ABOUT THIS COLLECTION

This open access archive contains publications from University of Arizona faculty, researchers and staff, primarily open-access versions of formally published journal articles. The collection includes published articles and final accepted manuscripts submitted by UA faculty under the UA Open Access Policy. The collection also includes books, book chapters, book reviews, presentations, data, and other scholarly materials submitters have chosen to make available in the repository.


HOW TO SUBMIT

  • Log in to the repository using your NetID and password
  • Click the "Submissions" link in the left sidebar (under "My Account")
  • Start a new submission in the UA Faculty Publications collection.
  • Library staff will check publisher policies, including embargo periods related to your submission.
  • You will receive an email with a persistent link to your submission when it is approved.

QUESTIONS?

Contact open-access@email.arizona.edu with your questions about the UA Faculty Publications collection.

Recent Submissions

  • Field Distance Effects of Fipronil and Chlorfenapyr as Soil Termiticides Against the Desert Subterranean Termite, Heterotermes aureus (Blattodea: Rhinotermitidae)

    Baker, Paul B; Miguelena, Javier G; Univ Arizona, Dept Entomol (UNIV ESTADUAL FEIRA SANTANA, 2020-04-18)
    A desirable trait of termiticides is that they suppress termite activity at a distance from the site of application. Fipronil and chlorfenapyr are two non-repellent termiticides that display delayed toxicity and are therefore good candidates for yielding distance effects. We assessed their effects as soil-applied termiticides for the management of the desert subterranean termite, Heterotermes aureus (Snyder), under field conditions in southern Arizona. Our approach involved recording termite activity within field experimental grids consisting of termite-monitoring stations at selected distances from a termiticide application perimeter. Fipronil-treated plots experienced large and significant reductions in termite presence and abundance relative to controls in stations immediately adjacent to treated soil. However, there was no evidence of reductions in termite activity in stations further away from the soil treatment. In contrast, termite abundance and presence in stations decreased relatively to controls after chlorfenapyr application in whole experimental grids and in several grid sections spatially separated from treated soil. These reductions were especially evident in the five central stations surrounded by the treatment perimeter and in the furthest set of stations. The spatial pattern of changes in chlorfenapyr plots was consistent with termiticide transfer as a mechanism behind distance effects. The impact of fipronil and chlorfenapyr on termite populations in our study suggest that they can both be useful for the management of H. aureus, although each might be suited for different management goals. Our results also suggest that perimeter treatments alone are not sufficient to accomplish full control of large H. aureus infestations.
  • Investigation of a Monte Carlo simulation and an analytic-based approach for modeling the system response for clinical I-123 brain SPECT imaging

    Auer, Benjamin; Zeraatkar, Navid; De Beenhouwer, Jan; Kalluri, Kesava; Kuo, Philip; Furenlid, Lars R.; King, Michael A.; Univ Arizona, Dept Med Imaging (SPIE-INT SOC OPTICAL ENGINEERING, 2019-05-28)
    The use of accurate system response modeling has been proven to be an essential key of SPECT image reconstruction, with its usage leading to overall improvement of image quality. The aim of this work was to investigate the imaging performance using an XCAT brain perfusion phantom of two modeling strategies, one based on analytic techniques and the other one based on GATE Monte-Carlo simulation. In addition, an efficient forced detection approach to improve the overall simulation efficiency was implemented and its performance was evaluated. We demonstrated that accurate modeling of the system matrix generated by Monte-Carlo simulation for iterative reconstruction leads to superior performance compared to analytic modeling in the case of clinical I-123 brain imaging. It was also shown that the use of the forced detection approach provided a quantitative and qualitative enhancement of the reconstruction.
  • On the shape of forward transit time distributions in low-order catchments

