Browsing by Author "Bradshaw, Stephen"

Now showing 1 - 7 of 7
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    Determining the Nanoflare Heating Frequency of an X-Ray Bright Point Observed by MaGIXS
    (IOP Publishing, 2024) Mondal, Biswajit; Athiray, P. S.; Winebarger, Amy R.; Savage, Sabrina L.; Kobayashi, Ken; Bradshaw, Stephen; Barnes, Will; Champey, Patrick R.; Cheimets, Peter; Dudík, Jaroslav; Golub, Leon; Mason, Helen E.; McKenzie, David E.; Moore, Christopher S.; Madsen, Chad; Reeves, Katharine K.; Testa, Paola; Vigil, Genevieve D.; Warren, Harry P.; Walsh, Robert W.; Zanna, Giulio Del
    Nanoflares are thought to be one of the prime candidates that can heat the solar corona to its multimillion kelvin temperature. Individual nanoflares are difficult to detect with the present generation of instruments, but their presence can be inferred by comparing simulated nanoflare-heated plasma emissions with the observed emission. Using HYDRAD coronal loop simulations, we model the emission from an X-ray bright point (XBP) observed by the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), along with the nearest available observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) and the X-Ray Telescope (XRT) on board the Hinode observatory. The length and magnetic field strength of the coronal loops are derived from the linear force-free extrapolation of the observed photospheric magnetogram by the Helioseismic and Magnetic Imager on board SDO. Each loop is assumed to be heated by random nanoflares, whose magnitude and frequency are determined by the loop length and magnetic field strength. The simulation results are then compared and matched against the measured intensity from AIA, XRT, and MaGIXS. Our model results indicate the observed emission from the XBP under study could be well matched by a distribution of nanoflares with average delay times 1500–3000 s. Further, we demonstrate the high sensitivity of MaGIXS and XRT for diagnosing the heating frequency using this method, while AIA passbands are found to be the least sensitive.
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    Diagnosing the Frequency of Energy Deposition in the Magnetically-Closed Solar Corona
    (2019-04-18) Barnes, Will Thomas; Bradshaw, Stephen
    The solar corona, the outermost layer of the Sun's atmosphere, is heated to temperatures in excess of one million Kelvin, nearly three orders of magnitude greater than the surface of the Sun. This so-called "coronal heating problem" has occupied the field of solar astrophysics for over seventy years and is one of the most important open questions in astronomy as a whole. While it is generally agreed that the continually stressed coronal magnetic field plays a role in producing these million-degree temperatures, the exact mechanism responsible for transporting this stored energy to the coronal plasma is yet unknown. Nanoflares, small-scale bursts of energy likely resulting from the frequent reconnection of twisted magnetic field lines, have long been proposed as a candidate for heating the non-flaring corona, especially in areas of high magnetic activity. However, a direct detection of heating by nanoflares has proved difficult due to their faint, transient nature and as such, properties of this proposed heating mechanism remain largely unconstrained. In this thesis, I use a hydrodynamic model of the coronal plasma combined with a sophisticated forward modeling approach and machine learning classification techniques to predict signatures of nanoflare heating and compare these predictions to real observational data. In particular, the focus of this work is constraining the frequency with which nanoflares occur on a given magnetic field line in non-flaring active regions. First, I give an introduction to the structure of the solar atmosphere and coronal heating, discuss the hydrodynamics of coronal loops, and provide an overview of the important emission mechanisms in a high-temperature, optically-thin plasma. Then, I describe the forward modeling pipeline for predicting time-dependent, multi-wavelength emission over an entire active region. Next, I use a hydrodynamic model of a single coronal loop to predict signatures of "very hot" plasma produced by nanoflares. I find that several effects, including flux limiting, nonequilibrium ionization, and nanoflare duration, are likely to affect the observability of this direct signature of nanoflare heating. Then, I use the forward modeling code described above to simulate time-dependent, multi-wavelength AIA emission from active region NOAA 1158 for a range of nanoflare frequencies and find that signatures of the heating frequency persist in multiple observable quantities. Finally, I use these predicted diagnostics to train a random forest classifier and apply this model to real AIA observations of NOAA 1158. In doing so, I am able to map the heating frequency, pixel by pixel, across the entire active region. Altogether, this thesis represents a critical step in systematically constraining the frequency of energy deposition in active regions. A novel component of this thesis is the development of a modular forward modeling pipeline, written in the Python programming language, that builds a "magnetic skeleton" from a three-dimensional field extrapolation, configures thousands of field-aligned hydrodynamic loop models, and computes arbitrary line-of-sight projections of the time-dependent, three-dimensional active region emission. The code is flexible and scalable and is openly-developed such that it may be used and improved by the larger solar physics community. Another novel component of this thesis is the use of machine learning to compare real observations and model results. By training a random forest classifier on predicted diagnostics, I am able to systematically and quantitatively assess observations in the context of multiple diagnostics in order to make an accurate prediction of the properties of the heating.
