arXiv subjects

Stuart Bale

Publications and source records attributed to Stuart Bale.

At least 19 recordsLinked to original sources

arXiv2026

Statistical Energy Budget of the Solar Wind with Parker Solar Probe and Solar Orbiter : 1 - Unbalanced Total Energy Radial Evolution

The magneto-hydrodynamic (MHD) description of the solar wind has long been regarded as one of the most successful frameworks for investigating solar wind heating and acceleration. Under a set of simplifying assumptions, it provides a reduced large-scale description that is widely used to study the energetic role of electromagnetic fluctuations in the solar wind energy budget. In practice, applying simplification to MHD can result in the loss of some physics compared to an exact set of equations. However, an observational energy budget can help us to verify whether any energetic information has been lost in the process of simplification. To do so, we directly test the validity of the large-scale non-linear ideal MHD energy budget with solar wind observations from Parker Solar Probe (PSP) and Solar Orbiter (SO). After verifying the reliability of the measurements through comparison with previous studies of similar solar wind properties, we test the conservation of the total energy predicted by the MHD theory. We find that the average total energy increases with radial distance by $56\%$ ($\pm9\%$) between 14 and 203 solar radii. The increase in kinetic energy is not sufficiently compensated by the decrease of the thermal and electromagnetic contributions. We investigate the major sources of observational uncertainty and find that they cannot account for the observed energy increase. These results suggest that the large-scale theoretical description may neglect an energetically significant contribution to solar wind acceleration. Assuming that such a contribution exists, we predict that this extra energy would scale with radial distance as $r^{-0.54\pm0.38}$.

astro-ph.SR↗

arXiv2026

A Singular Value Decomposition Framework for Jovian Radio Emissions from Parker Solar Probe

Jovian decametric and hectometric radio emissions provide critical insights into Jupiter's magnetospheric dynamics and its electrodynamic coupling with Io. Across 21 perihelion encounters since its first light in 2018, the Parker Solar Probe mission has repeatedly recorded high-resolution radio and plasma wave data near closest approach, as a result serving as a long-term distant observer of the Jovian system. However, extracting these relatively faint planetary signals from the continuous solar wind background presents a significant analytical challenge, as the data is routinely saturated by quasi-thermal plasma noise, spacecraft instrumental interference, and solar bursts. To overcome these observational barriers, we present a generalized empirical pipeline that utilizes Singular Value Decomposition coupled with a deterministic, dual-stage noise filtering process. By converting dynamic spectra into a periodic, phase-folded reference frame, this model isolates structured Jovian emissions from the stochastic heliospheric background, demonstrating empirical Signal-to-Noise Ratio improvements of $1.54 \pm 0.11$ dB, reaching up to $3.32$ dB. Additionally, a secondary ``Eigenfaces'' matrix factorization is applied to identify long-term global morphological trends and serve as a detection method for future Jovian emissions. This automated mathematical framework extracts transient planetary signals without relying on localized spatial constraints, establishing the viability of PSP as a Jovian radio observatory.

astro-ph.IM↗

arXiv2026

The FLARE Facility

The Facility for Laboratory Reconnection Experiments (FLARE) has been constructed to study magnetic reconnection in multiple X-line regimes relevant to space, astrophysical, and fusion plasmas. Building upon the successful design of the Magnetic Reconnection Experiment (MRX), FLARE features a larger physical volume, stronger magnetic fields, and an independent ohmic heating drive to significantly extend the accessible parameter space, targeting Lundquist numbers up to S ~ 10^5 and normalized system sizes up to \lambda ~ 10^3. This paper details the facility's core engineering components, including the primary vacuum vessel, internal flux cores, highly segmented external coil systems, modular capacitor banks, and the safety interlock and control architecture. An initial diagnostic suite is presented, comprising high-resolution 2D magnetic probe arrays, triple Langmuir probes, a fully fiber-coupled interferometer, ion Doppler spectroscopy, and fast camera imaging. Initial operations demonstrate the device's experimental flexibility and reliability, successfully executing symmetric push-pull reconnection, spheromak merging, and asymmetric downstream configurations. Currently operating within "Stage 2.5" with S ~ 2,500 and \lambda ~ 60 for anti-parallel reconnection, FLARE provides immediate access to the multiple X-line regimes. Planned hardware upgrades, advanced diagnostic additions, and integration with fully kinetic simulations will further expand its capabilities as it transitions into a collaborative user facility for the broader plasma science community.

physics.plasm-ph↗

arXiv2026

On the Radial Evolution of the Solar Wind : The Source Alignment Method Applied to Parker Solar Probe and Solar Orbiter Observations

