Publication Search Results

ID: 159338
Type: article
Authors: Allen, R. C.; Ho, G. C.; Mason, G. M.; Li, G.; Jian, L. K.; Vines, S. K.; Schwadron, N. A.; Joyce, C. J.; Bale, S. D.; Bonnell, J. W.; Case, Anthony W.; Christian, E. R.; Cohen, C. M. S.; Desai, M. I.; Filwett, R.; Goetz, K.; Harvey, P. R.; Hill, M. E.; Kasper, Justin C.; Korreck, Kelly E.; Lario, D.; Larson, D.; Livi, R.; MacDowall, R. J.; Malaspina, D. M.; McComas, D. J.; McNutt, R.; Mitchell, D. G.; Paulson, Kristoff W.; Pulupa, M.; Raouafi, N.; Stevens, Michael L.; Whittlesey, P. L.; Wiedenbeck, M.
Abstract: The addition of Parker Solar Probe (PSP) to the Heliophysics System Observatory has allowed for the unprecedented ability to study Corotating Interaction Regions (CIRs) at multiple radial distances without significant temporal/longitudinal variations. On September 19, 2019, PSP observed a CIR at ∼0.5 au when it was nearly radially aligned with the Solar Terrestrial Relations Observatory Ahead (STEREO A) spacecraft at ∼1 au, allowing for an unambiguous assessment of the radial evolution of a single CIR. Bulk plasma and magnetic field signatures of the CIR evolve in a fashion characteristic to previous observations; however, the suprathermal ions are enhanced over a larger longitudinal range at PSP than at STEREO A, although at much lower intensities. The longitudinal spread appears to be largely a consequence of magnetic field line topology at CIRs between the compressed slow solar wind upstream and high speed stream following the CIR, underscoring the importance of the large scale topology of these structures.

ID: 158546
Type: article
Authors: Bowen, T. A.; Bale, S. D.; Bandyopadhyay, R.; Bonnell, J. W.; Case, Anthony; Chasapis, A.; Chen, C. H. K.; Curry, S.; Dudok de Wit, T.; Goetz, K.; Goodrich, K.; Gruesbeck, J.; Halekas, J.; Harvey, P. R.; Howes, G. G.; Kasper, J. C.; Korreck, Kelly; Larson, D.; Livi, R.; MacDowall, R. J.; Malaspina, D. M.; Mallet, A.; McManus, M. D.; Page, B.; Pulupa, M.; Raouafi, N.; Stevens, Michael L.; Whittlesey, P.
Abstract: While not specifically designed as a planetary mission, NASA's Parker Solar Probe (PSP) mission uses a series of Venus gravity assists (VGAs) in order to reduce its perihelion distance. These orbital maneuvers provide the opportunity for direct measurements of the Venus plasma environment at high cadence. We present first observations of kinetic scale turbulence in the Venus magnetosheath from the first two VGAs. In VGA1, PSP observed a quasi-parallel shock, beta similar to 1 magnetosheath plasma, and a kinetic range scaling of k(-2.9). VGA2 was characterized by a quasi-perpendicular shock with beta similar to 10, and a steep k(-3.4) spectral scaling. Temperature anisotropy measurements from VGA2 suggest an active mirror mode instability. Significant coherent waves are present in both encounters at sub-ion and electron scales. Using conditioning techniques to exclude these electromagnetic wave events suggests the presence of developed sub-ion kinetic turbulence in both magnetosheath encounters. Key Points . Observations from Parker Solar Probe reveal kinetic scale turbulence in the Venus magnetosheath Differences in kinetic range spectral indices between flyby-encounters are possibly due to shock geometry and kinetic plasma instabilities

ID: 159337
Type: article
Authors: Goodrich, Katherine A.; Bonnell, John W.; Curry, Shannon; Livi, Roberto; Whittlesey, Phyllis; Mozer, Forrest; Malaspina, David; Halekas, Jasper; McManus, Michael; Bale, Stuart; Bowen, Trevor; Case, Anthony; Dudok de Wit, Thierry; Goetz, Keith; Harvey, Peter; Kasper, Justin; Larson, Davin; MacDowall, Robert; Pulupa, Marc; Stevens, Michael
Abstract: Depressions in magnetic field strength, commonly referred to as magnetic holes, are observed ubiquitously in space plasmas. Subproton scale magnetic holes with spatial scales smaller than or on the order of a proton gyroradius, are likely supported by electron current vortices, rotating perpendicular to the ambient magnetic field. While there are numerous accounts of subproton scale magnetic holes within the Earth's magnetosphere, there are few, if any, reported observations in other space plasma environments. We present the first evidence of subproton scale magnetic holes in the Venusian magnetosheath. During Parker Solar Probe's first Venus Gravity Assist, the spacecraft crossed the planet's bow shock and subsequently observed the Venusian magnetosheath. The FIELDS instrument suite onboard the spacecraft achieved magnetic and electric field measurements of magnetic hole structures. The electric fields associated with magnetic depressions are consistent with electron current vortices with amplitudes on the order of 1 μA/m².

