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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 kλd = 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. [ABSTRACT FROM AUTHOR]

A variety of energy sources, ranging from dynamic processes, such as magnetic reconnection and waves, to quasi-steady terms, such as plasma pressure, may contribute to the acceleration of the solar wind. We utilize a combination of charged particle and magnetic field observations from the Parker Solar Probe (PSP) to attempt to quantify the steady-state contribution of the proton pressure, the electric potential, and the wave energy to the solar wind proton acceleration observed by PSP between 13.3 and ∼100 solar radii (R ☉). The proton pressure provides a natural kinematic driver of the outflow. The ambipolar electric potential acts to couple the electron pressure to the protons, providing another definite proton acceleration term. Fluctuations and waves, while inherently dynamic, can act as an additional effective steady-state pressure term. To analyze the contributions of these terms, we utilize radial binning of single-point PSP measurements, as well as repeated crossings of the same stream at different distances on individual PSP orbits (i.e., fast radial scans). In agreement with previous work, we find that the electric potential contains sufficient energy to fully explain the acceleration of the slower wind streams. On the other hand, we find that the wave pressure plays an increasingly important role in the faster wind streams. The combination of these terms can explain the continuing acceleration of both slow and fast wind streams beyond 13.3 R ☉. [ABSTRACT FROM AUTHOR]

Robust in situ magnetic field measurements are critical to understanding the various mechanisms that couple mass, momentum, and energy throughout our solar system. However, the spacecraft on which magnetometers are often deployed contaminate the magnetic field measurements via onboard subsystems including reaction wheels and magnetorquers. Two magnetometers can be deployed at different distances from the spacecraft to determine an approximation of the interfering field for subsequent removal, but constant data streams from both magnetometers can be impractical due to power and telemetry limitations. Here we propose a method to identify and remove time‐varying magnetic interference from sources such as reaction wheels using statistical decomposition and convolutional neural networks, providing high‐fidelity magnetic field data even in cases where dual‐sensor measurements are not constantly available. For example, a measurement interval from the Parker Solar Probe outboard magnetometer experienced a 95.1% reduction in reaction wheel interference following application of the proposed technique. Plain Language Summary: Measurements of magnetic fields captured by instruments onboard spacecraft are necessary to further our understanding of the solar system. These measurements are often contaminated by magnetic noise from the host spacecraft, substantially reducing the fidelity of the data. One common method to reduce the impact of these interfering magnetic fields utilizes a pair of magnetometers deployed at different distances from the main body of the spacecraft. The difference between the two sensors allows for an approximation of the interfering fields to be calculated for subsequent removal. However, many missions cannot afford to send data from both instruments back to Earth. This manuscript proposes a method for the mitigation of magnetic interference caused by the host spacecraft, even with limited data from the magnetometer pair. Specifically, signal decomposition techniques and machine learning are used to isolate and remove the interfering magnetic fields. For example, the proposed method applied to a measurement interval from the Parker Solar Probe outboard magnetometer enabled a 95% reduction in magnetic interference. Key Points: Local magnetic interference is a common issue faced by in situ magnetometersGradiometer measurements have historically been required to denoise in situ magnetometer dataStatistical decomposition and machine learning enable denoising even with limited gradiometer data [ABSTRACT FROM AUTHOR]

Context. Whistler waves are electromagnetic waves produced by electron-driven instabilities, which 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 whistler waves 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 analyzed the cross-spectral matrices of magnetic field fluctuations 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 quasiparallel to the background magnetic field; however, a significant part (3%) of the 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 provide 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 implications of our results for the scattering of the strahl in the solar wind. [ABSTRACT FROM AUTHOR]

We utilize observations from the Parker Solar Probe (PSP) to study the radial evolution of the solar wind in the inner heliosphere. We analyze electron velocity distribution functions observed by the Solar Wind Electrons, Alphas, and Protons suite to estimate the coronal electron temperature and the local electric potential in the solar wind. From the latter value and the local flow speed, we compute the asymptotic solar wind speed. We group the PSP observations by asymptotic speed, and characterize the radial evolution of the wind speed, electron temperature, and electric potential within each group. In agreement with previous work, we find that the electron temperature (both local and coronal) and the electric potential are anticorrelated with wind speed. This implies that the electron thermal pressure and the associated electric field can provide more net acceleration in the slow wind than in the fast wind. We then utilize the inferred coronal temperature and the extrapolated electric + gravitational potential to show that both electric field driven exospheric models and the equivalent thermally driven hydrodynamic models can explain the entire observed speed of the slowest solar wind streams. On the other hand, neither class of model can explain the observed speed of the faster solar wind streams, which thus require additional acceleration mechanisms. [ABSTRACT FROM AUTHOR]

