −5/3 power-law index, depending on the partitioning of power among the various wave modes, and this partitioning may change with wavenumber.

The purpose of this paper is to present a novel optimal theoretical framework based on potential flow theory in ideal gas dynamics, which provides a smooth extrapolation of Parker’s steady solar wind model to the unsteady case. The viability of this framework is illustrated by providing the first ever systematic theoretical formulation to successfully address the long-standing open issue of regularization of the singularity associated with the Parker sonic critical point (where the solar wind flow velocity equals the speed of sound in the gas) in the linear stability problem of Parker’s steady solar wind solution. This development involves going outside the framework of the linear perturbation problem and incorporating the dominant nonlinearities in this dynamical system, and hence provides an appropriate nonlinear recipe to regularize this singularity. The stability of Parker’s steady wind solution is found to also extend to the neighborhood of the Parker sonic critical point upon analyzing the concomitant nonlinear problem. The new theoretical framework given here seems, therefore, to have the potential to provide a viable basis for future formulations addressing various theoretical aspects of the unsteady version of Parker’s steady solar wind model.

A well-known shortcoming of single-spacecraft spectral analysis is that only the 1D wavenumber spectrum can be observed, assuming the characteristic wave propagation speed is much smaller than the solar wind flow speed. This limitation has motivated an extended debate about whether fluctuations observed in the solar wind are waves or structures. Multispacecraft analysis techniques can be used to calculate the wavevector independent of the observed frequency, thus allowing one to study the frequency–wavenumber spectrum of turbulence directly. The dispersion relation for waves can be identified, which distinguishes them from nonpropagating structures. We use magnetic field data from the four Magnetospheric Multiscale (MMS) spacecraft to measure the frequency–wavenumber spectrum of solar wind turbulence based on the k-filtering and phase differencing techniques. Both techniques have been used successfully in the past for the Earth’s magnetosphere, although applications to solar wind turbulence have been limited. We conclude that the solar wind turbulence intervals observed by MMS show features of nonpropagating structures that are associated with frequencies close to zero in the plasma rest frame. However, there is no clear evidence of propagating Alfvén waves that have a nonzero rest-frame frequency. The lack of waves may be due to instrument noise and spacecraft separation. Our results support the idea of turbulence dominated by quasi-2D structures.

Observations of the young solar wind by the Parker Solar Probe (PSP) mission reveal the existence of intense plasma wave bursts with frequencies between 0.05 and 0.20f ce (tens of hertz up to ∼300 Hz) in the spacecraft frame. The wave bursts are often collocated with inhomogeneities in the solar wind magnetic field, such as local dips in magnitude or sudden directional changes. The observed waves are identified as electromagnetic whistler waves that propagate either sunward, anti-sunward, or in counter-propagating configurations during different burst events. Being generated in the solar wind flow, the waves experience significant Doppler downshift and upshift of wave frequency in the spacecraft frame for sunward and anti-sunward waves, respectively. Their peak amplitudes can be larger than 2 nT, where such values represent up to 10% of the background magnetic field during the interval of study. The amplitude is maximum for propagation parallel to the background magnetic field. We (i) evaluate the properties of these waves by reconstructing their parameters in the plasma frame, (ii) estimate the effective length of the PSP electric field antennas at whistler frequencies, and (iii) discuss the generation mechanism of these waves.