Magnetic switchbacks, large-amplitude, localized Alfvenic like rotations of the solar wind magnetic field, have been the subject of intensive investigation, with approximately 200 refereed papers published in the last decade. Yet, fundamental controversies persist regarding whether switchbacks can be described with Ideal MHD (magnetohydrodynamic) physics or Hall-MHD physics and whether their origin is at the solar surface or in the solar wind. To settle these controversies, we present Parker Solar Probe electric field measurements between 13 and 40 solar radii, which show that switchbacks have non-zero electric fields in the plasma frame, a finding that definitively settles the physics controversy by proving that switchbacks are Hall-MHD, not Ideal MHD, structures. Along with these electric fields, there are enhanced Poynting vectors having three components with similar magnitudes that exist only inside the switchbacks. These facts contradict the view of switchbacks as simple outward-propagating pulses. Together, they resolve one controversy by showing that switchbacks in the young solar wind are a non-MHD process. They contribute to the second (source) controversy by identifying switchbacks as sites of active, in-situ, evolution. These findings provide a new framework for understanding energy transport and dissipation in astrophysical plasmas.

Magnetic switchbacks (SB) – 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 the same local magnetic field angle, we test whether SBs sample an anisotropic cascade from different viewing angles or host intrinsically amplified wave activity. We find that the transverse magnetic power δB_⊥ is systematically enhanced inside switchbacks 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.

A major goal of solar physics is understanding the transition of the medium from the closed-loop magnetic configuration of the corona to the open structure of the heliospheric current sheet. The evolution of solar wind streamers, an essential component of this transition, has been observed in-situ for the first time by measuring a pass through a streamer stalk at 11.7 solar radii. A plasma rest frame DC electric field, reaching an amplitude of 400 mV/m, was observed as the magnetic field changed from +1100 nT to -1100 nT. This electric field violates the frozen in condition so it must be understood via the non-MHD Hall term of the Generalized Ohm's Law. Two plasma regimes containing the large electric field were observed. In the first regime, the magnetic field was large, the ion beta was small ( 0.1), the ratio of the ion skin depth to ion gyroradius was large (>10), and the current of 0.3 mA/m2 was carried by electrons. In the second regime, the magnetic field was small, the ion beta was large ( 10), the ion skin depth was equal to the ion gyroradius, and the 10 mA/m2 current was carried by the ions. Also, measurements during 15 such crossings showed that the plasma flow speed inside the current sheets exceeded that outside six times and the 326 km/sec average speed inside the current sheets exceeded the 266 km/sec outside. Such findings challenge the traditional consensus that streamers are the source of the "slow" solar wind.

Near the 10 solar radius perihelion of Parker Solar Probe encounter 24, a confined region containing an enhanced plasma density (25,000 per cubic centimeter) and broadband electrostatic waves was encountered. The solar wind velocity (200 kilometers per second) and ion temperature (25 electron Volts) were significantly reduced as compared to their values in the ambient solar wind. Because the polarity of the radial magnetic field did not change sign on the two sides of the crossing and the crossed region contained a double-peaked plasma structure, the spacecraft must have passed near the base of a pseudo-streamer. In the plasma frame, an electric field as large as 400 millivolts per meter was detected during the crossing. This field is not associated with an ExB drift because it is observed in the plasma rest frame. Instead, it is balanced by the Generalized Ohm's Law terms whose current dependent term associated with this electric field was less than 1 milliampere per square meter, corresponding to a drift velocity less than 2.5 kilometers per second. The region also contained a turbulent plasma with density fluctuations as large as 0.3, suggesting that the resistive term in the Generalized Ohm's Law could be significant. In addition, both the density and electric field had non-zero slopes as functions of time, suggesting that the pressure gradient term in the Generalized Ohm's Law also mattered. This large electric field in the plasma rest frame may be the first and largest such field ever reported.