ID: 159340
Type: article
Authors: Klein, K. G.; Verniero, J. L.; Alterman, B.; Bale, S.; Case, Anthony; Kasper, J. C.; Korreck, Kelly; Larson, D.; Lichko, E.; Livi, R.; McManus, M.; Martinović, M.; Rahmati, A.; Stevens, Michael; Whittlesey, P.
Abstract: The hot and diffuse nature of the Sun's extended atmosphere allows it to persist in non-equilibrium states for long enough that wave-particle instabilities can arise and modify the evolution of the expanding solar wind. Determining which instabilities arise, and how significant a role they play in governing the dynamics of the solar wind, has been a decades-long process involving in situ observations at a variety of radial distances. With new measurements from the Parker Solar Probe (PSP), we can study what wave modes are driven near the Sun, and calculate what instabilities are predicted for different models of the underlying particle populations. We model two hours-long intervals of PSP/SPAN-i measurements of the proton phase-space density during the PSP's fourth perihelion with the Sun using two commonly used descriptions for the underlying velocity distribution. The linear stability and growth rates associated with the two models are calculated and compared. We find that both selected intervals are susceptible to resonant instabilities, though the growth rates and kinds of modes driven unstable vary depending on whether the protons are modeled using one or two components. In some cases, the predicted growth rates are large enough to compete with other dynamic processes, such as the nonlinear turbulent transfer of energy, in contrast with relatively slower instabilities at larger radial distances from the Sun.

ID: 159628
Type: article
Authors: Shi, Chen; Zhao, Jinsong; Huang, Jia; Wang, Tieyan; Wu, Dejin; Chen, Yu; Hu, Qiang; Kasper, Justin C.; Bale, Stuart D.
Abstract: Small-scale flux ropes (SFRs) are common in the interplanetary environment. However, previous identification procedures generally discard SFRs with medium and high Alfvénicity, which are thought to be Alfvénic waves or Alfvénic structures. This paper first identifies an SFR event with medium Alfvénicity in the inner heliosphere (at ∼0.2 au) using Parker Solar Probe measurements. We find Alfvénic waves that arise inside SFR based on high correlations between the magnetic field and velocity fluctuations. We also observe quasi-monochromatic electromagnetic waves with frequencies f that are usually larger than the local proton cyclotron frequency at the leading and trailing edges of this SFR. These waves are well explained by the outward-propagating ion-cyclotron waves, which have wave frequencies ∼0.03-0.3 Hz and wavelengths ∼60-2000 km in the plasma frame. Moreover, we show that the power spectral density of the magnetic field in SFR middle region follows the power-law distribution, where the spectral index changes from -1.5 (f ≲ 1 Hz) to -3.3 (f ≳ 1 Hz). These findings would motivate developing an automated program to identify SFRs with medium and high Alfvénicity from Alfvénic waves/structures.

ID: 155841
Type: article
Authors: Adhikari, L.; Zank, G. P.; Zhao, L. -L; Kasper, Justin C.; Korreck, Kelly E.; Stevens, Michael; Case, Anthony W.; Whittlesey, P.; Larson, D.; Livi, R.; Klein, K. G.
Abstract: The Parker Solar Probe (PSP) achieved its first orbit perihelion on 2018 November 6, reaching a heliocentric distance of about 0.165 au (35.55 R_☉). Here, we study the evolution of fully developed turbulence associated with the slow solar wind along the PSP trajectory between 35.55 R_☉ and 131.64 R_☉ in the outbound direction, comparing observations to a theoretical turbulence transport model. Several turbulent quantities, such as the fluctuating kinetic energy and the corresponding correlation length, the variance of density fluctuations, and the solar wind proton temperature are determined from the PSP Solar Wind Electrons Alphas and Protons (SWEAP) plasma data along its trajectory between 35.55 R_☉ and 131.64 R_☉. The evolution of the PSP derived turbulent quantities are compared to the numerical solutions of the nearly incompressible magnetohydrodynamic (NI MHD) turbulence transport model recently developed by Zank et al. We find reasonable agreement between the theoretical and observed results. On the basis of these comparisons, we derive other theoretical turbulent quantities, such as the energy in forward and backward propagating modes, the total turbulent energy, the normalized residual energy and cross-helicity, the fluctuating magnetic energy, and the correlation lengths corresponding to forward and backward propagating modes, the residual energy, and the fluctuating magnetic energy.

