Theses and Dissertations at Montana State University (MSU)
Permanent URI for this communityhttps://scholarworks.montana.edu/handle/1/732
Browse
9 results
Search Results
Item Modeling and understanding coronal loop dynamics during solar flares(Montana State University - Bozeman, College of Letters & Science, 2020) Unverferth, John Edward, IV; Chairperson, Graduate Committee: Dana W. Longcope; Dana Longcope was a co-author of the article, 'Effects of the canopy and flux tube anchoring on evaporation flow of a solar flare' in the journal 'The astrophysical journal' which is contained within this dissertation.; Dana Longcope was a co-author of the article, 'Modeling observable differences in flare loop evolution due to reconnection location and current sheet structure' in the journal 'The astrophysical journal' which is contained within this dissertation.; Dana Longcope was a co-author of the article, 'Flux tube interactions as a cause for sub-alfvenic reconnection outflow' submitted to the journal 'The astrophysical journal' which is contained within this dissertation.Magnetic reconnection is widely considered to be the mechanism behind solar flares. Models powered by reconnection manage to explain many of the observational features seen in a flare. However, these models miss or contradict important elements of the flare. Here we consider three effects overlooked by models. First, the role played by the magnetic canopy in determining the chromospheric response in a flare. Second, how variations in magnetic field strength along the current sheet impact the evolution of are loops. Third, how flux tube interactions with the current sheet can lead to sub-Alfvenic motion, bringing dynamics in line with observations. These three effects were investigated with the use of one dimensional and thin flux tube models. This allowed for the dynamics to be considered independent of the reconnection process that generated the flux tubes. The canopy interaction revealed that the creation of an expansion followed by a constriction, a chamber in the flux tube, leads to multiple solutions. The solutions include smooth flow and standing shocks in the chamber. The standing shock increases the emission of the flow, as well as slowing it to subsonic speeds. The shocked solution shifts the ensemble of flux tubes to have a distribution that would indicate slower speeds than expected. The structure of the current sheet magnetic field leaves a signature on the flux tube. Each case leads to a difference in emission. Retraction through a constricting field creating a plug of material leading to a bright emission in the apex. This contrasts with retraction through an expanding field which generates high temperatures, but as a fainter emission. The interaction of drag in the current sheet allowed for the retraction to proceed at slower rates. The slower retraction matches observations of features in flares more accurately. The slower retraction also increases the brightness of the synthetic current sheet. This increased brightness brings the current sheet closer to the observed brightness. These investigations found that there was benefit to considering these additional effects. Each one of these effects was found to bring the models more in line to what observations note.Item The structure of energy-extracting black hole magnetospheres(Montana State University - Bozeman, College of Letters & Science, 2019) Thoelecke, Kevin; Chairperson, Graduate Committee: Yves U. IdzerdaSpinning black holes can store enormous amounts of rotational energy. Efficiently extracting that rotational energy can lead to significant energy outflows capable of powering very high energy astrophysical phenomena, such as gamma-ray bursts and active galactic nuclei. Black holes are unique in that they do not exist as physical objects in the same way a rock, planet, or star exists; instead, black holes exist only as spacetime curvature. As such processes for extracting a black hole's rotational energy are largely unique to black holes. This work explores one such process, the extraction of a black hole's rotational energy via an appropriately configured magnetosphere. Both analytic perturbation techniques and numerical codes are developed in order to solve for thousands of energy-extracting black hole magnetospheres. Those magnetospheres broadly sample the relevant solution space, allowing correlations to be drawn between different rates of black hole rotational energy and angular momentum extraction and global magnetosphere structure. The most fundamental behavior discovered is that magnetospheres that extract the most energy per unit angular momentum direct that energy away from the black hole's rotational axis, while magnetospheres that extract the least amount of energy per unit angular momentum direct that energy into jet-like structures aligned with the black hole's rotational axis. Exploration of the solutions obtained also suggests that magnetospheres