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Astrophysics and Astrononmy
Gravitational Lensing and Quasars
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
13.07.2017
Co-author:
Carina Fian, Evencio Mediavilla
Abstract:
We describe principles of gravitational lensing, macrolensing and microlensing.
The physics of Quasars is shortly discussed and then quasar macro- and microlensing by galaxies is described. Own observations consist of photometric observations and spectroscopic observations.
We show how from the lensing parameters quantities like the Hubble constant, the mass of the lensing galaxy, the size of the accretion disc and the mass of the supermassive black hole at the center of the quasar can be derived.
Multiwavelength study of penumbral decay using GREGOR, VTT, DST, NST, and Hinode
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
19.01.2017
Co-author:
C. Denker, H. Balthasar, C. Kuckein, Reza Rezai, M. Sobotka, N. Deng, H. Wang, A. Tritschler, M. Collados, A. Diercke, S.J. González Manrique
Abstract:
Active region NOAA 12597 emerged on 22 September 2016 in southern hemisphere.
The region was observed two days later on 24 September 2016 with GREGOR, VTT,
NST, and Hinode during a campaign organized as part of the SOLARNET initiative
for coordinated observing campaigns. The leading spot of the region was observed
for the next four days. We obtained high-resolution imaging, spectroscopic, and
spectropolarimetric data in various spectral lines covering the photosphere as
well as the chromosphere. These data were complemented by synoptic line-of-sight
magnetograms and continuum images obtained with the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory (SDO) and slit-jaw images
from the Interface Region Imaging Spectrograph (IRIS). We will present the
dataset taken on 24 September 2016 containing the leading spot. The sunspot was
at its maximum growth and slowly started to disintegrate at the time of GREGOR
and VTT observations followed by the observations from Hinode and NST a few
hours later. We will discuss the photospheric and chromospheric flow fields
along with the magnetic fields during the penumbral decay of a large penumbral
sector. The penumbral filaments do not simply vanish but intermingle with the
nearby granules and even temporarily form darkened areas resembling umbral cores
filled with umbral dots.
Seismic diagnosis from gravity modes strongly affected by rotation
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
14.07.2016
Co-author:
Stéphane Mathis, François Lignières, Jérôme Ballot, Pierre-Marie Culpin
Abstract:
Most of the information we have about the internal rotation of stars comes from modes that are weakly affected by rotation, for instance thanks to rotational splittings.
In contrast, we present here a method (based on the asymptotic theory of Prat et al. 2016, A&A, 587, A110) which allows us to analyze the signature of rotation where its effect is the most important, that is in low-frequency modes that are strongly affected by rotation.
Deep Meridional Flow Inversions with Spherical Born Kernels and Time-Distance Helioseismology
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
09.05.2017
Co-author:
Shukur Kholikov, Jason Jackiewicz, Markus Roth
Abstract:
In this study, we present first inversion results for deep meridional flow using spherical Born approximation kernels and time-distance helioseismology.
The computation of Born approximation kernels for flows has only recently become available in spherical geometry. Compared to the ray approximation, the Born approximation is considered to provide a more realistic model of the advection and scattering processes in the solar interior, which are captured in travel time measurements.
We first validate this method using artificial data from a linear 3D simulation of solar interior wave propagation. We find that the prediction of the Born approximation model coincides well with the simulated data.
We then perform standard SOLA inversions of the solar meridional flow. First, inversion results of the simulated data are discussed and compared to the original flow profile included in the simulation.
Finally, we apply the validated method to GONG data spanning periods of low, medium and high solar activity (2001-2003, 2004-2006, and 2007-2009). The results are discussed and compared to literature.
Validating Spherical Born Kernels for Meridional Flows
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
07.09.2015
Co-author:
Markus Roth, Jason Jackiewicz
Abstract:
We present the current status of an undergoing validation of a recently developed model for computing spherical Born approximation sensitivity functions for flows. In a first step, power spectra and reference cross-correlations from the model and a simulation of Hartlep et al. (2013) are matched. Some difficulties in obtaining such a match are discussed. In a second step, travel times from the forward model and from the simulation, which includes a standard meridional flow profile, are to be compared. The analysis procedure including the use of phase-speed filters is identical to the one employed in Jackiewicz et al. (2015). Furthermore, we present a novel approach for a fast computation of integrated sensitivity functions which can be used for interpreting rotationally symmetric flows such as differential rotation and meridional flow.
Welcome Presentation of the Solarnet III / HELAS VII / SpaceInn Conference
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
31.08.2015
Abstract:
These are the slides of the opening of the Solarnet III / HELAS VII / SpaceInn Conference in Freiburg, August 31, 2015.
SpaceInn - Results and Impact
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
11.07.2016
Co-author:
SpaceInn Board
Abstract:
The collaborative project "SpaceInn - Exploitation of Space Data for Innovative Helio- and Asteroseismolgy" is funded under the European Union’s Seventh Framework Programme from January 2013 – December 2016. The European Helio- and Asteroseismology Network (HELAS) has initiated this project with the mission to build on the existing European strength in the field of time-domain stellar physics. In the last three years SpaceInn activities aimed to secure optimal use of the existing and planned data, from space and from the ground, in helio- and asteroseismology in order to improve access and scientific exploitation of the existing data.