ID: 156386
Type: article
Authors: Agapitov, O. V.; Wit, T. Dudok de; Mozer, F. S.; Bonnell, J. W.; Drake, J. F.; Malaspina, D.; Krasnoselskikh, V.; Bale, S.; Whittlesey, P. L.; Case, Anthony W.; Chaston, C.; Froment, C.; Goetz, K.; Goodrich, K. A.; Harvey, P. R.; Kasper, J. C.; Korreck, Kelly E.; Larson, D. E.; Livi, R.; MacDowall, Robert J.; Pulupa, M.; Revillet, C.; Stevens, Michael; Wygant, J. R.
Abstract: Observations by the Parker Solar Probe mission of the solar wind at ∼35.7 solar radii reveal the existence of whistler wave packets with frequencies below 0.1 f_ce (20-80 Hz in the spacecraft frame). These waves often coincide with local minima of the magnetic field magnitude or with sudden deflections of the magnetic field that are called switchbacks. Their sunward propagation leads to a significant Doppler frequency downshift from 200-300 to 20-80 Hz (from 0.2 to 0.5 f_ce). The polarization of these waves varies from quasi- parallel to significantly oblique with wave normal angles that are close to the resonance cone. Their peak amplitude can be as large as 2-4 nT. Such values represent approximately 10% of the background magnetic field, which is considerably more than what is observed at 1 au. Recent numerical studies show that such waves may potentially play a key role in breaking the heat flux and scattering the Strahl population of suprathermal electrons into a halo population.

ID: 155836
Type: article
Authors: Allen, R. C.; Lario, D.; Odstrcil, D.; Ho, G. C.; Jian, L. K.; Cohen, C. M. S.; Badman, S. T.; Jones, S. I.; Arge, C. N.; Mays, M. L.; Mason, G. M.; Bale, S. D.; Bonnell, John W.; Case, Anthony W.; Christian, E. R.; de Wit, T. Dudok; Goetz, K.; Harvey, P. R.; Henney, C. J.; Hill, M. E.; Kasper, Justin C.; Korreck, Kelly E.; Larson, D.; Livi, R.; MacDowall, R. J.; Malaspina, D. M.; McComas, D. J.; McNutt, R.; Mitchell, D. G.; Pulupa, M.; Raouafi, N.; Schwadron, N.; Stevens, Michael L.; Whittlesey, P. L.; Wiedenbeck, M.
Abstract: Several fast solar wind streams and stream interaction regions (SIRs) were observed by the Parker Solar Probe (PSP) during its first orbit (2018 September─2019 January). During this time, several recurring SIRs were also seen at 1 au at both L1 (Advanced Composition Explorer (ACE) and Wind) and the location of the Solar Terrestrial Relations Observatory-Ahead (STEREO-A). In this paper, we compare four fast streams observed by PSP at different radial distances during its first orbit. For three of these fast stream events, measurements from L1 (ACE and Wind) and STEREO-A indicated that the fast streams were observed by both PSP and at least one of the 1 au monitors. Our associations are supported by simulations made by the ENLIL model driven by GONG-(ADAPT-)WSA, which allows us to contextualize the inner heliospheric conditions during the first orbit of PSP. Additionally, we determine which of these fast streams are associated with an SIR and characterize the SIR properties for these events. From these comparisons, we find that the compression region associated with the fast-speed streams overtaking the preceding solar wind can form at various radial distances from the Sun in the inner heliosphere inside 0.5 au, with the suprathermal ion population (energies between 30 and 586 keV) observed as isolated enhancements suggesting localized acceleration near the SIR stream interface at ∼0.3 au, which is unlike those seen at 1 au, where the suprathermal enhancements extend throughout and behind the SIR. This suprathermal enhancement extends further into the fast stream with increasing distance from the Sun.

