645 research outputs found
New Directions for Public Speaking: The Perfect Pendulum Swings
The author will present her personal and frank ideas of what she sees as a dangerous trend in public speaking events. A loose analogy of forensics to a swinging pendulum will first be explained. Then the author will explain what she sees as the current state of public speaking events, and finally, an attempt to predict some outcomes for the future of public speaking events and some suggestions for our consideration. The aim of this paper is to make us think: where are we in this quest for excellence in public speaking events? How did we get to where we are and where do we go from here
Multiple scattering of waves by a pair of gravitationally stratified flux tubes
We study the near-field coupling of a pair of flux tubes embedded in a
gravitationally stratified environment. The mutual induction of the near-field
{\it jackets} of the two flux tubes can considerably alter the scattering
properties of the system, resulting in sizable changes in the magnitudes of
scattering coefficients and bizarre trends in the phases. The dominant length
scale governing the induction zone turns out to be approximately half the
horizontal wave length of the incident mode, a result that fits in quite
pleasantly with extant theories of scattering. Higher- flux tubes are
more strongly coupled than weaker ones, a consequence of the greater role that
the near-field jacket modes play in the such tubes. We also comment on the
importance of incorporating the effects of multiple scattering when studying
the effects of mode absorption in plage and interpreting related scattering
measurements. That the near-field plays such an important role in the
scattering process lends encouragement to the eventual goal of observationally
resolving sub-wavelength features of flux tubes using techniques of
helioseismology.Comment: ApJ, submitted, 15 pages 7 figure
A Novel Approach to Resonant Absorption of the Fast MHD Eigenmodes of a Coronal Arcade
The arched eld lines forming coronal arcades are often observed to undulate as magne- tohydrodynamic (MHD) waves propagate both across and along the magnetic eld. These waves are most likely a combination of resonantly coupled fast magnetoacoustic waves and Alfv\' en waves. The coupling results in resonant absorption of the fast waves, converting fast wave energy into Alfv\' en waves. The fast eigenmodes of the arcade have proven difficult to compute or derive analytically, largely because of the mathematical complexity that the coupling introduces. When a traditional spectral decomposition is employed, the discrete spectrum associated with the fast eigenmodes is often subsumed into the continuous Alfv \'en spectrum. Thus fast eigenmodes, become collective modes or quasi-modes. Here we present a spectral decomposition that treats the eigenmodes as having real frequencies but complex wavenumbers. Using this procedure we derive dispersion relations, spatial damping rates, and eigenfunctions for the resonant, fast eigenmodes of the arcade. We demonstrate that resonant absorption introduces a fast mode that would not exist otherwise. This new mode is heavily damped by resonant absorption, only travelling a few wavelengths before losing most of its energy
Superbubble evolution including the star-forming clouds: Is it possible to reconcile LMC observations with model predictions?
Here we present a possible solution to the apparent discrepancy between the
observed properties of LMC bubbles and the standard, constant density bubble
model. A two-dimensional model of a wind-driven bubble expanding from a
flattened giant molecular cloud is examined. We conclude that the expansion
velocities derived from spherically symmetric models are not always applicable
to elongated young bubbles seen almost face-on due to the LMC orientation. In
addition, an observational test to differentiate between spherical and
elongated bubbles seen face-on is discussed.Comment: 25 pages, 7 figures, accepted to ApJ (September, 1999 issue
Acoustic Power Absorption and its Relation with Vector Magnetic Field of a Sunspot
The distribution of acoustic power over sunspots shows an enhanced absorption
near the umbra--penumbra boundary. Earlier studies revealed that the region of
enhanced absorption coincides with the region of strongest transverse potential
field. The aim of this paper is to (i) utilize the high-resolution vector
magnetograms derived using Hinode SOT/SP observations and study the
relationship between the vector magnetic field and power absorption and (ii)
study the variation of power absorption in sunspot penumbrae due to the
presence of spine-like radial structures. It is found that (i) both potential
and observed transverse fields peak at a similar radial distance from the
center of the sunspot, and (ii) the magnitude of the transverse field, derived
from Hinode observations, is much larger than the potential transverse field
derived from SOHO/MDI longitudinal field observations. In the penumbra, the
radial structures called spines (intra-spines) have stronger (weaker) field
strength and are more vertical (horizontal). The absorption of acoustic power
in the spine and intra-spine shows different behaviour with the absorption
being larger in the spine as compared to the intra-spine.Comment: 18 pages, 7 figures, In Press Solar Physics, Topical Issue on
Helio-and-Astroseismolog
Structure-property relationships in ionic liquids: Influence of branched and cyclic groups on vaporization enthalpies of imidazolium-based ILs
© 2015 Elsevier Ltd. Ionic liquids (ILs) with branched and cyclic substituents are seldom studied in the literature, and as such there are little to no data characterizing their thermophysical properties. ILs with branched and cyclic substituents are just as convenient to synthesize and study as their counterparts with linear substituents, but the effects of these substituents on IL properties are not yet well-defined due to the preference for linear substituents. Standard molar vaporization enthalpies of six imidazolium based ionic liquids [Rmim][NTf2] with iso-alkyl and cyclic substituents (R = iso-propyl, iso-butyl, sec-butyl, methylcyclopropyl, cyclopentyl and methylcyclohexyl) were derived from quartz-crystal microbalance (QCM) method. Enthalpies of vaporization measured at elevated temperatures have been adjusted to the reference temperature 298 K and tested for consistency by comparison with the homomorphy alkane, alkylbenzenes and alkyl-imidazoles. It was found that vaporization enthalpies of ILs with the iso-alkyl and cyclic groups are generally on the same level within (±2 to 3) kJ · mol-1 significantly compared to the analogous ILs with the imidazolium cation substituted with the linear alkyl substituents of the same chain length. These findings are useful for the quick estimation of vaporization enthalpies of various substituted IL cations (e.g. pyrrolidinium, ammonium, pyridinium, etc.)
