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One of the driving objectives behind the Parker Solar Probe (PSP) mission is to cross into the magnetically closed corona and to observe the coronal plasma in situ. The transition from open to closed coronal phenomena can be a nuanced distinction, but in gross terms the key boundary is the "Alfvén Point." This is the surface at which the bulk flow speed, u , of the solar wind is equal to the phase speed of the transverse Alfvén wave, cA . In encounter 2, the Fields and SWEAP experiments measured dimensionless bulk flow speeds as low as u/cA = 1.3 ± 0.1. Based on inner heliospheric scalings with distance from the first two encounters, lower estimates for the slow wind Alfvén Point were produced, ranging from 14 to 26 R⊙ , and the likelihood that PSP would first cross the Alfvén point some time in encounters 3-5 was estimated to be 25-50%. In this paper, we repeat the analysis for encounters 3-5 and report the following findings: (1) the Alfvén point has not yet been crossed for any extended (~10 minute) period prior to encounter 6, (2) speeds as low as u/cA = 1.15 ± 0.10 were observed over an extended period during encounter 4, (3) the Alfvén point associated with certain well-measured slow streams in encounters 3-5 was estimated from 5 to 18 R⊙ , and (4) it is highly likely that PSP will spend extended parts of perihelion 8 in the sub-Alfvénic wind, thus achieving a major mission milestone in early 2021 if not before.

In the early hours of June 13, 2020, just after the end of its 5 th solar encounter, the Faraday Cup on-board Parker Solar Probe (PSP) registered the passage of a prominent solar wind structure embedded in highly variable plasma and characterized by regions with distinct plasma conditions (speed, density, magnetic field, etc) and wave activity. From remote sensing observations, using spacecraft at different heliospheric locations, several transients were observed ejected from the Sun, including a coronal mass ejection (CME) oriented away from PSP that may have also influenced the solar wind conditions through which the structure in question has propagated. In this work, we report the PSP-SPC observations around this structure and identify possible solar sources relying on multi-spacecraft observations. We also characterized the propagation and evolution of the CME observed on June 12, 2020 and the role it played influencing the conditions of the unusual variable ambient solar wind.

The induced magnetosphere of Venus presents an obstacle to the solar wind. At the flow-obstacle interface, plasma is slowed, deflected, and heated. Based on observations of Earth's bow shock and magnetosheath, kinetic plasma processes are expected to play an important role in these dynamics at Venus. However, these processes have gone largely undetected at Venus due to the small number of measurements at sufficiently high sampling cadences. Using data from a Parker Solar Probe encounter with Venus, this study demonstrates the existence of kinetic-scale electric field structures, including plasma double layers, at the outer edge of the Venusian magnetosheath. Many of the double layers have signatures of developed two-stream instabilities and phase space holes. Double layer spatial scales are consistent with those reported at Earth. Estimated potential drops are similar to the electron temperature change across the bow shock, consistent with double layers driven by mixing of inhomogeneous plasmas at the magnetosheath boundary. A large number of distinct double layers are found in few burst captures, implying that double layers, and the waves that they drive, have higher amplitudes relative to other high frequency bow shock and magnetosheath waves at Venus as compared to Earth. These are the first direct observations of plasma double layers beyond near-Earth space, demonstrating that kinetic plasma processes are active in diverse plasma environments, and can be identified when high cadence observations are available.

Whistler waves were widely observed during the first solar encounters of Parker Solar Probe. The interaction of these electromagnetic waves with the strahl electrons is known to affect the heat flux and PSP brings the first opportunity to study them in the young solar wind. We present several examples of whistler wave analysis during PSP's 1st encounter. We mainly use cross spectra (full spectral matrices from 20 Hz - 4.5 kHz) of the magnetic field, as measured by the Search-Coil Magnetometer (SCM) that is part of the FIELDS experiment. In this study, we 1) check the spectral matrices by comparing them to waveforms (for the few low-frequency whistlers), 2) determine their wave properties (planarity, ellipticity, polarization) and 3) connect these properties to their signatures in the electric field (EFI electric field antennas) and 4) eventually determine the wave characteristics using the solar wind magnetic field (MAG Fluxgate Magnetometer) and density from QTN (Quasi-Thermal Noise from FIELDS) and SWEAP observations.

