3,236 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
Departure of high temperature iron lines from the equilibrium state in flaring solar plasmas
The aim of this study is to clarify if the assumption of ionization
equilibrium and a Maxwellian electron energy distribution is valid in flaring
solar plasmas. We analyze the 2014 December 20 X1.8 flare, in which the
\ion{Fe}{xxi} 187~\AA, \ion{Fe}{xxii} 253~\AA, \ion{Fe}{xxiii} 263~\AA\ and
\ion{Fe}{xxiv} 255~\AA\ emission lines were simultaneously observed by the EUV
Imaging Spectrometer onboard the Hinode satellite. Intensity ratios among these
high temperature Fe lines are compared and departures from isothermal
conditions and ionization equilibrium examined. Temperatures derived from
intensity ratios involving these four lines show significant discrepancies at
the flare footpoints in the impulsive phase, and at the looptop in the gradual
phase. Among these, the temperature derived from the
\ion{Fe}{xxii}/\ion{Fe}{xxiv} intensity ratio is the lowest, which cannot be
explained if we assume a Maxwellian electron distribution and ionization
equilibrium, even in the case of a multi-thermal structure. This result
suggests that the assumption of ionization equilibrium and/or a Maxwellian
electron energy distribution can be violated in evaporating solar plasma around
10~MK.Comment: 10 pages, 4 figures, to appear in Ap
Fe XIII emission lines in active region spectra obtained with the Solar Extreme-Ultraviolet Research Telescope and Spectrograph
Recent fully relativistic calculations of radiative rates and electron impact
excitation cross sections for Fe {\sc xiii} are used to generate emission-line
ratios involving 3s3p--3s3p and
3s3p--3s3p3d transitions in the 170--225 \AA and 235--450 \AA
wavelength ranges covered by the Solar Extreme-Ultraviolet Research Telescope
and Spectrograph (SERTS). A comparison of these line ratios with SERTS active
region observations from rocket flights in 1989 and 1995 reveals generally very
good agreement between theory and experiment. Several new Fe {\sc xiii}
emission features are identified, at wavelengths of 203.79, 259.94, 288.56 and
290.81 \AA. However, major discrepancies between theory and observation remain
for several Fe {\sc xiii} transitions, as previously found by Landi (2002) and
others, which cannot be explained by blending. Errors in the adopted atomic
data appear to be the most likely explanation, in particular for transitions
which have 3s3p3d D as their upper level. The most useful Fe
{\sc xiii} electron density diagnostics in the SERTS spectral regions are
assessed, in terms of the line pairs involved being (i) apparently free of
atomic physics problems and blends, (ii) close in wavelength to reduce the
effects of possible errors in the instrumental intensity calibration, and (iii)
very sensitive to changes in N over the range 10--10
cm. It is concluded that the ratios which best satisfy these conditions
are 200.03/202.04 and 203.17/202.04 for the 170--225 \AA wavelength region, and
348.18/320.80, 348.18/368.16, 359.64/348.18 and 359.83/368.16 for 235--450 \AA.Comment: 11 pages, 8 figures, 10 tables, MNRAS, in pres
Emission lines of Fe X in active region spectra obtained with the Solar Extreme-ultraviolet Research Telescope and Spectrograph
Fully relativistic calculations of radiative rates and electron impact
excitation cross sections for Fe X are used to derive theoretical emission-line
ratios involving transitions in the 174-366 A wavelength range. A comparison of
these with solar active region observations obtained during the 1989 and 1995
flights of the Solar Extreme-ultraviolet Research Telescope and Spectrograph
(SERTS) reveals generally very good agreement between theory and experiment.
Several Fe X emission features are detected for the first time in SERTS
spectra, while the transition at 195.32 A is identified for the first time (to
our knowledge) in an astronomical source. The most useful Fe X electron density
diagnostic line ratios are assessed to be 175.27/174.53 and 175.27/177.24,
which both involve lines close in wavelength and free from blends, vary by
factors of 13 between Ne = 1E8 and 1E13 cm-3, and yet show little temperature
sensitivity. Should these lines not be available, then the 257.25/345.74 ratio
may be employed to determine Ne, although this requires an accurate evaluation
of the instrument intensity calibration over a relatively large wavelength
range. However, if the weak 324.73 A line of Fe X is reliably detected, the use
of 324.73/345.74 or 257.25/324.73 is recommended over 257.25/345.74.Comment: 11 pages, 10 figures, MNRAS in pres
Cosmogenic-neutron activation of TeO2 and implications for neutrinoless double-beta decay experiments
Flux-averaged cross sections for cosmogenic-neutron activation of natural
tellurium were measured using a neutron beam containing neutrons of kinetic
energies up to 800 MeV, and having an energy spectrum similar to that of
cosmic-ray neutrons at sea-level. Analysis of the radioisotopes produced
reveals that 110mAg will be a dominant contributor to the cosmogenic-activation
background in experiments searching for neutrinoless double-beta decay of
130Te, such as CUORE and SNO+. An estimate of the cosmogenic-activation
background in the CUORE experiment has been obtained using the results of this
measurement and cross-section measurements of proton activation of tellurium.
Additionally, the measured cross sections in this work are also compared with
results from semi-empirical cross-section calculations.Comment: 11 pages, 5 figure
Dust in the Ionized Medium of the Galaxy: GHRS Measurements of Al III and S III
We present interstellar absorption line measurements of the ions S III and Al
III towards six stars using archival Goddard High Resolution Spectrograph data.
