1,336 research outputs found
The size distribution of magnetic bright points derived from Hinode/SOT observations
Context. Magnetic Bright Points (MBPs) are small-scale magnetic features in
the solar photosphere. They may be a possible source of coronal heating by
rapid footpoint motions that cause magnetohydrodynamical waves. The number and
size distribution are of vital importance in estimating the small
scale-magnetic-field energy. Aims. The size distribution of MBPs is derived for
G-band images acquired by the Hinode/SOT instrument. Methods. For
identification purposes, a new automated segmentation and identification
algorithm was developed. Results. For a sampling of 0.108 arcsec/pixel, we
derived a mean diameter of (218 +- 48) km for the MBPs. For the full resolved
data set with a sampling of 0.054 arcsec/pixel, the size distribution shifted
to a mean diameter of (166 +- 31) km. The determined diameters are consistent
with earlier published values. The shift is most probably due to the different
spatial sampling. Conclusions. We conclude that the smallest magnetic elements
in the solar photosphere cannot yet be resolved by G-band observations. The
influence of discretisation effects (sampling) has also not yet been
investigated sufficiently.Comment: Astronomy and Astrophysics, Volume 498, Issue 1, 2009, pp.289-29
Propagating waves in polar coronal holes as seen by SUMER and EIS
To study the dynamics of coronal holes and the role of waves in the
acceleration of the solar wind, spectral observations were performed over polar
coronal hole regions with the SUMER spectrometer on SoHO and the EIS
spectrometer on Hinode. Using these observations, we aim to detect the presence
of propagating waves in the corona and to study their properties. The
observations analysed here consist of SUMER spectra of the Ne VIII 770 A line
(T = 0.6 MK) and EIS slot images in the Fe XII 195 A line (T = 1.3 MK). Using
the wavelet technique, we study line radiance oscillations at different heights
from the limb in the polar coronal hole regions. We detect the presence of long
period oscillations with periods of 10 to 30 min in polar coronal holes. The
oscillations have an amplitude of a few percent in radiance and are not
detectable in line-of-sight velocity. From the time distance maps we find
evidence for propagating velocities from 75 km/s (Ne VIII) to 125 km/s (Fe
XII). These velocities are subsonic and roughly in the same ratio as the
respective sound speeds. We interpret the observed propagating oscillations in
terms of slow magneto-acoustic waves. These waves can be important for the
acceleration of the fast solar wind.Comment: 5 pages, 7 figures Accepted as Astronomy and Astrophysics Lette
Relationship between non-thermal electron energy spectra and GOES classes
We investigate the influence of the variations of energy spectrum of
non-thermal electrons on the resulting GOES classes of solar flares. Twelve
observed flares with various soft to hard X-ray emission ratios were modelled
using different non-thermal electron energy distributions. Initial values of
the flare physical parameters including geometrical properties were estimated
using observations. We found that, for a fixed total energy of non-thermal
electrons in a flare, the resulting GOES class of the flare can be changed
significantly by varying the spectral index and low energy cut-off of the
non-thermal electron distribution. Thus, the GOES class of a flare depends not
only on the total non-thermal electrons energy but also on the electron beam
parameters. For example, we were able to convert a M2.7 class solar flare into
a merely C1.4 class one and a B8.1 class event into a C2.6 class flare. The
results of our work also suggest that the level of correlation between the
cumulative time integral of HXR and SXR fluxes can depend on the considered HXR
energy range.Comment: 8 pages, 5 figures, Astronomy and Astrophysics (accepted, March 2009
Seeing-Induced Errors in Solar Doppler Velocity Measurements
Imaging systems based on a narrow-band tunable filter are used to obtain
Doppler velocity maps of solar features. These velocity maps are created by
taking the difference between the blue- and red-wing intensity images of a
chosen spectral line. This method has the inherent assumption that these two
images are obtained under identical conditions. With the dynamical nature of
the solar features as well as the Earth's atmosphere, systematic errors can be
introduced in such measurements. In this paper, a quantitative estimate of the
errors introduced due to variable seeing conditions for ground-based
observations is simulated and compared with real observational data for
identifying their reliability. It is shown, under such conditions, that there
is a strong cross-talk from the total intensity to the velocity estimates.
These spurious velocities are larger in magnitude for the umbral regions
compared to the penumbra or quiet-sun regions surrounding the sunspots. The
variable seeing can induce spurious velocities up to about 1 km/s It is also
shown that adaptive optics, in general, helps in minimising this effect.Comment: 14 page
CME liftoff with high-frequency fragmented type II burst emission
Aims: Solar radio type II bursts are rarely seen at frequencies higher than a
few hundred MHz. Since metric type II bursts are thought to be signatures of
propagating shock waves, it is of interest to know how these shocks, and the
type II bursts, are formed. In particular, how are high-frequency, fragmented
type II bursts created? Are there differences in shock acceleration or in the
surrounding medium that could explain the differences to the "typical" metric
type IIs? Methods: We analyse one unusual metric type II event in detail, with
comparison to white-light, EUV, and X-ray observations. As the radio event was
associated with a flare and a coronal mass ejection (CME), we investigate their
connection. We then utilize numerical MHD simulations to study the shock
structure induced by an erupting CME in a model corona including dense loops.
