1,735 research outputs found
Bayesian least squares deconvolution
Aims. To develop a fully Bayesian least squares deconvolution (LSD) that can
be applied to the reliable detection of magnetic signals in noise-limited
stellar spectropolarimetric observations using multiline techniques. Methods.
We consider LSD under the Bayesian framework and we introduce a flexible
Gaussian Process (GP) prior for the LSD profile. This prior allows the result
to automatically adapt to the presence of signal. We exploit several linear
algebra identities to accelerate the calculations. The final algorithm can deal
with thousands of spectral lines in a few seconds. Results. We demonstrate the
reliability of the method with synthetic experiments and we apply it to real
spectropolarimetric observations of magnetic stars. We are able to recover the
magnetic signals using a small number of spectral lines, together with the
uncertainty at each velocity bin. This allows the user to consider if the
detected signal is reliable. The code to compute the Bayesian LSD profile is
freely available.Comment: 8 pages, accepted for publication in A&
Signatures of the impact of flare ejected plasma on the photosphere of a sunspot light-bridge
We investigate the properties of a sunspot light-bridge, focusing on the
changes produced by the impact of a plasma blob ejected from a C-class flare.
We observed a sunspot in active region NOAA 12544 using spectropolarimetric
raster maps of the four Fe I lines around 15655 \AA\ with the GREGOR Infrared
Spectrograph (GRIS), narrow-band intensity images sampling the Fe I 6173 \AA\
line with the GREGOR Fabry-P\'erot Interferometer (GFPI), and intensity broad
band images in G-band and Ca II H band with the High-resolution Fast Imager
(HiFI). All these instruments are located at the GREGOR telescope at the
Observatorio del Teide, Tenerife, Spain. The data cover the time before,
during, and after the flare event. The analysis is complemented with
Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI)
data from the Solar Dynamics Observatory (SDO). The physical parameters of the
atmosphere at differents heights were inferred using spectral-line inversion
techniques. We identify photospheric and chromospheric brightenings, heating
events, and changes in the Stokes profiles associated to the flare eruption and
the subsequent arrival of the plasma blob to the light bridge, after traveling
along an active region loop. The measurements suggest that these phenomena are
the result of reconnection events driven by the interaction of the plasma blob
with the magnetic field topology of the light bridge.Comment: Accepted for publication in A&
Validating the effect of fuel moisture content by a multivalued operator in a simplified physical fire spread model
Fuel moisture content (FMC) plays a significant role in wildfire behavior and rate of spread (ROS). In addition, FMC is a highly dynamic factor and very vulnerable to climate variations. Understanding the effect of FMC on the behavior of fire spread models is crucial, and detailed analysis of specific aspects of complex models is a very effective way to improve them. The simplified physical fire spread model PhyFire considers the effect of FMC in a novel way, involving a multivalued maximal monotone operator. Several numerical experiments have been carried out to confirm that the behavior of the ROS simulated with PhyFire involving FMC is as expected in the reviewed literature: an exponential decrease in fire ROS compared to FMC, for different scenarios, considering different fuel types, terrain slopes and wind speeds. PhyFire performs very accurately, proving that the multivalued operator used is suitable and consistent
Paleomagnetic and paleoenvironmental implications of magnetofossil occurrences in late Miocene marine sediments from the Guadalquivir Basin, SW Spain
Although recent studies have revealed more widespread occurrences of magnetofossils in pre-Quaternary sediments than have been previously reported, their significance for paleomagnetic and paleoenvironmental studies is not fully understood. We present a paleo- and rock-magnetic study of late Miocene marine sediments recovered from the Guadalquivir Basin (SW Spain). Well-defined paleomagnetic directions provide a robust magnetostratigraphic chronology for the two studied sediment cores. Rock magnetic results indicate the dominance of intact magnetosome chains throughout the studied sediments. These results provide a link between the highest-quality paleomagnetic directions and higher magnetofossil abundances. We interpret that bacterial magnetite formed in the surface sediment mixed layer and that these magnetic particles gave rise to a paleomagnetic signal in the same way as detrital grains. They, therefore, carry a magnetization that is essentially identical to a post-depositional remanent magnetization, which we term a bio-depositional remanent magnetization. Some studied polarity reversals record paleomagnetic directions with an apparent 60-70 kyr recording delay. Magnetofossils in these cases are interpreted to carry a biogeochemical remanent magnetization that is locked in at greater depth in the sediment column. A sharp decrease in magnetofossil abundance toward the middle of the studied boreholes coincides broadly with a major rise in sediment accumulation rates near the onset of the Messinian salinity crisis (MSC), an event caused by interruption of the connection between the Mediterranean Sea and the Atlantic Ocean. This correlation appears to have resulted from dilution of magnetofossils by enhanced terrigenous inputs that were driven, in turn, by sedimentary changes triggered in the basin at the onset of the MSC. Our results highlight the importance of magnetofossils as carriers of high-quality paleomagnetic and paleoenvironmental signals even in dominantly terrigenous sediments.This study was funded by the Guadaltyc project (MINECO,
CGL2012–30875), ARC grant DP120103952, and NSFC grant
41374073
3D GPS velocity field and its implications on the present-day postorogenic deformation of the Western Alps and Pyrenees