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    Evidence for Impulsive Heating of Active Region Coronal Loops
    (2013-07-24) Reep, Jeffrey; Bradshaw, Stephen; Alexander, David; Ecklund, Karl
    We present observational and numerical evidence supporting the theory of impulsive heating of the solar corona. We have run numerical simulations solving the hydrodynamic equations for plasma confined to a magnetic flux tube, for the two distinct cases of steady and impulsive heating. We find that steady heating cannot explain the observed amount of low-temperature plasma in active regions on the sun. The results for impulsive heating closely match those of the observations. The ratio of heating time to cooling time predominantly determines the observed temperature distribution of the plasma. We have also identified an observational bias in calculating intensities of spectral lines in previous studies, which causes an under-estimation of low-temperature plasma. We predict Doppler shifts in the observed line emission that are in agreement with observations, and which may serve as a diagnostic of the strength of heating. We conclude that impulsive heating of active region coronal loops is more likely than steady heating.
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    High performance high-order numerical methods: applications in ocean modeling
    (2015-08-27) Gandham, Rajesh; Warburton, Timothy; Symes, William; Bradshaw, Stephen; Beatrice, Riviere
    This thesis presents high-order numerical methods for time-dependent simulations of oceanic wave propagation on modern many-core hardware architecture. Simulation of the waves such as tsunami, is challenging because of the varying fluid depths, propagation in many regions, requirement of high resolution near the shore, complex nonlinear wave phenomenon, and necessity of faster than real-time predictions. This thesis addresses issues related to stability, accuracy, and efficiency of the numerical simulation of these waves. For the simulation of tsunami waves, a two-dimensional nonlinear shallow water PDE model is considered. Discontinuous Galerkin (DG) methods on unstructured triangular meshes are used for the numerical solution of the model. These methods are not stable for nonlinear problems. To address the stability of these methods, a total variational bounded slope limiter in conjunction with a positive preserving scheme is developed, in particular for unstructured triangular meshes. Accuracy and stability of the methods are verified with test cases found in literature. These methods are also validated using 2004 Indian Ocean tsunami data to demonstrate faster than real-time simulation capability for practical problems using a commodity workstation. For accurate modeling of tsunami and ocean waves, in general, a three-dimensional hydrostatic incompressible Navier-Stokes model along with free surface conditions is considered. DG discretizations on unstructured prismatic elements are used for the numerical solutions. These prismatic elements are obtained by extruding the triangular meshes from ocean free surface to the ocean bottom. The governing equations are represented in a fixed sigma coordinate reference system. The limiting procedure, time-stepping method, accelerated implementations are adopted from two-dimensional formulations. The runtime performance of this three-dimensional method is compared with the performance of the two-dimensional shallow water model, to give an estimate of computational overhead in moving forward to three-dimensional models in practical ocean modeling applications. A GPU accelerated unsmooth aggregation algebraic method is developed. Algebraic multi-grid method is used as a linear solver in many engineering applications such as computational fluid dynamics. The developed method involves a setup stage and a solution stage. This method is parallelized for both stages unlike most of the methods that are parallelized only for the solution stage. Efficiency of the setup is crucial in these applications since the setup has to be performed many times. The efficiency of the method is demonstrated using a sequence of downsized problems. The computational kernels are expressed in an extensive multi-threading library OCCA. Using OCCA, the developed implementations achieve portability across various hardware architectures such as GPUs, CPUs, and multi-threading programming models OpenCL, CUDA, and OpenMP. The optimal performance of these kernels across various thread models and hardware architecture is compared.
