Highly Efficient Modeling of Dynamic Coronal Loops

Observational and theoretical evidence suggests that coronal heating is impulsive and occurs on very small cross-field spatial scales. A single coronal loop could contain a hundred or more individual strands that are heated quasi-independently by nanoflares. It is therefore an enormous undertaking to model an entire active region or the global corona. Three-dimensional MHD codes have inadequate spatial resolution, and 1D hydro codes are too slow to simulate the many thousands of elemental strands that must be treated in a reasonable representation. Fortunately, thermal conduction and flows tend to smooth out plasma gradients along the magnetic field, so "0D models" are an acceptable alternative. We have developed a highly efficient model called Enthalpy-Based Thermal Evolution of Loops (EBTEL) that accurately describes the evolution of the average temperature, pressure, and density along a coronal strand. It improves significantly upon earlier models of this type--in accuracy, flexibility, and capability. It treats both slowly varying and highly impulsive coronal heating; it provides the differential emission measure distribution, DEM(T), at the transition region footpoints; and there are options for heat flux saturation and nonthermal electron beam heating. EBTEL gives excellent agreement with far more sophisticated 1D hydro simulations despite using four orders of magnitude less computing time. It promises to be a powerful new tool for solar and stellar studies.Comment: 34 pages, 8 figures, accepted by Astrophysical Journal (minor revisions of original submitted version

Hornworts of Australia: Three New Anthoceros L. (Anthocerotaceae) Species from New South Wales.

Three new species of Anthoceros sect. Fusiformes are described, all occurring on the North West Slopes and Plains of New South Wales and all sharing spore characteristics of a smooth unornamented strip either side of the triradiate mark on the proximal face. Illustrations and a distribution map are provided for all species, along with a key to Australian Anthocerotaceae

Homogenization of the Equations Governing the Flow Between a Slider and a Rough Spinning Disk

We have analyzed the behavior of the flow between a slider bearing and a hard-drive magnetic disk under two types of surface roughness. For both cases the length scale of the roughness along the surface is small as compared to the scale of the slider, so that a homogenization of the governing equations was performed. For the case of longitudinal roughness, we derived a one-dimensional lubrication-type equation for the leading behavior of the pressure in the direction parallel to the velocity of the disk. The coefficients of the equation are determined by solving linear elliptic equations on a domain bounded by the gap height in the vertical direction and the period of the roughness in the span-wise direction. For the case of transverse roughness the unsteady lubrication equations were reduced, following a multiple scale homogenization analysis, to a steady equation for the leading behavior of the pressure in the gap. The reduced equation involves certain averages of the gap height, but retains the same form of the usual steady, compressible lubrication equations. Numerical calculations were performed for both cases, and the solution for the case of transverse roughness was shown be in excellent agreement with a corresponding numerical calculation of the original unsteady equations

A new approach for modelling chromospheric evaporation in response to enhanced coronal heating : II. Non-uniform heating

This project has received funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1 and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No 647214).We proposed that the use of an approximate “jump condition” at the solar transition region permits fast and accurate numerical solutions of the one dimensional hydrodynamic equations when the corona undergoes impulsive heating. In particular, it eliminates the need for the very short timesteps imposed by a highly resolved numerical grid. This paper presents further examples of the applicability of the method for cases of non-uniform heating, in particular, nanoflare trains (uniform in space but non-uniform in time) and spatially localised impulsive heating, including at the loop apex and base of the transition region. In all cases the overall behaviour of the coronal density and temperature shows good agreement with a fully resolved one dimensional model and is significantly better than the equivalent results from a 1D code run without using the jump condition but with the same coarse grid. A detailed assessment of the errors introduced by the jump condition is presented showing that the causes of discrepancy with the fully resolved code are (i) the neglect of the terms corresponding to the rate of change of total energy in the unresolved atmosphere; (ii) mass motions at the base of the transition region and (iii) for some cases with footpoint heating, an over-estimation of the radiative losses in the transition region.PostprintPeer reviewe