ID: 156231
Type: article
Authors: Badman, Samuel T.; Bale, Stuart D.; Martínez Oliveros, Juan C.; Panasenco, Olga; Velli, Marco; Stansby, David; Buitrago-Casas, Juan C.; Réville, Victor; Bonnell, John W.; Case, Anthony W.; Dudok de Wit, Thierry; Goetz, Keith; Harvey, Peter R.; Kasper, Justin C.; Korreck, Kelly E.; Larson, Davin E.; Livi, Roberto; MacDowall, Robert J.; Malaspina, David M.; Pulupa, Marc; Stevens, Michael L.; Whittlesey, Phyllis L.
Abstract: We compare magnetic field measurements taken by the FIELDS instrument on board Parker Solar Probe (PSP) during its first solar encounter to predictions obtained by potential field source surface (PFSS) modeling. Ballistic propagation is used to connect the spacecraft to the source surface. Despite the simplicity of the model, our results show striking agreement with PSP's first observations of the heliospheric magnetic field from ∼0.5 au (107.5 R_☉) down to 0.16 au (35.7 R_☉). Further, we show the robustness of the agreement is improved both by allowing the photospheric input to the model to vary in time, and by advecting the field from PSP down to the PFSS model domain using in situ PSP/Solar Wind Electrons Alphas and Protons measurements of the solar wind speed instead of assuming it to be constant with longitude and latitude. We also explore the source surface height parameter (R_SS) to the PFSS model, finding that an extraordinarily low source surface height (1.3─1.5 R_☉) predicts observed small-scale polarity inversions, which are otherwise washed out with regular modeling parameters. Finally, we extract field line traces from these models. By overlaying these on extreme ultraviolet images we observe magnetic connectivity to various equatorial and mid-latitude coronal holes, indicating plausible magnetic footpoints and offering context for future discussions of sources of the solar wind measured by PSP.

ID: 155842
Type: article
Authors: Bandyopadhyay, Riddhi; Goldstein, M. L.; Maruca, B. A.; Matthaeus, W. H.; Parashar, T. N.; Ruffolo, D.; Chhiber, R.; Usmanov, A.; Chasapis, A.; Qudsi, R.; Bale, Stuart D.; Bonnell, John W.; Dudok de Wit, Thierry; Goetz, Keith; Harvey, Peter R.; MacDowall, Robert J.; Malaspina, David M.; Pulupa, Marc; Kasper, Justin C.; Korreck, Kelly E.; Case, Anthony W.; Stevens, Michael; Whittlesey, P.; Larson, D.; Livi, R.; Klein, K. G.; Velli, M.; Raouafi, N.
Abstract: Direct evidence of an inertial-range turbulent energy cascade has been provided by spacecraft observations in heliospheric plasmas. In the solar wind, the average value of the derived heating rate near 1 au is $\sim {10}^{3}\,{\rm{J}}\,{\mathrm{kg}}^{-1}\,{{\rm{s}}}^{-1}$ , an amount sufficient to account for observed departures from adiabatic expansion. Parker Solar Probe, even during its first solar encounter, offers the first opportunity to compute, in a similar fashion, a fluid- scale energy decay rate, much closer to the solar corona than any prior in situ observations. Using the Politano─Pouquet third-order law and the von Kármán decay law, we estimate the fluid-range energy transfer rate in the inner heliosphere, at heliocentric distance R ranging from 54 R_☉ (0.25 au) to 36 R_☉ (0.17 au). The energy transfer rate obtained near the first perihelion is about 100 times higher than the average value at 1 au, which is in agreement with estimates based on a heliospheric turbulence transport model. This dramatic increase in the heating rate is unprecedented in previous solar wind observations, including those from Helios, and the values are close to those obtained in the shocked plasma inside the terrestrial magnetosheath.

