Browsing by Author "Bradshaw, Stephen J"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Hydrodynamic Modeling of Heating Processes in Solar Flares
    (2014-10-09) Reep, Jeffrey; Bradshaw, Stephen J; Alexander, David; Warburton, Tim
    This thesis examines the heating of the solar atmosphere due to energy release in solar flares. A one-dimensional hydrodynamic model, which solves the equations of conservation of mass, momentum, and energy along a magnetic flux tube, is described in detail and employed to study the dynamic response of the solar atmosphere to large amounts of energy release from the magnetic field. A brief introduction to the solar atmosphere and solar flares, from both observational and theoretical perspectives, is given. Then, the hydrodynamic model is described, along with derivations of energy deposition due to a beam of highly energetic electrons colliding with the ambient atmosphere (and their implementation in the model is explained). Using this model of heating along with RHESSI-derived parameters of observed flares, the sensitivity of the GOES flare classification to the parameters of the electron beam (the non-thermal energy, the power-law index of the electron distribution, and the low-energy cut-off) are examined, and clear correlations are determined. Next, the response of the atmosphere to heating due to isoenergetic beams of electrons are studied, to elucidate the importance of electrons at different energy. It is found that at high total energy fluxes, the energy of individual electrons are unimportant, but that at lower fluxes, lower energy electrons are significantly more efficient at heating the atmosphere and driving chromospheric evaporation than high energy electrons. It is also found that the threshold for explosive evaporation is strongly dependent on the cut-off energy, as well as the beam flux. A case study of a well-observed flare is performed. The flare, a C-class flare that occurred on 28 November 2002, was modeled for various cases of heating due to a beam of electrons, in situ coronal heating, and a hybrid model that combines both forms of heating. It is found that the observation of X-ray source heights seen with RHESSI are most consistent with a hybrid model. The results indicate that the energy must be partitioned between thermal and kinetic energy, and the implications are discussed. This work is then summarized, and future avenues of research are discussed. Improvements that can be made to the model, the forward modeling of emission, and comparisons to observations are discussed.
  • Thumbnail Image
    Item
    The Source and Time Variability of the Slow Solar Wind
    (2015-04-24) Mueller, Brandon Anthony; Bradshaw, Stephen J; Alexander, David; Si, Qimiao
    This thesis examines a proposed source of the slow solar wind at the boundaries of active regions. It shows the time variability observed in the wind can be produced by time-variable heating in such regions. To understand how these structures contribute to observed outflows, we adapt a two-fluid hydrodynamic model of the solar corona. Adjusting model parameters within their physical bounds allows us to reproduce the expected values for the slow wind and gain a better understanding of the energy deposition along coronal flux tubes. We use forward models to compare with observations from the Extreme Ultraviolet Imaging Spectrometer aboard Hinode. Non-thermal line broadening and Doppler shifts are calculated for quantitative comparisons. The simulations including non-equilibrium ionization indicate equilibrium ionization cannot be assumed in many cases. Successful reproduction of remote sensing observations leaves us optimistic about future comparisons with in situ measurements from the space missions Solar Probe+ and Solar Orbiter.
  • Thumbnail Image
    Item
    Understanding the energy balance of TR structures observed by IRIS in non-equilibrium emission
    (2019-04-18) Bahauddin, Shah Mohammad; Bradshaw, Stephen J
    The corona, the outer atmosphere of the Sun, is a multi-million degree plasma, nearly three orders magnitude hotter than the visible surface. The exact mechanism by which the corona is heated is still the subject of debate, but possibilities include magnetic reconnection and magnetohydrodynamic waves. Studying the thin boundary layer connecting the cooler chromosphere to the hotter corona, named the TR, is an important step toward understanding mass and energy transport from the chromosphere to the corona. Thus, spectral emissions from the cool (< 1 MK) loop-like structures in this region are in need of extensive study and analysis. Because observations lack sufficient spatial resolution, this type of structure was called the “unresolved fine structure”, which is now considered resolved by the Interface Region Imaging Spectrograph (IRIS). In the active TR of the Sun, IRIS has observed loop-like structures with intermittent brightenings which are thought to originate from impulsive heating. In this thesis, the author present evidence of magnetic field line braiding and reconnection mediated brightenings of TR loops using IRIS slit-jaw images and spectral data, complemented by the EUV channels of the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). The set of observables used to characterize the brightenings consists of diagnostics of temperature, density, line broadening, and Doppler-shift on a pixel-by-pixel basis. The characterization scheme is extended by accumulating time dependent differential emission measure (DEM) distributions to define the nature of the spatial heating profile and frequency. A field-aligned hydrodynamic simulation and a forward modeling code, designed to generate synthetic observations from numerical experiments for comparison with real data, are employed. Non-equilibrium ionization is included in the computation of synthetic spectra. In addition, the relatively high-density TR plasma requires the inclusion of density-dependent dielectronic recombination rates to calculate the ion populations and the emission line intensities. We show that the observations and the numerical experiments are consistent with reconnection mediated impulsive heating at the braiding sites of multi-stranded TR loops. The combination of observation and numerical analysis will provide the building blocks of time-dependent 3D models of these loops and their contribution to active region emission which will, in turn, help us to understand the energy balance of these structures and may shed light on the long standing coronal heating problem: “Why is the Sun’s corona so much hotter than the surface?”.