The main goals of the project include:
-Establish coordinated archives of space- and ground-based data, as well as of the results of the analyses of these data. This includes tools for efficient data access.
-Secured long-term preservation of these, often unique, data.
-Coordinated utilization of the data, resulting in a much improved understanding of solar structure, dynamics and activity, as well as of stellar structure and evolution
-An increased awareness of the field, amongst the general public and at all levels of the educational system, throughout Europe.
In this talk I will present all these activities and the main outcome of the project.
The Solar Physics Research Integrated Network Group – SPRING
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
08.10.2015
Co-author:
Frank Hill[2], Michael Thompson[3], Sanjay Gusain[1,2]
Abstract:
SPRING is a project to develop a geographically distributed network of instrumentation to obtain synoptic solar observations. Building on the demonstrated success of networks to provide nearly-continuous long-term data for helioseismology, SPRING will provide data for a wide range of solar research areas. Scientific objectives include internal solar dynamics and structure; wave transport in the solar atmosphere; the evolution of the magnetic field over the activity cycle; irradiance fluctuations; and space weather origins. Anticipated data products include simultaneous full-disk multi-wavelength Doppler and vector magnetic field images; filtergrams in H-Alpha, CaK, and white light; and PSPT-type irradiance support. The data will be obtained with a duty cycle of around 90% and at a cadence no slower than one minute. The current concept is a multi-instrument platform installed in at least six locations, and which will also provide context information for large-aperture solar telescopes such as EST and the DKIST. There is wide support for the idea within the EU and the US solar research communities. The project is in the early planning stages, and we are open to and looking for participants in the science and instrument definition.
The need for synoptic solar observations from the ground
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
20.10.2015
Abstract:
Synoptic observations are indispensable in studies of long-term effects pertinent to variation in solar radiative output, space weather and space climate, as well as for understanding the physics of global processes taking place on our nearest star. Synoptic data also allow putting the Sun in the context of stellar evolution. Historically, the main-stay of such observations has been ground-based, although the improving longevity of space-borne instruments puts some space missions into the category of synoptic facilities. Space- and ground-based (synoptic) observations are complementary to each other; neither is inferior or superior to the other. Ground-based facilities can have a long-term (50 years+) operations horizon, and in comparison with their space-based counterparts, they are less expensive to operate and have fewer restrictions on international collaboration and data access. The instruments can be serviced, upgraded, and cross-calibrated to ensure the continuity and uniformity of long-term data series. New measurements could be added in response to changes in understanding the solar phenomena. Some drawbacks such as day-night cycle and the variable atmospheric seeing can be mitigated e.g., by creating the global networks and by employing the adaptive optics. Furthermore, the ground-based synoptic observations can serve as a backbone and a back-up to space-based observations.
In my talk I will review some existing ground-based synoptic facilities, describe plans for future networks, and outline the current efforts in strengthening the international collaboration in synoptic solar observations from the ground.
Two-scale Analysis of Solar Magnetic Helicity
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
23.11.2017
Co-author:
G. J. D. Petrie, & N. K.:Singh
Abstract:
The solar magnetic helicity has opposite signs not only in the two hemispheres, but also at large and small length scales. The latter can be detected by computing magnetic helicity spectra, but this must be done separately in each hemisphere. Here we utilize a two-scale method from mean-field dynamo theory that allows us to compute magnetic helicity spectra as a function of two different wavenumbers: one corresponding to rapidly varying scale and one corresponding to a slowly varying one. We generalize this method to spherical harmonics and compute in that way global magnetic helicity spectra for that part of the field that shows a global dipolar symmetry. We present results from simple one-dimensional model calculations, three-dimensional dynamo simulations, and the two-dimensional magnetic field from synaptic vector magnetograms.
Global helioseismic measurement of meridional circulation and differential rotation from mode eigenfunction perturbations
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
09.09.2015
Abstract:
I review and discuss recent helioseismic measurements of the meridional circulation and solar rotation from analysis of mode eigenfunction perturbations. Results obtained from analysis of MDI data covering 2006-2010 and from HMI data covering 2010-1014 are shown. The method is verified using numerical simulations. The results are compared to other helioseismic measurements of the meridional flow.
Accurate mean density and surface gravity of Delta Scuti stars using Asteroseismology
Natural Sciences (Astrophysics and Astrononmy)
Date of upload:
19.07.2016
Co-author:
M. J. P. F. G. Monteiro, S. Martín, Z. Guo, D. R. Reese, J. C. Suárez, J. Pascual-Granado, A. Moya, R. Garrido
Abstract:
In the work we present here, we empirically demonstrate that a simple relation exists between a periodic pattern present in the frequency spectra of Delta Scuti stars and their mean density. This relation is homologous to that of solar-type stars. But the most important result is that this relation is indeed independent of the rotational velocity, becoming an ideal tool to constrain the pyhisical parameters of the star, even at high rotation rates.
Moreover, we pushed the data one step further. Using also an estimation of the luminosity of each object, we demonstrate that it is possible, once the periodic pattern is determined, to derived the surface gravity of the star. The typical uncertainty we found for this quantity is the same as with high resolution spectroscopic data. This result might settle the degeneracy problem of the surface gravity determination with the spectra of A-type stars.
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