Stellar Activity and Coronal Heating: an overview of recent results

Observations of the coronae of the Sun and of solar-like stars provide complementary information to advance our understanding of stellar magnetic activity, and of the processes leading to the heating of their outer atmospheres. While solar observations allow us to study the corona at high spatial and temporal resolution, the study of stellar coronae allows us to probe stellar activity over a wide range of ages and stellar parameters. Stellar studies therefore provide us with additional tools for understanding coronal heating processes, as well as the long-term evolution of solar X-ray activity. We discuss how recent studies of stellar magnetic fields and coronae contribute to our understanding of the phenomenon of activity and coronal heating in late-type stars.Comment: Accepted for publication on Philosophical Transactions A. 29 pages, 5 figure

DISSOLVED OXYGEN AND NUTRIENT CYCLING IN CHESAPEAKE BAY: AN EXAMINATION OF CONTROLS AND BIOGEOCHEMICAL IMPACTS USING RETROSPECTIVE ANALYSIS AND NUMERICAL MODELS

Hypoxia, or the condition of low dissolved oxygen levels, is a topic of interest throughout aquatic ecology. Hypoxia has both realized and potential impacts on biogeochemical cycles and many invertebrate and vertebrate animal populations; the majority of the impacts being negative. It is apparent that the extent and occurrence of hypoxic conditions has been on the rise globally, despite a handful of reductions due to management success stories. Efforts to curb the development of hypoxia are well underway in many aquatic ecosystems worldwide, where oxygen levels are a key target for water quality management. Long-term increases in the volume of seasonal bottom-water hypoxia have been observed in Chesapeake Bay. Although there is evidence for the occurrence of low oxygen conditions following initial European habitation of the Chesapeake watershed, as well as direct observations of anoxia prior to the mid 20th century large-scale nutrient load increases, it is clear that hypoxic volume has increased over the last 50 years. Surprisingly, the volume of hypoxia observed for a given nutrient load has doubled since the mid-1980s, suggesting the importance of hypoxia controls beyond nutrient loading alone. I conducted a suite of retrospective data analyses and numerical modeling studies to understand the controls on and consequences of hypoxia in Chesapeake Bay over multiple time and space scales. The doubling of hypoxia per unit TN load was associated with an increase in bottom-water inorganic nitrogen and phosphorus concentrations, suggesting the potential for a positive feedback, where hypoxia-induced increases in N and P recycling support higher summer algal production and subsequent O2 consumption. I applied a two-layer sediment flux model at several stations in Chesapeake Bay, which revealed that hypoxic conditions substantially reduce coupled nitrification-denitrification and phosphorus sorption to iron oxyhydroxides, leading to the elevated sediment-water N and P fluxes that drive this feedback. An analysis of O2 dynamics during the winter-spring indicate that the day of hypoxia onset and the rate of March-May water-column O2 depletion are most strongly correlated to chlorophyll-a concentrations in bottom water; this suggests that the spring bloom drives early season O2 depletion. Metrics of winter-spring O2 depletion were un-correlated with summer hypoxic volumes, however, suggesting that other controls (including physical forcing and summer algal production) are important. I used a coupled hydrodynamic-biogeochemical model for Chesapeake Bay to quantify the extent to which summer algal production is necessary to maintain hypoxia throughout the summer, and that nutrient load-induced increases in hypoxia are driven by elevated summer respiration in the water-column of lower-Bay regions

Isolated Traumatic Expressive Aphasia

[West J Emerg Med. 2011;12(1):141.

Factors Regulating Variability in Water Quality and Net Biogeochemical Fluxes in the Patuxent River Estuary

Net biogeochemical production and transport rates for several variables were computed for the Patuxent River estuary from 1985 to 2003 using a box model. Monthly rate estimates were analyzed for temporal patterns and variability in response to climatic factors and nutrient management. The middle estuary was the most productive estuarine region and was characterized by strong pelagic-benthic coupling. Phytoplankton biomass in this region peaked in spring as fueled by seaward nutrient inputs. Nutrients regenerated from decomposition of this spring bloom were required to support summer productivity. Improvements of sewage treatment in the watershed resulted in declining point source nutrient loads to the estuary, but water quality did not improve in the mesohaline estuary. Poor water quality in the middle estuary was maintained by persistent non-point nutrient loads, while degrading water quality in the lower estuary correlated with increasing DIN inputs from Chesapeake Bay, high river flow, and declining herbivorous grazing

