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Events

Kolloquium

Our colloquium takes place, unless otherwise specified, on Mondays at 15:00 CET. The format is hydrid: both in-person and remote. Regardless, all colloquium will be broadcasted via Zoom. Link will be provided in this website. and it will be available to all participants. People without relationship to our institute can only attend via Zoom. If you have suggestions write to: colloquia@

UPCOMING SEMINARS:  

  • Monday 12.05.2025 at 15:00 CET
    • Joao da Silva Santos, NSO
    • Title: Magnetic Reconnection in a Compact Magnetic Dome: Peculiar Emissions and High-velocity Plasma Flows (TBC)
       
  • Monday 13.01.2025 at 15:00 CET
    • Ioannis Kontogiannis, ETH/IRSOL
    • Title: TBA
       

  • Wednesday 15.01.2025 at 15:00 CET

    • Alex Pietrow, IAP
    • Title: TBA

     

PAST SEMINARS: 

  • Monday 09.12.2024 at 15:00 CET
    • Paul Charbonneau, Henri Lamarre, University of Montreal
    • Title: Flares as Magnetohydrodynamical Avalanches ?
    • Abstract: Avalanches of elementary reconnection events in stressed coronal magnetic structures overlying solar active regions offer a natural explanation for the  power-law form characterizing the observed frequency distribution of flare energy. So far, this rather bold statement is supported primarily by (relatively) simple lattice-based avalanche models, operating on simple discrete evolutionary rules loosely inspired by magnetohydrodynamics. In this talk we report on preliminary results regarding the search for avalanching behavior in magnetohydrodynamical (MHD) numerical simulations of chromospheric dynamics driven  by photospheric convection. Specifically, we work on numerical outputs from radiative MHD simulations carried out with the BIFROST code, as well as MHD simulations of a coronal magnetic arcade subjected to footpoint twisting, using the PLUTO code. Various measures of energy dissipation events in these simulations exhibit power-laws in their frequency distributions, but we will argue that the most reliable and unambiguous indicator of avalanching behavior  is the so-called spreading exponent, which measures the growth rate  of energy dissipation events. We will briefly discuss the potential of such MHD simulations in providing guidance towards designing  better physically motivated evolutionary rules for existing avalanche models used for flare prediction.
       
  • Monday 02.12.2024 at 11:00 CET
    • Itahiza Domiguez Cerdeña, Spanish National Geographical Institute (IGN)
    • Title: Overview of the 2021 La Palma volcanic eruption and the future effects on the astronomical observations of the Canary Islands
       
  • Monday 04.11.2024 at 15:00 CET
    • Mark Rast, CU Boulder
    • Title: Acoustic wave sources: Where are we/they?
    • Abstract: The source of solar acoustic oscillations has been the focus of research since the early days of p-modes observations. Deductions have been made based on the solar p-mode spectra and measurements of wave fluxes. New analysis techniques may allow, for the first time, direct identification of individual wave sources on the Sun. This talk will review those past efforts, these new developments, and the implications of recent DKIST observations.
       
  • Monday 28.10.2024 at 15:00 CET
    • Dmitrii Kolotkov, University of Warwick
    • Title: Space-time signatures of fast wave trains in the solar corona
    • Abstract: The highly-filamented nature of the coronal plasma plays a decisive role in the development and observational manifestation of various dynamic processes in the corona, such as magnetohydrodynamic waves and oscillations. Broadband fast magnetoacoustic waves, guided by coronal plasma non-uniformities, are known to be subject to strong geometric dispersion, which results in the formation of quasi-periodic fast-propagating (QFP) wave trains. Such wave trains are directly resolved in the imaging observations in the extreme ultraviolet band and indirectly in, for example, microwaves and low-frequency radio, from the low to high corona, up to a few solar radii. In addition to their significant seismological potential, QFP wave trains serve as important tools for understanding the magnetic connectivity channels, energy and mass transport in the corona. This talk presents the development of three distinct phases of a QFP wave train in coronal plasma structures with steep cross-field density profiles: a long-period quasi-periodic phase, where the oscillation period gradually shortens over time; a multi-periodic (peloton) phase, in which harmonics with distinctly different periods coexist; and a short-lived periodic (Airy) phase. The appearance of these phases is attributed to a non-monotonic dependence of the fast wave group speed on the parallel wavenumber. In the Morlet wavelet spectra, this corresponds to a characteristic boomerang-shaped pattern, with two well-pronounced arms at shorter and longer periods. On the other hand, measuring the QFP wave trains’ field-aligned group speed itself is challenging due to the strong dispersion and rapid evolution of the wave train's envelope. We demonstrate that the group speed of QFP wave trains in the corona can be assessed through the propagation speed of the wave train's effective centre of mass along the waveguide (as potential observable). The discrepancy between the centroid speed as the group speed measure and the phase speed at the corresponding wavelength is shown to reach 70\%, which is crucial for the energy flux estimations.
       
