30,977 research outputs found
Velocity measurements for a solar active region fan loop from Hinode/EIS observations
The velocity pattern of a fan loop structure within a solar active region
over the temperature range 0.15-1.5 MK is derived using data from the EUV
Imaging Spectrometer (EIS) on board the Hinode satellite. The loop is aligned
towards the observer's line-of-sight and shows downflows (redshifts) of around
15 km/s up to a temperature of 0.8 MK, but for temperatures of 1.0 MK and above
the measured velocity shifts are consistent with no net flow. This velocity
result applies over a projected spatial distance of 9 Mm and demonstrates that
the cooler, redshifted plasma is physically disconnected from the hotter,
stationary plasma. A scenario in which the fan loops consist of at least two
groups of "strands" - one cooler and downflowing, the other hotter and
stationary -- is suggested. The cooler strands may represent a later
evolutionary stage of the hotter strands. A density diagnostic of Mg VII was
used to show that the electron density at around 0.8 MK falls from 3.2 x 10^9
cm^-3 at the loop base, to 5.0 x 10^8 cm^-3 at a projected height of 15 Mm. A
filling factor of 0.2 is found at temperatures close to the formation
temperature of Mg VII (0.8 MK), confirming that the cooler, downflowing plasma
occupies only a fraction of the apparent loop volume. The fan loop is rooted
within a so-called "outflow region" that displays low intensity and blueshifts
of up to 25 km/s in Fe XII 195.12 A (formed at 1.5 MK), in contrast to the
loop's redshifts of 15 km/s at 0.8 MK. A new technique for obtaining an
absolute wavelength calibration for the EIS instrument is presented and an
instrumental effect, possibly related to a distorted point spread function,
that affects velocity measurements is identified.Comment: 42 pages, 15 figures, submitted to Ap
Dynamic and Stagnating Plasma Flow Leading to Magnetic Flux Tube Collimation
Highly collimated, plasma-filled magnetic flux tubes are frequently observed
on galactic, stellar and laboratory scales. We propose that a single, universal
magnetohydrodynamic pumping process explains why such collimated, plasma-filled
magnetic flux tubes are ubiquitous. Experimental evidence from carefully
diagnosed laboratory simulations of astrophysical jets confirms this assertion
and is reported here. The magnetohydrodynamic process pumps plasma into a
magnetic flux tube and the stagnation of the resulting flow causes this flux
tube to become collimated.Comment: to be published in PRL; color figures on electronic versio
The Cool ISM in Elliptical Galaxies. II. Gas Content in the Volume - Limited Sample and Results from the Combined Elliptical and Lenticular Surveys
We report new observations of atomic and molecular gas in a volume limited
sample of elliptical galaxies. Combining the elliptical sample with an earlier
and similar lenticular one, we show that cool gas detection rates are very
similar among low luminosity E and SO galaxies but are much higher among
luminous S0s. Using the combined sample we revisit the correlation between cool
gas mass and blue luminosity which emerged from our lenticular survey, finding
strong support for previous claims that the molecular gas in ellipticals and
lenticulars has different origins. Unexpectedly, however, and contrary to
earlier claims, the same is not true for atomic gas. We speculate that both the
AGN feedback and merger paradigms might offer explanations for differences in
detection rates, and might also point towards an understanding of why the two
gas phases could follow different evolutionary paths in Es and S0s. Finally we
present a new and puzzling discovery concerning the global mix of atomic and
molecular gas in early type galaxies. Atomic gas comprises a greater fraction
of the cool ISM in more gas rich galaxies, a trend which can be plausibly
explained. The puzzle is that galaxies tend to cluster around
molecular-to-atomic gas mass ratios near either 0.05 or 0.5.Comment: 37 pages, including 4 tables and 12 figures. Accepted for publication
in the Astrophysical Journa
Playing with nonuniform grids
Numerical experiments with discretization methods on nonuniform grids are presented for the convection-diffusion equation. These show that the accuracy of the discrete solution is not very well predicted by the local truncation error. The diagonal entries in the discrete coefficient matrix give a better clue: the convective term should not reduce the diagonal. Also, iterative solution of the discrete set of equations is discussed. The same criterion appears to be favourable.
Coronal magnetic field measurement using loop oscillations observed by Hinode/EIS
We report the first spectroscopic detection of a kink MHD oscillation of a solar coronal structure by the Extreme-Ultraviolet Imaging Spectrometer (EIS) on the Japanese Hinode satellite. The detected oscillation has an amplitude of 1 kms−1 in the Doppler shift of the FeXII 195 Å spectral line (1.3 MK), and a period of 296 s. The unique combination of EIS’s spectroscopic and imaging abilities
enables us to measure simultaneously the mass density and length of the oscillating loop. This enables us to measure directly the magnitude of the local magnetic field, the fundamental coronal plasma parameter, as 39 ± 8 G, with unprecedented accuracy. This proof of concept makes EIS an exclusive instrument for the full scale implementation of the MHD coronal seismological technique
Derivatives and inequalities for order parameters in the Ising spin glass
Identities and inequalities are proved for the order parameters, correlation
functions and their derivatives of the Ising spin glass. The results serve as
additional evidence that the ferromagnetic phase is composed of two regions,
one with strong ferromagnetic ordering and the other with the effects of
disorder dominant. The Nishimori line marks a crossover between these two
regions.Comment: 10 pages; 3 figures; new inequalities added, title slightly change
GREAT: the SOFIA high-frequency heterodyne instrument
We describe the design and construction of GREAT, the German REceiver for
Astronomy at Terahertz frequencies operated on the Stratospheric Observatory
for Infrared Astronomy (SOFIA). GREAT is a modular dual-color heterodyne
instrument for highresolution far-infrared (FIR) spectroscopy. Selected for
SOFIA's Early Science demonstration, the instrument has successfully performed
three Short and more than a dozen Basic Science flights since first light was
recorded on its April 1, 2011 commissioning flight.
We report on the in-flight performance and operation of the receiver that -
in various flight configurations, with three different detector channels -
observed in several science-defined frequency windows between 1.25 and 2.5 THz.
The receiver optics was verified to be diffraction-limited as designed, with
nominal efficiencies; receiver sensitivities are state-of-the-art, with
excellent system stability. The modular design allows for the continuous
integration of latest technologies; we briefly discuss additional channels
under development and ongoing improvements for Cycle 1 observations.
GREAT is a principal investigator instrument, developed by a consortium of
four German research institutes, available to the SOFIA users on a
collaborative basis
Simulations of Metal Enrichment in Galaxy Clusters by AGN Outflows
We assess the importance of AGN outflows with respect to the metal enrichment
of the intracluster medium (ICM) in galaxy clusters. We use combined N-body and
hydrodynamic simulations, along with a semi-numerical galaxy formation and
evolution model. Using assumptions based on observations, we attribute outflows
of metal-rich gas initiated by AGN activity to a certain fraction of our model
galaxies. The gas is added to the model ICM, where the evolution of the
metallicity distribution is calculated by the hydrodynamic simulations. For the
parameters describing the AGN content of clusters and their outflow properties,
we use the observationally most favorable values. We find that AGNs have the
potential to contribute significantly to the metal content of the ICM or even
explain the complete abundance, which is typically ~0.5 Z_sun in core regions.
Furthermore, the metals end up being inhomogeneously distributed, in accordance
with observations.Comment: 7 pages, 6 figures, accepted for publication in A&
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