We report observations of reconnection exhausts in the Heliospheric Current Sheet (HCS) during Parker Solar Probe Encounters 08 and 07, at 16 Rs and 20 Rs, respectively. Heliospheric current sheet (HCS) reconnection accelerated protons to almost twice the solar wind speed and increased the proton core energy by a factor of ∼3, due to the Alfvén speed being comparable to the solar wind flow speed at these near‐Sun distances. Furthermore, protons were energized to super‐thermal energies. During E08, energized protons were found to have leaked out of the exhaust along separatrix field lines, appearing as field‐aligned energetic proton beams in a broad region outside the HCS. Concurrent dropouts of strahl electrons, indicating disconnection from the Sun, provide further evidence for the HCS being the source of the beams. Around the HCS in E07, there were also proton beams but without electron strahl dropouts, indicating that their origin was not the local HCS reconnection exhaust. Plain Language Summary: Magnetic reconnection in current sheets is a universal plasma process that converts magnetic energy into particle energy. The process is important in many laboratory, solar, and astrophysical plasmas. The heliospheric current sheet (HCS), which originates from the Sun and extends throughout the heliosphere, is the largest current sheet in the solar system. One of the surprises of the Parker Solar Probe mission is the finding that magnetic reconnection is almost always active in the near‐Sun HCS, despite its enormous scales. In this paper, we report direct evidence showing that reconnection in the HCS close to the Sun can be a source of energetic protons observed in the solar wind. The reason protons can be accelerated to high energies (to tens of kilo‐electronvolts) is because the available magnetic energy per particle is high close to the Sun. This finding is important because the source of energetic protons in the heliosphere is unknown. Key Points: Large available magnetic energy per particle led to significant proton acceleration by reconnection in the near‐Sun heliospheric current sheet (HCS) at 16 and 20 RsProton beams and strahl electron dropouts in separatrices are evidence for HCS being a source of energetic protons seen outside the HCSEnergetic protons beams outside the HCS also exist without strahl electron dropouts. Their origin is unlikely to be the local HCS exhaust [ABSTRACT FROM AUTHOR]

Parker Solar Probe is the first mission to probe in situ the innermost heliosphere, revealing an exceptionally dynamic and structured outer solar corona. Its payload includes a search‐coil magnetometer (SCM) that measures up to three components of the fluctuating magnetic field between 3 Hz and 1 MHz. After more than 3 years of operation, the SCM has revealed a multitude of different wave phenomena in the solar wind. Here we present an overview of some of the discoveries made so far. These include oblique and sunward propagating whistler waves that are important for their interaction with energetic electrons, the first observation of the magnetic signature associated with escaping electrons during dust impacts, the first observation of the magnetic field component for slow extraordinary wave modes during type III radio burst events, and more. This study focuses on the major observations to date, including a description of the instrument and lessons learned. Plain Language Summary: The search‐coil magnetometer (SCM) search‐coil magnetometer on Parker Solar Probe is an instrument that measures fluctuations of the magnetic field in the solar wind. The instrument covers frequencies ranging from 3 Hz to 1 MHz. After 3 years of operation the SCM has revealed a wealth of different wave phenomena. Here, we describe some of the highlights. These include whistler waves that propagate oblique to the magnetic field, the first observation of the magnetic signature associated with the impact of dust particles, and the first observation of the magnetic signature of high‐frequency coherent waves that are associated with solar radio bursts. Key Points: We present an overview of 3 years of results from the search‐coil magnetometer (SCM) on Parker Solar ProbeThe SCM reveals the magnetic signature of dust impacts and of slow extraordinary waves associated with type III radio burstsThe SCM reveals in detail oblique whistlers and the radial evolution of turbulence at kinetic scales [ABSTRACT FROM AUTHOR]