    Heidbüchel, Ingo; Yang, Jie; Musolff, Andreas; Troch, Peter; Ferré, Ty; Fleckenstein, Jan H.; Univ Arizona, Dept Hydrol & Atmospher Sci (COPERNICUS GESELLSCHAFT MBH, 2020-06-03)
    Transit time distributions (TTDs) integrate information on timing, amount, storage, mixing and flow paths of water and thus characterize hydrologic and hydrochemical catchment response unlike any other descriptor. Here, we simulate the shape of TTDs in an idealized low-order catchment and investigate whether it changes systematically with certain catchment and climate properties. To this end, we used a physically based, spatially explicit 3-D model, injected tracer with a precipitation event and recorded the resulting forward TTDs at the outlet of a small ( - 6000 m(2)) catchment for different scenarios. We found that the TTDs can be subdivided into four parts: (1) early part - controlled by soil hydraulic conductivity and antecedent soil moisture content, (2) middle part - a transition zone with no clear pattern or control, (3) later part - influenced by soil hydraulic conductivity and subsequent precipitation amount, and (4) very late tail of the breakthrough curve - governed by bedrock hydraulic conductivity. The modeled TTD shapes can be predicted using a dimensionless number: higher initial peaks are observed if the inflow of water to a catchment is not equal to its capacity to discharge water via subsurface flow paths, and lower initial peaks are connected to increasing available storage. In most cases the modeled TTDs were humped with nonzero initial values and varying weights of the tails. Therefore, none of the best-fit theoretical probability functions could describe the entire TTD shape exactly. Still, we found that generally gamma and log-normal distributions work better for scenarios of low and high soil hydraulic conductivity, respectively.
  • Analysis of the Internal Structure of the Streamer Blowout Observed by the Parker Solar Probe During the First Solar Encounter

    Nieves-Chinchilla, Teresa; Szabo, Adam; Korreck, Kelly E.; Alzate, Nathalia; Balmaceda, Laura A.; Lavraud, Benoit; Paulson, Kristoff; Narock, Ayris A.; Wallace, Samantha; Jian, Lan K.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    In this paper, we present an analysis of the internal structure of a coronal mass ejection (CME) detected by in situ instruments on board the Parker Solar Probe (PSP) spacecraft during its first solar encounter. On 2018 November 11 at 23:53 UT, the FIELDS magnetometer measured an increase in strength of the magnetic field as well as a coherent change in the field direction. The SWEAP instrument simultaneously detected a low proton temperature and signatures of bidirectionality in the electron pitch angle distribution (PAD). These signatures are indicative of a CME embedded in the slow solar wind. Operating in conjunction with PSP was the STEREO A spacecraft, which enabled the remote observation of a streamer blowout by the SECCHI suite of instruments. The source at the Sun of the slow and well-structured flux rope was identified in an overlying streamer, the details of which are described in Korreck et al. Our detailed inspection of the internal transient structure magnetic properties suggests high complexity in deviations from an ideal flux rope 3D topology. Reconstructions of the magnetic field configuration reveal a highly distorted structure consistent with the highly elongated "bubble" observed remotely. A double-ring substructure observed in the SECCHI-COR2 field of view (FOV) is suggestive of a double internal flux rope. Furthermore, we describe a scenario in which mixed topology of a closed flux rope is combined with the magnetically open structure, which helps explain the flux dropout observed in the measurements of the electron PAD. Our justification for this is the plethora of structures observed by the EUV imager (SECCHI-EUVI) in the hours preceding the streamer blowout evacuation. Finally, taking advantage of the unique observations from PSP, we explore the first stages of the effects of coupling with the solar wind and the evolutionary processes in the magnetic structure. We found evidence of bifurcated current sheets in the structure boundaries, suggestive of magnetic reconnection. Our analysis of the internal force imbalance indicates that internal Lorentz forces continue to dominate the evolution of the structure in the COR2 FOV and serve as the main driver of the internal flux rope distortion detected in situ at PSP solar distance.
  • CME-associated Energetic Ions at 0.23 au: Consideration of the Auroral Pressure Cooker Mechanism Operating in the Low Corona as a Possible Energization Process

