Andrea Larosa

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.

The velocity distribution functions (VDFs) of space plasma typically present non-Maxwellian shapes due to the very low level of collisisionality. The small scale gradients of the VDFs could be the key feature to explain heating and dissipation, inibiting the revesibility of the energy exchange between fields and particles once a significant level of complexity is achieved.In this work, we investigate the solar wind protons VDFs fine features and their relation to different measures of the real space turbulent cascade. We explore different solar wind regimes and heliocentrice distances by using both Parker Solar Probe and Solar Orbiter data.These results, suggestive of the presence of a dual velocity-real space cascade, contribute to a better understanding of turbulence in space plasmas.

Magnetic switchbacks are fluctuations in the solar wind in which the interplanetary magnetic field sharply deflects away from its background direction so as to create folds in magnetic field lines while remaining of roughly constant magnitude. The magnetic field and velocity fluctuations are extremely well correlated in a way corresponding to Alfvénic fluctuations propagating away from the Sun. For a background field which is nearly radial this causes an outwardly propagating jet to form. Switchbacks and their characteristic velocity jets have recently been observed to be nearly ubiquitous by Parker Solar Probe with in situ measurements in the inner heliosphere within 0.3 AU. Their prevalence, substantial energy content, and potentially fundamental role in the dynamics of the outer corona and solar wind motivate the significant research efforts into their understanding. Here we review the in situ measurements of these structures (primarily by Parker Solar Probe). We discuss how they are identified and measured, and present an overview of the primary observational properties of these structures, both in terms of individual switchbacks and their collective arrangement into “patches”. We identify both properties for which there is a strong consensus and those that have limited or qualified support and require further investigation. We identify and collate several open questions and recommendations for future studies.

Shocks driven by coronal mass ejections (CMEs) are the most powerful accelerators of gradual solar energetic particles (SEPs) in the inner heliosphere. On 2023 March 13, a halo CME, as seen from the Solar and Heliospheric Observatory (SOHO) and the Sun TErrestrial Relations Observatory (STEREO), gave rise to a strong SEP event. In this work, we aim to analyze this CME-driven shock from multiple spacecraft, using both remote-sensing observations from STEREO-A/COR2 and in situ data from the Parker Solar Probe (PSP), Solar Orbiter (SolO), and Wind. In order to determine its direction of propagation and kinematic properties, we model the shock geometry using STEREO-A/COR2 and SOHO Large Angle and Spectrometer Coronagraph (LASCO)/C3 observations as an expanding ellipsoid. The density compression ratio of the shock is determined by fitting the brightness profile from the coronagraphic images with that obtained from raytracing simulations of a double-Gaussian shock-density profile. We compare physical quantities such as compression ratio and Alfvénic Mach number derived from remote-sensing observations with in situ measurements by PSP, SolO, STEREO-A, and Wind. From STEREO-A/COR2, we determine the compression ratio around the entire shock front in the corona, finding significant inhomogeneities that can impact the values found during in situ crossings. Following the evolution of the parameters characterizing the CME from the source to space, we find that closer to the Sun both the gas compression ratio and the Alfvénic Mach number remain almost constant, while they increase at larger radial distances. This indicates a nontrivial evolution of the shock parameters during its journey through the interplanetary space.

Context. Energetic particles in interplanetary space are normally measured on timescales that are long compared to the ion gyroperiod. Such observations by necessity average out the microphysics associated with the acceleration and transport of 10 s–100 s keV particles. Aims. We aim to investigate previously unseen nonequilibrium features that only become observable at very high time resolution and discuss possible explanations for these features. Methods. We used unprecedentedly high time-resolution data that were acquired by the in situ instruments on Solar Orbiter in the vicinity of two interplanetary shocks observed on 29 Nov 2023 07:51:17 UTC and 30 Nov 2023 10:47:26 UTC at ∼0.83 astronomical units from the Sun. Results. The solar-wind proton-beam population follows the magnetic field instantaneously and on timescales that are significantly shorter than a gyro-period. Energetic particles, despite sampling large volumes of space, vary on remarkably short timescales, typically on the order of the convection time of their gyro-radius. Nonequilibrium features such as bump-on-tail distributions of energetic particles are formed by small-scale magnetic structures in the interplanetary magnetic field. Conclusions. High time-resolution observations show previously unobserved microphysics in the vicinity of two traveling interplanetary shocks, including ion reflection at a current sheet, which may explain where ions are reflected in shock acceleration.

Magnetic switchbacks are fluctuations in the solar wind in which the interplanetary magnetic field sharply deflects away from its background direction so as to create folds in magnetic field lines while remaining of roughly constant magnitude. The magnetic field and velocity fluctuations are extremely well correlated in a way corresponding to Alfvénic fluctuations propagating away from the Sun. For a background field which is nearly radial this causes an outwardly propagating jet to form. Switchbacks and their characteristic velocity jets have recently been observed to be nearly ubiquitous by Parker Solar Probe with in situ measurements in the inner heliosphere within 0.3 AU. Their prevalence, substantial energy content, and potentially fundamental role in the dynamics of the outer corona and solar wind motivate the significant research efforts into their understanding. Here we review the in situ measurements of these structures (primarily by Parker Solar Probe). We discuss how they are identified and measured, and present an overview of the primary observational properties of these structures, both in terms of individual switchbacks and their collective arrangement into ``patches''. We identify both properties for which there is a strong consensus and those that have limited or qualified support and require further investigation. We identify and collate several open questions and recommendations for future studies.