The properties of the solar wind, as measured in situ throughout the heliosphere, depend both on the characteristics of its coronal source and on the intrinsic processes governing its interplanetary evolution. Recently, radial and Parker spiral alignment techniques have been applied to Parker Solar Probe (PSP) and Solar Orbiter (SO) observations to investigate the radial evolution of the same solar wind parcel. These studies have shown that the solar wind can undergo significant acceleration even beyond its primary acceleration region (i.e., above 15 solar radii). However, such radial and Parker spiral alignments are rare in practice, which limits the statistical significance and general applicability of the results. We introduce a new source alignment technique designed to overcome these limitations. Using magnetic backmapping, we associate similar solar wind streams observed by the two spacecraft based on the proximity of their photospheric footpoints, combined with additional in-situ stream similarity criteria. Applying the source alignment method to PSP and SO observations, we identify a total of 548 alignment intervals, each lasting 30 minutes. By constructing statistics over all alignments, we find that the solar wind speed increases by an average of 45% per radial decade (approximately 147 km/s) between the two probes. This result demonstrates that solar wind acceleration in the inner heliosphere remains significant compared to that occurring below 15 solar radii. Among the different studied plasma parameters, the radial evolution of the electron temperature and plasma density, show the strongest anti-correlation with the increase in bulk velocity.

astro-ph.SR↗

arXiv2026

Parker Solar Probe Observations of Compound Reconnection Exhaust Boundaries and Mirror-Mode Structures in the Near-Sun Heliospheric Current Sheet

Magnetic reconnection is a fundamental physical process that can drive rapid conversion of magnetic energy into plasma bulk flows, thermal heating, and particle acceleration in space and astrophysical plasmas. Classical reconnection theory predicts that the Alfvenic reconnection exhausts are bounded by pairs of slow-mode shocks. However, identifying and characterizing these shocks through in situ spacecraft observations remains a challenge. Here we report Parker Solar Probe (PSP) observations of a reconnection exhaust embedded in the heliospheric current sheet (HCS) at a heliocentric distance of 12.2 R_O. The reconnection exhaust is bounded on both boundaries by compound magnetic structures rather than a pair of pure slow shocks. Each boundary consists of a rapidly evolving, steep inner slow shock, whose Mach numbers and shock-normal angles change significantly within several minutes, and an outer, gradual compound structure which comprises a slow shock and a rotational discontinuity. These slow shocks are quasi-perpendicular and are accompanied by enhanced proton perpendicular heating. Deep within the reconnection exhaust, high perpendicular temperature together with large plasma beta trigger mirror instability and generate mirror-mode structures. These observations provide new insights into the structure of reconnection exhaust boundaries and their role in energy conversion in the near-Sun plasma.

astro-ph.SR↗

arXiv2025

Electric fields and waves in the Venus nightside magnetosphere

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. 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 f ield 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. These results describe the first step in the proper technique for future measurements of electric fields in shadow.

physics.space-ph↗

arXiv2024

Ion-Scale Solitary Structures in the Solar Wind Observed by Solar Orbiter and Parker Solar Probe

We investigate a class of ion-scale magnetic solitary structures in the solar wind, characterized by distinct magnetic field enhancements and bipolar rotations over spatial scales of several proton inertial lengths. These structures are revisited using high-resolution data from the Solar Orbiter and Parker Solar Probe missions. Using a machine learning-based method, we identified nearly a thousand such structures, providing new insights into their evolution and physical properties. Statistical analysis shows that these structures are more abundant closer to the Sun, with occurrence rates peaking around (30 - 40, R_sun) and decreasing farther out. High-cadence measurements reveal that these structures are predominantly found in low-beta (beta <= 1) environments, with consistent fluctuations in density, velocity, and magnetic field. Magnetic field enhancements are often accompanied by plasma density drops, which, under near pressure balance, limit field increases. This leads to small fractional field enhancements near the Sun (approximately 0.01 at 20 R_sun), making detection challenging. Magnetic field variance analysis indicates that these structures are primarily oblique to the local magnetic field. Alfv\'enic velocity-magnetic field correlations suggest that most of these structures, unlike most near-Sun solar wind fluctuations, exhibit sunward-directed Alfv\'enic polarization in the plasma frame. We compare these findings with previous studies, discussing possible generation mechanisms and their implications for the turbulent cascade in the near-Sun Alfv\'enic solar wind. While these structures might be Alfv\'enic solitons, vortices, or flux ropes, we refrain from a definitive classification pending further evidence. Further high-resolution observations and simulations are needed to fully understand their origins and impacts.