    Mitchell, D. G.; Giacalone, J.; Allen, R. C.; Hill, M. E.; McNutt, R. L.; McComas, D. J.; Szalay, J. R.; Schwadron, N. A.; Rouillard, A. P.; Bale, S. B.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    We draw a comparison between a solar energetic particle event associated with the release of a slow coronal mass ejection close to the Sun, and the energetic particle population produced in high current density field-aligned current structures associated with auroral phenomena in planetary magnetospheres. We suggest that this process is common in CME development and lift off in the corona, and may account for the electron populations that generate Type III radio bursts, as well as for the prompt energetic ion and electron populations typically observed in interplanetary space.
  • The Enhancement of Proton Stochastic Heating in the Near-Sun Solar Wind

    Martinović, Mihailo M.; Klein, Kristopher G.; Kasper, Justin C.; Case, Anthony W.; Korreck, Kelly E.; Larson, Davin; Livi, Roberto; Stevens, Michael; Whittlesey, Phyllis; Chandran, Benjamin D. G.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    Stochastic heating (SH) is a nonlinear heating mechanism driven by the violation of magnetic moment invariance due to large-amplitude turbulent fluctuations producing diffusion of ions toward higher kinetic energies in the direction perpendicular to the magnetic field. It 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 SH rate Q(perpendicular to) at radial distances from the Sun as close as 0.16 au, using measurements from the first two Parker Solar Probe encounters. Our results for both the amplitude and radial trend of the heating rate, Q(perpendicular to) proportional to r(-2.5), agree with previous results based on the Helios data set at heliocentric distances from 0.3 to 0.9 au. Also in agreement with previous results, Q(perpendicular to) is significantly larger in the fast solar wind than in the slow solar wind. We identify the tendency in fast solar wind for cuts of the core proton velocity distribution transverse to the magnetic field to exhibit a flattop shape. The observed distribution agrees with previous theoretical predictions for fast solar wind where SH is the dominant heating mechanism.
  • Identification of Magnetic Flux Ropes from Parker Solar Probe Observations during the First Encounter

    Zhao, L.-L.; Zank, G. P.; Adhikari, L.; Hu, Q.; Kasper, J. C.; Bale, S. D.; Korreck, K. E.; Case, A. W.; Stevens, M.; Bonnell, J. W.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    The Parker Solar Probe (PSP) observed an interplanetary coronal mass ejection (ICME) event during its first orbit around the Sun, among many other events. This event is analyzed by applying a wavelet analysis technique to obtain the reduced magnetic helicity, cross helicity, and residual energy, the first two of which are magnetohydrodynamics (MHD) invariants. Our results show that the ICME, as a large-scale magnetic flux rope, possesses high magnetic helicity, very 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 during the period from 2018 October 22 to November 21, which are intrinsic to quasi-two-dimensional MHD turbulence in the solar wind. Multiple structures with durations between 8 and 300 minutes are identified from PSP in situ spacecraft measurements. The location and scales of these structures are characterized by wavelet spectrograms of the normalized reduced magnetic helicity, normalized cross helicity, and normalized residual energy. Transport theory suggests that these small-scale magnetic flux ropes may contribute to the acceleration of charged particles through magnetic reconnection processes, and the dissipation of these structures may be important for understanding the coronal heating processes.
  • Kinetic-scale Spectral Features of Cross Helicity and Residual Energy in the Inner Heliosphere

    Vech, Daniel; Kasper, Justin C.; Klein, Kristopher G.; Huang, Jia; Stevens, Michael L.; Chen, Christopher H. K.; Case, Anthony W.; Korreck, Kelly; Bale, Stuart D.; Bowen, Trevor A.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    In this work, we present the first results from the flux angle (FA) operation mode of the Faraday Cup instrument on board the Parker Solar Probe (PSP). The FA mode allows rapid measurements of phase space density fluctuations close to the peak of the proton velocity distribution function with a cadence of 293 Hz. This approach provides an invaluable tool for understanding kinetic-scale turbulence in the solar wind and solar corona. We describe a technique to convert the phase space density fluctuations into vector velocity components and compute several turbulence parameters, such as spectral index, residual energy, and cross helicity during two intervals when the FA mode was used in PSP's first encounter at 0.174 au distance from the Sun.
  • Magnetic Field Kinks and Folds in the Solar Wind