ID: 155849
Type: article
Authors: Bandyopadhyay, Riddhi; Matthaeus, W. H.; Parashar, T. N.; Chhiber, R.; Ruffolo, D.; Goldstein, M. L.; Maruca, B. A.; Chasapis, A.; Qudsi, R.; McComas, D. J.; Christian, E. R.; Szalay, J. R.; Joyce, C. J.; Giacalone, J.; Schwadron, N. A.; Mitchell, D. G.; Hill, M. E.; Wiedenbeck, M. E.; McNutt, R. L., Jr.; Desai, M. I.; Bale, Stuart D.; Bonnell, John W.; de Wit, Thierry Dudok; Goetz, Keith; Harvey, Peter R.; MacDowall, Robert J.; Malaspina, David M.; Pulupa, Marc; Velli, M.; Kasper, Justin C.; Korreck, Kelly E.; Stevens, Michael; Case, Anthony W.; Raouafi, N.
Abstract: Observations at 1 au have confirmed that enhancements in measured energetic-particle (EP) fluxes are statistically associated with "rough" magnetic fields, i.e., fields with atypically large spatial derivatives or increments, as measured by the Partial Variance of Increments (PVI) method. One way to interpret this observation is as an association of the EPs with trapping or channeling within magnetic flux tubes, possibly near their boundaries. However, it remains unclear whether this association is a transport or local effect; i.e., the particles might have been energized at a distant location, perhaps by shocks or reconnection, or they might experience local energization or re- acceleration. The Parker Solar Probe (PSP), even in its first two orbits, offers a unique opportunity to study this statistical correlation closer to the corona. As a first step, we analyze the separate correlation properties of the EPs measured by the Integrated Science Investigation of the Sun (IS☉IS) instruments during the first solar encounter. The distribution of time intervals between a specific type of event, i.e., the waiting time, can indicate the nature of the underlying process. We find that the IS☉IS observations show a power-law distribution of waiting times, indicating a correlated (non-Poisson) distribution. Analysis of low-energy (∼15 ─ 200 keV/nuc) IS☉IS data suggests that the results are consistent with the 1 au studies, although we find hints of some unexpected behavior. A more complete understanding of these statistical distributions will provide valuable insights into the origin and propagation of solar EPs, a picture that should become clear with future PSP orbits.

ID: 156983
Type: article
Authors: Berčič, Laura; Larson, Davin; Whittlesey, Phyllis; Maksimović, Milan; Badman, Samuel T.; Landi, Simone; Matteini, Lorenzo; Bale, Stuart D.; Bonnell, John W.; Case, Anthony W.; Dudok de Wit, Thierry; Goetz, Keith; Harvey, Peter R.; Kasper, Justin C.; Korreck, Kelly E.; Livi, Roberto; MacDowall, Robert J.; Malaspina, David M.; Pulupa, Marc; Stevens, Michael L.
Abstract: The shape of the electron velocity distribution function plays an important role in the dynamics of the solar wind acceleration. Electrons are normally modeled with three components, the core, the halo, and the strahl. We investigate how well the fast strahl electrons in the inner heliosphere preserve the information about the coronal electron temperature at their origin. We analyzed the data obtained by two missions, Helios, spanning the distances between 65 and 215 R_S, and Parker Solar Probe (PSP), reaching down to 35 R_S during its first two orbits around the Sun. The electron strahl was characterized with two parameters: pitch-angle width (PAW) and the strahl parallel temperature (T_s∥). PSP observations confirm the already reported dependence of strahl PAW on core parallel plasma beta (β_ec\parallel). Most of the strahl measured by PSP appear narrow with PAW reaching down to 30°. The portion of the strahl velocity distribution function aligned with the magnetic field is for the measured energy range well described by a Maxwellian distribution function. T_s∥ was found to be anticorrelated with the solar wind velocity and independent of radial distance. These observations imply that T_s∥ carries the information about the coronal electron temperature. The obtained values are in agreement with coronal temperatures measured using spectroscopy, and the inferred solar wind source regions during the first orbit of PSP agree with the predictions using a PFSS model.

ID: 156221
Type: article
Authors: Bowen, Trevor A.; Mallet, Alfred; Huang, Jia; Klein, Kristopher G.; Malaspina, David M.; Stevens, Michael; Bale, Stuart D.; Bonnell, John W.; Case, Anthony W.; Chandran, Benjamin D. G.; Chaston, C. C.; Chen, Christopher H. K.; Dudok de Wit, Thierry; Goetz, Keith; Harvey, Peter R.; Howes, Gregory G.; Kasper, Justin C.; Korreck, Kelly E.; Larson, Davin; Livi, Roberto; MacDowall, Robert J.; McManus, Michael D.; Pulupa, Marc; Verniero, J. L.; Whittlesey, Phyllis
Abstract: Understanding the physical processes in the solar wind and corona that actively contribute to heating, acceleration, and dissipation is a primary objective of NASA's Parker Solar Probe (PSP) mission. Observations of circularly polarized electromagnetic waves at ion scales suggest that cyclotron resonance and wave─particle interactions are dynamically relevant in the inner heliosphere. A wavelet-based statistical study of circularly polarized events in the first perihelion encounter of PSP demonstrates that transverse electromagnetic waves at ion resonant scales are observed in 30─50% of radial field intervals. Average wave amplitudes of approximately 4 nT are measured, while the mean duration of wave events is on the order of 20 s; however, long- duration wave events can exist without interruption on hour-long timescales. Determination of wave vectors suggests propagation parallel/antiparallel to the mean magnetic field. Though ion-scale waves are preferentially observed during intervals with a radial mean magnetic field, we show that measurement constraints, associated with single spacecraft sampling of quasi-parallel waves superposed with anisotropic turbulence, render the measured coherent ion-wave spectrum unobservable when the mean magnetic field is oblique to the solar wind flow; these results imply that the occurrence of coherent ion-scale waves is not limited to a radial field configuration. The lack of radial scaling of characteristic wave amplitudes and duration suggests that the waves are generated in situ through plasma instabilities. Additionally, observations of proton distribution functions indicate that temperature anisotropy may drive the observed ion-scale waves.

