Lucas Colomban

Magnetic switchbacks (SBs)—the localized magnetic structures with magnetic field direction inclined at an angle θ relative to the background B ₀ —in the young solar wind have been associated with enhanced ion-scale wave activity and local plasma heating. It remains debated whether the apparent wave power increase is intrinsic or mainly caused by sampling geometry. In this work, we analyze magnetic and electric field fluctuations measured by Parker Solar Probe, focusing on the 0.1–3 f _cp frequency band that spans the transition from the MHD inertial range to ion-kinetic scales. By decomposing magnetic fluctuations into field-aligned and transverse components and comparing SB and non-SB intervals at comparable magnetic field orientations, we test whether the enhanced wave power can be explained solely by geometric sampling of an anisotropic turbulent cascade. We find that the transverse magnetic power δB _⊥ is systematically enhanced inside SBs across a wide range of magnetic field rotation angles θ . The enhancement persists even at small and intermediate deflections, where geometric projection alone predicts weak power, indicating an intrinsic origin beyond sampling geometry. The inertial-range spectral indices also remain similar between SB and non-SB intervals despite the enhanced wave power inside SBs, suggesting that the underlying turbulence cascade is largely preserved. This excess δB _⊥ coincides with elevated proton temperatures and enhanced electric field fluctuations, supporting the interpretation that SBs act as localized sites of cross-scale energy transfer and ion-scale dissipation in the near-Sun solar wind.

Diffusive shock acceleration, at shocks from coronal mass ejections to supernova remnant blast waves, presupposes a scattering wave field that the accelerated particles themselves maintain. This self-regulation has not been resolved in situ. We report Parker Solar Probe observations of a fast (≃2800 km s ⁻¹ ), near-parallel interplanetary shock at 0.24 au on 2023 March 13 and separate its upstream wave field into four families, a classification not made before at a fast shock near the Sun. Right-hand and left-hand circularly polarized families over a common wavenumber band, with a field-aligned linearly polarized family, are cyclotron resonant with the suprathermal-to-MeV protons streaming from the shock: the beam drives the field that scatters it, and the measured mean free path, half the precursor scale, leaves the beam anisotropic enough to sustain the drive. Outside this loop lies a weak, oblique, linearly polarized component, a few percent of the wave power, resolved here for the first time at an in situ foreshock. Its in-phase density and field-magnitude fluctuations identify the compressive part as fast magnetosonic and shift the cyclotron resonance energies of the resonant families by up to 13% along the precursor. Acceleration at shocks inside 0.3 au is governed upstream, in a foreshock the shock builds for itself.

Interactions entre les ondes de sifflement et les électrons suprathermiques du vent solaire : observations de Solar Orbiter et de Parker Solar Probe: L'évolution de la fonction de distribution des électrons du vent solaire avec la distance héliocentrique présente des caractéristiques encore inexpliquées. Nous pouvons citer l'augmentation de la largeur angulaire du faisceau d'électrons suprathermiques aligné avec le champ magnétique (Strahl). Cette augmentation est associée à une augmentation de la densité relative des électrons suprathermiques plus isotrope du halo et à la diminution rapide du flux de chaleur. Dans cette thèse, nous cherchons à préciser le rôle des interactions ondes-particules dans cette dynamique et en particulier à savoir si les ondes de sifflement peuvent expliquer l'augmentation observée de la largeur angulaire du Strahl. Pour ce faire, nous effectuons d'abord une analyse statistique de ces ondes avec Solar Orbiter et Parker Solar Probe (PSP), entre 0,2 et 1 UA. 110 000 paquets d'ondes sont détectés et caractérisés dans le référentiel du plasma. L'écrasante majorité des ondes ont un angle de propagation (θ) quasi-aligné avec le champ magnétique. Au-delà de 0,3 UA, les ondes sont alignées avec le Strahl (y compris lors des "switchbacks"), et il n'y a pratiquement pas d'ondes obliques ( θ∼70°). Ces dernières ont pourtant été prédites par de nombreuses études et sont théoriquement très efficaces pour diffuser les électrons du Strahl. À 0,2 UA, environ la moitié des ondes sont "counter-streaming" et la proportion d'ondes obliques reste très faible. Les caractéristiques des ondes sont ensuite utilisées pour calculer les coefficients de diffusion dans le cadre de la théorie quasi-linéaire. Ces coefficients sont intégrés, en utilisant l'occurrence des ondes, afin de déduire l'effet global des siffleurs sur les électrons suprathermiques. Au-delà de 0,3 UA, les ondes de sifflement peuvent expliquer l'augmentation observée de la largeur angulaire du Strahl et sont efficaces pour isotropiser le halo. Nous montrons que la diffusion du Strahl est due à des ondes légèrement obliques, θ∈[15,45]°. Près de 0,2 UA, les ondes counter-streaming diffusent les électrons de Strahl plus efficacement que les ondes alignées avec le Strahl de deux ordres de grandeur. Par ailleurs, un problème technique avec une bobine du fluxmètre tri-axial (SCM) de PSP à la fin de l'orbite 1 nous a empêché de déduire directement les propriétés de polarisation des ondes de sifflement pour les orbites suivantes. Nous proposons une technique utilisant les champs électromagnétiques disponibles pour reconstruire les composantes manquantes (et donc les propriétés de polarisation des ondes) en négligeant le champ électrique parallèle au champ magnétique. Cette technique est applicable avec les hypothèses suivantes : (i) la fréquence de l'onde dans le repère du plasma est petite par rapport à la fréquence cyclotronique électronique ; (ii) un petit angle de propagation par rapport au champ magnétique ; et (iii) une grande vitesse de phase par rapport à la vitesse du vent solaire perpendiculaire au champ magnétique. La méthode ne peut pas être appliquée si le champ magnétique est aligné avec la bobine du SCM concernée. Nous validons notre méthode en utilisant les mesures en mode rafale effectuées lors de l'orbite 1. Les conditions de reconstruction sont satisfaites pour 80% des siffleurs détectés. Cette méthode devrait permettre de révéler pour la première fois les propriétés de polarisation des ondes sifflantes jusqu'à 10 rayons solaires (∼0.05 UA). Ce travail est une étape importante pour la prise en compte des interactions ondes-particules dans les modèles cinétiques et pour améliorer notre description du vent solaire.