Stochastic heating, a non-linear heating mechanism driven by the violation of magnetic moment invariance due to large-amplitude turbulent fluctuations producing energy diffusion perpendicular to the magnetic field, is frequently invoked as a mechanism responsible for the heating of ions in the solar wind. Here, we quantify for the first time the proton stochastic heating rate Q⊥ at radial distances down to 0.16 au, using measurements from the first two Parker Solar Probe encounters. We find satisfying agreement for both the amplitude and radial trend of the heating rate, Q⊥ ∼ r-2.5, calculated using the Helios data set at distances from 0.3 to 0.9 au. In agreement with previous results, Q⊥ is significantly larger in the fast compared to slow solar wind. We identify the tendency in fast solar wind of cuts of the core proton velocity distribution transverse to the magnetic field to exhibit a flat-top shape. The observed distribution agrees with previous theoretical predictions for fast solar wind where stochastic heating is the dominant heating mechanism.

Solar Wind temperature at 1 AU exhibits statistical correlation with the magnetic structure, wherein regions with high temperature are found to be associated with coherent structures [1]. Using Parker Solar Probe (PSP) data from the first encounter, we studied this correlation between the magnetic field structure, measured using the Partial Variance of Increments (PVI) [2], and the radial temperature of the ionized hydrogen atoms (protons). For the magnetic field, we used the low cadence data from FIELDS instrument and for proton temperature we used the moments data from SWEAP. We observed that the probability distribution function (PDF) of events with high PVIs have a higher median temperature than those with lower PVI, implying the presence of heating mechanism in the solar wind, associated with turbulence driven structures. [1] Osman, K. T., Matthaeus, W. H., Greco, A., & Servidio, S.2011, ApJ, 727, L11 [2] A. Greco, W. H. Matthaeus, S,. Perri, K. T. Osman, S. Servidio, M. Wan and P. Dmitruk, Space Sci Rev., 214, 1 (2018)

During the Parker Solar Probe's (PSP) first perihelion pass, the spacecraft reached to within a heliocentric distance of ~37 solar radii and observed magnetic and flow structures characterized by sharp gradients. As we try and understand these intermittent coronal structures better, an important property to examine is their degree of correlation. To this end, we use the well-tested Partial Variance of Increments (PVI) technique [1] to identify intermittent events in FIELDS and SWEAP observations of magnetic and velocity fields. We then examine the distributions of waiting times between events with varying separation and PVI levels. We find power-law distributions, suggesting a high degree of correlation that may originate in a clustering process, as opposed to a random distribution produced by a memory-less Poisson process [2]. We also find that waiting times between events with separations larger than inertial-range scales follow a power-law close to -1, hinting at a possible connection with observations of "1/f noise" associated with signals originating near the source solar surface [3]. The present study complements the one by Dudok de Wit et al., which focuses on the waiting times between the observed "switchbacks" in the radial magnetic field. [1] Greco et al. Space Sci. Rev. (2018) 214:1 [2] Greco et al. Phys. Rev. E (2009) 80, 046401 [3] Matthaeus & Goldstein PRL (1986) 57, 4

Parker Solar Probe has revealed the presence of large numbers of discrete Alfvenic impulses in the near-Sun solar wind with an anti-Sunward sense of propagation. These are similar to those previously observed near 1 AU and in high speed streams over the Sun's poles and at 60 solar radii. At 35 solar radii, however, they are typically shorter and sharper than seen elsewhere. In addition, these spikes occur in "patches" and there are also clear periods within the same stream when they do not occur. While the velocity fluctuations associated with these spikes are typically under 100 km/s due to the rather low Alfven speeds in the streams observed by Probe to date, these are still associated with large angular deflections of the magnetic field - and these deflections are not isotropic. We discuss the scales, amplitudes and orientations of these structures and their links to other properties measured by Probe, such as the bulk plasma flow. We also discuss how these new observations, combined with those from earlier missions, provide evidence for the possible origins of these events and in particular whether they are the long-sought interplanetary signature of discrete reconnection jets in the solar corona.

Over the course of the first two orbits of Parker Solar Probe (PSP), the spacecraft has made solar wind and interplanetary magnetic field measurements inside 0.25 AU for the first time. Being this close to the Sun allows a view of the solar wind much closer to the source of its initial acceleration than has been previously observed. At this distance, smaller features have not yet interacted with the surrounding plasma, and are still clearly visible with in situ data. Observations from these first two orbits show an extraordinary number of transient features that appear as "jets" in the plasma data. These jets consist of temporally short (seconds to 10s of minutes) spikes in the solar wind speed that promptly return to the baseline wind speed. The velocity spikes are accompanied by "switchbacks" in the magnetic field, where the radial component of the magnetic field briefly changes sign and the electron strahl direction follows the field reversal. This study characterizes the statistical properties of these jets (e.g., sizes, durations, and flow directions) to develop a picture of the overall structure of the jets and to assess whether they may have a significant influence over the dynamics of solar wind in the inner heliosphere.

