Browsing by Author "Gonnermann, Helge"

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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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    Impacts of biochar concentration and particle size on hydraulic conductivity and DOC leaching of biochar-sand mixtures
    (Elsevier, 2016) Liu, Zuolin; Dugan, Brandon; Masiello, Caroline A.; Barnes, Rebecca T.; Gallagher, Morgan E.; Gonnermann, Helge
    The amendment of soil with biochar can sequester carbon and alter hydrologic properties by changing physical and chemical characteristics of soil. To understand the effect of biochar amendment on soil hydrology, we measured the hydraulic conductivity (K) of biochar–sand mixtures as well as dissolved organic carbon (DOC) in leachate. Specifically, we assessed the effects of biochar concentration and particle size on K and amount of DOC in the soil leachate. To better understand how physical properties influenced K, we also measured the skeletal density of biochars and sand, and the bulk density, the water saturation, and the porosity of biochar–sand mixtures. Our model soil was sand (0.251–0.853 mm) with biochar rates from 2 to 10 wt% (g biochar/g total soil × 100%). As biochar (<0.853 mm) concentration increased from 0 to 10 wt%, K decreased by 72 ± 3%. When biochar particle size was equal to, greater than, and less than particle size of sand, we found that biochar in different particle sizes have different effects on K. For a 2 wt% biochar rate, K decreased by 72 ± 2% when biochar particles were finer than sand particles, and decreased by 15 ± 2% when biochar particles were coarser than sand particles. When biochar and sand particle size were comparable, we observed no significant effect on K. We propose that the decrease of K through the addition of fine biochar was because finer biochar particles filled spaces between sand particles, which increased tortuosity and reduced pore throat size of the mixture. The decrease of K associated with coarser biochar was caused by the bimodal particle size distribution, resulting in more compact packing and increased tortuosity. The loss of biochar C as DOC was related to both biochar rate and particle size. The cumulative DOC loss was 1350% higher from 10 wt% biochar compared to pure sand. This large increase reflected the very small DOC yield from pure sand. In addition, DOC in the leachate decreased as biochar particle size increased. For all treatments, the fraction of carbon lost as DOC ranged from 0.06 to 0.18 wt% of biochar. These experiments suggest that mixing sandy soils with biochar is likely to reduce infiltration rates, holding water near the surface longer with little loss of biochar-derived carbon to groundwater and streams.