Forrest S. Mozer

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.

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.

On November 6, 2024, the Parker Solar Probe flew past Venus to make the first accurate electric field measurement in the nightside Venusian magnetosphere. To achieve this result, the electric field antennas were current biased in a way never before experienced by an electric field detector at Venus. This biasing requirement, that the positive bias current in the Venus shadow be about equal to the electron thermal current, is discussed and illustrated. About one minute of useful electric field data in the eight minute nightside magnetosphere crossing was obtained, during which the only feature observed was a few Hz signal. This result, along with the magnetic field measurements, showed that there were few if any electromagnetic waves, such as low frequency electromagnetic turbulence or whistlers, in the nightside crossing. Instead, a few Hertz, purely electrostatic signal was found. This suggests that the interaction of the solar wind with an unmagnetized body having an ionosphere may be different from that of previously studied magnetized bodies. In the sunlit flanks, many electromagnetic wave modes were observed. An additional result of this research is development of an improved algorithm for biasing electric field antennas in the Sun’s shadow which improves on guessing the bias current as done in this research.

On November 6, 2024, the Parker Solar Probe flew past Venus to make the first accurate electric field measurement in the nightside Venusian magnetosphere. To achieve this result, the electric field antennas were current biased in a way never before experienced by an electric field detector at Venus. This biasing requirement, that the positive bias current in the Venus shadow be about equal to the electron thermal current, is discussed and illustrated. About one minute of useful electric field data in the eight minute nightside magnetosphere crossing was obtained, during which the only feature observed was a few Hz signal. This result, along with the magnetic field measurements, showed that there were few if any electromagnetic waves, such as low frequency electromagnetic turbulence or whistlers, in the nightside crossing. Instead, a few Hertz, purely electrostatic signal was found. This suggests that the interaction of the solar wind with an unmagnetized body having an ionosphere may be different from that of previously studied magnetized bodies. In the sunlit flanks, many electromagnetic wave modes were observed.

Type III radio bursts are signatures of the fluxes of near-relativistic electrons ejected during solar flares. These bursts are frequently observed by spacecraft such as the Parker Solar Probe. It has been 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 backreaction 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.

We present analysis of broadband electrostatic fluctuations observed on the Magnetospheric Multiscale satellite around the ramp of one quasi‐parallel and one quasi‐perpendicular interplanetary shock. In both shocks, wave packets are shown to be overall similar. They have plasma frame speeds mostly less than 200 km/s, comparable with the ion‐acoustic speed, wavelengths of 100–500 m or 10–50 Debye lengths, and electrostatic potential amplitudes mostly within 0.01–0.3 V or 0.1%–1% of the local electron temperature. Most wave packets are polarized nearly parallel to the local magnetic field, though nearly perpendicular polarizations also occur in both shocks. Based on the revealed parameters, we interpret the electrostatic fluctuations as ion‐acoustic waves. The revealed wave properties are compared with those reported at Earth's bow shock and potential sources of this wave activity are discussed.

On 2024 March 30, the Parker Solar Probe crossed the heliospheric current sheet at 13 solar radii while encountering two distinctly different regions at sharp boundaries on several occasions. The two regions had very different plasma densities, electric-field spectra, and magnetic-field geometries. In one region the strahl flowed only along the direction from the Sun, and there were relatively few particles at pitch angles near 90°, while in the other region the strahl flowed both toward and away from the Sun, and there were relatively many particles at pitch angles near 90°. These different properties are interpreted as being due to the spacecraft being in the heliospheric current sheet on long open magnetic field lines in the case of unidirectional strahl flow and the spacecraft being in coronal loops having much shorter closed magnetic field lines in the case with bidirectional strahl flow. The two regions intermingled on time scales less than 100 ms to create a complex magnetic field geometry. Broadband waves were observed in the open field-line regions, while narrowband electrostatic harmonic waves were observed in both regions. These harmonic frequencies correlated with the proton plasma frequency, f _pp, with the lowest frequency at ∼0.1f _pp. This result, plus the field-aligned electric field waves and plasma density fluctuations, requires that the observed electrostatic mode and associated harmonics were ion acoustic waves.

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.

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.