The formation of a collisionless shock is the result of a balance between nonlinear steepening and processes that counteract this steepening. Dispersive shocks are shocks in which dispersive processes counterbalance the front steepening and are formed when the dispersive spatial scale exceeds scales associated with resistive processes. Oblique dispersive shocks are characterized by a phase standing wave precursor adjacent to the magnetic ramp. The whistler critical Mach number is defined as the maximum Mach number for which a linear whistler wave can phase stand upstream of the shock front. It was widely accepted that if the Mach number exceeds , linear whistler waves propagating along the shock normal are not able to “phase stand” in the upstream flow, and “…the shock will be initiated by a monotonic ramp.” (Kennel et al., 1985, https://doi.org/10.1029/gm034p0001 ). In this study, we present results of numerical simulations and observations of shocks with that reveal the occurrence of an alternative scenario. For both the shock resulting from kinetic particle‐in‐cell simulations and that observed by MMS, the propagation direction of the precursor deviates from the shock normal direction. As a result, the velocity of the surface of constant phase along the shock normal exceeds the phase speed of these waves. It is shown that the propagation of the surface of constant phase along the shock normal occurs at a velocity that is nearly equal to the shock speed. Hence, these waves are “phase standing along the shock normal” in spite of .

During the total solar eclipse in the United States on 8 April 2024, we observed the amplitude and phase of Very Low Frequency (VLF) signals from five U.S. Navy VLF transmitters using a novel geometry of VLF receivers deployed along and across the totality path. Nine receiver sites (four of them inside the totality path) were deployed to collect the data in different transmitter‐receiver configurations relative to the totality path, which intersected with the radio propagation paths from the Cutler, MA (NAA, 24 kHz) and the LaMoure, ND (NML, 25.2 kHz) Navy transmitters. The transmitters themselves experienced 98.7% (NAA) and 68% (NML) solar obscuration at 75 km altitude. The novelty of the observations is the near‐total obscuration of one of the transmitters and observations of several radio propagation paths closely aligned to the path of totality. This configuration enabled observations of the effects of the Moon's shadow progression from Texas to Maine for over three hours on a total of 27 radio paths (with coverage from 55% to 100%) with transmitter‐to‐receiver distances ranging from 780 to 7,700 km. Both positive and negative (5–10 dB) amplitude changes were observed by receivers throughout the eclipse period. The observed phase changes were mostly negative. The unique observations of VLF propagation along the totality path produced a temporally dynamic quasiperiodic response in amplitude that can be used to determine the gradients and spatial scales of the eclipse effect on the lower ionosphere. For observations within 780 km of NAA, the eclipse produced a 13 dB surge in amplitude.

