661 research outputs found
Evidence of quiet Sun chromospheric activity related to an emerging small-scale magnetic loop
Aims: We investigate the temporal evolution of magnetic flux emergence in the
quiet Sun atmosphere close to disk center. Methods: We combine high-resolution
SoHO/MDI magnetograms with TRACE observations taken in the 1216 {\AA} channel
in order to analyze the temporal evolution of an emerging small-scale magnetic
loop and its traces in the chromosphere. Results: At first place, we find
signatures of flux emergence very close to the edge of a supergranular network
boundary located at disk center. The new emerging flux appears first in the MDI
magnetograms in form of an asymmetric bipolar element, i.e. the patch with
negative polarity is roughly two-times weaker than the corresponding patch with
opposite polarity. The average values of magnetic flux and magnetic flux
densities reach 1.6 x 10^18 Mx, -8.5 x 10^17 Mx, and 55 Mx cm^-2, -30 Mx cm^-2,
respectively. The spatial distance between the opposite polarity patches of the
emerged feature increases from about 2.5" to 5.0" during the lifetime of the
loop which was not longer than 36 min. A more precise lifetime-estimate of the
feature was not possible because of a gap in the temporal sequence of the MDI
magnetograms. The chromospheric response to the emerged magnetic dipole occurs
~ 9 minutes later with respect to the photospheric magnetograms. It consists of
a quasi-periodic sequence of time-localized brightenings visible in the 1216
{\AA} TRACE channel apparent for ~ 14 minutes and being co-spatial with the
axis connecting the two patches of opposite magnetic polarity. Conclusions: We
identify the observed event as a small-scale magnetic loop emerging at
photospheric layers and subsequently rising up to the chromosphere. We discuss
the possibility that the fluctuations detected in the chromospheric emission
probably reflect magnetic field oscillations which propagate to the
chromosphere in form of waves.Comment: 6 pages, 4 figures, 1 table, Astronomy and Astrophysics, in pres
Near-infrared spectropolarimetry of a delta-spot
Sunspots harboring umbrae of both magnetic polarities within a common
penumbra (delta-spots) are often but not always related to flares. We present
first near-infrared (NIR) observations (Fe I 1078.3 nm and Si I 1078.6 nm
spectra) obtained with the Tenerife Infrared Polarimeter (TIP) at the Vacuum
Tower Telescope (VTT) in Tenerife on 2012 June 17, which afford accurate and
sensitive diagnostics to scrutinize the complex fields along the magnetic
neutral line of a delta-spot within active region NOAA 11504. We examine the
vector magnetic field, line-of-sight (LOS) velocities, and horizontal proper
motions of this rather inactive delta-spot. We find a smooth transition of the
magnetic vector field from the main umbra to that of opposite polarity
(delta-umbra), but a discontinuity of the horizontal magnetic field at some
distance from the delta-umbra on the polarity inversion line. The magnetic
field decreases faster with height by a factor of two above the delta-umbra.
The latter is surrounded by its own Evershed flow. The Evershed flow coming
from the main umbra ends at a line dividing the spot into two parts. This line
is marked by the occurrence of central emission in the Ca II 854.2 nm line.