ID: 155833
Type: article
Authors: Case, Anthony W.; Kasper, Justin C.; Stevens, Michael L.; Korreck, Kelly E.; Paulson, Kristoff; Daigneau, Peter; Caldwell, David; Freeman, Mark; Henry, Thayne; Klingensmith, Brianna; Bookbinder, J. A.; Robinson, Miles; Berg, Peter; Tiu, Chris; Wright, K. H., Jr.; Reinhart, Matthew J.; Curtis, David; Ludlam, Michael; Larson, Davin; Whittlesey, Phyllis; Livi, Roberto; Klein, Kristopher G.; Martinović, Mihailo M.
Abstract: Solar Probe Cup (SPC) is a Faraday cup instrument on board NASA's Parker Solar Probe (PSP) spacecraft designed to make rapid measurements of thermal coronal and solar wind plasma. The spacecraft is in a heliocentric orbit that takes it closer to the Sun than any previous spacecraft, allowing measurements to be made where the coronal and solar wind plasma is being heated and accelerated. The SPC instrument was designed to be pointed directly at the Sun at all times, allowing the solar wind (which is flowing primarily radially away from the Sun) to be measured throughout the orbit. The instrument is capable of measuring solar wind ions with an energy between 100 and 6000 V (protons with speeds from 139 to 1072 km s⁻¹). It also measures electrons with an energy/charge between 100 and 1500 V. SPC has been designed to have a wide dynamic range that is capable of measuring protons and alpha particles at the closest perihelion (9.86 solar radii from the center of the Sun) and out to 0.25 au. Initial observations from the first orbit of PSP indicate that the instrument is functioning well.

ID: 156705
Type: article
Authors: Chaston, C. C.; Bonnell, J. W.; Bale, S. D.; Kasper, Justin C.; Pulupa, M.; Dudok de Wit, T.; Bowen, T. A.; Larson, D. E.; Whittlesey, P. L.; Wygant, J. R.; Salem, C. S.; MacDowall, R. J.; Livi, R. L.; Vech, D.; Case, Anthony W.; Stevens, Michael L.; Korreck, Kelly E.; Goetz, K.; Harvey, P. R.; Malaspina, D. M.
Abstract: Field and plasma variations during the first perihelion pass of the Parker Solar Probe (PSP) from 53 into 35 solar radii (R _S ) from the Sun and over a frequency range in the spacecraft frame (f _SC) from 0.0002 to 0.2 Hz are decomposed into constituent magnetohydrodynamic (MHD) modes. The analysis operates on measurements of the MHD variables recorded between impulsive, large amplitude rotations of the magnetic field to reveal the dominance of a broad spectrum of shear Alfvén waves propagating antiparallel (backward) to the background magnetic field ( B ₀) with a significant fraction of spectral energy density in the backward slow mode and, to a lesser extent, fast mode waves. While all three MHD modes provide Poynting flux directed outward from the Sun the impulsive rotations of B ₀ from inward to outward radial orientations provide intervals of outward and inward propagation in the plasma frame, respectively. This morphology is suggestive of outward wave propagation from a near Sun source along kinked field lines that provide alternating radial B ₀ orientations as the magnetic field is advected with the flow over the spacecraft. Shear Alfvén and slow mode spectral energy density is generally largest within intervals of reversed radial B ₀, while the fast mode tends to occur outside these regions, albeit with lower intensity. The spectral energy density in the forward propagating modes increases with f _SC above 0.01 Hz, which is suggestive of back scatter in the plasma frame of the dominant backward modes from the radial field reversals and associated inhomogeneities of the embedded plasmas.