  • Monday 14.10.2024 at 15:00 CET.
    • Jaime de la Cruz Rodriguez, University of Stockholm
    • Title: A high spatial resolution view to the chromospheric heating problem on the Sun
    • Abstract: The outer layers of the Sun, the photosphere, chromosphere and corona, are shaped by a complex balance between gravity, magnetic forces and forces originating from gas pressure gradients. The resulting stratification is very dynamic and finely structured. Waves are excited by convection and propagate through these three layers where the gas density drops by more than seven orders of magnitude from the photosphere to the corona. Furthermore, the chromosphere is not in radiative equilibrium with the photosphere: it is much hotter than radiative equilibrium models predict, which raises the long-standing question of "what is heating the outer layers of the Sun". Waves and magnetic fields play a fundamental role in the transport of energy from the photosphere to the chromosphere, where this energy can be dissipated. Using results from advanced empirical models, in this seminar I will show some observational data are helping us to set constraints on the processes that heat the chromosphere.
       
  • Wednesday 25.09.2024 at 14:00 CET
    • Sebastian Castellanos Duran (MPS)
    • Title: On the common appearance of superstrong magnetic fields in bipolar light bridges
    • Abstract: Bipolar light bridges (BLBs) are bright features in sunspots located between two umbrae with opposite magnetic polarity. Recent observations revealed intriguing cases of BLBs with very strong magnetic fields of the order of 8.2 kG, which is at least twice the typical values measured in sunspot umbrae. Since these observations were only a handful, it is a question of whether BLBs with extraordinarily strong fields are very rare. We used the most extensive set of spectropolarimetric observations of sunspots with BLBs compiled so far, consisting of data acquired with Hinode/SOT-SP. We analyzed these data using a state-of-the-art inversion technique, which accounts for the data degradation caused by the intrinsic PSF of the telescope. These data are compiled in the so-called MODEST catalog. We identified 98 individual BLBs within 51 distinct sunspot groups. Since 66% of the identified BLBs were observed multiple times, our sample contained a total of 630 spectropolarimetric scans. Our analysis showed that 89% of the (individual) BLBs contain magnetic fields stronger than 4.0 kG, at the optical depth of maximum magnetic sensitivity with even higher field strengths in deeper layers. We also found that BLBs display a unique continuum intensity and field strength combination, forming a population well-separated from the umbrae and the penumbrae. 
       
  • Monday 23.09.2024 at 15:00 CET
    • Elena Petrova, KU Lueven
    • Title: Exploring high-frequency transverse waves in the chromosphere
    • Abstract:  Recent advancements in ground- and space-based solar telescopes have facilitated the detection of pervasive incompressible transverse waves, present in both the chromosphere and corona. Observations from the Solar Orbiter HRIEUV have extended the power-law behavior of energy versus frequency to the range of higher frequencies, revealing their significant potential in terms of the energy budget. In this study, our objective is to examine the power-law behavior of the detected transverse waves in the chromosphere and contrast it with the one in the corona. This analysis offers valuable insights into understanding the propagation of energy through the layers of the solar atmosphere. To accomplish this objective, we utilize the MiHI instrument, an integral field spectrograph renowned for its ability to achieve simultaneous ultra-high resolution across spatial, temporal, and spectral domains. Centered around the H$\alpha$ line, it provides observations of the chromosphere layer. A vast network of chromospheric fibrils traverses the field of view captured by MiHI, where we detect short-period oscillation events with periods of order of tens of seconds. Moreover, high resolution in the spectral dimension enables the inference of velocities from line asymmetries using the bisector method. This is one of the rare instances where we have access to both intensity and Doppler velocity maps, allowing for their simultaneous analysis.
       
  • Monday 08.07.2024 at 15:00 CET
    • Laura Hayes, ESA
    • Title: Solar flares and quasi-periodic pulsations: New insights from Solar Orbiter.
    • Abstract: Solar flares are among the most intense manifestations of solar activity, and one of the major drivers of space weather. Over the course of tens of minutes, solar flares can release energies reaching 10^25 J, resulting in emission observable across the electromagnetic spectrum, particularly in X-ray wavelengths. An integral feature of both solar and stellar flares is the occurrence of quasi-periodic pulsations (QPPs), which appear as oscillatory patterns in emission with varying periods from several seconds to minutes. These pulsations are most clearly evident during the impulsive phase of flares, observed prominently in non-thermal hard X-ray and microwave emissions, and provide significant insights into the particle acceleration at the flare site. Recent advancements in observational capabilities, notably from ESA’s Solar Orbiter, have broadened our perspective, revealing the presence of QPPs across an extended wavelength range, including thermal soft X-ray, extreme ultraviolet (EUV), and decimetric radio bursts. Given that QPPs are directly linked to all aspects of flaring energy release, their observations provide clues to the physical processes operating in the flare site. In this presentation, I will discuss an overview of multi-wavelength observations of solar flares and QPP signatures together with current interpretations of the phenomena. In particular, I’ll focus on how observations now available with Solar Orbiter provide new insights into solar flares 
       