most compatible with nearby accreting matter can very naturally launch jets, implying that black hole energy extraction and jet launching are likely to be concurrent and common features of astrophysical black hole magnetospheres.Item Thermionic emission from synthetic coal slags and MHD electrode materials(Montana State University - Bozeman, College of Letters & Science, 1978) Wilson, Mark LowellItem Hall effect and electrical conductivity studies of some MHD and fuel cell related materials(Montana State University - Bozeman, College of Letters & Science, 1978) Snyder, Stuart CodyItem Propagating disturbances in the lower solar corona(Montana State University - Bozeman, College of Letters & Science, 2003) Wills-Davey, Meredith JenningsItem Thermal expansion under load of candidate material for MHD preheaters(Montana State University - Bozeman, College of Letters & Science, 1980) Halvorson, Alan LesterItem Electrical conductivity of MHD coal slags to 2025 K(Montana State University - Bozeman, College of Letters & Science, 1978) Westpfahl, David JohnItem Patchy reconnection in the solar corona(Montana State University - Bozeman, College of Letters & Science, 2011) Guidoni, Silvina Esther; Chairperson, Graduate Committee: Dana W. LongcopeMagnetic reconnection in plasmas, a process characterized by a change in connectivity of field lines that are broken and connected to other ones with different topology, owes its usefulness to its ability to unify a wide range of phenomena within a single universal principle. There are newly observed phenomena in the solar corona that cannot be reconciled with two-dimensional or steady-state standard models of magnetic reconnection. Supra-arcade downflows (SADs) and supra-arcade downflowing loops (SADLs) descending from reconnection regions toward solar post-flare arcades seem to be two different observational signatures of retracting, isolated reconnected flux tubes with irreducible three-dimensional geometries. This dissertation describes work in refining and improving a novel model of patchy reconnection, where only a small bundle of field lines is reconnected across a current sheet (magnetic discontinuity) and forms a reconnected thin flux tube. Traditional models have not been able to explain why some of the observed SADs appear to be hot and relatively devoid of plasma. The present work shows that plasma depletion naturally occurs in flux tubes that are reconnected across nonuniform current sheets and slide trough regions of decreasing magnetic field magnitude. Moreover, through a detailed theoretical analysis of generalized thin flux tube equations, we show that the addition to the model of pressure-driven parallel dynamics, as well as temperature-dependent, anisotropic viscosity and thermal conductivity is essential for self-consistently producing gas-dynamic shocks inside reconnected tubes that heat and compress plasma to observed temperatures and densities. The shock thickness can be as long as the entire tube and heat can be conducted along tube's legs, possibly driving chromospheric evaporation. We developed a computer program that solves numerically the thin flux tube equations that govern the retraction of reconnected tubes. Simulations carried out with this program corroborate our theoretical predictions. A comparison of these simulations with fully three-dimensional magnetohydrodynamic simulations is presented to assess the validity of the thin flux tube model. We also present an observational method based on total emission measure and mean temperature to determine where in the current sheet a tube was reconnected.Item Magnetohydrodynamic shocks near rotating black holes(Montana State University - Bozeman, College of Letters & Science, 2003) Rilett, Darrell Jon; Chairperson, Graduate Committee: Sachiko Tsuruta; William Hiscock (co-chair)The theory of general relativistic magnetohydrodynamic standing shock formation is analyzed for accreting MHD plasma in a rotating, stationary, and axisymmetric black hole magnetosphere. All postshock physical quantities are expressed in terms of the relativistic compression ratio. The compression ratio is a solution of a seventh degree polynomial, incorporating the jump conditions, that is to be solved simultaneously with an equation for the polytropic index of the postshock plasma. Then the downstream state of the shocked plasma is determined entirely in terms of preshock quantities. Slow and fast magnetosonic shock solutions are analyzed for both equatorial and non-equatorial accretion flows. Shock categories for fast and slow shocks are developed, based on conserved quantities. These categories relate the initial conditions of a preshock flow to the spin of the black hole and can be used as a predictor of shock strength and location. We show that shocks may produce a hot region close to the horizon that could be applied to the generation mechanism of the iron fluorescence line from a Seyfert nucleus.