ID: 155829
Type: article
Authors: Chen, C. H. K.; Bale, S. D.; Bonnell, John W.; Borovikov, D.; Bowen, T. A.; Burgess, D.; Case, Anthony W.; Chandran, B. D. G.; de Wit, T. Dudok; Goetz, K.; Harvey, P. R.; Kasper, Justin C.; Klein, K. G.; Korreck, Kelly E.; Larson, D.; Livi, R.; MacDowall, R. J.; Malaspina, D. M.; Mallet, A.; McManus, M. D.; Moncuquet, M.; Pulupa, M.; Stevens, Michael L.; Whittlesey, P.
Abstract: The first two orbits of the Parker Solar Probe spacecraft have enabled the first in situ measurements of the solar wind down to a heliocentric distance of 0.17 au (or 36 ${R}_{\odot }$ ). Here, we present an analysis of this data to study solar wind turbulence at 0.17 au and its evolution out to 1 au. While many features remain similar, key differences at 0.17 au include increased turbulence energy levels by more than an order of magnitude, a magnetic field spectral index of −3/2 matching that of the velocity and both Elsasser fields, a lower magnetic compressibility consistent with a smaller slow-mode kinetic energy fraction, and a much smaller outer scale that has had time for substantial nonlinear processing. There is also an overall increase in the dominance of outward-propagating Alfvénic fluctuations compared to inward-propagating ones, and the radial variation of the inward component is consistent with its generation by reflection from the large-scale gradient in Alfvén speed. The energy flux in this turbulence at 0.17 au was found to be ∼10% of that in the bulk solar wind kinetic energy, becoming ∼40% when extrapolated to the Alfvén point, and both the fraction and rate of increase of this flux toward the Sun are consistent with turbulence-driven models in which the solar wind is powered by this flux.

ID: 155826
Type: article
Authors: Chhiber, Rohit; Goldstein, M. L.; Maruca, B. A.; Chasapis, A.; Matthaeus, W. H.; Ruffolo, D.; Bandyopadhyay, R.; Parashar, T. N.; Qudsi, R.; de Wit, T. Dudok; Bale, S. D.; Bonnell, John W.; Goetz, K.; Harvey, P. R.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.; Kasper, Justin C.; Korreck, Kelly E.; Case, Anthony W.; Stevens, Michael L.; Whittlesey, P.; Larson, D.; Livi, R.; Velli, M.; Raouafi, N.
Abstract: During the Parker Solar Probe's (PSP) first perihelion pass, the spacecraft reached within a heliocentric distance of ∼37 R_☉ and observed numerous magnetic and flow structures characterized by sharp gradients. To better understand these intermittent structures in the young solar wind, an important property to examine is their degree of correlation in time and space. To this end, we use the well-tested partial variance of increments (PVI) technique to identify intermittent events in FIELDS and SWEAP observations of magnetic and proton-velocity fields (respectively) during PSP's first solar encounter, when the spacecraft was within 0.25 au from the Sun. We then examine distributions of waiting times (WT) between events with varying separation and PVI thresholds. We find power-law distributions for WT shorter than a characteristic scale comparable to the correlation time of the fluctuations, suggesting a high degree of correlation that may originate in a clustering process. WT longer than this characteristic time are better described by an exponential, suggesting a random memory- less Poisson process at play. These findings are consistent with near- Earth observations of solar wind turbulence. The present study complements the one by Dudok de Wit et al., which focuses on WT between observed "switchbacks" in the radial magnetic field.

ID: 157654
Type: article
Authors: Cohen, C. M. S.; Christian, E. R.; Cummings, A. C.; Davis, A. J.; Desai, M. I.; Giacalone, J.; Hill, M. E.; Joyce, C. J.; Labrador, A. W.; Leske, R. A.; Matthaeus, W. H.; McComas, D. J.; McNutt, R. L., Jr.; Mewaldt, R. A.; Mitchell, D. G.; Rankin, J. S.; Roelof, E. C.; Schwadron, N. A.; Stone, E. C.; Szalay, J. R.; Wiedenbeck, M. E.; Allen, R. C.; Ho, G. C.; Jian, L. K.; Lario, D.; Odstrcil, D.; Bale, S. D.; Badman, S. T.; Pulupa, M.; MacDowall, R. J.; Kasper, Justin C.; Case, Anthony W.; Korreck, Kelly E.; Larson, D. E.; Livi, Roberto; Stevens, Michael L.; Whittlesey, Phyllis
Abstract: The Parker Solar Probe was launched on 2018 August 12 and completed its second orbit on 2019 June 19 with perihelion of 35.7 solar radii. During this time, the Energetic Particle Instrument-Hi (EPI-Hi, one of the two energetic particle instruments comprising the Integrated Science Investigation of the Sun, IS☉IS) measured seven proton intensity increases associated with stream interaction regions (SIRs), two of which appear to be occurring in the same region corotating with the Sun. The events are relatively weak, with observed proton spectra extending to only a few MeV and lasting for a few days. The proton spectra are best characterized by power laws with indices ranging from -4.3 to -6.5, generally softer than events associated with SIRs observed at 1 au and beyond. Helium spectra were also obtained with similar indices, allowing He/H abundance ratios to be calculated for each event. We find values of 0.016-0.031, which are consistent with ratios obtained previously for corotating interaction region events with fast solar wind ≤600 km s^-1. Using the observed solar wind data combined with solar wind simulations, we study the solar wind structures associated with these events and identify additional spacecraft near 1 au appropriately positioned to observe the same structures after some corotation. Examination of the energetic particle observations from these spacecraft yields two events that may correspond to the energetic particle increases seen by EPI-Hi earlier.