    Tenerani, Anna; Velli, Marco; Matteini, Lorenzo; Réville, Victor; Shi, Chen; Bale, Stuart D.; Kasper, Justin C.; Bonnell, John W.; Case, Anthony W.; Dudok de Wit, Thierry; et al. (IOP PUBLISHING LTD, 2020-02-03)
    Parker Solar Probe (PSP) observations during its first encounter at 35.7 R-circle dot have shown the presence of magnetic field lines that are strongly perturbed to the point that they produce local inversions of the radial magnetic field, known as switchbacks. Their counterparts in the solar wind velocity field are local enhancements in the radial speed, or jets, displaying (in all components) the velocity-magnetic field correlation typical of large amplitude Alfven waves propagating away from the Sun. Switchbacks and radial jets have previously been observed over a wide range of heliocentric distances by Helios, Wind, and Ulysses, although they were prevalent in significantly faster streams than seen at PSP. Here we study via numerical magnetohydrodynamics simulations the evolution of such large amplitude Alfvenic fluctuations by including, in agreement with observations, both a radial magnetic field inversion and an initially constant total magnetic pressure. Despite the extremely large excursion of magnetic and velocity fields, switchbacks are seen to persist for up to hundreds of Alfven crossing times before eventually decaying due to the parametric decay instability. Our results suggest that such switchback/jet configurations might indeed originate in the lower corona and survive out to PSP distances, provided the background solar wind is sufficiently calm, in the sense of not being pervaded by strong density fluctuations or other gradients, such as stream or magnetic field shears, that might destabilize or destroy them over shorter timescales.
  • Seed Population Preconditioning and Acceleration Observed by the Parker Solar Probe

    Schwadron, N. A.; Bale, S.; Bonnell, J.; Case, A.; Christian, E. R.; Cohen, C. M. S.; Cummings, A. C.; Davis, A. J.; Dudok de Wit, T.; de Wet, W.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    A series of solar energetic particle (SEP) events was observed by the Integrated Science Investigation of the Sun (ISeIS) on the Parker Solar Probe (PSP) during the period from 2019 April 18 through 24. The PSP spacecraft was located near 0.48 au from the Sun on Parker spiral field lines that projected out to 1 au within similar to 25 degrees of the near-Earth spacecraft. These SEP events, though small compared to historically large SEP events, were among the largest observed thus far in the PSP mission and provide critical information about the space environment inside 1 au during SEP events. During this period, the Sun released multiple coronal mass ejections (CMEs). One of these CMEs observed was initiated on 2019 April 20 at 01:25 UTC, and the interplanetary CME (ICME) propagated out and passed over the PSP spacecraft. Observations by the Electromagnetic Fields Investigation show that the magnetic field structure was mostly radial throughout the passage of the compression region and the plasma that followed, indicating that PSP did not directly observe a flux rope internal to the ICME, consistent with the location of PSP on the ICME flank. Analysis using relativistic electrons observed near Earth by the Electron, Proton and Alpha Monitor on the Advanced Composition Explorer demonstrates the presence of electron seed populations (40-300 keV) during the events observed. The energy spectrum of the IS circle dot S-observed proton seed population below 1 MeV is close to the limit of possible stationary-state plasma distributions out of equilibrium. IS circle dot IS observations reveal the enhancement of seed populations during the passage of the ICME, which likely indicates a key part of the preacceleration process that occurs close to the Sun.
  • Anticorrelation between the Bulk Speed and the Electron Temperature in the Pristine Solar Wind: First Results from the Parker Solar Probe and Comparison with Helios