Magnetic switchbacks, often observed in the near-Sun solar wind, have received increased interest in recent years due to their potential role in mediating the heating and acceleration of the solar wind, but their origin remains debated. In this work, we present a coordinated observation of a switchback cluster by BepiColombo (0.35 au) and Solar Orbiter (0.67 au), obtained during the alignment between 2021 October 6 and 8, which enabled the first direct investigation of switchback evolution across heliocentric distances. In particular, the stream observed by the spacecraft can be tracked back to the boundary of an equatorial coronal hole. Large-scale switchbacks exhibit strong sublinear expansion, thus appearing almost unevolved in morphology during the propagation when the spacecraft cutting-angle effect is taken into account. The stable magnetic configuration of the analyzed switchbacks suggests that they can be identified as small-scale flux ropes. Indeed, for shear-driven instabilities triggered by stream interaction with the background slow wind, short-living (one eddy turnover time, τ ∼ 1 hr) switchbacks would be expected compared to the travel time from BepiColombo to Solar Orbiter (∼38 hr). These findings provide critical insights on switchback origin and evolution, potentially constraining future phenomenologies on their formation. A useful consequence of our observations is that statistical analyses on switchback evolution should always account for the cutting-angle effect.

In space plasmas, the rarity of collisions leads to complex structures in the velocity space where a turbulent cascade of the velocity distribution function (VDF) fluctuations is thought to occur. Previous studies have explored this phenomenon using the Hermite decomposition of the ion VDF in both magnetosheath data and numerical simulations. In this work, we investigate the Hermite spectrum of the ion VDFs measured by the Parker Solar Probe in the inner heliosphere. We analyze a super-Alfvénic stream at a radial distances of R ≈ 28 R _⊙ and a sub-Alfvénic stream at R ≈ 11 R _⊙ , the former characterized by a prevalence of VDFs with suprathermal beams (also known as hammerhead distributions). The Hermite analysis is also compared with various proxies of energization and dissipation, in order to establish a connection between turbulent cascades in real space and those in the velocity space. A qualitative agreement between the energization proxies and the Hermite analysis is observed. The results are suggestive of the presence of a dual cascade in real and velocity space.

We investigate a class of ion-scale magnetic solitary structures in the solar wind, characterized by distinct magnetic field enhancements and bipolar rotations over spatial scales of several proton inertial lengths. These structures are revisited using high-resolution data from the Solar Orbiter and Parker Solar Probe missions. Using a machine learning–based method, we identified nearly a thousand such structures, providing new insights into their evolution and physical properties. Statistical analysis shows that these structures are more abundant closer to the Sun, with occurrence rates peaking around 30−40 R _⊙ and decreasing farther out. High-cadence measurements reveal that these structures are predominantly found in low-beta ( β ≤ 1) environments, with consistent fluctuations in density, velocity, and magnetic field. Magnetic field enhancements are often accompanied by plasma density drops, which, under near-pressure balance, limit field increases. This leads to small fractional field enhancements near the Sun (approximately 0.01 at 20 R _⊙ ), making detection challenging. Magnetic field variance analysis indicates that these structures are primarily oblique to the local magnetic field. Alfvénic velocity–magnetic field correlations suggest that most of these structures, unlike most near-Sun solar wind fluctuations, exhibit sunward-directed Alfvénic polarization in the plasma frame. We compare these findings with previous studies, discussing possible generation mechanisms and their implications for the turbulent cascade in the near-Sun Alfvénic solar wind. While these structures might be Alfvénic solitons, vortices, or flux ropes, we refrain from a definitive classification pending further evidence. Further high-resolution observations and simulations are needed to fully understand their origins and impacts.

Shocks driven by coronal mass ejections (CMEs) are the most powerful accelerators of gradual solar energetic particles (SEPs) in the inner heliosphere. On 2023 March 13, a halo CME, as seen from the Solar Heliospheric Observatory (SoHO) and the Sun TErrestrial Relations Observatory (STEREO), gave rise to a strong SEP event. In this work, we aim to analyze this CME-driven shock from multiple spacecraft, using both remote sensing observations from STEREO-A/COR2 and in-situ data from Parker Solar Probe (PSP), Solar Orbiter (SolO), and Wind. In order to determine its direction of propagation and kinematic properties, we model the shock geometry using STEREO-A/COR2 and SoHO/LASCO/C3 observations as an expanding ellipsoid. The density compression ratio of the shock is determined by fitting the brightness profile from the coronagraphic images with that obtained from raytracing simulations of a double-Gaussian shock density profile. We compare physical quantities such as compression ratio and Alfvénic Mach number derived from remote sensing observations with in-situ measurements by PSP, SolO, STEREO-A, and Wind. From STEREO-A/COR2, we determine the compression ratio around the entire shock front in the corona, finding significant non-homogeneities that can impact the values found during in-situ crossings. Following the evolution of the parameters characterizing the CME from the source to space, we find that closer to the Sun, both the gas compression ratio and the Alfvénic Mach number remain almost constant, while they increase at larger radial distances. This indicates a non-trivial evolution of the shock parameters during its journey through the interplanetary space.