X-ray Flares of EV Lac: Statistics, Spectra, Diagnostics

We study the spectral and temporal behavior of X-ray flares from the active M-dwarf EV Lac in 200 ks of exposure with the Chandra/HETGS. We derive flare parameters by fitting an empirical function which characterizes the amplitude, shape, and scale. The flares range from very short (<1 ks) to long (10 ks) duration events with a range of shapes and amplitudes for all durations. We extract spectra for composite flares to study their mean evolution and to compare flares of different lengths. Evolution of spectral features in the density-temperature plane shows probable sustained heating. The short flares are significantly hotter than the longer flares. We determined an upper limit to the Fe K fluorescent flux, the best fit value being close to what is expected for compact loops.Comment: 9 pages; 9 figures; latex/emulateapj style; Submitted to The Astrophysical Journa

Development of a submerged aquatic vegetation growth model in the coupled ocean-atmosphere-wave-sediment transport (COAWST v3.4) model

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Kalra, T. S., Ganju, N. K., & Testa, J. M. Development of a submerged aquatic vegetation growth model in the coupled ocean-atmosphere-wave-sediment transport (COAWST v3.4) model. Geoscientific Model Development, 13(11), (2020): 5211-5228, doi:10.5194/gmd-13-5211-2020.The coupled biophysical interactions between submerged aquatic vegetation (SAV), hydrodynamics (currents and waves), sediment dynamics, and nutrient cycling have long been of interest in estuarine environments. Recent observational studies have addressed feedbacks between SAV meadows and their role in modifying current velocity, sedimentation, and nutrient cycling. To represent these dynamic processes in a numerical model, the presence of SAV and its effect on hydrodynamics (currents and waves) and sediment dynamics was incorporated into the open-source Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) model. In this study, we extend the COAWST modeling framework to account for dynamic changes of SAV and associated epiphyte biomass. Modeled SAV biomass is represented as a function of temperature, light, and nutrient availability. The modeled SAV community exchanges nutrients, detritus, dissolved inorganic carbon, and dissolved oxygen with the water-column biogeochemistry model. The dynamic simulation of SAV biomass allows the plants to both respond to and cause changes in the water column and sediment bed properties, hydrodynamics, and sediment transport (i.e., a two-way feedback). We demonstrate the behavior of these modeled processes through application to an idealized domain and then apply the model to a eutrophic harbor where SAV dieback is a result of anthropogenic nitrate loading and eutrophication. These cases demonstrate an advance in the deterministic modeling of coupled biophysical processes and will further our understanding of future ecosystem change.This is University of Maryland Center for Environmental Contribution no. 5909

X-ray Diagnostics of Grain Depletion in Matter Accreting onto T Tauri Stars

Recent analysis of high resolution Chandra X-ray spectra has shown that the Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this result, we point out the utility of the Ne/O ratio as a discriminant for accretion-related X-rays from T Tauri stars, and for probing the measure of grain-depletion of the accreting material in the inner disk. We apply the Ne/O diagnostic to the classical T Tauri stars BP Tau and TW Hya--the two stars found to date whose X-ray emission appears to originate, at least in part, from accretion activity. We show that TW Hya appears to be accreting material which is significantly depleted in O relative to Ne. In constrast, BP Tau has an Ne/O abundance ratio consistent with that observed for post-T Tauri stars. We interpret this result in terms of the different ages and evolutionary states of the circumstellar disks of these stars. In the young BP Tau disk (age 0.6 Myr) dust is still present near the disk corotation radius and can be ionized and accreted, re-releasing elements depleted onto grains. In the more evolved TW Hya disk (age 10 Myr), evidence points to ongoing coagulation of grains into much larger bodies, and possibly planets, that can resist the drag of inward-migrating gas, and accreting gas is consequently depleted of grain-forming elements.Comment: 13 pages, 1 Figure, ApJ Letters, in pres

A Spatiotemporal Synthesis of High-Resolution Salinity Data with Aquaculture Applications

Technological advancement and the desire to better monitor shallow habitats in the Chesapeake Bay, Maryland, United States led to the initiation of several high-resolution monitoring programs such as ConMon (short for “Continuous Monitoring”) measuring oxygen, salinity, and chlorophyll-a at a 15-minute frequency. These monitoring efforts have yielded an enormous volume of data and insight into the condition of the tidal water of the Bay. But this information is underutilized in documenting the fine-scale variability of water quality, which is critical in identifying the link between water quality and ecological responses, partly due to the challenges in integrating monitoring data collected at different frequencies and locations. In a project to understand the environmental suitability of aquaculture sites and the future potential overlap between aquaculture and submerged aquatic vegetation, we developed a spatiotemporal synthesis of ConMon data with data from long-term, fixed-station seasonal monitoring. Here, we present our generalized additive model-based approach to predict salinity at high frequency (15 minutes) and fine spatial resolution (~100 meters) in the Maryland portion of the Bay, its major tributaries, and the shallow tidal creeks that exchange with the tributaries. Predictive performance was validated to be 1 PSU (practical salinity unit) in root mean square error using de novo monitoring. The resulting data provide insights into the environmental suitability of aquaculture, specifically the sensitivity of the Easter oyster (Crassostrea virginica) to low salinity stress. The spatiotemporal synthesis approach has potential applications for integrated monitoring and potential linkage with high-resolution water quality models for shallow habitats

