arXiv subjects

S. Wallace

Publications and source records attributed to S. Wallace.

15 recordsLinked to original sources

arXiv2025

NASA/NOAA MOU Annex Final Report: Evaluating Model Advancements for Predicting CME Arrival Time

The purpose of this project was to assess improvements in CME arrival time forecasts at Earth using the Air Force Data Assimilative Photospheric Flux Transport (ADAPT) model driven by data from the Global Oscillation Network Group (GONG) ground observatories. These outputs are then fed into the coupled Wang-Sheeley-Arge (WSA) - ENLIL model and compared to an operational version of WSA-ENLIL (without ADAPT). SWPC selected a set of 38 historical events over the period of five years from 2012--2014 (33 events) and 2017--2019 (5 events). The overall three-year project consisted of multiple simulation validation studies for the entire event set (1292 simulations): (a) benchmark single map (operational version prior to May 2019) (b) time-dependent sequence of GONG maps driving WSA-ENLIL with 4 different model settings (c) single test simulation of a time-dependent sequence of GONG maps driving ADAPT-WSA-ENLIL (d) single GONG map driving ADAPT-WSA-ENLIL (e) time-dependent sequence of GONG maps driving ADAPT-WSA-ENLIL. We report that for all 38 events, within each model version/settings combination, the CME arrival time error decreased by 0.2 to 0.9 hours when using a sequence of time-dependent zero-point corrected magnetograms compared to using single magnetogram input. Overall, for all events, when using the older uncorrected magnetograms, the CME arrival time error increased for all new model versions/settings combination compared to the benchmark. Notably for the 5 events in the period 2017--2019 when more reliable zero-point corrected magnetograms were available, the ADAPT-WSA-ENLIL (median arrival realization) CME arrival time error decreased against all benchmarks. In this report we also discuss replicating the operational model, challenges in detecting CME arrival in simulations, and comparing zero-point corrected and uncorrected magnetogram inputs.

physics.space-ph↗

arXiv2023

Ensemble Forecasts of Solar Wind Connectivity to 1 Rs using ADAPT-WSA

The solar wind which arrives at any location in the solar system is, in principle, relatable to the outflow of solar plasma from a single source location. This source location, itself usually being part of a larger coronal hole, is traceable to 1 Rs along the Sun's magnetic field, in which the entire path from 1 Rs to a location in the heliosphere is referred to as the solar wind connectivity. While not directly measurable, the connectivity between the near-Earth solar wind is of particular importance to space weather. The solar wind solar source region can be obtained by leveraging near-sun magnetic field models and a model of the interplanetary solar wind. In this article we present a method for making an ensemble forecast of the connectivity presented as a probability distribution obtained from a weighted collection of individual forecasts from the combined Air Force Data Assimilative Photospheric Flux Transport - Wang Sheeley Arge (ADAPT-WSA) model. The ADAPT model derives the photospheric magnetic field from synchronic magnetogram data, using flux transport physics and ongoing data assimilation processes. The WSA model uses a coupled set of potential field type models to derive the coronal magnetic field, and an empirical relationship to derive the terminal solar wind speed observed at Earth. Our method produces an arbitrary 2D probability distribution capable of reflecting complex source configurations with minimal assumptions about the distribution structure, prepared in a computationally efficient manner.

astro-ph.SR↗

arXiv2023

The 17 April 2021 widespread solar energetic particle event

Context. A solar eruption on 17 April 2021 produced a widespread Solar Energetic Particle (SEP) event that was observed by five longitudinally well-separated observers in the inner heliosphere at heliocentric distances of 0.42 to 1 au: BepiColombo, Parker Solar Probe, Solar Orbiter, STEREO A, and near-Earth spacecraft. The event produced relativistic electrons and protons. It was associated with a long-lasting solar hard X-ray flare and a medium fast Coronal Mass Ejection (CME) with a speed of 880 km/s driving a shock, an EUV wave as well as long-lasting radio burst activity showing four distinct type III burst. Methods. A multi-spacecraft analysis of remote-sensing and in-situ observations is applied to attribute the SEP observations at the different locations to the various potential source regions at the Sun. An ENLIL simulation is used to characterize the interplanetary state and its role for the energetic particle transport. The magnetic connection between each spacecraft and the Sun is determined. Based on a reconstruction of the coronal shock front we determine the times when the shock establishes magnetic connections with the different observers. Radio observations are used to characterize the directivity of the four main injection episodes, which are then employed in a 2D SEP transport simulation. Results. Timing analysis of the inferred SEP solar injection suggests different source processes being important for the electron and the proton event. Comparison among the characteristics and timing of the potential particle sources, such as the CME-driven shock or the flare, suggests a stronger shock contribution for the proton event and a more likely flare-related source of the electron event. Conclusions. We find that in this event an important ingredient for the wide SEP spread was the wide longitudinal range of about 110 degrees covered by distinct SEP injections.