The solar wind strahl is a narrow, magnetic field aligned, suprathermal electron beam observed in the energy range from ~50 eV to ~1 keV that carries heat flux away from the solar corona. Because of their small gyro-radius, high gyro frequency, and large mean free path, strahl electrons are excellent tracers of magnetic topology. It is important to understand their behavior in order to better understand the global coronal structure. We present observations of the strahl made by the SPAN-Electron sensors of the SWEAP instrument on Parker Solar Probe (PSP) from 35 to 50 Rsun to show that, in general, the strahl width is narrower at closer heliocentric distances. However, there is considerable variation in strahl width as a function of plasma parameters such as plasma beta, collisional age, solar wind speed and plasma density. Furthermore the beam width tends to get narrower with increasing electron energy. The strahl width is controlled by competing mechanisms: The mirror force of the diverging solar magnetic field tends to narrow the beam whereas scattering processes such as wave particle interactions and Coulomb collisions tend to widen the beam. Using the strahl characteristics measured inside of 50 Rs, we will address the relative importance of these mechanisms.

The unique location of Parker Solar Probe offers the chance to observe solar radio emission close to the source, measuring signals which are less affected by scattering and refraction than signals observed at 1 AU. We present PSP/FIELDS Radio Frequency Spectrometer (RFS) observations in the inner heliosphere, comparing occurrence rates, amplitudes, and cross spectral properties of solar radio bursts from the first two PSP encounters. During the first encounter, only a few small bursts were observed. During the second encounter, copious Type III radio bursts were observed throughout the entire encounter, including intervals of radio storms where bursts occurred almost continuously. Notably, several strong Type III bursts during the second encounter display signatures of circular polarization from ∼ 6-16

The Parker Solar Probe (PSP) is taking first-of-its kind measurements of the inner heliosphere, paving the way for a new understanding of processes that drive heating in the solar corona and beyond. In its first two orbits of the Sun, PSP reached as close as 0.17 AU (37 solar radii) from the Sun's center. At such close distances, instruments on PSP are uniquely suited to identifying discrete streams of fast solar wind thanks to minimal turbulent mixing between fast streams and any surrounding slow wind. Furthermore, at perihelion PSP's angular speed exceeds that of solar rotation, meaning that the probe is able to sample a given solar longitude over a radial range of as much as 20 solar radii. Taking advantage of these capabilities of PSP, we used data from its first two orbits taken by the Solar Wind Electrons Alphas and Protons (SWEAP) Solar Probe Cup as well as magnetic field data taken by the FIELDS suite to search for solar wind streams crossed by PSP at multiple points near perihelion. We then characterized the radial temperature profile of these streams to better understand the thermal behavior of the solar wind outside the solar corona. This work was supported by the NSF-REU solar physics program at SAO, grant number AGS-1560313.

The SWEAP/SPAN-Electron sensors on Parker Solar Probe (PSP) measure the 3D distributions of solar wind electrons, including total flux, velocity, and temperature, in the inner heliosphere. Strahl electrons, which are the high energy, beam-like component of the solar wind electron population, flow outward from the sun along magnetic field lines, and thus serve as excellent tracers of magnetic topology and magnetic solar connectivity. Comparing strahl electron pitch angle distributions (PADs) to PSP/FIELDS magnetic field data, we find that, during the numerous intervals in PSP's first and second encounters where the sense of the magnetic field briefly reverses, the strahl electrons likewise reverse direction and briefly flow sunward. The strahl PADs, in agreement with measurements from the Solar Probe Cup (SPC) and SPAN-Ion instruments, therefore indicate that these intervals, termed "switchbacks", are likely not related to large-scale solar magnetic field polarity reversals, but are kinks in the magnetic field originating elsewhere and propagating outward from the sun. This work also characterizes the strahl electron population during switchback intervals compared to non-switchback periods.

The first two orbits of Parker Solar Probe have provided us with the closest ever in situ measurements to the Sun, enabling solar wind turbulence to be measured down to a heliocentric distance of 35.7 solar radii for the first time. Here, we present measurements of MHD inertial range turbulence at this distance and its radial evolution out to 1 AU. We show how the turbulence spectrum changes with distance, including its spectral index, magnetic compressibility and cross-helicity (imbalance of outward and inward energy fluxes). Power levels at perihelion are several orders of magnitude larger than at 1 AU, and the radial variation of these depends on both scale and the component being considered (inward vs outward). We also show how the large scale end of the inertial range (correlation scale, break scale) changes with distance, moving to smaller scales at smaller radial distances down to 35.7 Rs. These properties are compared to models of Alfvenic turbulence and solar wind evolution to determine both the fundamentals of MHD turbulence, and the role it plays in the generation and evolution of the solar wind.