Testing EUV/X-ray Atomic Data for the Solar Dynamics Observatory

The Atmospheric Imaging Assembly (AIA) and the Exteme-ultraviolet Variability Experiment (EVE) onboard the Solar Dynamics Observatory include spectral windows in the X-ray/EUV band. Accuracy and completeness of the atomic data in this wavelength range is essential for interpretation of the spectrum and irradiance of the solar corona, and of SDO observations made with the AIA and EVE instruments. Here we test the X-ray/EUV data in the CHIANTI database to assess their completeness and accuracy in the SDO bands, with particular focus on the 94A and 131A AIA passbands. Given the paucity of solar observations adequate for this purpose, we use high-resolution X-ray spectra of the low-activity solar-like corona of Procyon obtained with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). We find that while spectral models overall can reproduce quite well the observed spectra in the soft X-ray range ll 130A, they significantly underestimate the observed flux in the 50-130A wavelength range. The model underestimates the observed flux by a variable factor ranging from \approx 1.5, at short wavelengths below \sim50A, up to \approx5-7 in the \sim 70-125A range. In the AIA bands covered by LETGS, i.e. 94A and 131A, we find that the observed flux can be underestimated by large factors (\sim 3 and \sim 1.9 respectively, for the case of Procyon presented here). We discuss the consequences for analysis of AIA data and possible empirical corrections to the AIA responses to model more realistically the coronal emission in these passbands.Comment: 11 pages, 9 figures, accepted for publication on Ap

Spatiotemporal variability of light attenuation and net ecosystem metabolism in a back-barrier estuary

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ganju, N. K., Testa, J. M., Suttles, S. E., & Aretxabaleta, A. L. Spatiotemporal variability of light attenuation and net ecosystem metabolism in a back-barrier estuary. Ocean Science, 16(3), (2020): 593-614, doi:10.5194/os-16-593-2020.Quantifying system-wide biogeochemical dynamics and ecosystem metabolism in estuaries is often attempted using a long-term continuous record at a single site or short-term records at multiple sites due to sampling limitations that preclude long-term monitoring. However, differences in the dominant primary producer at a given location (e.g., phytoplankton versus benthic producers) control diel variations in dissolved oxygen and associated ecosystem metabolism, and they may confound metabolic estimates that do not account for this variability. We hypothesize that even in shallow, well-mixed estuaries there is strong spatiotemporal variability in ecosystem metabolism due to benthic and water-column properties, as well as ensuing feedbacks to sediment resuspension, light attenuation, and primary production. We tested this hypothesis by measuring hydrodynamic properties, biogeochemical variables (fluorescent dissolved organic matter – fDOM, turbidity, chlorophyll a fluorescence, dissolved oxygen), and photosynthetically active radiation (PAR) over 1 year at 15 min intervals at paired channel (unvegetated) and shoal (vegetated by eelgrass) sites in Chincoteague Bay, Maryland–Virginia, USA, a shallow back-barrier estuary. Light attenuation (KdPAR) at all sites was dominated by turbidity from suspended sediment, with lower contributions from fDOM and chlorophyll a. However, there was significant seasonal variability in the resuspension–shear stress relationship on the vegetated shoals, but not in adjacent unvegetated channels. This indicated that KdPAR on the shoals was mediated by submerged aquatic vegetation (SAV) and possibly microphytobenthos presence in the summer, which reduced resuspension and therefore KdPAR. We also found that gross primary production (Pg) and KdPAR were significantly negatively correlated on the shoals and uncorrelated in the channels, indicating that Pg over the vegetated shoals is controlled by a feedback loop between benthic stabilization by SAV and/or microphytobenthos, sediment resuspension, and light availability. Metabolic estimates indicated substantial differences in net ecosystem metabolism between vegetated and unvegetated sites, with the former tending towards net autotrophy in the summer. Ongoing trends of SAV loss in this and other back-barrier estuaries suggest that these systems may also shift towards net heterotrophy, reducing their effectiveness as long-term carbon sinks. With regards to temporal variability, we found that varying sampling frequency between 15 min and 1 d resulted in comparable mean values of biogeochemical variables, but extreme values were missed by daily sampling. In fact, daily resampling minimized the variability between sites and falsely suggested spatial homogeneity in biogeochemistry, emphasizing the need for high-frequency sampling. This study confirms that properly quantifying ecosystem metabolism and associated biogeochemical variability requires characterization of the diverse estuarine environments, even in well-mixed systems, and demonstrates the deficiencies introduced by infrequent sampling to the interpretation of spatial variability.This study was funded by the USGS Coastal and Marine Geology Program and the Department of the Interior Hurricane Sandy Recovery program (GS2-2D)