    Maksimovic, M.; Bale, S. D.; Berčič, L.; Bonnell, J. W.; Case, A. W.; Dudok de Wit, T.; Goetz, K.; Halekas, J. S.; Harvey, P. R.; Issautier, K.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    We discuss the solar wind electron temperatures T-e as measured in the nascent solar wind by Parker Solar Probe during its first perihelion pass. The measurements have been obtained by fitting the high-frequency part of quasithermal noise spectra recorded by the Radio Frequency Spectrometer. In addition we compare these measurements with those obtained by the electrostatic analyzer discussed in Halekas et al. These first electron observations show an anticorrelation between T-e and the wind bulk speed V: this anticorrelation is most likely the remnant of the wellknown mapping observed at 1 au and beyond between the fast wind and its coronal hole sources, where electrons are observed to be cooler than in the quiet corona. We also revisit Helios electron temperature measurements and show, for the first time, that an in situ (T-e, V) anticorrelation is well observed at 0.3 au but disappears as the wind expands, evolves, and mixes with different electron temperature gradients for different wind speeds.
  • Observations of the 2019 April 4 Solar Energetic Particle Event at the Parker Solar Probe

    Leske, R. A.; Christian, E. R.; Cohen, C. M. S.; Cummings, A. C.; Davis, A. J.; Desai, M. I.; Giacalone, J.; Hill, M. E.; Joyce, C. J.; Krimigis, S. M.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    A solar energetic particle event was detected by the Integrated Science Investigation of the Sun (IS circle dot IS) instrument suite on Parker Solar Probe (PSP) on 2019 April 4 when the spacecraft was inside of 0.17 au and less than 1 day before its second perihelion, providing an opportunity to study solar particle acceleration and transport unprecedentedly close to the source. The event was very small, with peak 1 MeV proton intensities of similar to 0.3 particles (cm(2) sr s MeV)(-1), and was undetectable above background levels at energies above 10 MeV or in particle detectors at 1 au. It was strongly anisotropic, with intensities flowing outward from the Sun up to 30 times greater than those flowing inward persisting throughout the event. Temporal association between particle increases and small brightness surges in the extreme-ultraviolet observed by the Solar TErrestrial RElations Observatory, which were also accompanied by type III radio emission seen by the Electromagnetic Fields Investigation on PSP, indicates that the source of this event was an active region nearly 80 degrees east of the nominal PSP magnetic footpoint. This suggests that the field lines expanded over a wide longitudinal range between the active region in the photosphere and the corona.
  • Predicting the Solar Wind at the Parker Solar Probe Using an Empirically Driven MHD Model

    Kim, T. K.; Pogorelov, N. V.; Arge, C. N.; Henney, C. J.; Jones-Mecholsky, S. I.; Smith, W. P.; Bale, S. D.; Bonnell, J. W.; Dudok de Wit, T.; Goetz, K.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    Since its launch on 2018 August 12, Parker Solar Probe (PSP) has completed its first and second orbits around the Sun, having reached down to 35.7 solar radii at each perihelion. In anticipation of the exciting new data at such unprecedented distances, we have simulated the global 3D heliosphere using an MHD model coupled with a semi-empirical coronal model using the best available photospheric magnetograms as input. We compare our heliospheric MHD simulation results with in situ measurements along the PSP trajectory from its launch to the completion of the second orbit, with particular emphasis on the solar wind structure around the first two solar encounters. Furthermore, we show our model prediction for the third perihelion, which occurred on 2019 September 1. Comparison of the MHD results with PSP observations provides new insights into solar wind acceleration. Moreover, PSP observations reveal how accurately the Air Force Data Assimilative Photospheric flux Transport-Wang-Sheeley-Arge-based predictions work throughout the inner heliosphere.
  • Energetic Particle Observations from the Parker Solar Probe Using Combined Energy Spectra from the IS⊙IS Instrument Suite