The extended solar corona at 10–30 solar radii is essentially devoid of all waves below 100 kHz other than triggered ion acoustic waves (TIAWs), which consist of a low-frequency (LF) electromagnetic wave at a frequency of a few Hz coupled to one or more electrostatic waves at a few hundred Hz, such that the amplitudes of the high-frequency (HF) waves peak at a fixed phase of each LF wave period. All the waves in a TIAW event travel at the same phase speed, which is found to be 150 km s ^−1 (the ion acoustic speed was about 100 km s ^−1 ). It has not been possible to explain the TIAW as a resonant wave–wave interaction, so a nonresonant interaction has been considered in which the loss of energy by the LF wave is used to both heat the electrons and grow the HF waves. Evidence in support of this explanation is described, and a particle-in-cell simulation that discusses this process is summarized. This simulation demonstrates wave–particle interactions driven by the enhanced LF fluctuations, which subsequently modify the proton velocity distribution function and create conditions favorable for the growth of HF waves. This interplay between a pair of waves, mediated by modifications of plasma parameters and energy conversion, represents a significant nonlinear process in plasma physics, the study of which will deepen the understanding of energy transfer, wave generation, and plasma dynamics in diverse astrophysical environments.

Understanding the mechanisms governing energy dissipation and localized heating in collisionless astrophysical plasmas remain a fundamental challenge to the standard framework of Ideal Magnetohydrodynamics (MHD). In these non-equilibrium environments, macro-scale kinetic energy is typically converted into micro-scale heat within intense, localized current sheets that develop throughout the plasma volume. Magnetic switchbacks (SBs)—abrupt, large-amplitude radial magnetic field rotations that dominate the young solar wind—provide an ideal in-situ laboratory to study this cross-scale coupling, yet controversy persists (approximately 200 refereed papers published in the last decade) regarding whether they are passive fluid structures or active sites of local kinetic dynamics. Here we present high-resolution electric and magnetic field measurements from the Parker Solar Probe between 13 and 40 solar radii that reveal the systematic presence of intense plasma-frame electric fields, Hall currents, and three-dimensional Poynting flux confined within these switchback boundary layers. These observations provide direct evidence that the "frozen-in" condition of Ideal MHD is consistently violated, demonstrating that SBs must be modeled using a Hall-MHD framework. Crucially, the discovery of confined, highly dynamic, circulating, and frequently sunward-directed Poynting flux rules out simple outward-propagating fluid models, establishing that switchbacks undergo profound local evolution during their propagation. Furthermore, the observed 1/r2 radial scaling of the plasma-frame electric field provides a critical new constraint for heliospheric energy transport. These results show that magnetic switchbacks are primary, active sites of non-ideal energy dissipation, offering a new paradigm for understanding turbulent heating across magnetized stellar and astrophysical outflows.

Aims. To investigate the magnetic field geometry and waves in the region near the Sun where the heliospheric current sheet is formed. Methods. One good example of apparent open and closed field lines was found and its fields and plasmas were analyzed. Results. The radial component of the magnetic field (the Z-component) measured on the Parker Solar Probe (PSP) changed sign between 12:00 and 13:00 UT on March 30, 2024, when the spacecraft was at 13 solar radii. This sign change may have occurred because the spacecraft crossed the heliospheric current sheet on long open magnetic field lines or it may have occurred because the spacecraft crossed from one side of the equator to the other on much shorter closed magnetic field lines. During this crossing, two distinct regions having different magnetic field geometries, strahl flows, plasma densities, and electric field spectra were observed and identified as regions with open and closed magnetic field lines, respectively. The two regions intermingled on time scales less than 100 milliseconds to create a complex magnetic field geometry. The waves observed in both regions were electrostatic and composed of wide band signatures (in the open field lines regions) and well-structured frequency harmonics (in both the open and closed field lines regions). These harmonic frequencies correlated with the proton plasma frequency, fpp, with the lowest frequency at ~0.1fpp. This result plus the field aligned electric field perturbations and plasma density fluctuations, require that the observed intense electrostatic mode and associated harmonics were ion acoustic waves. The absence of broadband electrostatic signals in closed field line regions is explained by the lower (than in open field line regions) hot and core electron density and higher ratio of the electron plasma frequency to the electron gyrofrequency, suppressing wave generation.

The largest electric fields between 18 and 30 solar radii are in narrow band waves simultaneously observed at a few Hz (somewhat above the local proton gyrofrequency) and a few hundred Hz (far below the lower hybrid frequency), with the higher frequency wave triggered at specific phases of the lower frequency wave. This wave pair, called triggered ion acoustic waves (TIAW), has been shown to both be physical and to occur at times of electron heating. A theory of electron heating and acceleration by the low frequency wave has been presented. While this theory and the TIAW results strongly suggest the presence of low frequency electric fields that are parallel to the local magnetic field, such fields have not been directly observed. In this paper, such parallel electric field observations are reported and TIAW are further described to conclude that they occur during about 75 percent of the Parker Solar Probe passes through 18 to 30 solar radii and, when present, they are the dominant wave signal, lasting for hours. In the presence of these parallel electric fields, electrons are heated while, in their absence, there is no electron heating. That there is no heating between 18 and 30 solar radii in the absence of TIAW is a most significant result because it invalidates other proposed mechanisms that predict heating in this radial range all of the time.

