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Our kolloquium takes place, unless otherwise specified, on Mondays at 16:00.


  • Monday 23.05.2022
    • Dr. Lena Khomenko. Instituto de Astrofisica de Canarias, IAC (La Laguna, Spain)
    • Title: Towards multi-fluid simulations of the solar chromosphere
    • Abstract: Solar chromosphere has been at the focus of solar physics studies for decades. Chromosphere is a transition layer between the interior and exterior of the Sun where the temperature starts to increase further from the surface, but the heating mechanisms responsible for this increase are still unknown. The chromospheric dynamics is defined by magnetic fields. Chromospheric plasma is strongly stratified, weakly ionised and not completely collisionally coupled. The latter circumstances need to be taken into account in theoretical models of the chromosphere. In this talk I will overview our recent results of the modeling of the solar chromosphere, comparing a more standard single-fluid approach and a more novel multi-fluid approach. I will describe the challenges posted by the multi-fluid modeling that still need to be overcome. Finally, I will talk about the observational effort in detecting the dynamical decoupling of the neutral and ionized species in the solar chromosphere.


          • Monday 13.06.2022
            • Dr. Maria Lukicheva. Max Planck Institut für Sonnensystemforschung (Göttingen, Germany)
          • Monday 20.06.2022
            • Dr. Ryan Campbell. Queen's University (Belfast, UK)
          • Monday 27.06.2022
            • Dr. Joao da Silva Santos. National Solar Observatory (Boulder, USA)
          • Monday 04.07.2022
            • Dr. Juie Shetye. New Mexico State University (Las Cruces, NM, USA)
          • Monday 18.07.2022
            • Dr. Oliver J. Hall. ESTEC (Noordwijk, The Netherlands)

          PAST SEMINARS:

          • Tuesday 05.04.2022 at 16:00
            • Dr. David Orozco Suarez. Instituto de Astrofisica de Andalucia, IAA (Granada, Spain)
            • Title: CMAG: A Coronal MAGnetograph mission for studying the inner corona magnetic fields
            • Abstract: The detailed study of the Sun's inner coronal magnetic fields is one of the technological challenges of solar physics today since it requires measuring the four Stokes components in forbidden coronal emission lines whose polarization signals can differ orders of magnitude in their amplitude. In this talk I will present an overview of the main challenges for determining the inner corona magnetic fields and how they can be addressed from space. In particular, I will present preliminary concept of the CMAG (Coronal MAGnetograph), a space mission designed to obtain maps of the coronal magnetic fields with unprecedented spatial and temporal resolutions by means of a dedicated imaging spectropolarimeter whose external occulter is placed about 400 meters away. CMAG baseline consists of two different spacecraft in formation flight. The first one is meant to act as an external occulter by carrying a dedicated disk. The second one would be a small satellite with a refracting telescope and an internal occulter to image the solar inner corona measuring all four Stokes parameters at given wavelength samples of selected emission spectral lines from the corona.
          • Monday 11.04.2022 at 16:00
            • Dr. Mariarita Murabito. INAF Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Roma (Italy)
            • Title: On the origin of magnetic perturbations associated to the FIP effect
            • Abstract: Observations of the photosphere, chromosphere, and corona combined with magnetic field modeling of one of the biggest sunspots of the 24 solar cycle, revealed that regions of high FIP bias plasma in the corona were magnetically linked to the locations of the intrinsic magnetic oscillations in the solar chromosphere. In order to characterize the driver of the oscillations, we analyzed the relation between the spatial distribution of the magnetic wave power and the overall field geometry and plasma parameters obtained from the multi-height spectropolarimetric non-local thermodynamic equilibrium (NLTE) inversions. In correspondence with the locations where the magnetic wave energy is observed at chromospheric heights, we found evidence in support of locally excited acoustic waves that, after crossing the equipartition layer located close to the umbra-penumbra boundary at photospheric heights, are converted into magnetic-like waves. These results indicate a direct connection between sunspot chromospheric activity and observable changes in coronal plasma composition, demonstrating the power of high resolution, multi-height studies of the solar atmosphere that will become the gold standard in the era of DKIST. 
          • Monday 25.04.2022
            • Dr. Jaime de la Cruz Rodriguez. Stockholm University (Stockholm, Sweden)
            • Title: Setting observational constraints to the chromospheric heating problem.
            • Abstract: The solar atmosphere is hotter than predicted by assuming radiative equilibrium. This is most obviously evidenced by the high temperature of the solar corona, but the bulk of the energy deposition happens already down in the much cooler chromosphere. While in recent years we have gain detailed understanding of many important processes that must be at work in the chromosphere, also from numerical simulations, their exact contribution to the total energy budget remains unclear. Chromospheric heating or cooling can be estimated by calculating the radiative losses whenever a model atmosphere is available. Most comparisons between simulations and observations have used canonical values of radiative losses that have been derived from 1D models of spatio-temporal averages of solar spectra (e.g., FAL / VAL models). Such approach cannot capture the high complexity and fine structures that is observed in high resolution observations. In this talk, I will discuss the accuracy and challenges associated with the methods that we use to infer radiative losses from observations. I will also present our first spatially resolved maps of the radiative losses derived from very high spatial resolutions observations acquired with the Swedish 1-m Solar Telescope in the Ca II K, Ca II 8542 and Fe I 6301/6302 lines.
          • Monday 02.05.2022
            • Dr. Arturo López Ariste. IRAP, Université de Toulouse, (Toulouse, France)
            • Title: Spectropolarimetric Imaging of Red SuperGiants. Doing solar physics in other stars.
            • Abstract: The unexpected recent discovery of linear polarisation in the spectra of Betelgeuse led us to a new imaging technique of the photosphere of this and other red supergiant stars. At present, we are able to periodically produce 3-dimensional images of the convection patterns of these stars. We have been able to measure typical scales and lifetimes of granulation. We also measured supersonic radial velocities, confirming the results from 3D simulations. In the last work, we have been able to measure the radial velocity profile of granulation and found some interesting cases in which this velocity is constant up to large heights. This points to an unknown force, compensating gravity, and potentially responsible for the high rates of mass loss that characterise the last stage of the lives of these stars. Through these techniques we are abandoning the exploration of stars as unresolved objects, and enter the era where we examine them with spatial resolution, as for the Sun. We do solar physics in other stars.
          • Monday 09.05.2022
            • Dr. Ivan Milic. University of Colorado (Boulder, USA)
            • Title: How to train your inversion
            • Abstract: Spectropolarimetric inversions are the method of choice for quantitative diagnostics of the solar atmosphere. They allow us to go from three-dimensional cubes of Stokes parameters (x,y, wavelength), to pseudo-three-dimensional cubes of physical parameters (x,y, optical depth). The process involves non-linear least-squares fitting of very complicated forward models and is subject to many pitfalls, limitations, and systematic errors. The first part of the talk will present the inversion process, focusing on the limitations and computational assumptions that lead to different systematic errors and biases. We will review state-of-the-art approaches that aim to rectify some of these problems. We will then discuss a few examples where we use MHD simulations as a testbed for the inversions. Finally, we will present some simple but robust applications of neural networks to spectropolarimetric inversions.
          • Monday 16.05.2022
            • Dr. Ioannis Kontogiannis. Leibniz Institute for Astrophysics Potsdam, AIP (Potsdam, Germany)
            • Title: Magnetic properties of active regions and eruptive activity: recent results and future directions
            • Abstract: The active regions that produce major flares and coronal mass ejections (CMEs) exhibit common morphological characteristics, such as δ-spots, filaments, sigmoids, and strongly sheared magnetic PILs. These are indicative of complex magnetic configurations, associated with strong electric currents and storing huge amounts of free magnetic energy and helicity. Using this empirical knowledge and appropriate observations we calculate parameters/predictors that can help us distinguish between quiet, flare-, and CME-productive (eruptive) active regions. In the era of near real time, uninterrupted, high-quality observations from space, and large, statistically significant samples, machine learning methods accommodate the processing of data of increasing volume and complexity. These comprise not only photospheric magnetograms, but also EUV images, spectra, and time series and, thus, our inventory of data sources and feature extraction methods expands. Additionally, exploratory research has focused on the study of active regions in terms of these magnetic parameters shedding more light on the mechanisms that produce flares and CMEs and leading to new magnetic predictors and prediction models. In this talk, I will review these efforts to parameterize the characteristics of eruptive active regions and I will discuss some recently introduced and promising magnetic parameters, as well as future prospects.