Test Excavations at the Culebra Creek Site, 41BX126, Bexar County, Texas

Archaeological test excavations were undertaken at 4IBX126 on Culebra Creek to offset the impact from a proposed Texas Department of Transportation (TxDOT) highway improvement project on Loop 1604 in northwest Bexar County. Archaeological investigations were conducted in three field seasons: the first two seasons were conducted by TxDOT archaeologists and the third was directed by personnel from the Center for Archaeological Research (CAR) of The University of Texas at San Antonio. During the three projects, 55 hand-dug units, 29 backhoe trenches, 36 shovel tests, and eight Gradall trenches were excavated. Seventeen features were recorded; 25 radiocarbon assays were conducted; over 59,000 lithic artifacts were recovered and analyzed; 1,655 liters of sediment float samples were processed; 3,337 kg of burned rock were analyzed; and nearly 300 g of fatmal material and 25 archaeomagnetic samples were analyzed. The testing revealed utilization of the site in the Early, Middle, and Late Archaic periods. The analysis of materials and results of all three field efforts are presented in this single volume. Geoarchaeological investigations show that four terraces (TO, Tl, T2, and T3) in the immediate site area accumulated from the Late Pleistocene through the Holocene. Five Stratigraphic Units (I-V) make up these terraces and overlap one another. The T2 terrace is composed of Stratigraphic Units IT, ill, and IV, while the Tl terrace consists mostly of Stratigraphic Units IV and V. Archaeological materials were discovered in situ within the Tl and T3 terraces and primarily within Stratigraphic Units ill and IV. Radiocarbon assays indicate that Stratigraphic Unit IV formed between at least 4000-2000 B.P., Stratigraphic Unit ill accumulated between approximately 11,500-4000 B.P., and Stratigraphic Unit IT was accreting at least 17,500 years ago. Too little evidence exists to determine the full time ranges of sediment accumulation, and whether significant temporal gaps exist between the sedimentation of these geological units. Archaeological excavations focused on three separate areas: A, B, and C. Area A is a new right-of-way east of the existing right-of-way. Excavations in this area defined a Late Archaic Montell component dating to approximately 2700 B.P. These materials include two burned rock features in situ within Unit IV on the scarp of the T2 terrace. This area probably was occupied by a small residential group during the Late Archaic period. Area B is east of Loop 1604 in the existing right-of-way and on the T2 terrace. Area B contains a Middle Archaic Nolan component in the upper portion of Stratigraphic Unit ill, below a Late Archaic burned rock midden with a central subsurface oven in Unit IV. Area C is in the existing right-of-way west of Loop 1604. Excavations in this area investigated the possibility of an intact Early Archaic occupation; however, no evidence of one was found. In Area B, the Nolan component consisted of lithic artifacts scattered among small burned rock features that probably served as hearths. This component is radiocarbon dated to approximately 4600 B.P. The Late Archaic burned rock midden was apparently used between 4000 B.P. and 2000 B.P. Subfeatures within the central oven indicate multiple cooking events. Ethnographic evidence suggests earth ovens contained food wrapped with insulating material over a layer of hot rocks heated by a coal bed. This was capped with dirt to seal the oven. When cooking was complete, the earth cap is removed to reach the food. CAR conducted earth-oven hot-rock experiments which indicated that local limestone could be used once or at the most twice. Local hot-rock cooking should generate a great deal of burned limestone debris. The framework ofthe feature at 41BX126 represents the cap and rock heating-element dumpings from separate cooking events as well as a few small intact burned rock features that served as ovens or hearths. At the base of the midden were a few depressions that may represent borrow pits used to obtain sediment for the central oven cap. Mixing of temporally distinct artifacts from the Nolan and later occupations occurs in and beyond the midden due to sediment excavation and transportation across the site, and redeposition of materials through erosion of materials off the framework

Analysis and Modeling of Two Flare Loops Observed by AIA and EIS

We analyze and model an M1.0 flare observed by SDO/AIA and Hinode/EIS to investigate how flare loops are heated and evolve subsequently. The flare is composed of two distinctive loop systems observed in EUV images. The UV 1600 \AA emission at the feet of these loops exhibits a rapid rise, followed by enhanced emission in different EUV channels observed by AIA and EIS. Such behavior is indicative of impulsive energy deposit and the subsequent response in overlying coronal loops that evolve through different temperatures. Using the method we recently developed, we infer empirical heating functions from the rapid rise of the UV light curves for the two loop systems, respectively, treating them as two big loops of cross-sectional area 5\arcsec by 5\arcsec, and compute the plasma evolution in the loops using the EBTEL model (Klimchuk et al. 2008). We compute the synthetic EUV light curves, which, with the limitation of the model, reasonably agree with observed light curves obtained in multiple AIA channels and EIS lines: they show the same evolution trend and their magnitudes are comparable by within a factor of two. Furthermore, we also compare the computed mean enthalpy flow velocity with the Doppler shift measurements by EIS during the decay phase of the two loops. Our results suggest that the two different loops with different heating functions as inferred from their footpoint UV emission, combined with their different lengths as measured from imaging observations, give rise to different coronal plasma evolution patterns captured both in the model and observations.Comment: Accepted for publication in Ap

Coronal loop widths and pressure scale heights

The scale heights of stratification and the widths of steady solar coronal loops exhibit properties unexplained by standard theory: observed scale heights are often much greater than static theory predicts, while the nearly-constant widths of loop emission signatures defy theoretical expectations for large flux tubes in stratified media. In this work we relate the cross-sectional profile of a coronal flux tube to its density scale height in steady-state plasma flow regimes. Steady flows may shorten or lengthen the scale height according to how the tube cross-sectional area varies with arclength. In a near-potential corona the flux tubes are expected to be sufficiently expansive in many active regions for scale heights to be increased by steady flows. On the other hand, cases where scale lengths are actually increased to observed sizes form a small part of the solution space, close to regimes where density profiles reverse. Therefore, although steady flows are the only steady process known to be capable of extending scale heights significantly, they are not expected to be not responsible for the majority of extended active region scale heights