3,709 research outputs found
The Area Distribution of Solar Magnetic Bright Points
Magnetic Bright Points (MBPs) are among the smallest observable objects on
the solar photosphere. A combination of G-band observations and numerical
simulations is used to determine their area distribution. An automatic
detection algorithm, employing 1-dimensional intensity profiling, is utilized
to identify these structures in the observed and simulated datasets. Both
distributions peak at an area of 45000 km, with a sharp decrease
towards smaller areas. The distributions conform with log-normal statistics,
which suggests that flux fragmentation dominates over flux convergence.
Radiative magneto-convection simulations indicate an independence in the MBP
area distribution for differing magnetic flux densities. The most commonly
occurring bright point size corresponds to the typical width of intergranular
lanes.Comment: Astrophysical Journal, accepte
Propagating Wave Phenomena Detected in Observations and Simulations of the Lower Solar Atmosphere
We present high-cadence observations and simulations of the solar
photosphere, obtained using the Rapid Oscillations in the Solar Atmosphere
imaging system and the MuRAM magneto-hydrodynamic code, respectively. Each
dataset demonstrates a wealth of magneto-acoustic oscillatory behaviour,
visible as periodic intensity fluctuations with periods in the range 110-600 s.
Almost no propagating waves with periods less than 140s and 110s are detected
in the observational and simulated datasets, respectively. High concentrations
of power are found in highly magnetised regions, such as magnetic bright points
and intergranular lanes. Radiative diagnostics of the photospheric simulations
replicate our observational results, confirming that the current breed of
magneto-hydrodynamic simulations are able to accurately represent the lower
solar atmosphere. All observed oscillations are generated as a result of
naturally occurring magnetoconvective processes, with no specific input driver
present. Using contribution functions extracted from our numerical simulations,
we estimate minimum G-band and 4170 Angstrom continuum formation heights of 100
km and 25 km, respectively. Detected magneto-acoustic oscillations exhibit a
dominant phase delay of -8 degrees between the G-band and 4170 Angstrom
continuum observations, suggesting the presence of upwardly propagating waves.
More than 73% of MBPs (73% from observations, 96% from simulations) display
upwardly propagating wave phenomena, suggesting the abundant nature of
oscillatory behaviour detected higher in the solar atmosphere may be traced
back to magnetoconvective processes occurring in the upper layers of the Sun's
convection zone.Comment: 13 pages, 9 figures, accepted into Ap
A non-LTE abundance analysis of the post-AGB star ROA 5701
An analysis of high-resolution Anglo-Australian Telescope (AAT)/ University
College London Echelle Spectrograph (UCLES) optical spectra for the ultraviolet
(UV)-bright star ROA 5701 in the globular cluster omega Cen (NGC 5139) is
performed, using non-local thermodynamic equilibrium (non-LTE) model
atmospheres to estimate stellar atmospheric parameters and chemical
composition. Abundances are derived for C, N, O, Mg, Si and S, and compared
with those found previously by Moehler et al. We find a general metal
underabundance relative to young B-type stars, consistent with the average
metallicity of the cluster. Our results indicate that ROA 5701 has not
undergone a gas-dust separation scenario as previously suggested. However, its
abundance pattern does imply that ROA 5701 has evolved off the AGB prior to the
onset of the third dredge-up.Comment: 9 pages, 2 figures. Accepted for publication in MNRAS (Online Early
Reducing seed viability of flaxleaf fleabane, feathertop Rhodes grass and awnless barnyard grass
In the sub-tropical grain region of Australia,
cotton and grains systems are now dominated by flaxleaf fleabane (Conyza bonariensis (L.) Cronquist), feathertop Rhodes grass (Chloris virgata Sw.) and awnless barnyard grass (Echinochloa colona (L.) Link). While control of these weed species is best achieved when they are young, previous studies have shown a potential for reducing seed viability and minimising seed bank replenishment by applying
herbicides when plants are reproductive.
Pot trials were established over two growing seasons to examine the effects of 2,4-D, 2,4-D + picloram, glyphosate and glufosinate which had been successful on other species, along with paraquat and haloxyfop (grasses only). Herbicides were applied at ¾ field rates in an attempt not to kill the plants. Flaxleaf fleabane plants were sprayed at two growth stages (budding and flowering) and the grasses were sprayed at two stages (late tillering/booting and flowering).
Spraying flaxleaf fleabane at flowering reduced seed viability to 0% (of untreated) in all treatments except glyphosate (51%) and 2,4-D + picloram (8%). Seed viability was not reduced with the first and second regrowths with the exception of 2,4-D + picloram where viability was reduced to 20%. When sprayed at budding only 2,4-D + picloram reduced seed viability in both trials.
Spraying the grasses at late tillering/booting did not reduce viability except for glufosinate on awnless barnyard grass (50%). Applying herbicides at flowering resulted in 0% seed viability in awnless barnyard grass from glufosinate, paraquat and glyphosate and 0% viability in feathertop Rhodes grass for glufosinate. These herbicides were less effective on heads that emerged and flowered after spraying, only slightly reducing seed viability.
These trials have shown that attempts to reduce seed viability have potential, however flaxleaf fleabane and feathertop Rhodes grass are able to regrow and will need on-going monitoring and control measures
Electron-Ion Recombination on Grains and Polycyclic Aromatic Hydrocarbons
With the high-resolution spectroscopy now available in the optical and
satellite UV, it is possible to determine the neutral/ionized column density
ratios for several different elements in a single cloud. Assuming ionization
equilibrium for each element, one can make several independent determinations
of the electron density. For the clouds for which such an analysis has been
carried out, these different estimates disagree by large factors, suggesting
that some process (or processes) besides photoionization and radiative
recombination might play an important role in the ionization balance. One
candidate process is collisions of ions with dust grains.
Making use of recent work quantifying the abundances of polycyclic aromatic
hydrocarbon molecules and other grains in the interstellar medium, as well as
recent models for grain charging, we estimate the grain-assisted ion
recombination rates for several astrophysically important elements. We find
that these rates are comparable to the rates for radiative recombination for
conditions typical of the cold neutral medium. Including grain-assisted ion
recombination in the ionization equilibrium analysis leads to increased
consistency in the various electron density estimates for the gas along the
line of sight to 23 Orionis. However, not all of the discrepancies can be
eliminated in this way; we speculate on some other processes that might play a
role. We also note that grain-assisted recombination of H+ and He+ leads to
significantly lower electron fractions than usually assumed for the cold
neutral medium.Comment: LaTeX(12 pages, 8 figures, uses emulateapj5.sty, apjfonts.sty);
submitted to ApJ; corrected typo
Optimization of double pulse pumping for Ni-like Sm x-ray lasers
We report a systematic study of double pulse pumping of the Ni-like Sm x-ray laser at 73 Angstrom, currently the shortest wavelength saturated x-ray laser. It is found that the Sm x-ray laser output can change by orders of magnitude when the intensity ratio of the pumping pulses and their relative delay are varied. Optimum pumping conditions are found and interpreted in terms of a simple model. (C) 1999 American Institute of Physics. [S0021-8979(99)07102-9]
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