  • Wednesday 03.07.2024 at 15:30 CET
    • Javier Fernandez, National Tsing Hua University, Taiwan (R.O.C.)
    • Title: The Taiwan Automated Telescope (TAT) Network Project and its Application in Asteroseismology
    • Abstract: The Taiwan Automated Telescope (TAT) Network is a global ground-based network of identical and fully automated optical telescopes. These telescopes are located at appropriate longitudes around the world to reduce diurnal gaps and to provide long data sets. The automatization of these telescopes was developed in the headquarters of the network, which is located in the Physics Department of the National Tsing Hua University, Taiwan. We present the application of the TAT network to investigate pulsations in two low-amplitude delta Scuti stars, V830 Her and HD 163032. The TAT network was used to measure photometric pulsations of two low-amplitude Scuti stars: V830 Her and HD 163032. The four-consecutive-year data were obtained from two stations of the network: Teide Observatory (Spain) and Maidanak Observatory (Uzbekistan). Several new analysis methods were used to reduce the noise in the measured light curves and improve the precision of measured mode parameters.
       
  • Monday 01.07.2024 at 15:00 CET
    • Hans-Guenther Ludwig, University of Heidelberg
    • Title: 3D magneto-hydrodynamical model atmospheres: the influence of surface magnetism on the center-to-limb variation of solar-type stars
    • Abstract: Observations indicate that magnetic activity has an influence on the center-to-limb variation (CLV) of solar-type stars. In my presentation, I briefly review the magnetic activity of the Sun as template of activity of solar-type stars in general. I further review the observations conducted with the Kepler space observatory on transiting exo-planetary systems that motivated my project on the CLV. With the help of 3D magneto-hydrodynamical (MHD) model atmospheres a theoretical description of the observation is attempted. I describe the properties of the MHD models relevant for the project. MHD models are constructed which differ in the mean magnetic field strength. They are considered as elements suitable to build-up the stellar surface. Their radiative output for various stellar limb angles is calculated and compared to the measurements. A partial correspondence is achieved, however, discrepancies remain that may be of observational or theoretical origin.
       
  • Wednesday 26.06.2024 at 15:00 CET
    • Volker Bothmer, Georg-August-Universität Göttingen
      Institut für Astrophysik und Geophysik
    • Title: Parker Solar Probe - A mission to „touch“ the Sun
    • Abstract: Since launch on 12 August 2018 NASA‘s Parker Solar Probe has completed 19 orbits around the Sun in June 2024 and has delivered data from hitherto unreached regions close to the Sun. As first spacecraft ever it has flown through the solar corona in April 2021, making it a historic space mission. After the seventh Venus flyby in November 2024, it will reach its planned closest approach of the Sun on December 24, 2024, at a distance below 10 solar radii. Parker Solar Probe has already provided a treasure worth of data from the birth regions of the solar wind, coronal mass ejections and solar energetic particles which have led to fundamental new insights into the physics of the solar corona and its effects on interplanetary space. Beyond that the mission has provided new views of planet Venus and the dust distribution within the Sun-Earth-system. This presentation will provide an insight into the current state of the mission, its spectacular new observations and a summary of the mission’s science highlights.  
       
  • Monday 17.06.2024 at 15:00 CET
    • Damien Fournier, Max Planck Institute for Solar System Research Göttingen
    • Title: Imaging the solar interior with time-distance helioseismology

    • Abstract: The acoustic waves propagating in the solar interior contain information about the internal structure of our closest star. To extract this information, a relation between the structure parameters and the observations (such as wave travel times) needs to be derived. Then, the inverse problem needs to be solved in order to recover parameters such as large-scale flows (differential rotation, meridional circulation). In this talk, I will present both the forward and inverse problems using numerical simulations as well as observations. The observations are usually corrected for various systematic errors such as center-to-limb variations. In the last part of my talk, I will present recent theoretical development to better understand these errors by modeling the effect of radiative transfer on the solar oscillations.