astro-ph.SR↗

arXiv2019

Estimating Total Open Heliospheric Magnetic Flux

Over the solar-activity cycle, there are extended periods where significant discrepancies occur between the spacecraft-observed total (unsigned) open magnetic flux and that determined from coronal models. In this article, the total open heliospheric magnetic flux is computed using two different methods and then compared with results obtained from in-situ interplanetary magnetic-field observations. The first method uses two different types of photospheric magnetic-field maps as input to the Wang Sheeley Arge (WSA) model: i) traditional Carrington or diachronic maps, and ii) Air Force Data Assimilative Photospheric Flux Transport model synchronic maps. The second method uses observationally derived helium and extreme-ultraviolet coronal-hole maps overlaid on the same magnetic-field maps in order to compute total open magnetic flux. The diachronic and synchronic maps are both constructed using magnetograms from the same source, namely the National Solar Observatory Kitt Peak Vacuum Telescope and Vector Spectromagnetograph. The results of this work show that the total open flux obtained from observationally derived coronal holes agrees remarkably well with that derived from WSA, especially near solar minimum. This suggests that, on average, coronal models capture well the observed large-scale coronal-hole structure over most of the solar cycle. Both methods show considerable deviations from total open flux deduced from spacecraft data, especially near solar maximum, pointing to something other than poorly determined coronal-hole area specification as the source of these discrepancies.

astro-ph.SR↗

arXiv2013

Photometric Survey to Search for Field sdO Pulsators

We present the results of a campaign to search for subdwarf O (sdO) star pulsators among bright field stars. The motivation for this project is the recent discovery by Randall et al. (2011), of four rapidly pulsating sdO stars in the globular cluster Omega Cen, with Teff near 50,000 K, 5.4 < log g < 6.0, and hydrogen-rich atmospheres. The only previously known sdO pulsator is significantly hotter at 68,500 K and log g = 6.1. All of the sdO pulsators identified so far are fainter than 17.4 in the V band and, thus, are poor candidates for an in-depth follow-up with asteroseismology. We therefore obtained high S/N light curves and spectroscopy for a number of field sdO stars to attempt to discover bright counterparts to these stars, particularly the Omega Cen pulsators. Our primary sample consisted of 19 sdO stars with hydrogen-rich atmospheres, log N(He)/N(H) < -1.0, effective temperatures in the range 40,000 K < Teff < 67,000 K, and surface gravities 5.3 < log g < 6.1. We also observed 17 additional helium-rich sdO stars with log N(He)/N(H) > -0.1 and similar temperatures and gravities. To date, we have found no detectable pulsations at amplitudes above 0.08% (4 times the mean noise level) in any of the 36 field sdO stars that we observed. The presence of pulsations in Omega Cen sdO stars and their apparent absence in seemingly comparable field sdO stars is perplexing. While very suggestive, the significance of this result is difficult to assess more completely right now due to remaining uncertainties about the temperature width and purity of the Omega Cen instability strip and the existence of any sdO pulsators with weaker amplitudes than the current detection limit in globular clusters.

astro-ph.SR↗

arXiv2010

The excited hadron spectrum in lattice QCD using a new method of estimating quark propagation

Progress in determining the spectrum of excited baryons and mesons in lattice QCD is described. Large sets of carefully-designed hadron operators have been studied and their effectiveness in facilitating the extraction of excited-state energies is demonstrated. A new method of stochastically estimating the low-lying effects of quark propagation is proposed which will allow reliable determinations of temporal correlations of single-hadron and multi-hadron operators.

hep-lat↗

arXiv2006

Baryon Operators and Baryon Spectroscopy

The issues involved in a determination of the baryon resonance spectrum in lattice QCD are discussed. The variational method is introduced and the need to construct a sufficient basis of interpolating operators is emphasised. The construction of baryon operators using group-theory techniques is outlined. We find that the use both of quark-field smearing and link-field smearing in the operators is essential firstly to reduce the coupling of operators to high-frequency modes and secondly to reduce the gauge-field fluctuations in correlators. We conclude with a status report of our current investigation of baryon spectroscopy.

