Analytical determination of coronal parameters using the period ratio P₁/2P₂

<p>Context. In transverse coronal loop oscillations, two periodicities have been measured simultaneously and are interpreted as the fundamental kink mode (with period P1) and the first harmonic (with period P2). Deviations of the period ratio P1/2P2 from unity provide information about the extent of longitudinal structuring within the loop.</p> <p>Aims. Here we develop an analytical approximation that describes the shift in P1/2P2 in terms of the ratio L/Λc of the length 2L of a coronal loop and the density scale height Λc.</p> <p>Methods. We study the MHD wave equations in a low β plasma using the thin tube approximation. Disturbances are described by a differential equation which may be solved for various equilibrium density profiles, obtaining dispersion relations in terms of Bessel functions. These dispersion relations may be used to obtain analytical approximations to the periods P1 and P2. We also present a variational approach to determining the period ratio and show how the WKB method may be used.</p> <p>Results. Analytical approximations to the period ratio P1/2P2 are used to shed light on the magnitude of longitudinal structuring in a loop, leading to a determination of the density scale height. We apply our formula to the observations in Verwichte et al. (2004) and Van Doorsselaere et al. (2007), obtaining the coronal density scale height.</p> <p>Conclusions. Our simple formula and approximate approaches highlight a useful analytical tool for coronal seismology. We demonstrate that P1/2P2 is linked to the density scale height, with no need for estimates of other external parameters. Given the accuracy of current observations, our formula provides a convenient means of determining density scale heights.</p&gt

Comparison of Kinetic Models for Gas Damping of Moving Microbeams

Numerical investigations of the gas flow structure and the gas-damping force on moving and heated microbeams are carried out using the Navier-Stokes equations with first-order velocity-slip and temperature-jump boundary conditions (the NSSJ method) and two kinetic numerical techniques: the particle-based direct simulation Monte Carlo (DSMC) method, and a deterministic discrete-ordinate solution of the ellipsoidal statistical (ES) kinetic model equation. The gas-damping coefficients on a moving microbeam for quasi-static isothermal conditions are estimated by the three numerical methods for Kn = 0.1-1.0. The NSSJ simulations tend to overestimate the gas-damping coefficient for Knudsen numbers larger than 0.1, whereas the DSMC and ES kinetic approaches are in good agreement for the slip and transitional flow regimes. The flow structure and the Knudsen force are calculated using the ES kinetic model for a heated microbeam over a wide range of Knudsen numbers. The Knudsen force peaks in the transitional regime (Kn ≈ 2), and the numerically predicted variation of the force with Knudsen number is consistent with experimentally observed displacements of the heated microbeam

Kink oscillations of cooling coronal loops with variable cross-section

We study kink waves and oscillations in a thin expanding magnetic tube in the presence of flow. The tube consists of a core region and a thin transitional region at the tube boundary. In this region the plasma density monotonically decreases from its value in the core region to the value outside the tube. Both the plasma density and velocity of background flow vary along the tube and in time. Using the multiscale expansions we derive the system of two equations describing the kink oscillations. When there is no transitional layer the oscillations are described by the first of these two equations. We use this equation to study the effect of plasma density variation with time on kink oscillations of an expanding tube with a sharp boundary. We assume that the characteristic time of the density variation is much greater than the characteristic time of kink oscillations. Then we use the Wentzel-Kramer-Brillouin (WKB) method to derive the expression for the adiabatic invariant, which is the quantity that is conserved when the plasma density varies. The general theoretical results are applied to the kink oscillations of coronal magnetic loops. We consider an expanding loop with the half-circle shape and assume that the plasma temperature inside a loop decays exponentially with time. We numerically calculated the dependences of the fundamental mode frequency, the ratio of frequencies of the first overtone and fundamental mode, and the oscillation amplitude on time. We obtained that the oscillation frequency and amplitude increase and the frequency ratio decreases due to cooling. The amplitude increase is stronger for loops with a greater expansion factor. This effect is also more pronounced for higher loops. However, it is fairly moderate even for loops that are quite high