       

  • Monday 10.06.2024 at 15:00 CET
    • Fabio Riva, IRSOL
    • Title: Simulating small-scale dynamo action in stars
    • Abstract:  The origin of the ubiquitous small-scale magnetic field observed on the Sun can be attributed to the action of a small-scale turbulent dynamo (SSD). A similar process could self-sustain a considerable amount of magnetic energy also on stars other than the Sun. Here, we report on a set of magneto-hydrodynamics SSD simulations of stars of spectral type F5V, G2V, K2V, and K8V that we carried out with the radiative magnetohydrodynamic CO5BOLD code. This to open a wider perspective on the action of the SSD itself. Simulations are set-up to have approximately the same Reynolds and magnetic Reynolds numbers, thus disentangling the impact of the physical parameters (effective temperature and surface gravity) on the SSD action from numerical effects. The resulting SSD growth rates in SI units differ for the four stellar models, with the largest and smallest growth rate for the K2V and F5V model, respectively. This is due to different convective turnover times in the four simulations. Yet, the SSD field strengths reached in the saturation phases are similar in all models. At saturation, the spatial structure of the magnetic field resulting from SSD action is investigated. Magnetic fields self-organize at the stellar surface in the form of strong kG magnetic flux concentrations, leading to magnetic bright features. Yet, more than 90% of the magnetic flux through any of these stellar surfaces has a field strength less than 1 kG. Moreover, small-scale vortical flows are found to be abundant in the simulated atmospheres of all the investigated stars and signatures of torsional Alfvénic pulses associated with this swirling motion are identified.
       
  • Friday 31.05.2024 at **11:00** CET
    • Carlos Quintero Noda, Manuel Collados, Juan Carlos Trelles Arjona, IAC
    • Title: First light observations of the upgraded Gregor Infrared Spectrograph
       
  • Monday 06.05.2024 at 15:00 CET
    • Christopher Osborne, University of Glasgow (UK)
    • Title: Modelling and Interpreting Non-LTE Emission from Isolated Solar Structures
       
  • Friday 03.05.2024 at 15:00 CET
    • Benoit Tremblay (HAO, Boulder, US)
    • Title: Probing photospheric flows with deep learning and physically informed neural networks (tentative title)
       
  • Monday 22.04.2024 at 15:00 CET
    • Markus Roth, TLS (Germany)
    • Title: The Sun in Focus
    • Abstract: The Sun has served as a paradigm for astrophysics for many decades. Even though this star is the closest to us it still holds a multitude of secrets, and fascinates astronomers and physicists alike. With dedicated instruments in space and on the ground the various aspects of these secrets are revealed, ranging from the structure and dynamics of the outer solar atmosphere to the deep solar interior. In particular, through helio- and asteroseismology detailed inferences of the stellar internal structure, rotation, flows, and origins of magnetic activity can be obtained. Therefore in addition to local high-resolution observations,  research projects like SPRING (Solar Physics Research Integrated Network Group) aim to study the Sun in a synoptic way. With the possibility of linking then the processes inside the Sun to the ones in the atmosphere, solar physics plays a crucial role in understanding stellar magnetic activty and in investigating the origins of space weather. Solar research has therefore a growing relevance for astrophysics and beyond.
       
  • Monday 15.04.2024 at 15:00 CET
    • Nadia Kostogryz, MPS (Germany)
    • Title: Stellar Magnetic Fields and Limb Darkening: Unveiling Exoplanet Secrets
    • Abstract:  Transit light curves are affected by host star darkening towards the limb, impacting transit profile and depth. Thus, accurate determination of planetary radii and atmospheric composition requires solving the problem of limb darkening for stars with different parameters. However, there is a conundrum: multiple observations show significantly brighter stellar limbs as compared to all available stellar models.
      We propose that neglecting surface stellar magnetic fields is the main issue in models of the limb darkening. Magnetic fields form within stars and emerge on their surface. They affect properties of stellar atmospheres and therefore emergent radiation. Here, we model the effect of small-scale concentrations of surface magnetic field in low- and high- resolution stellar spectra using 3D radiative magnetohydrodynamics (MHD) code MURaM and radiative transfer code MPS-ATLAS. These small-scale concentrations of surface magnetic field create a rather uniform network on stellar surface. We found that including this network in our simulations allows reproducing stellar limb darkening deduced from the observations of Kepler and TESS transits, thus reconciling models and observations.
       