Modern spacecraft missions have an enormous problem: the instruments can sample extremely high-resolution measurements, but telemetry restrictions often prevent the collection of data at a sufficient time resolution to allow correlations at the wave period of interest. The Parker Solar Probe (PSP) / correlator is a joint venture between the Solar Wind Electron Alpha Proton particle instrument suite, SWEAP, and the electromagnetic fields instrument suite, FIELDS, which interfaces with a DPU called the SWEAP Electronics Module (SWEM). A single interface cable was placed between the SWEAP and FIELDS instruments through the Data Processing Unit/Time Domain Sampler (DPU/TDS), enabling a novel approach to explore wave-particle interactions in the near Sun environment. Particle pulses from any combination anodes from the electrostatic analyzers, called the Solar Probe ANalyzers (SPAN) are sent through a single channel to the FIELDS DPU. Wave-particle correlations can be seen up to 1 MHz, so both electron and ion scale kinetic physics can be studied. Particle count rates and field measurements are made simultaneously and captured in the same data packet eliminating potential timing errors. We also show an example of real data from laboratory calibration of the flight instrumentation PSP FIELDS/SWEAP particle correlator. During this measurement, the particle flux from an (ion) gun was modulated by a sinusoidal voltage applied to a modulation grid. Individual particles detected by the PSP/SPAN-Ion sensor were transmitted as short voltage pulses to the FIELDS/TDS instrument and recorded at extremely high time resolution. The same modulation signal was also provided to the FIELDS/TDS instrument. Complimentary ground-based analysis of wave-particle interactions will guide data downlink selection from the correlator and help develop this concept for future missions.

The Parker Solar Probe (PSP) has made two orbits of the Sun since its launch in August 2018. The PSP achieved its first orbit perihelion on November 6, 2018, reaching a heliocentric distance of about 0.16 au. Here, we study the evolution of fully developed turbulence associated with the slow solar wind along the PSP trajectory between ~ 0.16 au and ~ 0.7 au, comparing observations to a theoretical turbulence transport model. Several turbulence quantities, such as the energy in forward and backward propagating modes, the fluctuating magnetic and kinetic energy, the normalized residual energy, the normalized cross-helicity, and the correlation lengths are determined from the PSP measurements along its trajectory between ~0.16 to ~0.7 au. The evolution of the PSP derived turbulence quantities are compared to the numerical solutions of the theoretical nearly incompressible magnetohydrodynamic (NI MHD) turbulence transport model recently developed by Zank et al. 2017. We find good agreement between the theoretical and observed results, suggesting that such a turbulence transport model can explain the turbulent evolution in the slow wind along the PSP trajectory.

During its first solar encounters, the Parker Solar Probe mission discovered that the near-Sun solar wind is teeming with plasma waves and instabilities. One prominent plasma wave type appears Sunward of 50 solar radii, where intervals of significant plasma wave power near the electron cyclotron frequency (fce) are detected. Most of this wave power is concentrated near 0.7 fce, with strong harmonics extending above fce often observed. At least two distinct concurrent wave modes are identified (electron Bernstein waves and electrostatic whistler-mode waves). Wave intervals range in duration from a few seconds to hours. The amplitude and occurrence of these near-fce waves increases significantly with decreasing distance to the Sun, suggesting that they play an important role in the evolution of electron populations in the near-Sun plasma environment. Correlations are observed between the occurrence of these waves and properties of the population of electrons escaping the solar corona (the strahl), implying that these waves play a role in regulating the heat flux carried by these electrons. Finally, the occurrence of these near-fce waves is strongly correlated with specific solar wind magnetic field configurations and the level of magnetic turbulence at the location of the spacecraft, offering new insight into the macro-scale structure of the near-Sun solar wind.

Radio waves are strongly scattered in the solar wind, so that their apparent sources seem to be considerably larger and shifted compared to the actual sources. Since the effect of radio wave scattering depends on the spectrum of density turbulence, better understanding of the radio wave propagation provides indirect information on the density fluctuations. Here, we have analyzed 30 type III bursts detected by Parker Solar Probe to retrieve decay times as a function of frequency. We observed a significant deviation for frequencies above 1 MHz when compared to previous observations by the STEREO spacecraft. Next, we performed Monte Carlo simulations to study the role of scattering on time-frequency profiles of radio emissions. By comparing Parker Solar Probe observations and Monte Carlo simulations we predicted relative density fluctuations between 2.5 and 14 solar radii. Finally, we calculated relative density fluctuations measured in situ by Parker Solar Probe at characteristic scale times of Monte Carlo simulations during the perihelion #1 and perihelion #2, and compared them with STEREO predictions.