We present a new 3D GPS velocity solution for 182 sites for the region encompassing the Western Alps, Pyrenees, and southern France. The velocity field is based on a Precise Point Positioning (PPP) solution, to which we apply a common-mode filter, defined by the 26 longest time series, in order to correct for network-wide biases (reference frame, unmodeled large scale processes, ¿). We show that processing parameters, such as troposphere delay modeling, can lead to systematic velocity variations of 0.1 - 0.5 mm/yr affecting both accuracy and precision, especially for short (< 5 yr) time series. A velocity convergence analysis shows that minimum time-series lengths of ~3 years and ~5.5 years are required to reach a velocity stability of 0.5 mm/yr in the horizontal and vertical components, respectively. On average, horizontal residual velocities show a stability of ~0.2 mm/yr in the Western Alps, Pyrenees, and southern France. The only significant horizontal strain rate signal is in the western Pyrenees with up to 4 x 10-9 yr-1 NNE-SSW extension, whereas no significant strain rates are detected in the Western Alps (< 1 x 10-9 yr-1). In contrast, we identify significant uplift rates up to 2 mm/yr in the Western Alps but not in the Pyrenees (0.1 ± 0.2 mm/yr). A correlation between site elevations and fast uplift rates in the northern part of the Western Alps, in the region of the Wurmian ice cap, suggests that part of this uplift is induced by postglacial rebound. The very slow uplift rates in the southern Western Alps and in the Pyrenees could be accounted for by erosion-induced rebound
A near-IR line of Mn I as a diagnostic tool of the average magnetic energy in the solar photosphere
We report on spectropolarimetric observations of a near-IR line of Mn I
located at 15262.702 A whose intensity and polarization profiles are very
sensitive to the presence of hyperfine structure. A theoretical investigation
of the magnetic sensitivity of this line to the magnetic field uncovers several
interesting properties. The most important one is that the presence of strong
Paschen-Back perturbations due to the hyperfine structure produces an intensity
line profile whose shape changes according to the absolute value of the
magnetic field strength. A line ratio technique is developed from the intrinsic
variations of the line profile. This line ratio technique is applied to
spectropolarimetric observations of the quiet solar photosphere in order to
explore the probability distribution function of the magnetic field strength.
Particular attention is given to the quietest area of the observed field of
view, which was encircled by an enhanced network region. A detailed theoretical
investigation shows that the inferred distribution yields information on the
average magnetic field strength and the spatial scale at which the magnetic
field is organized. A first estimation gives ~250 G for the mean field strength
and a tentative value of ~0.45" for the spatial scale at which the observed
magnetic field is horizontally organized.Comment: 42 pages, 17 figures, accepted for publication in the Astrophysical
Journal. Figures 1 and 9 are in JPG forma
Lightning climatology of exoplanets and brown dwarfs guided by Solar system data
We highlight financial support of the European Community under the FP7 by an ERC starting grant number 257431. RAT thanks the Royal Astronomical Society (RAS) and the Physics Trust of the University of St Andrews for supporting his summer placement at the University of St Andrews.Clouds form on extrasolar planets and brown dwarfs where lightning could occur. Lightning is a tracer of atmospheric convection, cloud formation and ionization processes as known from the Solar system, and may be significant for the formation of prebiotic molecules. We study lightning climatology for the different atmospheric environments of Earth, Venus, Jupiter and Saturn. We present lightning distribution maps for Earth, Jupiter and Saturn, and flash densities for these planets and Venus, based on optical and/or radio measurements from the World Wide Lightning Location Network and Sferics Timing and Ranging Network radio networks, the Lightning Imaging Sensor/Optical Transient Detector satellite instruments, the Galileo, Cassini, New Horizons and Venus Express spacecraft. We also present flash densities calculated for several phases of two volcano eruptions, Eyjafjallajökull's (2010) and Mt Redoubt's (2009). We estimate lightning rates for sample, transiting and directly imaged extrasolar planets and brown dwarfs. Based on the large variety of exoplanets, six categories are suggested for which we use the lightning occurrence information from the Solar system. We examine lightning energy distributions for Earth, Jupiter and Saturn. We discuss how strong stellar activity may support lightning activity. We provide a lower limit of the total number of flashes that might occur on transiting planets during their full transit as input for future studies. We find that volcanically very active planets might show the largest lightning flash densities. When applying flash densities of the large Saturnian storm from 2010/11, we find that the exoplanet HD 189733b would produce high lightning occurrence even during its short transit.Publisher PDFPeer reviewe
Decadal variability of European sea level extremes in relation to the solar activity
This study investigates the relationship between decadal changes in solar activity and sea level extremes along the European coasts and derived from tide gauge data. Autumn sea level extremes vary with the 11 year solar cycle at Venice as suggested by previous studies, but a similar link is also found at Trieste. In addition, a solar signal in winter sea level extremes is also found at Venice, Trieste, Marseille, Ceuta, Brest, and Newlyn. The influence of the solar cycle is also evident in the sea level extremes derived from a barotropic model with spatial patterns that are consistent with the correlations obtained at the tide gauges. This agreement indicates that the link to the solar cycle is through modulation of the atmospheric forcing. The only atmospheric regional pattern that showed variability at the 11 year period was the East Atlantic pattern
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