  • Monday 08.04.2024 at 15:00 CET
    • María Jesús Martínez González, Instituto Astrofisica de Canarias (Spain)
    • Title: Magnetism and activity. From the Sun to stars
    • Abstract: Stars are the building blocks of astrophysics, yet very explicit questions on stellar magnetism are still unsolved. How magnetic fields form? How do they evolve? Are they relic fossils or do they regenerate from plasma motions through dynamo action? How does dynamos actually work? How do they reconnect? What mechanism/s underlie coronal heating? From my point of view, solving such fundamental questions requires the complementary study of the Sun and other stars. Extremely detailed physical models can be tested on the Sun in spectroscopic, spatial, and temporal detail. But the Sun is a moderate, average example of a middle-aged main-sequence star, in no way special nor extreme, and presents only a small range of atmospheric behaviors. Stars with different masses, ages, rotation periods, and chemical composition need to be understood as well. During my postdoctoral position at the Meudon Observatory, I worked closely to Prof. Meir Semel, who was an excellent Solar Physicist and the father of stellar spectropolarimetry. In this talk I will show (mainly through my work from the past 15 years) how stellar observations can help understanding the Sun’s magnetism and viceversa.
  • Monday 25.03.2024 at 15:00 CET
    • Stefan Hofmeister, Columbia University (USA)
    • Title: Forecasting Space Weather
    • Abstract: Space Weather research has a direct impact on society, but also allows scientists to develop models and thereby test their understanding of the Sun-Earth system. In this talk, I will shortly speak about Space Weather impacts on society and the general setup of models, but focus afterwards on key issues we have to resolve in order to make the models accurate. Particularly, we need deeper collaborations between the fields of spectropolarimetry in the photosphere, global models of the corona, the solar wind acceleration, and the solar wind propagation from the Sun to Earth.
       
  • Tuesday 19.03.2024 at 15:00 CET
    • Damien Przybylski, Max Planck Institute for Solar System Research (Germany)
    • Title: The MURaM chromospheric extension
    • Abstract: The study of the solar chromosphere is complicated by its highly dynamic and structured nature and the necessary inclusion of NLTE physics. Additionally, the theoretical treatment is complicated, and previous models fail to reproduce observed features of chromospheric spectral lines. We extend the MURaM code to include the physics required to treat the solar chromosphere, following the Bifrost code. We produce models of the quiet sun and investigate the behavior of the modeled chromosphere. We synthesize a number of diagnostics formed in the chromosphere and show a close match to the average profiles of the observed special lines.
       
  • Monday 18.03.2024 at 15:00 CET
    • Theodosios Chatzistergos, Max Planck Institute for Solar System Research (Germany)
    • Title: Historical solar observations for understanding past solar magnetism
    • Abstract: Reconstructing past variations in irradiance is crucial for climate studies, especially considering the limited coverage of direct Total Solar Irradiance (TSI) measurements over time. Models have proven effective in this regard, attributing variability on timescales longer than a day to the evolution of the solar surface magnetic field. Consequently, these models require accurate data to capture the contributions of solar surface magnetic features, including sunspots and faculae. While sunspot data extend back to the 1600s, facular data are notably scarcer. Although historical archives of Ca II K and Hα observations provide the necessary information for understanding faculae and past solar magnetism, their utilization has been hindered by many challenges. In particular, the existing archives contain observations performed with different observational characteristics (for instance bandpasses ranging between 0.1 and 9A). In this presentation, I will give an overview of our recent work on Ca II K and Hα observations as well as sunspot data to improve our understanding of past solar magnetism and solar irradiance variations.
       
  • Monday 11.03.2024 at 15:00 CET
    • Dusan Vukadinovic, Max Planck Institute for Solar System Research (Germany)
    • Title: Inferring atomic line parameters from solar spectra
    • Abstract: Many solar spectral lines have poorly determined atomic parameters, such as the transition probability or the central wavelength, especially in the near-ultraviolet (NUV) range. We present a novel approach to exploit high-angular-resolution spectropolarimetric observations of the Sun to obtain precise atomic parameters of spectral lines, required for the proper analysis of solar and stellar observations. The inversion-based approach requires that unique (global) values of atomic parameters must represent spectral line profiles originating from very different solar atmospheres within an observed field of view very well. To test this method, we analyzed synthetic spectra computed from magneto-hydrodynamic models of the solar atmosphere and compared the results to the previously used methods. The method is general enough to be applied to the spectral lines from the NUV to the infrared
      wavelengths. The method is able to retrieve reliable estimates of atomic parameters even for weak and blended spectral lines, thus extending the list of spectral lines with reliable atomic parameters used to analyze solar and stellar spectra. 
       
  • Monday 26.02.2024 at 15:00 CET
    • David Kuridze,  National Solar Observatory (USA)
    • Title: Fine-scale structure of the solar chromosphere with DKIST
    • Abstract: The fine-scale structure of the solar chromosphere is one of the oldest scientific topics in astrophysics. It is produced through the complex interaction between the plasma, radiation and the magnetic field. Despite a wealth of observations and numerical models, disentangling the processes that drive the chromospheric fine structure remains one of the most compelling problems in solar physics. Resolving properly fine-scale details of the chromosphere and measuring polarization signals remain one of the main observational challenges too. In this seminar, I will present results from our recent work where the fine-scale structure of the chromosphere and its magnetic environment were studied by analyzing broadband imaging and full Stokes spectropolarimetric observations obtained with the largest solar optical telescope, DKIST. The large aperture and state-of-the-art instrumentation afforded by DKIST, allowed us to observe a large field of view and resolve unprecedented fine-scale details at ~20 km spatial resolution in some instances. Our results demonstrate that Balmer-beta spectral line is an excellent diagnostic to identify and track the enigmatic fine-scale structure of the chromosphere. The results also showed that the morphological characteristics of the chromospheric fine structures (fibrils) are defined by the topology of magnetic field in the lower solar atmosphere.
       