Dark Matter and the Chemical Evolution of Irregular Galaxies
We present three types of chemical evolution models for irregular galaxies:
closed-box with continuous star formation rates (SFRs), closed-box with
bursting SFRs, and O-rich outflow with continuous SFRs. We discuss the chemical
evolution of the irregular galaxies NGC 1560 and II Zw 33, and a ``typical''
irregular galaxy. The fraction of low-mass stars needed by our models is larger
than that derived for the solar vicinity, but similar to that found in globular
clusters. For our typical irregular galaxy we need a mass fraction of about 40%
in the form of substellar objects plus non baryonic dark matter inside the
Holmberg radius, in good agreement with the results derived for NGC 1560 and II
Zw 33 where we do have an independent estimate of the mass fraction in non
baryonic dark matter. Closed-box models are better than O-rich outflow models
in explaining the C/O and Z/O observed values for our typical irregular galaxy.Comment: 14 pages, 2 figure, uses emulateapj.sty package. ApJ in press. New
models were added. The order of Tables has been correcte
Time-dependent suppression of oscillatory power in evolving solar magnetic fields
Oscillation amplitudes are generally smaller within magnetically active regions like sunspots and plage when compared to their surroundings. Such magnetic features, when viewed in spatially resolved power maps, appear as regions of suppressed power due to reductions in the oscillation amplitudes. Employing high spatial- and temporal-resolution observations from the Dunn Solar Telescope (DST) in New Mexico, we study the power suppression in a region of evolving magnetic fields adjacent to a pore. By utilizing wavelet analysis, we study for the first time how the oscillatory properties in this region change as the magnetic field evolves with time. Image sequences taken in the blue continuum, G-band, Ca ii K, and Hα filters were used in this study. It is observed that the suppression found in the chromosphere occupies a relatively larger area, confirming previous findings. Also, the suppression is extended to structures directly connected to the magnetic region, and is found to get enhanced as the magnetic field strength increased with time. The dependence of the suppression on the magnetic field strength is greater at longer periods and higher formation heights. Furthermore, the dominant periodicity in the chromosphere was found to be anti-correlated with increases in the magnetic field strength
Newly identified properties of surface acoustic power
The cause of enhanced acoustic power surrounding active regions, the acoustic
halo, is not as yet understood. We explore the properties of the enhanced
acoustic power observed near disk center from 21 to 27 January 2002, including
AR 9787. We find that (i) there exists a strong correlation of the enhanced
high frequency power with magnetic-field inclination, with greater power in
more horizontal fields, (ii) the frequency of the maximum enhancement increases
along with magnetic field strength, and (iii) the oscillations contributing to
the halos show modal ridges which are shifted to higher wavenumber at constant
frequency in comparison to the ridges of modes in the quiet-Sun.Comment: 16 pages, 10 figures, submitted to solar physic
Fundamental-mode Oscillations of Two Coronal Loops within a Solar Magnetic Arcade
We analyze intensity variations, as measured by the Atmospheric Imaging Assembly in the 171 Ă… passband, in two coronal loops embedded within a single coronal magnetic arcade. We detect oscillations in the fundamental mode with periods of roughly 2 minutes and decay times of 5 minutes. The oscillations were initiated by interaction of the arcade with a large wavefront issuing from a flare site. Further, the power spectra of the oscillations evince signatures consistent with oblique propagation to the field lines and for the existence of a two-dimensional waveguide instead of a one-dimensional one
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