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    High Temperature Plasma Dynamics in Solar Flares
    (2021-08-13) Mandage, Revati Sudam; Bradshaw, Stephen; Alexander, David; Gonnermann, Helge
    Spatially resolved spectroscopic observations show wing enhancements and broadening in extreme ultraviolet emission from particular hot iron lines. Several physical processes ranging from plasma turbulence, magnetic perturbations to non-Gaussian ion populations, and non-thermal physics have been proposed to play a role in their formation. In this thesis I investigate in detail the role of plasma dynamics in spectral line shapes by studying the wing enhancements of Fe XXIII and XXIV observed during solar flares, using a field-aligned hydrodynamic model. First I examine how plasma dynamics in a single, monolithic flaring loop contributes to the formation of line asymmetry. This is done by running 35 simulations that use the observed values and their uncertainties for the driving electron beam parameters. Next I study the effect of flaring loop length on spectral line shape and broadening by running simulations with different loop lengths and the same beam parameters. The presence of sub-resolution structures, confirmed by increasingly high-resolution observations, and observational difficulties in isolating a monolithic loop from nearby loops, necessitate an investigation into the effect of superposed dynamics on some line asymmetries. Hence, I design multiloop models that are representative of three possible configurations of loops. Here I study how the resultant Fe XXIII spectral line profiles differ in each case and examine the differences between these multiloop models and the single loop model. I also briefly explore the role of a constant time delay in heating successive sub-loops of a multiloop configuration on the spectral line shape. The results show that the single loop model can successfully reproduces line asymmetries, and the loop length plays an important role in explaining some of the key observations such as the positive correlation between Doppler shifts and line widths, and broad but symmetric hot Fe lines. For a multiloop model with sub-loops of the same length, an imposed heating time delay is an important factor that significantly alters the line profile shape from the single loop case. Whereas, multiloop models with sub-loops of varied lengths predict significantly different line profiles, such as asymmetric lines for longer durations and with large blue-shifts, without the necessity of introducing time delays.
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    The Equilibrium of Coronal Loops Near Separatrices
    (IOP Publishing, 2023) Mason, Emily I.; Antiochos, Spiro K.; Bradshaw, Stephen
    We present numerical models from the field-aligned HYDrodynamics and RADiation code (HYDRAD) of a highly asymmetric closed coronal loop with near-singular expansion factor. This loop was chosen to simulate a coronal magnetic flux tube that passes close to a null point, as in the last set of closed loops under the fan surface of a coronal jet or a pseudostreamer. The loop has a very large cross section localized near the coronal null. The coronal heating was assumed to be uniform and steady. A siphon flow establishes itself within 4 hr of simulation time, flowing from the smaller-area footpoint to the larger-area footpoint, with high initial speeds dropping rapidly as the plasma approaches the null region. Observationally, this would translate to strong upflows on the order of 10 km s−1 from the footpoint rooted in the localized minority polarity, and weak downflows from the fan-surface footpoint on the order of a few kilometers per second, along with near-stationary plasma near the null region. We present the model results for two heating rates. In addition, we analyzed analogous Hinode EUV Imaging Spectrometer observations of null-point topologies, which show associated Doppler shifts in the plasma that correlate well with the simulation results in both direction and magnitude of the bulk velocity. We discuss the implications of our results for determining observationally the topology of the coronal magnetic field.
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    The First Flight of the Marshall Grazing Incidence X-Ray Spectrometer (MaGIXS)
    (IOP Publishing, 2023) Savage, Sabrina L.; Winebarger, Amy R.; Kobayashi, Ken; Athiray, P. S.; Beabout, Dyana; Golub, Leon; Walsh, Robert W.; Beabout, Brent; Bradshaw, Stephen; Bruccoleri, Alexander R.; Champey, Patrick R.; Cheimets, Peter; Cirtain, Jonathan; DeLuca, Edward E.; Zanna, Giulio Del; Dudík, Jaroslav; Guillory, Anthony; Haight, Harlan; Heilmann, Ralf K.; Hertz, Edward; Hogue, William; Kegley, Jeffery; Kolodziejczak, Jeffery; Madsen, Chad; Mason, Helen; McKenzie, David E.; Ranganathan, Jagan; Reeves, Katharine K.; Robertson, Bryan; Schattenburg, Mark L.; Scholvin, Jorg; Siler, Richard; Testa, Paola; Vigil, Genevieve D.; Warren, Harry P.; Watkinson, Benjamin; Weddendorf, Bruce; Wright, Ernest
    The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) sounding rocket experiment launched on 2021 July 30 from the White Sands Missile Range in New Mexico. MaGIXS is a unique solar observing telescope developed to capture X-ray spectral images of coronal active regions in the 6–24 Å wavelength range. Its novel design takes advantage of recent technological advances related to fabricating and optimizing X-ray optical systems, as well as breakthroughs in inversion methodologies necessary to create spectrally pure maps from overlapping spectral images. MaGIXS is the first instrument of its kind to provide spatially resolved soft X-ray spectra across a wide field of view. The plasma diagnostics available in this spectral regime make this instrument a powerful tool for probing solar coronal heating. This paper presents details from the first MaGIXS flight, the captured observations, the data processing and inversion techniques, and the first science results.