  • Monday 05.02.2024 at 15:00 CET
    • Nishant Singh,  Inter-University Centre for Astronomy and Astrophysics, Pune, India
    • Title: On magnetic fields and surface gravity mode of the Sun
    • Abstract: The Sun offers to be a unique laboratory where various predictions of the mechanisms responsible for the generation of the magnetic fields can be observationally tested. It has proved to be challenging to understand the origins of solar magnetism at large, viz, the global fields revealing a butterfly pattern, and at small scales, e.g., the appearance of sunspots and active regions. We will discuss some latest developments on these topics based on numerical and observational studies. It is likely that gaps in our understanding of stellar convection is the reason that the problem of solar/stellar magnetism has proved so challenging. Feasibility and potentials of magnetically induced perturbations on the solar surface gravity or the f-mode will also be discussed.
       
  • Monday 29.01.2024 at 15:00 CET
    • Franziska Zeuner,  IRSOL (Switzerland)
    • Title: The GREGOR Slow Polarization Modulator
    • Abstract: With an increasing interest in using the GREGOR telescope for high-sensitivity scattering polarization measurements, the need arose to decrease systematic errors limiting the zero-level accuracy in polarimetric measurements. To achieve this goal, a novel observation method is proposed to enhance the zero-level accuracy at GREGOR.The method is based on a slow modulation combined with fast modulation full Stokes polarimetry. Built on our experience with this method gained at the IRSOL telescope in Locarno, Switzerland, a newly designed slow modulator was implemented and tested at the GREGOR telescope in the last years. Here I will briefly present the method, and what we learned about its advantages and limitations when applied to high-sensitivity scattering polarization measurements at GREGOR.
       
  • Monday 09.10.2023 at 15:00 CET
    • Hans-Peter Doerr, Tautenburg Observatory (Germany)
    • Title: Towards high-fidelity spectropolarimetry from the ground

    • Abstract: This is because the problem is rooted in the rather short evolution time scale of the Sun which decreases as telescope resolution increases. To make things worse, image restoration is required to improve the fidelity of ground-based data, which restores resolution and contrast but brings out the noise as well.

      Two ways to approach this problem have been explored at the Max Planck Institute for Solar System Research: Integral field instruments capture an extended two-dimensional scene at once with no need to scan in any of the (x,y,lambda)-dimensions and thus allow to maximize the integration time for a given cadence. Wide-band slit spectropolarimeters allow a large number of spectral lines to be inverted simultaneously, effectively increasing the accuracy and sensitivity of the inferred atmospheric parameters. I will talk about the prototypes and instruments of that type (and their benefits and challenges) that have been built at MPS for the Swedish Solar Telescope and the Goode Solar Telescope and which are envisaged to play a key role for the science targeted with the European Solar Telescope.

