We present low-frequency (80 – 240 MHz) radio observations of coronal holes (CHs) made with the
Murchison Widefield Array
(MWA). CHs are expected to be dark structures relative to the background ...corona across the MWA bandwidth due to their low densities. However, we observe that multiple CHs near disk center transition from being dark structures at higher frequencies to bright structures at lower frequencies (
≲
145
MHz
). We compare our observations to synthetic images obtained using the software suite FORWARD, in combination with the magnetohydrodynamic algorithm outside a sphere (MAS) model of the global coronal magnetic field, density, and temperature structure. The synthetic images do not exhibit this transition, and we quantify the discrepancy as a function of frequency. We propose that the dark-to-bright transition results from refraction of radio waves into the low-density CH regions, and we develop a qualitative model based on this idea and the relative optical depths inside and outside a CH as a function of frequency. We show that opacity estimates based on the MAS model are qualitatively consistent with our interpretation, and we conclude that propagation and relative absorption effects are a viable explanation for the dark-to-bright transition of CHs from high to low frequencies.
We present an analysis of extreme-ultraviolet and soft X-ray emission detected toward Comet Lovejoy (C/2011 W3) during its post-perihelion traverse of the solar corona on 2011 December 16. ...Observations were recorded by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory and the X-Ray Telescope (XRT) aboard Hinode. A single set of contemporaneous images is explored in detail, along with prefatory consideration for time evolution using only the 171 Angstrom data. For each of the eight passbands, we characterize the emission and derive outgassing rates where applicable. As material sublimates from the nucleus and is immersed in coronal plasma, it rapidly ionizes through charge states seldom seen in this environment. The AIA data show four stages of oxygen ionization (O III-O VI) along with C IV, while XRT likely captured emission from O VII, a line typical of the corona. With a nucleus of at least several hundred meters upon approach to a perihelion that brought the comet to within 0.2 R sub(middot in circle) of the photosphere, Lovejoy was the most significant sungrazer in recent history. Correspondingly high outgassing rates on the order of 10 super(32.5) oxygen atoms per second are estimated. Assuming that the neutral oxygen comes from water, this translates to a mass-loss rate of ~9.5 x 10 super(9) g s super(-1), and based only on the 171 middot in circle observations, we find a total mass loss of ~10 super(13) g over the AIA egress. Additional and supporting analyses include a differential emission measure to characterize the coronal environment, consideration for the opening angle, and a comparison of the emission's leading edge with the expected position of the nucleus.
Abstract
Genomic maps of DNA G-quadruplexes (G4s) can help elucidate the roles that these secondary structures play in various organisms. Herein, we employ an improved version of a G-quadruplex ...sequencing method (G4-seq) to generate whole genome G4 maps for 12 species that include widely studied model organisms and also pathogens of clinical relevance. We identify G4 structures that form under physiological K+ conditions and also G4s that are stabilized by the G4-targeting small molecule pyridostatin (PDS). We discuss the various structural features of the experimentally observed G-quadruplexes (OQs), highlighting differences in their prevalence and enrichment across species. Our study describes diversity in sequence composition and genomic location for the OQs in the different species and reveals that the enrichment of OQs in gene promoters is particular to mammals such as mouse and human, among the species studied. The multi-species maps have been made publicly available as a resource to the research community. The maps can serve as blueprints for biological experiments in those model organisms, where G4 structures may play a role.
Despite the frequent coexistence of shear banding and wall slip in flowing wormlike micelle (WLM) solutions, the influence of wall slip on the spatiotemporal evolution of the flow field during shear ...banding has not been systematically explored. Here, shear-banded cylindrical Couette flows of WLMs described by the Germann–Cook–Beris model are simulated with wall slip. Two simple slip conditions – one proportional to shear rate and the other proportional to shear stress – motivated by datasets quantifying wall slip with shear banding are considered. In steady shear, introducing wall slip of either form delays the onset of shear banding and reduces the width of the high-shear-rate band, consistent with experiments. During shear startup, wall slip dampens the magnitude of the apparent shear rate, which extends the time for shear-banded flow to develop. The complex evolution of the flow field during shear-band formation produces large variations of the slip velocity, which enhance or diminish the degree of elastic recoil and flow reversal depending on the slip condition. This work demonstrates that the incorporation of wall slip into shear-banding simulations via simple slip boundary conditions can improve the qualitative agreement between models of shear banding in WLMs and experiments.
•Shear-banded flows of WLMs described by the GCB model are simulated with wall slip.•Wall slip is incorporated using two phenomenological slip boundary conditions.•Wall slip impacts the flow curve and certain shear-band characteristics.•Flow reversal during shear-band formation depends on the type of slip condition.