During the first two orbits of Parker Solar Probe (October 2018-May 2019), recurring solar wind stream flows were sequentially observed at one astronomical unit (AU) by ACE, at L1, and STEREO-A, ~108°-92° eastward from the Sun-Earth line. Compression regions characterized by magnetic field and density enhancements were observed at the leading edge of these solar wind streams. At one AU they were often accompanied by increases of energetic (>50 keV) ion intensities, as is typical of compression regions. In this paper, we associate the recurrent solar wind streams observed at one AU with those observed by the Parker Solar Probe (PSP). The association allows us to investigate the plasma and energetic particle properties of the interface between slow and fast streams at different solar distances. Our associations are supported by predictions made by the ENLIL model using GONG-ADAPT-WSA which allows us to contextualize the inner heliospheric conditions during PSP's first two orbits.

The Parker Solar Probe (PSP) mission serves as an opportunity to understand the connection between turbulent dissipation and particle heating and solar wind acceleration. Though inherently connected to plasma kinetic scales, the physical dynamics and in-situ observational signatures of dissipation typically depend on macroscale properties of the mean magnetic field. In order to observe anisotropic characteristics of magnetic field fluctuations at inertial and dissipation scales, a merged dataset consisting of fluxgate and search-coil magnetometer measurements has been developed for PSP/FIELDS. This work reports first observations from the PSP merged magnetic field data, focusing on anisotropic properties of dissipation scale phenomena: coherent electromagnetic ion cyclotron waves, spectral power distributions, intermittency, and magnetic helicity. Analyzing these observational signatures at kinetic scales will largely constrain the physical processes responsible for coronal heating and solar wind acceleration.

Switchbacks are among the most prominent features observed by Parker Solar Probe during its close encounter with the Sun (Bale et al., [2019]). These magnetic structures with reversed polarity have well defined boundaries, higher flow velocity with respect to the surrounding plasma, enhanced wave activity inside the structures and at the boundaries. Their duration varies from a few seconds to tens of minutes and more. We present an extensive statistical study of their properties, with a particular focus on their boundaries: orientation, type of discontinuity, presence of wave activity, velocity orientation with respect to the boundaries, and Poynting flux. This characterization is based both on wavefield data (FIELDS) and on particle data (SWEAP)

Solar wind protons regularly deviate from local thermodynamic equilibrium (LTE). Non-LTE features of the VDF can provide sources of free energy that drive instabilities. One such source of free energy is an excess temperature parallel to the local magnetic field, which may include a non-zero proton heat flux. We characterize this excess temperature with two drifting proton populations, one Maxwellian and one bi-. We compare proton beam and core measurements at two locations. At 1 AU, we report more than 20 years of measurements with the Wind/SWE Faraday cups. Below 0.3 AU, we also report their radial variation using measurements from the Parker Solar Probe SWEAP and Fields instruments.

The SWEAP/SPAN-Ion instrument on Parker Solar Probe (PSP) measures ion distribution functions in the Solar wind, and in addition is equipped with a time of flight analyser allowing heavier ions to be distinguished from protons. Despite a large proportion of the core ion distributions still being outside the instrument's field of view, it is possible to make good measurements of the proton and alpha moments, particularly of velocity and temperature, in directions dictated by the sensor geometry and field of view. Having carried out a careful subtraction of the small contamination in the instrument's alpha channel from protons, we present here first results on the alpha particle content in the solar wind, as observed by SPAN-Ion, during PSP's second encounter from 55 to 35 Solar radii. In particular we report on alpha to proton abundance ratios, both inside and outside the magnetic "switchbacks" observed by the FIELDS instrument suite, alpha particle velocities, and alpha to proton temperature ratios. Wavelet spectra of the FIELDS magnetic field measurements reveal an abundance of ion cyclotron waves. While these are most likely due to secondary proton beams, we examine whether or not a proton alpha drift instability is an additional driver of these waves.