As the Parker Solar Probe explores new regions of the inner heliosphere, it travels ever deeper into the electric potential of the Sun. In the near-Sun environment, a new feature of the electron distribution emerges, in the form of a deficit in the sunward suprathermal population. The lower boundary of this deficit forms a cutoff in phase space, at an energy determined by the electric potential drop between the observation point and the outer heliosphere. We explore the characteristics of the sunward deficit and the associated cutoff, as well as the properties of the plasma in which we observe them. The deficit occurs in ∼60%–80% of electron observations within ∼0.2 au, and even more frequently in plasma with low β, low collisional age, and a more anisotropic electron core population. At greater distances, the deficit rapidly disappears, as the suprathermal halo grows, with these two trends likely related. The cutoff energy varies linearly with the local electron core temperature, confirming a direct relationship to the ambipolar electric potential. Meanwhile, the cutoff width varies with β and collisional age, suggesting that energy diffusion plays a role in erasing the deficit. The nearly ubiquitous occurrence of the sunward deficit in the inner heliosphere suggests that we may need to reconsider the functional forms commonly used to represent electron distributions in this environment. [ABSTRACT FROM AUTHOR]

The solar wind is slowed, deflected, and heated as it encounters Venus's induced magnetosphere. The importance of kinetic plasma processes to these interactions has not been examined in detail, due to a lack of constraining observations. In this study, kinetic‐scale electric field structures are identified in the Venusian magnetosheath, including plasma double layers. The double layers may be driven by currents or mixing of inhomogeneous plasmas near the edge of the magnetosheath. Estimated double‐layer spatial scales are consistent with those reported at Earth. Estimated potential drops are similar to electron temperature gradients across the bow shock. Many double layers are found in few high cadence data captures, suggesting that their amplitudes are high relative to other magnetosheath plasma waves. These are the first direct observations of plasma double layers beyond near‐Earth space, supporting the idea that kinetic plasma processes are active in many space plasma environments. Plain Language Summary: Venus has no internally generated magnetic field, yet electric currents running through its ionized upper atmosphere create magnetic fields that push back against the flow of the solar wind. These induced fields cause the solar wind to slow and heat as the flow is deflected around Venus. This work reports observations of very small plasma structures that accelerate particles, identifiable by their characteristic electric field signatures, at the boundary where the solar wind starts to be deflected. These small plasma structures observed at Venus have been studied in near‐Earth space for decades but have never before been found near another planet. These structures are known to be important to the physics of strong electrical currents in space plasmas and the blending of dissimilar plasmas. Their identification at Venus is a strong demonstration that these small plasma structures are a universal plasma phenomena, at work in many plasma environments. Key Points: Plasma double layers are detected near the Venusian bow shockMultiple double layers are identified in a small amount of burst dataKinetic processes may help mediate interaction between the solar wind and induced magnetospheres [ABSTRACT FROM AUTHOR]

The current understanding of the characteristics of solar and inner heliospheric electron events is inferred almost entirely from observations made by spacecraft located at 1 astronomical unit (au). Previous observations within 1 au of the Sun, by the Helios spacecraft at ∼0.3–1 au, indicate the presence of electron events that are not detected at 1 au or may have merged during transport from the Sun. Parker Solar Probe's close proximity to the Sun at perihelion provides an opportunity to make the closest measurements yet of energetic electron events. We present an overview of measurements of electrons with energies between ∼17 keV and ∼1 MeV made by the Parker Solar Probe Integrated Science Investigation of the Sun (IS⊙IS) instrument suite during Encounter 2 (2019 March 31–April 10 with perihelion of ∼0.17 au), including several small electron events. We examine these events in the context of the electromagnetic and solar wind environment measured by the FIELDS and SWEAP instruments on Parker Solar Probe. We find most of these electron enhancements to be associated with type III radio emissions that reach the local plasma frequency and one enhancement that appears to be primarily associated with abrupt changes in the local magnetic field. Together, these associations suggest that these are indeed the first measurements of energetic electron events within 0.2 au. [ABSTRACT FROM AUTHOR]