hep-lat↗

arXiv2006

Towards a determination of the spectrum of QCD using a space-time lattice

Progress by the Lattice Hadron Physics Collaboration in determining the baryon and meson resonance spectrum of QCD using Monte Carlo methods with space-time lattices is described. The extraction of excited-state energies necessitates the evaluation of correlation matrices of sets of operators, and the importance of extended three-quark operators to capture both the radial and orbital structures of baryons is emphasized. The use of both quark-field smearing and link-field smearing in the operators is essential for reducing the couplings of the operators to the high-frequency modes and for reducing statistical noise in the correlators. The extraction of nine energy levels in a given symmetry channel is demonstrated, and identifying the continuum spin quantum numbers of the levels is discussed.

hep-lat↗

arXiv2005

Group-theoretical construction of extended baryon operators in lattice QCD

The design and implementation of large sets of spatially-extended, gauge-invariant operators for use in determining the spectrum of baryons in lattice QCD computations are described. Group-theoretical projections onto the irreducible representations of the symmetry group of a cubic spatial lattice are used in all isospin channels. The operators are constructed to maximize overlaps with the low-lying states of interest, while minimizing the number of sources needed in computing the required quark propagators. Issues related to the identification of the spin quantum numbers of the states in the continuum limit are addressed.

hep-lat↗

arXiv2004

Analysis of $N^{\star}$ spectra using matrices of correlation functions based on irreducible baryon operators

We present results for ground and excited-state nucleon masses in quenched lattice QCD using anisotropic lattices. Group theoretical constructions of local and nonlocal straight-link irreducible operators are used to obtain suitable sources and sinks. Matrices of correlation functions are diagonalized to determine the eigenvectors. Both chi-square fitting and Bayesian inference with an entropic prior are used to extract masses from the correlation functions in a given channel. We observe clear separation of the excited state masses from the ground state mass. States of spin >= 5/2 have been isolated by use of G_2 operators.

hep-lat↗

arXiv2004

Group-theoretical construction of extended baryon operators

The design and implementation of large sets of spatially extended baryon operators for use in lattice simulations are described. The operators are constructed to maximize overlaps with the low-lying states of interest, while minimizing the number of sources needed in computing the required quark propagators.

hep-lat↗

arXiv2004

Baryonic sources using irreducible representations of the double-covered octahedral group

Irreducible representations (IRs) of the double-covered octahedral group are used to construct lattice source and sink operators for three-quark baryons. The goal is to achieve a good coupling to higher spin states as well as ground states. Complete sets of local and nonlocal straight-link operators are explicitly shown for isospin 1/2 and 3/2 baryons. The orthogonality relations of the IR operators are confirmed in a quenched lattice simulation.

hep-lat↗

arXiv2003

Baryon operators and spectroscopy in lattice QCD

The construction of the operators and correlators required to determine the excited baryon spectrum is presented, with the aim of exploring the spatial and spin structure of the states while minimizing the number of propagator inversions. The method used to construct operators that transform irreducibly under the symmetries of the lattice is detailed, and the properties of example operators are studied using domain-wall fermion valence propagators computed on MILC asqtad dynamical lattices.

hep-lat↗

arXiv2003

Mass spectrum of N* and source optimization

We have computed correlation functions for nucleons and extracted the masses for positive- and negative-parities. Use of group theory plays an important role in obtaining sources that have good overlap to higher spin states and minimum contamination from unwanted states. In the simulation three distinct sources and corresponding sinks that transform according to the $G_1$ irreducible representations are tested and used to form matrices of correlation functions. Diagonalizations give us clear mass splittings between low-lying states and excited states for both parities.

hep-lat↗

arXiv2003

Baryonic Operators for Lattice Simulations

The construction of baryonic operators for determining the N* excitation spectrum is discussed. The operators are designed with one eye towards maximizing overlaps with the low-lying states of interest, and the other eye towards minimizing the number of sources needed in computing the required quark propagators. Issues related to spin identification are outlined. Although we focus on tri-quark baryon operators, the construction method is applicable to both mesons and penta-quark operators.

hep-lat↗