  • Monday 16.10.2023 at 15:00 CET
    • Daniel Ryan, University of Applied Sciences and Arts Northwestern Switzerland (Switzerland)
    • Title: Reconstructing the 3-D Evolution of a Solar Flare Thermal Looptop Source using Solar Orbiter/STIX and Hinode/XRT
    • Abstract: The 2020 launch of ESA's Solar Orbiter satellite has placed a solar X-ray imager far beyond Earth's orbit (STIX: Spectrometer/Telescope for Imaging X-rays).  This enables us to reconstruct the 3-D geometry of solar thermal X-ray sources for the first time by combining STIX observations with those of the Hinode's X-ray Telescope (XRT).  Solar thermal X-ray sources are crucial for understanding of solar flares.  Their locations reveal where the hottest plasma resides within the 3-D magnetic field structure, their motions indicate something about the continued evolution of that structure, and their volume affects how much energy they contain.  In this talk we reconstruct the 3-D evolution of the thermal looptop X-ray source of an M4-class solar flare and discuss the implication for its thermodynamic evolution.  We also discuss some of the challenges of 3-D reconstruction and its benefits over traditional methods of estimating 3-D source volumes from single-viewpoint observations.  Finally, we discuss other capabilities of Solar Orbiter's STIX instrument and to how it might help in your research.
  • Monday 23.10.2023 at 15:00 CET
    • Sijie Yu, New Jersey Institute of Technology (New Jersey, USA)
    • Title: Microwaves/X-ray Bursts in Solar Flares
    • Abstract: Solar flares are known for their ability to efficiently accelerate electrons, yet the detailed mechanisms underlying this phenomenon remain less understood due to limited diagnostics in and around the acceleration regions. By combining radio and X-ray observations, we can obtain complementary insights into the nonthermal electron acceleration during solar flares. This combination allows us to track the spatial and temporal evolution of nonthermal electrons across different energy ranges. When integrated with EUV solar observations, we can better understand electron acceleration processes in the context of other flare events, such as reconnection outflows. I will present some recent advancements in this area and discuss the potential of next-generation radio telescopes, with a focus on the capabilities of the Frequency Agile Solar Radiotelescope (FASR).
  • Monday 18.09.2023 at 15:00 CET
    • Iulia Ana Maria Chifu, University of Göttingen (Germany)
    • Title: Exploring solar wind and corona with Parker Solar Probe
    • Abstract: NASA’s Parker Solar Probe (PSP) mission was launched, shortly before the last solar cycle minimum, in August 2018. The main scientific goals of PSP are to help unravel the origin and structure of the solar corona and the processes responsible for the acceleration of the solar wind and of solar energetic particles. Until now PSP has accomplished 16 orbits around the Sun, approaching it to a distance of 13.4 solar radii and has become the first spacecraft to travel to the outer atmosphere of a star. PSP will reach its closest distance to the Sun on 24th December 2024 at a distance below 10 solar radii. Its technical design allowed to withstand the Sun’s heat and to operate the scientific instruments flawlessly. Each encounter has brought new spectacular data, sampling for the first-time structures of the solar corona and solar wind below the sub-alfvénic regime, from within coronal mass ejections (CMEs) and during crossings of coronal streamers with its embedded heliospheric current sheet (HCS). Beyond breakthroughs in solar and heliospheric physics, PSP has facilitated spectacular views of planets, asteroids, and comets, and the interplanetary distribution of dust particles. In my talk, I will introduce the basic mission concept and briefly summarise the new observations. The focus will be on the contribution of the CGAUSS (coronagraphic German and US Parker Solar Probe Survey) project to the WISPR (Wide-field Imager for Solar Probe) camera led by the University of Göttingen.
  • Monday 17.07.2023 at 15:00 CET
    • Prof. Mitchell Berger, Exeter University (UK)
    • Title: The geometrical and topological description of magnetic fields
    • Abstract: Magnetic fields have a strong influence on the behaviour of the solar interior and atmosphere, and the solar wind. Magnetic fields are important in planetary cores, in accretion disks and in interstellar space. An important part of magnetic field studies lies in understanding and quantifying their geometry and topology. In this talk I will review various methods of describing magnetic field geometry. The magnetic field lines within a 3-D region can be divided into subregions where the field lines do not diverge; these subregions are separated by separatrices or quasi-seperatrix layers. The twist and writhe of the field lines with a region add to its self-helicity. The linking and winding of fieldlines in different regions about each other leads to mutual helicity. Complex magnetic field structures can sometimes be further described using concepts from braid theory. I will discuss some applications to solar coronal heating and to the characterization of magnetic clouds in the solar wind
  • Wednesday 12.07.2023 at 15:00 CET
    • Prof. Arnab Rai Choudhuri, Indian Institute of Science (Bangalore, India)
    • Title: An enigmatic fluid flow inside the Sun: the meridional circulation
    • Abstract: On the solar surface, we find a continuous flow of material from the equator to the poles. To avoid material piling up at the poles, there has to be a back-flow somewhere underneath the solar surface, which is now mapped by helioseismology. This flow – known as the meridional circulation – plays an important role in the dynamo process producing the 11-year sunspot cycle. I shall discuss our present-day observational and theoretical understanding of this intriguing circulation. The theory of the meridional circulation is intimately connected with the theory of differential rotation and provides a clue about the formation of the near-surface shear layer. It is found that the meridional circulation becomes weaker at the time of the sunspot maximum. I shall present our recent work on modelling this theoretically.
  • Friday 14.07.2023 at 10:00 CET
    • Prof. Arnab Rai Choudhuri, Indian Institute of Science (Bangalore, India)
    • Title: How the Saha ionization equation was discovered.
    • Abstract: Although the Saha ionization equation is a standard topic in advanced statistical physics and most professional physicists would have some knowledge of it, the exact nature of Meghnad Saha’s contributions in this subject is not widely known. Based on an analysis of Saha’s original papers and other relevant papers by contemporary scientists, as well as other source materials such as letters exchanged between scientists, I shall discuss how Saha used the theory of thermal ionization to solve some of the most important astrophysics problems of that era.
  • Monday 03.07.2023 at 15:00 CET
    • Antonio Ferriz-Mas, University of Vigo (Spain)
    • Title: The role of the entropy stratification in the magnetic-flux-storage capacity of the tacocline
    • Abstract: At the interface between the convection zone and the underlying stably stratified radiative interior there is a thin layer of overshooting convection (‘overshoot layer’) in which the temperature gradient is subadiabatic. Below the convection zone there also exists a shear layer known as the ‘tachocline’, which represents a sharp transition between two distinct rotational regimes: the differentially rotating convection zone and the almost rigidly rotating radiative interior. The relative position between these two boundary layers - one mechanical and one thermal - determines the degree of subadiabaticityof the tachocline. Since the decade of 1980 many astrophysicists believe that the tachocline plays a fundamental role in the generation and storage of the toroidal magnetic flux that eventually becomes unstable and buoyantly rises to emerge at the stellar surface producing sunspots. In this talk I will discuss the role of the thermodynamic properties (and more specifically, the entropy stratification) of the overshoot layer in determining the stability of magnetic structures and, therefore, its capability to store magnetic flux. The entropy stratification is quantified by a dimensionless physical quantity called the superadiabaticity, δ. Tiny variations in δ (of the order of 10−4 or less) may determine global properties of the magnetic field at the solar surface. The connection between temporal variations in δ (which would alter the storage capacity of the tachocline) with the occurence of Maunder-minimum-like episodes and will be discussed
    •  
  • Monday 26.06.2023 at 15:00 CET (via zoom)
    • Antonie Strugarek, CEA Paris-Saclay, DAp-AIM (France)
    • Title: Detectability of magnetic star-planet interactions in compact exosystems