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.

Type III radio bursts are a signature of the flux of near-relativistic electrons ejected during solar flares. These bursts are frequently observed by spacecraft such as the Parker Solar Probe. It is traditionally believed that these electron beams generate Langmuir waves through the two-stream instability, which are then converted into electromagnetic waves. In this study, we revise that model by examining how the electron distribution becomes truncated due to the "time-of-flight" effect as the beam travels through a randomly inhomogeneous, and gently varying solar-wind plasma. Rather than the two-stream instability, this truncation destabilizes the distribution and leads to the generation of Langmuir waves via a linear instability; we confine our analysis to this linear regime and do not take into account the back reaction of the generated Langmuir waves on the electron distribution, which is nonlinear. The instability grows until slower electrons arrive and dampen the waves. Our qualitative analysis shows that the resulting wave intensity growth and decay closely match the intensity-time profile of observed Type III radio bursts at the fundamental frequency, supporting this modified theory.

The operating principles of a DC and low frequency electric field detector are developed, after which, examples of earlier important electric field measurements are presented, including, the first observation of parallel electric fields in the auroral acceleration region, the first observation of time domain structures in space, the first experimental verification of symmetric magnetic field reconnection, the first observations of triggered ion acoustic waves, and oblique whistlers that directly accelerate electrons. Future possible improvements in the electric field measurement technique are described.

Parker Solar Probe measurements have recently shown that coherent fast magnetosonic and Alfvén ion-cyclotron waves are abundant in the solar wind and can be accompanied by higher-frequency electrostatic fluctuations. In this letter we reveal the nonlinear process capable of channelling the energy of low-frequency electromagnetic to higher-frequency electrostatic fluctuations observed aboard Parker Solar Probe. We present Hall-MHD simulations demonstrating that low-frequency electromagnetic fluctuations can resonate with the ion-sound mode, which results in steepening of plasma density fluctuations, electrostatic spikes and harmonics in the electric field spectrum. The resonance can occur around the wavenumber determined by the ratio between local sound and Alfvén speeds, but only in the case of {\it oblique} propagation to the background magnetic field. The resonance wavenumber, its width and steepening time scale are estimated, and all indicate that the revealed two-wave resonance can frequently occur in the solar wind. This process can be a potential channel of energy transfer from cyclotron resonant ions producing the electromagnetic fluctuations to Landau resonant ions and electrons absorbing the energy of the higher-frequency electrostatic fluctuations.

We present a statistical analysis of electrostatic solitary waves observed aboard Magnetospheric Multiscale spacecraft in the Earth's magnetosheath. Applying single-spacecraft interferometry to several hundred solitary waves collected in about two minute intervals, we show that almost all of them have the electrostatic potential of positive polarity and propagate quasi-parallel to the local magnetic field with plasma frame velocities of the order of 100 km/s. The solitary waves have typical parallel half-widths from 10 to 100 m that is between 1 and 10 Debye lengths and typical amplitudes of the electrostatic potential from 10 to 200 mV that is between 0.01 and 1\% of local electron temperature. The solitary waves are associated with quasi-Maxwellian ion velocity distribution functions, and their plasma frame velocities are comparable with ion thermal speed and well below electron thermal speed. We argue that the solitary waves of positive polarity are slow electron holes and estimate the time scale of their acceleration, which occurs due to interaction with ions, to be of the order of one second. The observation of slow electron holes indicates that their lifetime was shorter than the acceleration time scale. We argue that multi-spacecraft interferometry applied previously to these solitary waves is not applicable because of their too-short spatial scales. The source of the slow electron holes and the role in electron-ion energy exchange remain to be established.