On the period ratio P₁/2P₂ in the oscillations of coronal loops

<p>Aims. With strong evidence of fast and slow magnetoacoustic modes arising in the solar atmosphere there is scope for improved determinations of coronal parameters through coronal seismology. Of particular interest is the ratio P<sub>1</sub>/2P<sub>2</sub>between the period P<sub>1</sub> of the fundamental mode and the period P<sub>2</sub> of its first harmonic; in an homogeneous medium this ratio is one, but in a more complex configuration it is shifted to lower values.</p> <p>Methods. We consider analytically the effects on the different magnetohydrodynamic modes of structuring and stratification, pointing out that transverse or longitudinal structuring or gravitational stratification modifies the ratio P<sub>1</sub>/2P<sub>2</sub>.</p> <p>Results. The deviations caused by gravity and structure are studied for the fast and slow modes. Structure along the loop is found to be the dominant effect.</p> <p>Conclusions. The departure of P<sub>1</sub>/2P<sub>2</sub> from unity can be used as a seismological tool in the corona. We apply our technique to the observations by Verwichte et al. (2004), deducing the density scale height in a coronal loop.</p&gt

Kelvin-Helmholtz instability in partially ionized compressible plasmas

The Kelvin-Helmholtz Instability (KHI) has been observed in the solar atmosphere. Ion-neutral collisions may play a relevant role for the growth rate and evolution of the KHI in solar partially ionized plasmas as in, e.g., solar prominences. Here, we investigate the linear phase of the KHI at an interface between two partially ionized magnetized plasmas in the presence of a shear flow. The effects of ion-neutral collisions and compressibility are included in the analysis. We obtain the dispersion relation of the linear modes and perform parametric studies of the unstable solutions. We find that in the incompressible case the KHI is present for any velocity shear regardless the value of the collision frequency. In the compressible case, the domain of instability depends strongly on the plasma parameters, specially the collision frequency and the density contrast. For high collision frequencies and low density contrasts the KHI is present for super-Alfvenic velocity shear only. For high density contrasts the threshold velocity shear can be reduced to sub-Alfvenic values. For the particular case of turbulent plumes in prominences, we conclude that sub-Alfvenic flow velocities can trigger the KHI thanks to the ion-neutral coupling.Comment: Accepted for publication in Ap

Борис Гесселевич (Григорьевич) Галкович как историк-картограф

Освещаются этапы жизни и творчества Б.Г. Галковича — автора или редактора около 700 исторических карт, в т. ч. в ряде атласов, в 13 томах «Всемирной истории», в 7 томах «Истории СССР с древнейших времён до наших дней» и др. Основы методологии исторической картографии изложены Б.Г. Галковичем на международных конференциях и в ряде статей. В приложении — список текстовых и картографических работ Б.Г. Галковича, опубликованных в 1950–1983 гг.The publication sheds light on the life and work of B.H. Halkovych, the author and editor of nearly 700 historical maps in atlases, 13 volumes of "The World History", 7 volumes of "The History of the USSR since Earliest Times up Today" etc. The methodological principles of historical cartography have been set out by B.H. Halkovych at international conferences and in numerous articles. The supplements contain a list of B.H. Halkovych’s text and cartographical works (published in 1950–1983)

Shed urinary ALCAM is an independent prognostic biomarker of three-year overall survival after cystectomy in patients with bladder cancer.