NASA's Parker Solar Probe (PSP) mission is currently investigating the local plasma environment of the inner heliosphere (<0.25 R⊙) using both in situ and remote sensing instrumentation. Connecting signatures of microphysical particle heating and acceleration processes to macroscale heliospheric structure requires sensitive measurements of electromagnetic fields over a large range of physical scales. The FIELDS instrument, which provides PSP with in situ measurements of electromagnetic fields of the inner heliosphere and corona, includes a set of three vector magnetometers: two fluxgate magnetometers (MAGs) and a single inductively coupled search‐coil magnetometer (SCM). Together, the three FIELDS magnetometers enable measurements of the local magnetic field with a bandwidth ranging from DC to 1 MHz. This manuscript reports on the development of a merged data set combining SCM and MAG (SCaM) measurements, enabling a high fidelity data product with an optimal signal‐to‐noise ratio. On‐ground characterization tests of complex instrumental responses and noise floors are discussed as well as application to the in‐flight calibration of FIELDS data. The algorithm used on PSP/FIELDS to merge waveform observations from multiple sensors with optimal signal‐to‐noise characteristics is presented. In‐flight analysis of calibrations and merging algorithm performance demonstrates a timing accuracy to well within the survey rate sample period of ∼340 μs. Key Points: We develop a technique to merge search‐coil and fluxgate magnetometer measurementsIn‐flight analysis shows that calibrations are accurate to within 100 μs [ABSTRACT FROM AUTHOR]

One of the discoveries of the Parker Solar Probe during its first encounters with the Sun is ubiquitous presence of relatively small-scale structures standing out as sudden deflections of the magnetic field. They were named "switchbacks" since some of them show a full reversal of the radial component of the magnetic field and then return to "regular" conditions. We carried out an analysis of three typical switchback structures having different characteristics: I. Alfvénic structure, where the variations of the magnetic field components take place while conserving the magnitude of the magnetic field; II. Compressional structure, where the magnitude of the field varies together with changes of its components; and III. Structure manifesting full reversal of the magnetic field, presumably Alfvén, which is an extremal example of a switchback. We analyzed the properties of the magnetic fields of these structures and of their boundaries. Observations and analyses lead to the conclusion that they represent localized twisted magnetic tubes moving with respect to surrounding plasma. An important feature is the existence of a relatively narrow boundary layer at the surface of the tube that accommodates flowing currents. These currents are closed on the surface of the structure and typically have comparable azimuthal and tube-axis-aligned components. They are supported by the presence of an effective electric field due to strong gradients of the density and ion plasma pressure. The ion beta is typically larger inside the structure than outside. The surface of the structure may also accommodate electromagnetic waves that assist particles in carrying currents. [ABSTRACT FROM AUTHOR]

NASA's Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products. [ABSTRACT FROM AUTHOR]

On 13 March 2010 an unusually long-duration event was observed by radio spectrographs onboard the STEREO-B and Wind spacecraft. The event started at about 13:00 UT and ended at approximately 06:00 UT on 14 March. The event presents itself as slow drifting, quasi-continuous emission in a very narrow frequency interval, with an apparent frequency drift from about 625 kHz to approximately 425 kHz. Using the Leblanc, Dulk, and Bougeret ( Solar Phys. 183, 165, ) interplanetary density model, we determined that the drift velocities of the radio source are ≈ 33 km s and ≈ 52 km s for 0.2 and 0.5 times the densities of Leblanc model, respectively, with a normalization density of 7.2 cm at 1 AU and assuming harmonic emission. A joint analysis of the radio direction-finding data, coronograph white-light observations and modeling revealed that the radio sources appear to be located in interaction regions with relatively high density and slow solar wind speed. [ABSTRACT FROM AUTHOR]

The STEREO/WAVES experiment is designed to measure the electric component of radio emission from interplanetary radio bursts and in situ plasma waves and fluctuations in the solar wind. Interplanetary radio bursts are generated from electron beams at interplanetary shocks and solar flares and are observed from near the Sun to 1 AU, corresponding to frequencies of approximately 16 MHz to 10 kHz. In situ plasma waves occur in a range of wavelengths larger than the Debye length in the solar wind plasma λ D ≈10 m and appear Doppler-shifted into the frequency regime down to a fraction of a Hertz. These phenomena are measured by STEREO/WAVES with a set of three orthogonal electric monopole antennas. This paper describes the electrical and mechanical design of the antenna system and discusses efforts to model the antenna pattern and response and methods for in-flight calibration. [ABSTRACT FROM AUTHOR]