Aims. We investigate processes associated with the generation of type III radiation using Parker Solar Probe measurements. Methods. We measured the amplitudes and phase velocities of electric and magnetic fields and their associated plasma density fluctuations. Results. 1. There are slow electrostatic waves near the Langmuir frequency and at as many as six harmonics, the number of which increases with the amplitude of the Langmuir wave. Their electrostatic nature is shown by measurements of the plasma density fluctuations. From these density fluctuations and the electric field magnitude, the k-value of the Langmuir wave is estimated to be 0.14 and k lambda(d) = 0.4. Even with the large uncertainty in this quantity (more than a factor of two), the phase velocity of the Langmuir wave was < 10 000 km/s. 2. The electromagnetic wave near the Langmuir frequency has a phase velocity lower than 50 000 km/s. 3. We cannot determine whether there are electromagnetic waves at the harmonics of the Langmuir frequency. If they existed, their magnetic field components would be below the noise level of the measurement. 4. The rapid (less than one millisecond) amplitude variations typical of the Langmuir wave and its harmonics are artifacts resulting from the addition of two waves, one of which has small frequency variations that arise because the wave travels through density irregularities. None of these results are expected in or consistent with the conventional model of the three-wave interaction of two counter-streaming Langmuir waves that coalesce to produce the type III wave. They are consistent with a new model in which electrostatic antenna waves are produced at the harmonics by radiation of the Langmuir wave, after which at least the first harmonic wave evolved through density irregularities such that its wave number decreased and it became the type III radiation.

In the interplanetary space solar wind plasma, whistler waves are observed in a wide range of heliocentric distance (from 20 solar radii (RS) to Jupiter's orbit). They are known to interact with solar wind suprathermal electrons (strahl and halo) and to regulate the solar wind heat flux through scattering the strahl electrons. We present the results of applying the technique to determine the whistler wave propagation direction to the spectral data continuously collected by the FIELDS instruments aboard Parker Solar Probe (PSP). The technique was validated based on the results obtained from burst mode magnetic and electric field waveform data collected during Encounter 1. We estimated the effective length of the PSP electric field antennas (EFI) for a variety of solar wind conditions in the whistler wave frequency range and utilized these estimates for determining the whistler wave properties during PSP Encounters 1-11. Our findings show that (1) the enhancement of the whistler wave occurrence rate and wave amplitudes observed between 25-35 RS is predominantly due to the sunward propagating whistler waves population associated with the switchback-related magnetic dips; (2) the anti-sunward or counter-propagating cases are observed at 30-40 RS; (3) between 40-50 RS, sunward and anti-sunward whistlers are observed with comparable occurrence rates; and (4) almost no sunward or counter-propagating whistlers were observed at heliocentric distances above 50 RS.

We present the analysis of 1,831 current sheets (CS) observed aboard four Cluster spacecraft in a pristine solar wind. Four-spacecraft estimates of the CS normal and propagation velocity are compared with different single-spacecraft estimates. The Minimum Variance Analysis (MVA) of the magnetic field is shown to be highly inaccurate in estimating the normal. The MVA normal often differs by more than 60 degrees from the normal obtained by multi-spacecraft timing method, likely due to ambient turbulent fluctuations. In contrast, the cross-product of magnetic fields at the CS boundaries delivers the normal with an uncertainty of less than 15 degrees at the confidence level of 90%. The CSs are essentially frozen into plasma flow, since their propagation velocity is consistent with local ion flow velocity within 20% at the confidence level of 90%. The single-spacecraft methodology based on the cross-product method and frozen-in assumption delivers the CS thickness and current density amplitude within 20% of their actual values at the confidence level of 90%. The CSs are kinetic-scale structures with half-thickness lambda from a few tenths to tens of local proton inertial length lambda p and scale-dependent shear angle and current density amplitude, Delta theta proportional to lambda/lambda p0.5 ${\Delta }\theta \propto {\left(\lambda /{\lambda }_{p}\right)}<^>{0.5}$ and J0 proportional to lambda/lambda p-0.5 ${J}_{0}\propto {\left(\lambda /{\lambda }_{p}\right)}<^>{-0.5}$. The classification of the CSs in terms of tangential and rotational discontinuities remains a challenge, because even the four-spacecraft normal has too large uncertainties to reveal the actual normal magnetic field component. The presented results will be valuable for the analysis of solar wind CSs, when only single-spacecraft measurements are available. Current sheets (CS) are sharp magnetic field rotations that could play a role in solar wind heating. These locally planar one-dimensional structures are highly likely produced by turbulence cascade, though some may originate in solar corona. Most of the studies of solar wind CSs were carried out using single-spacecraft measurements, the CS normal was typically computed using Minimum Variance Analysis (MVA), and the other CS properties were estimated using that normal. In this study we present multi-spacecraft analysis of 1,831 CSs observed aboard four Cluster spacecraft and determine the CS normal using four-spacecraft observations. We demonstrate that MVA is highly inaccurate in estimating the CS normal, while the cross-product of magnetic fields at the CS boundaries delivers accurate estimates of the normal. We show that the single-spacecraft methodology based on the cross-product normal and the assumption that solar wind CSs are frozen into local plasma flow provides accurate estimates of the CS thickness and current density. This single-spacecraft methodology assumes zero normal component of the magnetic field and does not allow classifying CSs in terms of tangential and rotational discontinuities, but we show that because of methodology uncertainties four-spacecraft observations do not allow carrying out such a classification either. Four-spacecraft observations are used to estimate the normal and propagation velocity for more than 1,800 current sheets at 1 AU MVA is highly inaccurate in estimating the current sheet normal, while the cross-product method delivers accurate estimates Single-spacecraft methodology based on the cross-product normal and frozen-in assumption is accurate in estimating current sheet properties