We examined the plasma velocity and magnetic field data in order to asses the validity of the Taylor hypothesis throughout the first two orbits of Parker Solar Probe. The Taylor hypothesis is of crucial importance for single-point observations, since it allows for the conversion of a measured time series into a spatial profile of a turbulent flow. In the case of the solar wind at 1AU, the validity of the Taylor hypothesis has generally been well established. However, this is not always the case for the Parker Solar Probe, since the velocity of the solar wind near the perihelion becomes comparable to the local Alfven speed of the plasma. Additionally, the spacecraft's orbital velocity is also of similar order for a part of the approach. Finally, turbulent velocity fluctuations can also become significant enough to undermine the frozen flow assumptions. We demonstrate that the validity of the Taylor hypothesis is less robust for the parts of the orbit below 40 solar radii. Additionally, local plasma conditions or increased turbulent fluctuations can impact the validity throughout the approach at larger distances.

The NASA Parker Solar Probe mission has made its first set of solar encounters at 35.7 solar radii. The first encounters reveal a solar wind rich in instabilities and kinetic physics. The FIELDS instrument on PSP measures copious, intermittent burstsof electron plasma (Langmuir) waves throughout Encounters 1 and 2. These waves are purely electrostatic, to the sensitivity of the measurement, and have wave (electric field) vectors aligned with the solar wind magnetic field. We suggest that these Langmuir waves are generated on a Landau resonance by electron beams with beam speeds of 5-10 v_th. The duration of the bursts suggests beams of ~100-10000 km across the magnetic field. We examine the statistics and occurence conditions. Langmuir waves during Encounter 1 are definitively not associated with solar type III radio bursts. FIELDS also measures signatures of ion cyclotron waves and kinetic scale turbulence, with indications for energy exchange processes in the inner corona.

The solar wind is known to be a hot, mostly-collisionless plasma that expands beyond the solar system domain. At closer distances to the Sun, the plasma is found to be less spatially uniform and further from local thermodynamic equilibrium. It is there that the Parker Solar Probe SWEAP experiment analyzes the kinetic properties of the plasma between different ion species in order to study different kinetic and thermodynamic processes involved in the solar wind evolution. The Solar Probe Cup (SPC) continuously measures fast, high-resolution measurements of the radial charge-flux distribution of solar wind ions. In these measurements, however, the hydrogen and helium components frequently overlap, making it very difficult to determine the density, velocity and temperature of the helium component. The Solar Probe ion Analyzer (SPAN-i), meanwhile, measures hydrogen and helium particle flux distributions in a variety of modes, including three-dimensional and mass-discriminating modes that are executed at varied cadences and resolutions. In this work, we combine simultaneous measurements of solar wind helium from the Faraday Cup and electrostatic analyzers in order to disentangle the two species in the SPC measurements. In so doing, we derive a rapid helium abundance ratio, relative speed, and temperature in the inner heliosphere. Using the helium abundance as a tracer of solar wind streams, we then compare the kinetic states of similar streams observed along the Parker Solar Probe encounter to their evolved states out at 1 AU with measurements from Wind spacecraft.

The Poynting flux, measured on the Parker Solar Probe during 10 days surrounding the perihelion pass on April 4, 2019, was large (~20,000 μwatts/m2) at 35 solar radii and nearly zero before and after perihelion at 50 solar radii. Its spatial divergence means that electromagnetic energy was converted to plasma energy that heated and accelerated the solar wind. Because the Poynting flux, which is electromagnetic energy flow, was comparable in magnitude to the kinetic energy flow, 0.5nmv3, near perihelion, the Poynting flux was adequate for performing this acceleration task. Properties of the MHD waves that comprised the Poynting flux are discussed, including their correlation with magnetic field switchbacks.

Adiabatic theory predicts that as the solar wind moves away from the Sun, electron temperature should increase in the direction parallel to the magnetic field, becoming highly anisotropic by 1 AU. Observations show on the contrary that electron temperatures remain relatively isotropic over this distance, suggesting some scattering mechanism, such as plasma instabilities driven by the temperature anisotropy, counteracts the magnetic focusing. During the first two orbits of Parker Solar Probe, the Solar Probe Analyzers (SPAN) measured electron distribution functions down to ~35 solar radii, allowing the characterization of the scattering process starting from near its onset. We compute, along with other important plasma diagnostics, the temperature anisotropy of the thermal and suprathermal solar wind electrons using a combination of numerical integration and model fitting, and discuss the dependence of these measurements on heliocentric distance and local plasma properties.

The proton-scale spectral break of magnetic field fluctuations is an important phenomenon in the solar wind. It is considered as the scale at which turbulent dissipation starts occurring and a transition to a kinetic turbulent cascade begins. In the cruise and encounter phases of its second orbit, Parker Solar Probe had enough resolution to continuously observe the radial dependence of the magnetic spectral break. This work reports observations of the proton-scale break near the Sun, and its evolution with solar radial distance (0.17~0.5 AU). We compare this to the plasma characteristic ion scales (proton gyroradius, proton inertial length, etc.), to relate the break to the possible mechanisms occurring at this scale. This analysis will help us to understand the origin and the development of the solar wind.