physics.space-ph↗

arXiv2024

LuSEE-Night power requirements and power generation strategy

The Lunar Surface Electromagnetics Experiment at Night (LuSEE-Night) is a project designed to investigate the feasibility of observing the Cosmic Dark Ages using an instrument on the lunar far-side. LuSEE-Night will measure the redshifted 21 cm transition of neutral hydrogen over a frequency range of 0.1-50 MHz, covering the redshift range 27 < z < 1100. The LuSEE-Night instrument is a radio frequency spectrometer, consisting of four horizontal monopole antennas, arranged to give wide zenith-pointing beams with two orthogonal linear polarizations. This combination of polarization, spectral, and angular sensitivity will be necessary to separate the cosmological signal from significantly stronger foreground emissions. LuSEE-Night will observe in drift scan during lunar night while the moon shields it from radio frequency interference from both the Earth and sun, and will transmit science and telemetry data back to Earth via an orbital relay during the lunar day. LuSEE-Night will have to operate in a challenging environment: its electronics must operate under hard radiation, the instrument must be thermally isolated during the cold 100~K lunar night, and have a thermal rejection path to survive the 390~K daytime temperature, and its photovoltaic and battery systems must provide sufficient power to operate during two weeks of lunar night. Furthermore, the instrument spectrometer must be powered throughout the lunar night using only a 7~kWh battery, due to mass limitations. Here we describe the power generation, storage, and delivery subsystems of the LuSEE-Night instrument, and the simulations which were performed to design the power subsystems and ensure instrument survival and operation throughout the long lunar night. We also describe the Concept of Operations (ConOps) developed for the LuSEE-Night mission, which derives from the power management simulations.

astro-ph.IM↗

arXiv2024

On the origin of the type-III radiation observed near the Sun

$\textit{AIMS.}$ To investigate processes associated with generation of type-III radiation. $\textit{METHODS.}$ Measure the amplitudes and phase velocities of Parker Solar Probe observed electric fields, magnetic fields, and plasma density fluctuations. $\textit{RESULTS.}$ (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 measurement 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 a large uncertainty in this quantity (more than a factor of two) the phase velocity of the Langmuir wave was <10,000 km/sec. (2) There is an electromagnetic wave near the Langmuir frequency having a phase velocity <50,000 km/sec. (3) Whether there are electromagnetic waves at the harmonics of the Langmuir frequency cannot be determined because, if they existed, their magnetic field components would be below the noise level of the measurement. (4) The rapid (less than one msec) amplitude variations typical of the Langmuir wave and its harmonics are artifacts resulting from addition of two waves, one of which has small frequency variations that arise from traveling 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.

astro-ph.SR↗

arXiv2023

Fundamental-harmonic pairs of interplanetary type III radio bursts

Type III radio bursts are not only the most intense but also the most frequently observed solar radio bursts. However, a number of their defining features remain poorly understood. Observational limitations, such as a lack of sufficient spectral and temporal resolution, have hindered a full comprehension of the emission process, especially in the hecto-kilometric wavelengths. Of particular difficulty is the ability to detect the harmonics of type III radio bursts. Hereafter we report for the first detailed observations of type III fundamental-harmonic pairs in the hecto-kilometric wavelengths, observed by the Parker Solar Probe. We present the statistical analysis of spectral characteristics and the polarization measurements of the fundamental-harmonic pairs. Additionally, we quantify various characteristic of the fundamental-harmonic pairs, such as the time-delay and time-profile asymmetry. Our report and preliminary analysis conclude that fundamental-harmonic pairs constitute a majority of all type III radio bursts observed during close encounters 6 -- 10 when the probe is in close proximity to the source region and propagation effects are less pronounced.

astro-ph.SR↗

arXiv2023

Embedded coherent structures from MHD to sub-ion scales in turbulent solar wind at 0.17 au