The ions Al III and S III trace heavily depleted and non-depleted elements,
respectively, in ionized gas. We use the photoionization code CLOUDY to derive
the ionization correction relating N(Al III)/N(S III) to the gas-phase
abundance [Al/S]_i in the ionized gas. For spectral types considered here, the
corrections are small and independent of the assumed ionization parameter.
Using the results of these photoionization models, we find [Al/S]_i = -1.0 in
the ionized gas towards three disk stars. These values of [Al/S]_i (=[Al/H]_i)
imply that Al-bearing grains are present in the ionized nebulae around these
stars. If the WIM of the Galaxy is photoionized by OB stars, our data for two
halo stars imply [Al/S]_i = -0.4 to -0.5 in the WIM and thus the presence of
dust grains containing Al in this important phase of the ISM. While
photoionization appears to be the most likely origin of the ionization for Al
III and S III, we cannot rule out confusion from the presence of hot,
collisionally ionized gas along two sightlines. We find that [Al/S]_i in the
ionized gas along the six sightlines is anti-correlated with the electron
density and average sightline neutral density. The degree of grain destruction
in the ionized medium of the Galaxy is not much higher than in the warm neutral
medium. The existence of grains in the ionized regions studied here has
important implications for the thermal balance of these regions. (Abstract
Abridged)Comment: 30 pages including 8 embedded tables and 8 embedded figures. Accepted
for publication in the Astrophysical Journa
Emission lines of Fe XI in the 257--407 A wavelength region observed in solar spectra from EIS/Hinode and SERTS
Theoretical emission-line ratios involving Fe XI transitions in the 257-407 A
wavelength range are derived using fully relativistic calculations of radiative
rates and electron impact excitation cross sections. These are subsequently
compared with both long wavelength channel Extreme-Ultraviolet Imaging
Spectrometer (EIS) spectra from the Hinode satellite (covering 245-291 A), and
first-order observations (235-449 A) obtained by the Solar Extreme-ultraviolet
Research Telescope and Spectrograph (SERTS). The 266.39, 266.60 and 276.36 A
lines of Fe XI are detected in two EIS spectra, confirming earlier
identifications of these features, and 276.36 A is found to provide an electron
density diagnostic when ratioed against the 257.55 A transition. Agreement
between theory and observation is found to be generally good for the SERTS data
sets, with discrepancies normally being due to known line blends, while the
257.55 A feature is detected for the first time in SERTS spectra. The most
useful Fe XI electron density diagnostic is found to be the 308.54/352.67
intensity ratio, which varies by a factor of 8.4 between N_e = 10^8 and 10^11
cm^-3, while showing little temperature sensitivity. However, the 349.04/352.67
ratio potentially provides a superior diagnostic, as it involves lines which
are closer in wavelength, and varies by a factor of 14.7 between N_e = 10^8 and
10^11 cm^-3. Unfortunately, the 349.04 A line is relatively weak, and also
blended with the second-order Fe X 174.52 A feature, unless the first-order
instrument response is enhanced.Comment: 9 pages, 5 figures, 13 tables; MNRAS in pres
Plant-soil feedbacks promote coexistence and resilience in multi-species communities
Both ecological theory and empirical evidence suggest that negative frequency dependent
feedbacks structure plant communities, but integration of these findings has been limited.
Here we develop a generic model of frequency dependent feedback to analyze coexistence
and invasibility in random theoretical and real communities for which frequency dependence
through plant-soil feedbacks (PSFs) was determined empirically. We investigated community
stability and invasibility by means of mechanistic analysis of invasion conditions and numerical
simulations. We found that communities fall along a spectrum of coexistence types ranging
from strict pair-wise negative feedback to strict intransitive networks. Intermediate community
structures characterized by partial intransitivity may feature “keystone competitors” which disproportionately
influence community stability. Real communities were characterized by stronger
negative feedback and higher robustness to species loss than randomly assembled
communities. Partial intransitivity became increasingly likely in more diverse communities.
The results presented here theoretically explain why more diverse communities are characterized
by stronger negative frequency dependent feedbacks, a pattern previously encountered
in observational studies. Natural communities are more likely to be maintained by strict negative
plant-soil feedback than expected by chance, but our results also show that community
stability often depends on partial intransitivity. These results suggest that plant-soil feedbacks
can facilitate coexistence in multi-species communities, but that these feedbacks may also initiate
cascading effects on community diversity following from single-species loss.DEB - 0919434, DEB - 1050237, DEB-1556664, DEB-173804
Discovery of spatial periodicities in a coronal loop using automated edge-tracking algorithms
A new method for automated coronal loop tracking, in both spatial and temporal domains, is presented. Applying this technique to TRACE data, obtained using the 171 Å filter on 1998 July 14, we detect a coronal loop undergoing a 270 s kink-mode oscillation, as previously found by Aschwanden et al. However, we also detect flare-induced, and previously unnoticed, spatial periodicities on a scale of 3500 km, which occur along the coronal loop edge. Furthermore, we establish a reduction in oscillatory power for these spatial periodicities of 45% over a 222 s interval. We relate the reduction in detected oscillatory power to the physical damping of these loop-top oscillations
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