We use measurements from the Parker Solar Probe in the inner heliosphere to decompose low frequency fluctuations in fields and plasmas into their constituent MHD modes. The decomposition is performed in frequency space in the spacecraft frame over a range extending from mHz up to that corresponding to the advection of the average bulk ion gyro-radii over the spacecraft. The decomposition reveals the outward propagation of a broad spectrum of shear Alfven waves along the background magnetic field with reversals in propagation in the plasma frame through 'switchback' intervals of reversed radial magnetic field. This morphology suggests propagation from a source closer to the sun along 'corrugated' field-lines that provide alternating radial orientations as the magnetic field is advected with the flow over the spacecraft. Locations of shear mode intensification are correlated with enhanced slow mode oscillations propagating along the magnetic field with the shear mode and over roughly the same frequency range. Curiously, during intervals of depressed magnetic field strength a significant fraction of the observed spectral energy density is comprised of fast mode waves propagating mostly inward in the plasma frame.

Ion temperature anisotropies are fundamental properties of plasma velocity distribution functions indicative of various unstable modes which often lead to energy dissipation through wave generation. Typically this quantity is directly measured using particle detectors spanning multiple look directions relative to the magnetic field. Using the Solar Probe Cup (SPC) on board Parker Solar Probe, we are able to infer the temperature anisotropy of the bulk solar wind plasma by examining the change in plasma flow angle across each scan through particle energy/charge. This gives us the possibility of measuring ion temperature anisotropy on the order of the instrument operation cadence, which is typically greater than 1 sample per second during each Solar encounter. Additionally, as the SPC looks directly at the Sun, we are able to measure the anisotropy at times when other instruments are unable to sample the full particle distribution due to limitations from look directions and shielding. We examine the behavior of this temperature anisotropy measurement relative to observed wave phenomena and other plasma parameters. To our knowledge, this use of SPC data is the first derivation of temperature anisotropy measurements using this method from a Faraday Cup instrument.

One of the driving objectives behind the Parker Solar Probe mission is to cross into the magnetically closed corona and to observe the coronal plasma in situ. The transition from open to closed coronal phenomena can be a nuanced distinction, but in gross terms the key boundary is clearly the "Alfvén Point"—the point where the magnetic and kinetic energy of the plasma are equal. This is not a point, of course, but a surface or layer—and almost certainly a fuzzy, highly corrugated one. It is inside of this surface that Alfvén waves propagate faster than the bulk flow of the wind, and it is thus inside of this surface that Alfvén waves can transport energy sunward. The presence of sunward Alfvén waves in the plasma unlocks modes of wave-particle energy exchange that may be key to the heating and acceleration of the solar wind, but are not accessible elsewhere in the heliosphere. In the years (and decades) leading up to the Parker Solar Probe mission, the location of the Alfvén point has been estimated from remote observations and from the scaling of in situ observables to lie anywhere from 10-30 solar radii above the surface of the sun. In this paper, we use inner heliospheric scalings from the first Parker Solar Probe encounters to estimate the Alfven Point in the streamer belt and coronal hole wind… and we reveal just how close the Parker Solar Probe has already come to skimming the Alfvén surface.

Alfv'enicity is an important concept in plasma dynamics, but its meaning is sometimes ambiguous, as it is used to imply different (related) constructs by different authors. There are three measures: Alfvén ratio r_A

The Parker Solar Probe observed a coronal mass ejection (CME) event during its first orbit around the sun. This event is analyzed by applying a wavelet analysis technique to the MHD invariants, namely the magnetic helicity, cross helicity, and residual energy. Our results show that the CME, as large scale magnetic flux rope, possess high magnetic helicity, relatively low cross helicity, and highly negative residual energy, thus pointing to a magnetic fluctuation dominated structure. Using the same technique, we also search for small-scale coherent magnetic flux rope structures, which are intrinsic to MHD turbulence in the solar wind during other periods. Multiple structures with duration between 8 and 360 minutes are identified from PSP in situ spacecraft measurements through analyzing the magnetic helicity, cross helicity and residual energy. The location and scales of these structures are characterized by wavelet spectra. Transport theory suggests that these small-scale magnetic flux ropes may be contribute to the acceleration of charged particles through reconnection processes, and the dissipation of these structures may be important for understanding the coronal heating processes.