We present statistical analysis of 16,903 current sheets (CSs) observed over 641 days aboard Ulysses spacecraft at 5 AU. We show that the magnetic field rotates across CSs through some shear angle, while only weakly varies in magnitude. The CSs are typically asymmetric with statistically different, though only by a few percent, magnetic field magnitudes at the CS boundaries. The data set is classified into about 90.6% non‐bifurcated and 9.4% bifurcated CSs. Most of the CSs are proton kinetic‐scale structures with the half‐thickness of non‐bifurcated and bifurcated CSs within respectively 200–2,000 km and 500–5,000 km or 0.5–5λp and 0.7–15λp in units of local proton inertial length. The amplitude of the current density, mostly parallel to magnetic field, is typically within 0.05–0.5 nA/m2 or 0.04–0.4JA in units of local Alfvén current density. The CSs demonstrate approximate scale‐invariance with the shear angle and current density amplitude scaling with the half‐thickness, and . The matching of the magnetic field rotation and compressibility observed within the CSs against those in ambient solar wind indicate that the CSs are produced by turbulence, inheriting its scale‐invariance and compressibility. The estimated asymmetry in plasma beta between the CS boundaries is shown to be insufficient to suppress magnetic reconnection through the diamagnetic drift of X‐line. The presented results will be of value for future comparative analysis of CSs observed at different distances from the Sun.

The heating of the solar wind has been shown to be correlated with certain ion acoustic waves. Here calculations of the heating are made, using the methods used previously for STEREO observations, which show that the strong damping of ion acoustic waves rapidly delivers their energy to the plasma of the solar wind. It is shown that heating by the observed waves is not only sufficient to produce the observed heating but can also provide much or all of the outward acceleration of the solar wind.

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.

Context. Whistler waves are electromagnetic waves produced by electron-driven instabilities, that in turn can reshape the electron distributions via wave-particle interactions. In the solar wind, they are one of the main candidates for explaining the scattering of the strahl electron population into the halo at increasing radial distances from the Sun and for subsequently regulating the solar wind heat flux. However, it is unclear what type of instability dominates to drive whistlers in the solar wind. Aims. Our goal is to study whistler wave parameters in the young solar wind sampled by Parker Solar Probe (PSP). The wave normal angle (WNA) in particular is a key parameter to discriminate between the generation mechanisms of these waves. Methods. We analyze the cross-spectral matrices of magnetic fieldfluctuations measured by the Search-Coil Magnetometer (SCM) and processed by the Digital Fields Board (DFB) from the FIELDS suite during PSP's first perihelion. Results. Among the 2701 wave packets detected in the cross spectra, namely individual bins in time and frequency, most were quasi-parallel to the background magnetic field but a significant part (3%) of observed waves had oblique (> 45°) WNA. The validation analysis conducted with the time-series waveforms reveal that this percentage is a lower limit. Moreover, we find that about 64% of the whistler waves detected in the spectra are associated with at least one magnetic dip. Conclusions. We conclude that magnetic dips provides favorable conditions for the generation of whistler waves. We hypothesize that the whistlers detected in magnetic dips are locally generated by the thermal anisotropy as quasi-parallel and can gain obliqueness during their propagation. We finally discuss the implication of our results for the scattering of the strahl in the solar wind.