Proteins involved in tumor cell migration can potentially serve as markers of invasive disease. Activated Leukocyte Cell Adhesion Molecule (ALCAM) promotes adhesion, while shedding of its extracellular domain is associated with migration. We hypothesized that shed ALCAM in biofluids could be predictive of progressive disease. ALCAM expression in tumor (n = 198) and shedding in biofluids (n = 120) were measured in two separate VUMC bladder cancer cystectomy cohorts by immunofluorescence and enzyme-linked immunosorbent assay, respectively. The primary outcome measure was accuracy of predicting 3-year overall survival (OS) with shed ALCAM compared to standard clinical indicators alone, assessed by multivariable Cox regression and concordance-indices. Validation was performed by internal bootstrap, a cohort from a second institution (n = 64), and treatment of missing data with multiple-imputation. While ALCAM mRNA expression was unchanged, histological detection of ALCAM decreased with increasing stage (P = 0.004). Importantly, urine ALCAM was elevated 17.0-fold (P < 0.0001) above non-cancer controls, correlated positively with tumor stage (P = 0.018), was an independent predictor of OS after adjusting for age, tumor stage, lymph-node status, and hematuria (HR, 1.46; 95% CI, 1.03-2.06; P = 0.002), and improved prediction of OS by 3.3% (concordance-index, 78.5% vs. 75.2%). Urine ALCAM remained an independent predictor of OS after accounting for treatment with Bacillus Calmette-Guerin, carcinoma in situ, lymph-node dissection, lymphovascular invasion, urine creatinine, and adjuvant chemotherapy (HR, 1.10; 95% CI, 1.02-1.19; P = 0.011). In conclusion, shed ALCAM may be a novel prognostic biomarker in bladder cancer, although prospective validation studies are warranted. These findings demonstrate that markers reporting on cell motility can act as prognostic indicators

Relativistically rotating dust

Dust configurations play an important role in astrophysics and are the simplest models for rotating bodies. The physical properties of the general--relativistic global solution for the rigidly rotating disk of dust, which has been found recently as the solution of a boundary value problem, are discussed.Comment: 18 pages, 11 figure

Inversion of physical parameters in solar atmospheric seismology

Magnetohydrodynamic (MHD) wave activity is ubiquitous in the solar atmosphere. MHD seismology aims to determine difficult to measure physical parameters in solar atmospheric magnetic and plasma structures by a combination of observed and theoretical properties of MHD waves and oscillations. This technique, similar to seismology or helio-seismology, demands the solution of two problems. The direct problem involves the computation of wave properties of given theoretical models. The inverse problem implies the calculation of unknown physical parameters, by means of a comparison of observed and theoretical wave properties. Solar atmospheric seismology has been successfully applied to different structures such as coronal loops, prominence fine structures, spicules, or jets. However, it is still in its infancy. Far more is there to come. We present an overview of recent results, with particular emphasis in the inversion procedure.Comment: 10 pages, 3 figures, review paper to appear in Astrophysics and Space Science Proceeding

Instrumental oscillations in RHESSI count rates during solar flares

Aims: We seek to illustrate the analysis problems posed by RHESSI spacecraft motion by studying persistent instrumental oscillations found in the lightcurves measured by RHESSI's X-ray detectors in the 6-12 keV and 12-25 keV energy range during the decay phase of the flares of 2004 November 4 and 6. Methods: The various motions of the RHESSI spacecraft which may contribute to the manifestation of oscillations are studied. The response of each detector in turn is also investigated. Results: We find that on 2004 November 6 the observed oscillations correspond to the nutation period of the RHESSI instrument. These oscillations are also of greatest amplitude for detector 5, while in the lightcurves of many other detectors the oscillations are small or undetectable. We also find that the variation in detector pointing is much larger during this flare than the counterexample of 2004 November 4. Conclusions: Sufficiently large nutation motions of the RHESSI spacecraft lead to clearly observable oscillations in count rates, posing a significant hazard for data analysis. This issue is particularly problematic for detector 5 due to its design characteristics. Dynamic correction of the RHESSI counts, accounting for the livetime, data gaps, and the transmission of the bi-grid collimator of each detector, is required to overcome this issue. These corrections should be applied to all future oscillation studies.Comment: 8 pages, 10 figure