Results: Our simulations show that the fragmented part of the type II burst can
be formed when a coronal shock driven by a mass ejection passes through a
system of dense loops overlying the active region.To produce fragmented
emission, the conditions for plasma emission have to be more favourable inside
the loop than in the interloop area. The obvious hypothesis, consistent with
our simulation model, is that the shock strength decreases significantly in the
space between the denser loops. The later, more typical type II burst appears
when the shock exits the dense loop system and finally, outside the active
region, the type II burst dies out when the changing geometry no longer favours
the electron shock-acceleration.Comment: 7 pages, 9 figures, A&A accepte
A Discovery of Rapid Optical Flares from Low-Luminosity Active Nuclei in Massive Galaxies
We report a serendipitous discovery of six very low-luminosity active
galactic nuclei (AGNs) only by optical variability in one-month baseline. The
detected flux variability is ~ 1-5% of the total luminosity of host galaxies.
Careful subtraction of host galaxy components in nuclear regions indicates that
the fractional variability (Delta F / F) of the nuclei is of order unity. At
least one of them is showing a compelling flaring activity within just a few
days, which appears to be quite different from previously known AGN
variability. We obtained spectroscopic data for the one showing the largest
flare and confirmed that it is in fact an AGN at z = 0.33 with an estimated
black hole mass of ~10^8 M_sun. As a possible interpretation, we suggest that
these activities are coming from the region around the black hole event
horizon, which is physically similar to the recently discovered near-infrared
flares of our Galactic nucleus. It is indicated that our Galaxy is not special,
and that surprisingly rapid flaring activity in optical/near-infrared bands may
be commonly hidden in nuclei of apparently normal galaxies with low Eddington
ratios, in contrast to the variability of well-studied luminous AGNs or
quasars.Comment: Accepted to ApJ Letter
Asymmetric Field Profile in Bose Glass Phase of Irradiated YBa2Cu3O7-d: Loss of Interlayer Coherence around 1/3 of Matching Field
Magneto-optical imaging in YBa2Cu3O7-d with tilted columnar defects (CD's)
shows an asymmetric critical-state field profile. The observed hysteretic shift
of the profile ridge (trough) from the center of the sample is explained by
in-plane magnetization originated from vortex alignment along CD's. The
extracted ratio of the in-plane to out-of-plane magnetization component has a
maximum at 1/5 of matching field () and disappears above ,
suggesting a reduction of interlayer coherence well bellow in the Bose
glass phase. Implications are discussed in comparison with the vortex liquid
recoupling observed in irradiated Bi2Sr2CaCu2O8+y.Comment: Revtex, 4 pages, 5 figures, also see a movie at
(http://www.ap6.t.u-tokyo.ac.jp/kitaka/Research/d-line/index_e.htm). This
manuscript will appear in Phys. Rev. Let
Subaru Spectroscopy of the Gravitational Lens HST 14176+5226: Implications for a Large Cosmological Constan
We present new optical spectroscopy of the lens elliptical galax in the
``Einstein Cross'' lens system HST 14176+5226, using the Faint Object Camera
and Spectrograph (FOCAS) of the Subaru t Our spectroscopic observations are
aimed at measuring the stella dispersion of the lens galaxy, located at high
redshift of z_L= as an important component to lens models. We have measured
this 230 +- 14 km s^{-1} (1 sigma) inside 0.35 effective radi based on the
comparison between the observed galaxy spectrum and templates of three G-K
giants by means of the Fourier cross-corr To extract the significance of this
information on the geometry universe which also affects the lensing of the
background image, to fit three different lens models to the available data of
the Provided that the lens galaxy has the structural and dynamical p (i.e., its
radial density profile, core radius, and velocity ani similar to those of local
elliptical galaxies, we calculate the function for the simultaneous
reproduction of both the observed and newly measured velocity dispersion of the
lens. Although the interval depends rather sensitively on the adopted lens
models o parameters, our experiments suggest the larger likelihood for a
cosmological constant, Omega_Lambda: formal 1 sigma lower Omega_Lambda in the
flat universe ranges 0.73 to 0.97, where lower limit is basically unavailable.
This method for determinin model is thus dependent on lens models but is
insensitive to oth ambiguities, such as the dust absorption or the evolutionary
eff galaxies. Exploring spectroscopic observations of more lens gala redshift
may minimize the model uncertainties and thus place a m constraint on
Omega_Lambda.Comment: 23 pages, 5 figures, accepted for publication in The Astronomical
Journa
Defect-unbinding and the Bose-glass transition in layered superconductors
The low-field Bose-glass transition temperature in heavy-ion irradiated
Bi_2Sr_2CaCu_2O_8+d increases progressively with increasing density of
irradiation-induced columnar defects, but saturates for densities in excess of
1.5 x10^9 cm^-2. The maximum Bose-glass temperature corresponds to that above
which diffusion of two-dimensional pancake vortices between different vortex
lines becomes possible, and above which the ``line-like'' character of vortices
is lost. We develop a description of the Bose-glass line that is in excellent
quantitative agreement with the experimental line obtained for widely different
values of track density and material parameters.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
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
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