ID: 155819
Type: article
Authors: Duan, Die; Bowen, Trevor A.; Chen, Christopher H. K.; Mallet, Alfred; He, Jiansen; Bale, Stuart D.; Vech, Daniel; Kasper, Justin C.; Pulupa, Marc; Bonnell, John W.; Case, Anthony W.; de Wit, Thierry Dudok; Goetz, Keith; Harvey, Peter R.; Korreck, Kelly E.; Larson, Davin; Livi, Roberto; MacDowall, Robert J.; Malaspina, David M.; Stevens, Michael L.; Whittlesey, Phyllis
Abstract: Magnetic field fluctuations in the solar wind are commonly observed to follow a power-law spectrum. Near proton-kinetic scales, a spectral break occurs that is commonly interpreted as a transition to kinetic turbulence. However, this transition is not yet entirely understood. By studying the scaling of the break with various plasma properties, it may be possible to constrain the processes leading to the onset of kinetic turbulence. Using data from the Parker Solar Probe, we measure the proton-scale break over a range of heliocentric distances, enabling a measurement of the transition from inertial to kinetic-scale turbulence under various plasma conditions. We find that the break frequency f_b increases as the heliocentric distance r decreases in the slow solar wind following a power law of f_b ∼ r^−1.11. We also compare this to the characteristic plasma ion scales to relate the break to the possible physical mechanisms occurring at this scale. The ratio f_b/f_c (f_c for Doppler-shifted ion cyclotron resonance scale) is close to unity and almost independent of plasma β_p. While f_b/f_ρ (f_ρ for Doppler-shifted proton thermal gyroradius) increases with β_p approaching to unity at larger β_p, f_b/f_d (f_d for Doppler-shifted proton inertial length) decreases with β_p from unity at small β_p. Due to the large comparable Alfvén and solar wind speeds, we analyze these results using both the standard and modified Taylor hypotheses, demonstrating the robust statistical results.

ID: 157589
Type: article
Authors: Finley, Adam J.; Matt, Sean P.; Réville, Victor; Pinto, Rui F.; Owens, Mathew; Kasper, Justin C.; Korreck, Kelly E.; Case, Anthony W.; Stevens, Michael L.; Whittlesey, Phyllis; Larson, Davin; Livi, Roberto
Abstract: The long-term evolution of the Sun's rotation period cannot be directly observed, and is instead inferred from trends in the measured rotation periods of other Sun-like stars. Assuming the Sun spins down as it ages, following rotation rate ∝ age^-1/2, requires the current solar angular momentum (AM) loss rate to be around 6 × 10³⁰ erg. Magnetohydrodynamic models, and previous observations of the solar wind (from the Helios and Wind spacecraft), generally predict a values closer to 1 × 10³⁰ erg or 3 × 10³⁰ erg, respectively. Recently, the Parker Solar Probe (PSP) observed tangential solar wind speeds as high as ∼50 km s^-1 in a localized region of the inner heliosphere. If such rotational flows were prevalent throughout the corona, it would imply that the solar wind AM-loss rate is an order of magnitude larger than all of those previous estimations. In this Letter, we evaluate the AM flux in the solar wind, using data from the first two orbits of PSP. The solar wind is observed to contain both large positive (as seen during perihelion), and negative AM fluxes. We analyze two solar wind streams that were repeatedly traversed by PSP; the first is a slow wind stream whose average AM flux fluctuates between positive and negative values, and the second is an intermediate speed stream that contains a positive AM flux (more consistent with a constant flow of AM). When the data from PSP are evaluated holistically, the average equatorial AM flux implies a global AM-loss rate of around (2.6-4.2) × 10³⁰ erg (which is more consistent with observations from previous spacecraft).