Switchbacks (SBs) are localized magnetic field deflections in the solar wind, marked by abrupt changes in the magnetic field direction relative to the ambient solar wind. Observations on board Parker Solar Probe (PSP) at heliocentric distances below 50 solar radii R _S showed that within SBs, perturbations in the magnetic field ( Δ B ) and the bulk solar wind velocity ( Δ V ) align, i.e., Δ B ∼ Δ V , producing enhanced radial velocity spikes. In this study, we examine the characteristics of SB boundaries, with particular attention to the role of boundary shear flow instabilities (Kelvin–Helmholtz instability; KHI) for surface wave phenomena based on the in situ magnetic field, plasma speed, and plasma density measurements from PSP. The results indicate that SB boundaries can be unstable for generating KHI-driven surface waves, suggesting that the wave activity observed at SB boundaries is caused by shear flow instabilities. In addition, the continued development of KHI may lead to boundary erosion, contributing to the radial evolution of SBs via structural weakening or broadening. However, when Δ B and Δ V are closely aligned, the boundary remains stable unless the velocity shear significantly exceeds the magnetic shear. Since the observed velocity shear typically ranges from 40% to 90% of magnetic shear, the instability condition is generally not satisfied. Thus, the configuration leading to the instability arises from deviations from precise alignment of Δ B and Δ V in the young solar wind, and the release of the KHI presumably leads to the formation of the Δ B and Δ V alignment observed at SB boundaries located at 35–55 R _S .

The Parker Solar Probe (PSP) mission has revealed frequent occurrences of switchbacks (SBs) and small-scale magnetic flux ropes (SMFRs) as prominent structures within the solar wind. These mesoscale features are observed across all heliocentric distances, with heightened activity in the young solar wind, such as successive SMFRs, blobs, and SBs using PSP in situ observations. One study, in particular, focuses on SMFRs observed during the intervals of PSP corotating with the Sun, which suggests a similar source of the observed solar wind. In this paper, we identified SBs at the boundaries of SMFRs as a regularly observed phenomenon and found instances where SBs and SMFRs co-occur, with the significance level α < 0.05. The SMFR-related SBs—observed at the leading and trailing edges of an SMFR—exhibit well-organized axial co-orientations, with their polarity flipping, meaning the radial direction remains constrained while the transversal field reverses. Furthermore, the axial field directions of SMFR-related SBs appear to be more closely connected than to another SB that is spatially closer and are linked to the SMFR orientation. Our analysis of their relative geometry, which examines the alignment between SBs and the SMFR axis, reveals a distinct tendency emphasizing their correlation, further supporting the idea that the axes of SMFR-related SBs are presumably determined by the SMFR orientation. Observations suggest that a fraction of SBs are spatially and temporally associated with SMFRs, implying that processes related to SMFR boundaries may contribute to SB formation or that SBs tend to develop in magnetic environments shaped by SMFRs.

We present statistics on the polarization properties of whistler waves in the solar wind down to 0.05 AU (10.75 RS) using data from Parker Solar Probe's first 17 encounters. These properties were previously uncharacterized due to a technical issue affecting one component of the search‐coil magnetometer after Encounter 1. Using a reconstruction technique, we show that whistlers are predominantly quasi‐parallel to the magnetic field across all distances and propagation directions, with 87% exhibiting a propagation angle relative to the background magnetic field below 25. Waves with above 25 are observed with significantly lower amplitudes. Sunward whistlers (including counter‐propagating (CP) cases, where anti‐sunward waves are observed at the same time) statistically exhibit higher normalized amplitudes and frequencies than anti‐sunward whistlers. We discuss generation mechanisms and wave‐particle interactions. Specifically, CP sunward and sunward waves interact via less efficient resonances with Strahl electrons ( eV).

In this study, we present the first-ever direct measurements of synchrotron-emitting heliospheric traveling shocks, intercepted by the Parker Solar Probe (PSP) during its close encounters. Given that much of our understanding of powerful astrophysical shocks is derived from synchrotron radiation, these observations by PSP provide an unprecedented opportunity to explore how shocks accelerate relativistic electrons and the conditions under which they emit radiation. The probe’s unparalleled capabilities to measure both electromagnetic fields and energetic particles with high precision in the near-Sun environment has allowed us to directly correlate the distribution of relativistic electrons with the resulting photon emissions. Our findings reveal that strong quasi-parallel shocks emit radiation at significantly higher intensities than quasi-perpendicular shocks due to the efficient acceleration of ultrarelativistic electrons. These experimental results are consistent with theory and recent observations of supernova remnant shocks and advance our understanding of shock physics across diverse space environments.