We study intermittent coherent structures in solar wind magnetic turbulence from MHD to kinetic plasma scales using Parker Solar Probe data during its first perihelion (at 0.17 au), when the satellite was in the Alfv\'enic slow wind of 340 km/s. The coherent structures are energetic events localized in time and covering wide range of scales. We detect them using Morlet wavelets. For the first time, we apply a multi-scale analyses in physical space to study these structures. At MHD scales within the inertial range, times scales $\tau \in (1, 10^{2} )$ s, we find (i) current sheets including switchback boundaries and (ii) Alfv\'en vortices. Within these events, there are embedded structures at smaller scales: typically Alfv\'en vortices at ion scales, $\tau \in (0.08, 1)$ s, and a compressible vortices at sub-ion scales, $\tau \in (8,80)$ ms. The number of coherent structures grows toward smaller scales: we observe about $\sim 200$ events during 5 h time interval at MHD scales, $\sim 10^{3}$ events ai ion scales and $\sim 10^{4}$ events at sub-ion scales. In general, there are multiple structures of ion and sub-ion scales embedded within one MHD structure. There are also examples of ion and sub-ion scales structures outside of MHD structures. To quantify the relative importance of different type of structures, we do a statistical comparison of the observed structures with the expectations of models of the current sheets and vortices. This comparison is based on amplitude anisotropy of magnetic fluctuations within the structures. The results show the dominance of Alfv\'en vortices at all scales in contrast to the widespread view of dominance of current sheets. This means that Alfv\'en vortices are important building blocs of solar wind turbulence.

astro-ph.SR↗

arXiv2023

Arguments for the physical nature of the triggered ion-acoustic waves observed on the Parker Solar Probe

Triggered ion-acoustic waves are a pair of coupled waves observed in the previously unexplored plasma regime near the Sun. They may be capable of producing important effects on the solar wind. Because this wave mode has not been observed or studied previously and it is not fully understood, the issue of whether it has a natural origin or is an instrumental artifact can be raised. This paper discusses this issue by examining 13 features of the data such as whether the triggered ion-acoustic waves are electrostatic, whether they are both narrow-band, whether they satisfy the requirement that the electric field is parallel to the k-vector, whether the phase difference between the electric field and the density fluctuations is 90 degrees, whether the two waves have the same phase velocity as they must if they are coupled, whether the phase velocity is that of an ion-acoustic wave, whether they are associated with other parameters such as electron heating, whether the electric field instrument otherwise performed as expected, etc. The conclusion reached from these analyses is that triggered ion-acoustic waves are highly likely to have a natural origin although the possibility that they are artifacts unrelated to processes occurring in the natural plasma cannot be eliminated. This inability to absolutely rule out artifacts as the source of a measured result is a characteristic of all measurements.

astro-ph.SR↗

arXiv2023

Effect of Spherical Polarization on the Magnetic Spectrum of the Solar Wind

Magnetic fluctuations in the solar wind are often observed to maintain constant magnitude of the magnetic field in a manner consistent with spherically-polarized large-amplitude Alfv\'en waves. We investigate the effect of spherical polarization on the magnetic spectral index through a statistical survey of magnetic fluctuations observed by Parker Solar Probe between 20$R_\odot$ and 200$R_\odot$. We find that deviations from spherical polarization, i.e., changes in $|\mathbf{B}|$ (compressive fluctuations) and one-dimensional discontinuities, have a dramatic effect on the scaling behavior of the turbulent fluctuations. We show that shallow $k^{-3/2}$ spectra are only observed for constant magnetic field strength, three-dimensional structures, which we identify as large amplitude Alfv\'en waves. The presence of compressive fluctuations coincides with a steepening of the spectrum up to $k^{-5/3}$. Steeper power law scalings approaching $k^{-2}$ are observed when the fluctuations are dominated by discontinuities. Near-sun fluctuations are found to be the most spherically polarized, suggesting that this spherical state is fundamental to the generation of the solar wind. With increasing distance from the Sun, fluctuations are found to become less three dimensional and more compressive, which may indicate the breakdown of the Alfv\'enic equilibrium state.

astro-ph.SR↗

arXiv2022

Triggered Ion-acoustic Waves

The Parker Solar Probe is in a solar orbit with a perihelion for orbit 12 at 13.3 solar radii. The electric field experiment on this satellite observes what we call triggered ion-acoustic waves as the most dominant wave mode above a few Hz within the solar radial distance of 15-25 solar radii. In this mode, a few Hz electrostatic wave is typically accompanied by bursts of a few hundred Hz wave whose bursts are phase locked with each low frequency wave period. Plasma density fluctuations with {\Delta}n/n~0.1 accompany these waves and they have no magnetic field component. The wave durations can be hours and their field and density fluctuations are nearly pure sine waves. They are identified as ion-acoustic waves. The low and high frequency waves are measured to have the same phase velocity within experimental uncertainties, which is a requirement associated with their phase locked relationship. From the measured wavelength, the potential associated with the low frequency wave is estimated to be ~10 Volts, which can result in electron heating via the Landau resonance that is in agreement with observations of the core electron temperature increases at times of such waves. Their phase locked relationship and pure frequency are surprising features that characterize a new regime of instability and evolution of ion-acoustic waves that may not have been reported previously. That these waves are an instrumental effect unrelated to natural processes is considered. While this is unlikely, the possibility that these waves are artificial cannot be rule out