Along this line, high chromospheric LOS-velocities of both signs appear. We
detect a shear flow within the horizontal flux transport velocities parallel to
the dividing line.Comment: 4 pages, will appear as Letter in Astronomy & Astrophysic
Magnetic loop emergence within a granule
We investigate the temporal evolution of magnetic flux emerging within a
granule in the quiet-Sun internetwork at disk center. We combined IR
spectropolarimetry performed in two Fe I lines at 1565 nm with
speckle-reconstructed G-band imaging. We determined the magnetic field
parameters by a LTE inversion of the full Stokes vector using the SIR code, and
followed their evolution in time. To interpret the observations, we created a
geometrical model of a rising loop in 3D. The relevant parameters of the loop
were matched to the observations where possible. We then synthesized spectra
from the 3D model for a comparison to the observations. We found signatures of
magnetic flux emergence within a growing granule. In the early phases, a
horizontal magnetic field with a distinct linear polarization signal dominated
the emerging flux. Later on, two patches of opposite circular polarization
signal appeared symmetrically on either side of the linear polarization patch,
indicating a small loop-like structure. The mean magnetic flux density of this
loop was roughly 450 G, with a total magnetic flux of around 3x10^17 Mx. During
the ~12 min episode of loop occurrence, the spatial extent of the loop
increased from about 1 to 2 arcsec. The middle part of the appearing feature
was blueshifted during its occurrence, supporting the scenario of an emerging
loop. The temporal evolution of the observed spectra is reproduced to first
order by the spectra derived from the geometrical model. The observed event can
be explained as a case of flux emergence in the shape of a small-scale loop.Comment: 10 pages, 13 figures; accepted for Astronomy and Astrophysics; ps and
eps figures in full resolution are available at
http://www.astro.sk/~koza/figures/aa2009_loop
Properties of the inner penumbral boundary and temporal evolution of a decaying sunspot
It was empirically determined that the umbra-penumbra boundaries of stable
sunspots are characterized by a constant value of the vertical magnetic field.
We analyzed the evolution of the photospheric magnetic field properties of a
decaying sunspot belonging to NOAA 11277 between August 28 - September 3, 2011.
The observations were acquired with the spectropolarimeter on-board of the
Hinode satellite. We aim to proof the validity of the constant vertical
magnetic-field boundary between the umbra and penumbra in decaying sunspots. A
spectral-line inversion technique was used to infer the magnetic field vector
from the full-Stokes profiles. In total, eight maps were inverted and the
variation of the magnetic properties in time were quantified using linear or
quadratic fits. We found a linear decay of the umbral vertical magnetic field,
magnetic flux, and area. The penumbra showed a linear increase of the vertical
magnetic field and a sharp decay of the magnetic flux. In addition, the
penumbral area quadratically decayed. The vertical component of the magnetic
field is weaker on the umbra-penumbra boundary of the studied decaying sunspot
compared to stable sunspots. Its value seem to be steadily decreasing during
the decay phase. Moreover, at any time of the shown sunspot decay, the inner
penumbra boundary does not match with a constant value of the vertical magnetic
field, contrary to what was seen in stable sunspots. During the decaying phase
of the studied sunspot, the umbra does not have a sufficiently strong vertical
component of the magnetic field and is thus unstable and prone to be
disintegrated by convection or magnetic diffusion. No constant value of the
vertical magnetic field was found for the inner penumbral boundary.Comment: Accepted for publication in Astronomy & Astrophysics, 6 pages, 7
figure
Surveillance des comportements face au VIH/sida chez les hommes ayant des rapports sexuels avec des hommes â Suisse, 1992-2004
Abstract.: Surveillance of HIV/AIDS â related behaviors among men who have sex with men â Switzerland, 1992-2004 Objective:: To update (1992-2004, Switzerland) trends in behaviors and opinions relative to HIV prevention among men who have sex with men. Methods:: A cross sectional survey, repeated five times since 1992, among readers of the gay press and members of gay associations. Results:: An increase was observed in the median number of partners (1994: 5; 2004: 7, p < 0.05), in the proportion of participants who practiced anal penetration with their stable partner (1992: 71%; 2004: 80%, p < 0.05) or with casual partners (1994: 59% 2004: 73% ; p < 0.05). Non systematic use of condoms with casual partners during anal penetration increased between 1994 and 2004 (9% to 20%; p < 0.05). Combination therapies gave rise to growing pessimism among participants regarding the decrease in preventive behaviors among homosexuals. Conclusions:: The slow trend towards a reduction of safer sex practices continues among men who have sex with men. It is necessary to strengthen prevention interventions in this populatio