NASA's Parker Solar Probe (PSP) has collected data for the first two encounters. We have modeled the solar corona and inner heliosphere of these encounters using the Alfvén Wave Solar atmosphere Model (AWSoM, van der Holst et al. 2014) with GONG-ADAPT magnetograms. AWSoM allows us to interpret the PSP data in the context of coronal heating via low-frequency Alfvén wave turbulence with partial wave reflection due to Alfvén speed gradients. In van der Holst et al. (2019), we made predictions for the first encounter. In our new simulations we show how improved partitioning of the wave dissipation to the electron and anisotropic proton temperatures and better grid design result in significantly improved comparison with the PSP data. We also show the simulated distance of PSP to the heliospheric current sheet, the magnetic connectivity of PSP to the photosphere, and whether PSP was in the fast or slow wind.

Identifying energy transfer mechanisms of the inner heliosphere has been an outstanding mystery for many decades. Unprecedented in-situ measurements from NASA's latest mission to the Sun, Parker Solar Probe (PSP), may shed light on an answer. The existence of non-Maxwellian ion velocity distribution functions in association with plasma wave growth (and decay) yields insight to key energy transfer mechanisms leading to solar wind thermalization. We present initial PSP observations during the second encounter within 35-50 solar radii supporting the presence of proton beams, measured by SPAN-I on the Solar Wind Electrons Alphas and Protons (SWEAP) instrument suite, associated with ion-cyclotron waves, as observed by the FIELDS instrument. Wavelet transformations applied to the magnetic field data reveal spectra approximate to the ion-cyclotron frequency. It is hypothesized that these wave modes were driven by the simultaneously measured secondary proton beams. During quiet periods where the magnetic field is radially aligned, we also show that the energy density is dominated by these waves. Further work will characterize the stability of the velocity distribution functions during the observed correlations between the proton beams and ion-cyclotron wave modes.

NASA's Parker Solar Probe (PSP) spacecraft launched on August 12, 2018, and reached its first of 24 perihelia (P1) on November 5th, 2018. We developed a 3D wave-turbulence-driven (WTD) magnetohydrodynamic (MHD) solution for the solar corona and inner heliosphere, driven by the then available observations of the Sun's photospheric magnetic field prior to this event, both as a test of our model's predictive capabilities as well as to aid in mission planning. We inferred that, in the days prior to first encounter, PSP was immersed in wind emanating from a well-established, positive-polarity northern polar coronal hole. During the encounter, however, field lines from the spacecraft mapped to a negative-polarity equatorial coronal hole, within which it remained for the entire encounter, before becoming magnetically connected to a positive-polarity equatorial coronal hole. In this presentation, we compare our MHD results with FIELDS, SWEAP, and WISPR observations made during P1, highlighting both agreements and disagreements. We use the model results to help interpret the PSP observations, and, in particular, provide a global context for them. From the model results, we can identify what types of solar wind PSP encountered, what the underlying magnetic structure was, and how complexities in the orbital trajectory can be interpreted within a global, inertial frame. Ultimately, the measurements returned by PSP can be used to constrain current theories for heating the solar corona and accelerating the solar wind, as well as improving the accuracy of the predictions.

Observations at 1 AU have reported direct evidence of an inertial-range energy cascade [1]. The average value of energy cascade rate in 1 AU solar wind plasma is around 1000 J/kg/s, which is shown to be sufficient to account for the heating of the solar-wind [2]. Parker Solar Probe (PSP) offers the first opportunity to estimate a similar, fluid-scale energy decay rate closer to the solar corona. Using a Politano-Pouquet [3] third-order law, we provide estimates of fluid-range energy cascade rate at 50 to 36 solar radii, during the first perihelion first encounter of PSP. Despite the cross-helicity being high in these regions of the heliosphere, there is an inertial-range cascade occurring. The energy transfer rate is at least 100 times higher than the average value at 1AU. Further, we estimate the global energy decay rate at the energy-containing scales using a Taylor-Karman decay phenomenology [4]. The von Karman energy decay estimates agree reasonably well with the third-order-law estimates. We also compare the two estimates with the heating rate obtained from a turbulence-based, global solar wind simulation [5]. [1] Sorriso-Valvo et al., Phys. Rev. Lett., 99, 115001 (2007) [2] MacBride et al., ApJ, 679, 1644 (2008) [3] Politano & Pouquet, GRL, 25, 273 (1998) [4] Wan et al., JFM, 697, 296315 (2012) [5] Usmanov et al., ApJ, 865, 25 (2018)