physics.plasm-ph↗

arXiv2022

Research Opportunities in Plasma Astrophysics

Major scientific questions and research opportunities are described on 10 unprioritized plasma astrophysics topics: (1) magnetic reconnection, (2) collisionless shocks and particle acceleration, (3) waves and turbulence, (4) magnetic dynamos, (5) interface and shear instabilities, (6) angular momentum transport, (7) dusty plasmas, (8) radiative hydrodynamics, (9) relativistic, pair-dominated and strongly magnetized plasmas, (10) jets and outflows. Note that this is a conference report from a Workshop on Opportunities in Plasma Astrophysics (WOPA, https://w3.pppl.gov/conferences/2010/WOPA/) in January 2010, that attracted broad representation from the community and was supported by the U.S. Department of Energy, National Aeronautics and Space Administration, National Science Foundation, American Physical Society's Topical Group for Plasma Astrophysics and Division of Plasma Physics, and Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas. Although there has been much planning and many developments in both science and infrastructure since the report was written, most of the motivation, priorities, problems and technical challenges discussed therein remain unaddressed and are relevant at the time of posting.

physics.plasm-ph↗

arXiv2021

An improved technique for measuring plasma density to high frequencies on the Parker Solar Probe

The correlation between the plasma density measured in space and the surface potential of an electrically conducting satellite body with biased electric field detectors has been recognized and used to provide density proxies. However, for Parker Solar Probe, this correlation has not produced quantitative density estimates over extended periods of time because it depends on the energy dependent exponential variation of the photoemission spectrum, the electron temperature, the ratio of the biased surface area to the conducting spacecraft surface area, the spacecraft secondary or thermal emission, the spacecraft distance from the Sun, etc. In this paper the density as a function of time and frequency to frequencies as high as the electron gyrofrequency is determined through least squares fits of a function of the spacecraft potential to the plasma density measured on the Parker Solar Probe. This function allows correction for the many effects on the spacecraft potential other than that due to the plasma density. Some examples of plasma density obtained from this procedure are presented.

physics.space-ph↗

arXiv2021

Core Electron Heating By Triggered And Ordinary Ion Acoustic Waves In The Solar Wind

Orbits six through nine of the Parker Solar Probe have been studied to show that solar wind core electrons emerged from 15 solar radii with temperatures that were constant to within ~10% although the solar wind speed varied from 300 to 800 km/sec. After leaving 15 solar radii, the core electrons were isotropically heated as much as a factor of two below 30 solar radii by triggered and natural ion acoustic waves. To distinguish this wave heating from processes occurring inside 15 solar radii, the electron temperature is modeled as having two components, the base temperature observed at 15 solar radii and the temperature resulting from wave interactions between 15 solar radii and the observing point. As further justification of this temperature model, there were about a dozen intervals during the four orbits, summing to about 250 hours of total time, during which there were few waves, TE/TI was small, and the core electron temperature was close to or slightly greater than the base temperature. Both this base temperature and the temperature resulting from wave heating decreased with radius due to the adiabatic expansion of the solar wind. The waves that produced the electron core heating were triggered [Mozer et al, 2021] or normal {Mozer et al, 2020A] ion acoustic waves. They are the dominant wave modes at frequencies greater than 100 Hz at solar distances between 15 and 30 solar radii.

astro-ph.SR↗

arXiv2021

On the origin of switchbacks observed in the solar wind

The origin of switchbacks in the solar wind is discussed in two classes of theory that differ in the location of the source being either in the transition region near the Sun or in the solar wind, itself. The two classes of theory differ in their predictions of the switchback rate as a function of distance from the Sun. To test these theories, one-hour averages of Parker Solar Probe data were summed over orbits three through seven. It is found that: 1. The average switchback rate was independent of distance from the Sun. 2. The average switchback rate increased with solar wind speed. 3. The switchback size perpendicular to the flow increased as R, the distance from the Sun, while the radial size increased as R2, resulting in a large and increasing switchback aspect ratio with distance from the Sun. 4. The switchback rotation angle did not depend on either the solar wind speed or the distance from the Sun. These results are consistent with switchback formation in the transition region because their increase of tangential size with radius compensates for the radial falloff of their equatorial density to produce switchback rates that are independent of distance from the Sun. This constant observed switchback rate is inconsistent with an in situ source. Once formed, the switchback size and aspect ratio, but not the rotation angle, increased with radial distance to at least 100 solar radii. In addition, quiet intervals between switchback patches occurred at the lowest solar wind speeds.

astro-ph.SR↗