    Joyce, C. J.; McComas, D. J.; Christian, E. R.; Schwadron, N. A.; Wiedenbeck, M. E.; McNutt, R. L.; Cohen, C. M. S.; Leske, R. A.; Mewaldt, R. A.; Stone, E. C.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    The Integrated Science Investigations of the Sun (IS circle dot IS) instrument suite includes two Energetic Particle instruments: EPI-Hi, designed to measure ions from similar to 1 to 200 MeV nuc(-1), and EPI-Lo, designed to measure ions from similar to 20 to similar to 15 MeV nuc(-1). We present an analysis of eight energetic proton events observed across the energy range of both instruments during Parker Solar Probe's (PSP) first two orbits in order to examine their combined energy spectra. Background corrections are applied to help resolve spectral breaks between the two instruments and are shown to be effective. In doing so we demonstrate that even in the early stages of calibration, IS circle dot IS is capable of producing reliable spectral observations across broad energy ranges. In addition to making groundbreaking measurements very near the Sun, IS circle dot IS also characterizes energetic particle populations over a range of heliocentric distances inside 1 au. During the first two orbits, IS circle dot IS observed energetic particle events from a single corotating interaction region (CIR) at three different distances from the Sun. The events are separated by two Carrington rotations and just 0.11 au in distance; however, the relationship shown between proton intensities and proximity of the spacecraft to the source region shows evidence of the importance of transport effects on observations of energetic particles from CIRs. Future IS circle dot IS observations of similar events over larger distances will help disentangle the effects of CIR-related acceleration and transport. We apply similar spectral analyses to the remaining five events, including four that are likely related to stream interaction regions (SIRs) and one solar energetic particle (SEP) event.
  • Properties of Suprathermal-through-energetic He Ions Associated with Stream Interaction Regions Observed over the Parker Solar Probe’s First Two Orbits

    Desai, M. I.; Mitchell, D. G.; Szalay, J. R.; Roelof, E. C.; Giacalone, J.; Hill, M. E.; McComas, D. J.; Christian, E. R.; Schwadron, N. A.; McNutt Jr., R. L.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    The Integrated Science Investigation of the Sun (IS circle dot IS) suite on board NASA's Parker Solar Probe (PSP) observed six distinct enhancements in the intensities of suprathermal-through-energetic (similar to 0.03-3 MeV nucleon(-1)) He ions associated with corotating or stream interaction regions (CIR or SIR) during its first two orbits. Our results from a survey of the time histories of the He intensities, spectral slopes, and anisotropies and the event-averaged energy spectra during these events show the following: (1) In the two strongest enhancements, seen at 0.35 and 0.85 au, the higher-energy ions arrive and maximize later than those at lower energies. In the event seen at 0.35 au, the He ions arrive when PSP was away from the SIR trailing edge and entered the rarefaction region in the highspeed stream. (2) The He intensities either are isotropic or show sunward anisotropies in the spacecraft frame. (3) In all events, the energy spectra between similar to 0.2 and 1 MeV nucleon(-1) are power laws of the form proportional to E-2. In the two strongest events, the energy spectra are well represented by flat power laws between similar to 0.03 and 0.4 MeV nucleon(-1) modulated by exponential rollovers between similar to 0.4 and 3 MeV nucleon(-1). We conclude that the SIR-associated He ions originate from sources or shocks beyond PSP's location rather than from acceleration processes occurring at nearby portions of local compression regions. Our results also suggest that rarefaction regions that typically follow the SIRs facilitate easier particle transport throughout the inner heliosphere such that low-energy ions do not undergo significant energy loss due to adiabatic deceleration, contrary to predictions of existing models.
  • Energetic Particle Increases Associated with Stream Interaction Regions

    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.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    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 circle dot 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.
  • The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere

    Chen, C. H. K.; Bale, S. D.; Bonnell, J. W.; Borovikov, D.; Bowen, T. A.; Burgess, D.; Case, A. W.; Chandran, B. D. G.; Dudok de Wit, T.; Goetz, K.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    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-circle dot). 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 Alfvenic 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 Alfven speed. The energy flux in this turbulence at 0.17 au was found to be similar to 10% of that in the bulk solar wind kinetic energy, becoming similar to 40% when extrapolated to the Alfven 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.
  • Ion-scale Electromagnetic Waves in the Inner Heliosphere

    Bowen, Trevor A.; Mallet, Alfred; Huang, Jia; Klein, Kristopher G.; Malaspina, David M.; Stevens, Michael; Bale, Stuart D.; Bonnell, J. W.; Case, Anthony W.; Chandran, Benjamin D. G.; et al. (IOP PUBLISHING LTD, 2020-02-06)
    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-scaley
  • Sharp Alfvénic Impulses in the Near-Sun Solar Wind

    Horbury, Timothy S.; Woolley, Thomas; Laker, Ronan; Matteini, Lorenzo; Eastwood, Jonathan; Bale, Stuart D.; Velli, Marco; Chandran, Benjamin D. G.; Phan, Tai; Raouafi, Nour E.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    Measurements of the near-Sun solar wind by the Parker Solar Probe have revealed the presence of large numbers of discrete Alfvenic impulses with an anti-sunward sense of propagation. These are similar to those previously observed near 1 au, 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; the timescale of these patches might be related to the rate at which the spacecraft magnetic footpoint tracks across the coronal hole from which the plasma originated. While the velocity fluctuations associated with these spikes are typically under 100 km s(-1), due to the rather low Alfven speeds in the streams observed by the spacecraft to date, these are still associated with large angular deflections of the magnetic field-and these deflections are not isotropic. These deflections do not appear to be related to the recently reported large-scale, pro-rotation solar wind flow. Estimates of the size and shape of the spikes reveal high aspect ratio flow-aligned structures with a transverse scale of approximate to 10(4) km. These events might be signatures of near-Sun impulsive reconnection events.
  • Small, Low-energy, Dispersive Solar Energetic Particle Events Observed by Parker Solar Probe

    Hill, M. E.; Mitchell, D. G.; Allen, R. C.; de Nolfo, G. A.; Vourlidas, A.; Brown, L. E.; Jones, S. I.; McComas, D. J.; McNutt, R. L.; Mitchell, J. G.; et al. (IOP PUBLISHING LTD, 2020-02-03)
    The Energetic Particle Instrument-Low Energy (EPI-Lo) experiment has detected several weak, low-energy (similar to 30-300 keV nucleon(-1)) solar energetic particle (SEP) events during its first two closest approaches to the Sun, providing a unique opportunity to explore the sources of low-energy particle acceleration. As part of the Parker Solar Probe (PSP) Integrated Science Investigation of the Sun (IS circle dot IS) suite, EPI-Lo was designed to investigate the physics of energetic particles; however, in the special lowest-energy "time-of-flight only" product used in this study, it also responds to solar photons in a subset of approximately sunward-looking apertures lacking special light-attenuating foils. During the first three perihelia, in a frame rotating with the Sun, PSP undergoes retrograde motion, covering a 17 degrees heliographic longitudinal range three times during the course of the similar to 11-day perihelion passes, permitting a unique spatial and temporal study into the location, correlation, and persistence of previously unmeasurable SEPs. We examine the signatures of these SEPs (during the first PSP perihelion pass only) and the connection to possible solar sources using remote observations from the Solar Dynamics Observatory (SDO), the Solar TErrestrial RElations Observatory (STEREO), and the ground-based Global Oscillation Network Group (GONG). The orientation of the Sun relative to STEREO, SDO, and GONG makes such identifications challenging, but we do have several candidates, including an equatorial coronal hole at a Carrington longitude of similar to 335 degrees. To analyze observations from EPI-Lo, which is a new type of particle instrument, we examine instrumental effects and provide a preliminary separation of the ion signal from the photon background.

View more