Multiepoch Radial Velocity Observations of L Dwarfs
We report on the development of a technique for precise radial-velocity
measurements of cool stars and brown dwarfs in the near infrared. Our technique
is analogous to the Iodine (I2) absorption cell method that has proven so
successful in the optical regime. We rely on telluric CH4 absorption features
to serve as a wavelength reference, relative to which we measure Doppler shifts
of the CO and H2O features in the spectra of our targets. We apply this
technique to high-resolution (R~50,000) spectra near 2.3 micron of nine L
dwarfs taken with the Phoenix instrument on Gemini-South and demonstrate a
typical precision of 300 m/s. We conduct simulations to estimate our expected
precision and show our performance is currently limited by the signal-to-noise
of our data. We present estimates of the rotational velocities and systemic
velocities of our targets. With our current data, we are sensitive to
companions with M sin i > 2MJ in orbits with periods less than three days. We
identify no companions in our current data set. Future observations with
improved signal-to-noise should result in radial-velocity precision of 100 m/s
for L dwarfs.Comment: Accepted for publication in ApJ, 24 pages, 7 figure
A multiwavelength radial velocity search for planets around the brown dwarf LP 944-20
The nearby brown dwarf LP 944-20 has been monitored for radial velocity
variability at optical and near-infrared wavelengths using the VLT/UVES and the
Keck/NIRSPEC spectrographs, respectively. The UVES radial velocity data
obtained over 14 nights spanning a baseline of 841 days shows significant
variability with an amplitude of 3.5 km s. The periodogram analysis of
the UVES data indicates a possible period between 2.5 hours and 3.7 hours,
which is likely due to the rotation of the brown dwarf. However, the NIRSPEC
data obtained over 6 nights shows an rms dispersion of only 0.36 km s
and do not follow the periodic trend. These results indicate that the
variability seen with UVES is likely to be due to rotationally modulated
inhomogeneous surface features. We suggest that future planet searches around
very low-mass stars and brown dwarfs using radial velocities will be better
conducted in the near-infrared than in the optical.Comment: accepted by ApJ Letter
Ca II 8542 \AA\ brightenings induced by a solar microflare
We study small-scale brightenings in Ca II 8542 \AA\ line-core images to
determine their nature and effect on localized heating and mass transfer in
active regions. High-resolution 2D spectroscopic observations of an active
region in the Ca II 8542 \AA\ line were acquired with the GFPI attached to the
1.5-meter GREGOR telescope. Inversions of the spectra were carried out using
NICOLE. We identified three brightenings of sizes up to 2"x2". We found
evidence that the brightenings belonged to the footpoints of a microflare (MF).
The properties of the observed brightenings disqualified the scenarios of
Ellerman bombs or IRIS bombs. However, this MF shared some common properties
with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs
and MFs are both apparent in chromospheric and coronal layers according to the
AIA channels, and (2) both show flaring arches with lifetimes of about 3.0-3.5
min and lengths of about 20". The inversions revealed heating by 600 K at the
footpoint location in the ambient chromosphere during the impulsive phase.
Connecting the footpoints, a dark filamentary structure appeared in the Ca II
line-core images. Before the start of the MF, the spectra of this structure
already indicated average blueshifts, meaning upward motions of the plasma
along the LOS. During the impulsive phase, these velocities increased up to
-2.2 km/s. Downflows dominated at the footpoints. However, in the upper
photosphere, slight upflows occurred during the impulsive phase. Hence,
bidirectional flows are present in the footpoints of the MF. Conclusions: We
detected Ca II brightenings that coincided with the footpoint location of an
MF. The MF event led to a rise of plasma in the upper photosphere, both before
and during the impulsive phase. Excess mass, previously raised to at most
chromospheric layers, slowly drained downward along arches toward the
footpoints of the MF.Comment: Accepted for publication in Astronomy & Astrophysics, 13 pages, 6
figures, 1 online movi
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