Wormlike micelles (WLMs) are elongated, self-assembled structures formed from amphiphilic molecules in solution. Although the structure of WLMs resembles that of polymers, WLMs are differentiated by ...their ability to break and recombine at rest and in response to deformations. This unique property has led to their ubiquitous use in a variety of applications such as consumer products and oilfield recovery. Additionally, entangled WLMs exhibit unique nonlinear flow behavior such as the formation of shear bands and other flow heterogeneities, which have garnered considerable scientific interest. While often formulated using small molecule ionic surfactants, WLMs can also form in solutions of amphiphilic block polymers. The most well-studied of these are poloxamers, ABA block polymers formed from two polyethylene oxide (PEO) end blocks and one polypropylene oxide (PPO) midblock. Poloxamers have the potential to form WLMs with a range of rheological properties due to the tunability block composition. The self-assembly of poloxamers into spherical micelles is well characterized, but the formation of poloxamer WLMs is poorly understood, leaving this class of WLMs underutilized.The first goal of this thesis is to formulate guidelines for varying poloxamer composition, temperature, salt type, and salt concentration to induce the formation of WLMs and tune their rheological properties. Small-angle neutron scattering, light transmittance measurements, and linear and nonlinear rheology were performed to characterize the temperature-induced rod formation, local micelle structure, and bulk mechanical properties of a large variety of poloxamer WLM formulations. This characterization revealed that the local microstructure of poloxamer WLMs is fairly insensitive to the poloxamer block composition, molecular weight, and the presence of salts, but the rheological properties varied greatly among formulations. Higher viscosity solutions were produced in poloxamers with higher molecular weights, lower PEO content, and added sodium chloride.Leveraging the insights from this self-assembly characterization, the second goal of this thesis is to study the nonlinear flow behavior of a highly elastic, high-viscosity WLM solution formed using poloxamers. These WLMs exhibited rheological behaviors typically observed in gels, including a yield stress and viscoelastic aging. Combined shear rheology and particle tracking velocimetry (rheo-PTV) measurements revealed this solution of WLMs formed shear bands in startup flows, accompanied by elastic instabilities and wall slip. The mechanism of shear-band formation was unlike any WLMs studied previously and more closely resembled the mechanism in yield stress fluids. Exploring the evolution of shear bands in this WLM gel with rheology similar to both canonical viscoelastic WLMs and yield stress fluids provided new insights on shear banding in both of these complex fluids. Shear bands in poloxamer WLMs are frequently accompanied by wall slip, which is also prevalent in nonlinear flow studies of many other WLMs. The third goal of this thesis is to explore the impact of wall slip on the spatiotemporal evolution of the flow field during shear banding. Cylindrical Couette flows of WLMs were simulated with the German-Cook-Beris (GCB) model and two phenomenological slip boundary conditions. Introducing wall slip was shown to delay the onset of shear banding and reduce the width of the high-shear-rate band, consistent with experiments. During shear band formation, the evolution of the flow field was sensitive to the form of the slip boundary condition; flow reversal prior to shear-band formation was enhanced with shear-rate-dependent wall slip and diminished with shear-stress-dependent wall slip. These results demonstrated that the qualitative agreement between shear-banding models of WLMs and experiments can be improved by incorporating wall slip into shear-banding simulations.The final goal of this thesis is to re-examine the peculiar shear-band formation in poloxamer WLM gels and verify the proposed yield-stress-driven shear banding mechanism. A new technique was developed to characterize flow heterogeneity that combines cessation of flow protocols with rheo-PTV. To successfully implement this technique, new methods to analyze rheo-PTV data were developed, which improved the calculation of the velocity by fitting entire particle trajectories described in cylindrical coordinates. Before performing experiments, theoretical flow problems were analyzed to demonstrate that fluid retraction accompanies stress relaxation in viscoelastic fluids with flow heterogeneity. The proposed evolution of flow heterogeneity in poloxamer WLM gels was confirmed by measuring fluid retraction in cessation of flow. This technique also revealed another poloxamer WLM formulation developed shear bands via the canonical shear-band formation mechanism. These findings demonstrate the utility of cessation of flow combined with PTV to characterize flow heterogeneity in viscoelastic complex fluids.Overall, this thesis combines computational and experimental approaches to gain a deeper understanding of the self-assembly and nonlinear flow behavior of WLMs formed from poloxamers. Fundamental insights are also uncovered about nonionic surfactant self-assembly, wall slip, shear banding, and flow heterogeneity in complex fluids in general. The unique rheology and highly tunable self-assembly of poloxamer WLMs make these solutions ideal candidates for future investigations about shear banding.
DYNAMICS OF THE TRANSITION CORONA Masson, Sophie; McCauley, Patrick; Golub, Leon ...
The Astrophysical journal,
06/2014, Letnik:
787, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Using a newly developed technique, we enhance the off-limb magnetic fine structures observed with the Atmospheric Imaging Assembly and identify a PS-like feature located close to the northern coronal ...hole. We first identify that the magnetic topology associated with the observation is a PS, null-point (NP) related topology bounded by the open field. By comparing the magnetic field configuration with the EUV emission regions, we determined that most of the magnetic flux associated with plasma emission are small loops below the PS basic NP and open field bounding the PS topology. The observed PS fine structures follow the dynamics of the magnetic field before and after reconnecting at the NP obtained by the interchange model. These morphological and dynamical similarities between the PS observations and the results from the simulation strongly suggest that the evolution of the PS, and in particular the opening/closing of the field, occurs via interchange/slipping reconnection at the basic NP of the PS.
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