    • Abstract: Close-in planets are thought to generally orbit in a sub-alfvénic stellar wind. The perturbations they excite in the stellar corona are able to travel upwind down to the stellar surface, and potentially induce observable phenomena in the visible, in X-rays, and in radio. The stellar global magnetic field is at the heart of star-planet magnetic interaction: its strength sets the magnetic energy available for the interaction, its shape determines the connection path between the star and the planet, and its temporal modulation (e.g. magnetic cycles) is at the source of an on/off behavior of the magnetic interaction.

      I will give an overview of our understanding of star-planet magnetic interactions and the existing scaling laws for their amplitude. I will then present specific studies of the 3D star-planet magnetic interactions, in particular in the Kepler-78 system and in HD 189733. I will show how stellar rotation, planetary orbit, and stellar magnetic topology come all in play to modulate the signal from star-planet magnetic interactions. By analysing such signal based on 3D numerical simulations, I will show that we can explain the difficulty to detect them with existing observational campaigns of HD 189733 (e.g. Cauley et al. 2018). Our results warrant dense spectroscopic observational campaigns, coupled to spectro-polarimetric campaigns to firmly detect star-planet magnetic interactions. Such firm detections would open up the possibilities to characterise the magnetic field of distant exoplanets on short orbit. 

  • Monday 19.06.2023 at 15:00 CET (via zoom)
    • Oliver Hall, European Space Agency (ESA, Noordwijk, Netherlands)
    • Title: Spinning stars in Kepler and TESS: from sound to starspots
    • Abstract: An explosion of space-based photometric data in the last decade from the Kepler and TESS missions has given us an unprecedented amount of information on stars across the HR diagram. On the main sequence this has led to an expansion of our understanding of how the rotation rates of Sun-like stars evolves as they age. This relationship between age and rotation, called gyrochronology can be used to age-date stars for various purposes, from measuring the ages of exoplanet hosts to determining groups of stars born together. In this talk I’ll go through two different ways we can measure stellar rotation – through asteroseismology and star spot variability – and how both contribute to a more comprehensive understanding of stars and stellar evolution.
  • Tuesday 13.06.2023 at 15:00 CET
    • Christoph Kuckein, Instituto de Astrofisica de Canarias (La Laguna, Spain)
    • Title: From bright points to flares, the capabilities of the GREGOR telescope.
    • Abstract: GREGOR, Europe's largest solar telescope, has undergone significant changes in both optics and instrument layout since its inauguration in 2012. Equipped with state-of-the-art instrumentation, GREGOR offers extensive capabilities for studying a wide range of solar physics topics. For example, it allows for the investigation of photospheric bright points, which are the smallest observable manifestations of the solar surface's magnetic field. By using its multiple instruments spanning the visible blue to the near-infrared spectral range, GREGOR enables the study of bright points at various atmospheric heights. Additionally, with the Sun's current activity, dynamic events like filament eruptions and flares have gained importance as targets for GREGOR observations. During this colloquium talk, I will present scientific results related to these topics and discuss the challenges associated with interpreting future spectropolarimetric flare observations using GREGOR.
  • Monday 05.06.2023 at 15:00 CET
    • Carlos Diaz Baso, Rosseland Center for Astrophysics (Oslo, Norway)
    • Title: Deep Learning 2.0: Combining physics-based models with data-driven solutions
    • Abstract: In the last decade, machine learning and neural networks have emerged as powerful tools to select and analyze relevant information from huge collections. By exploiting symmetries and patterns in the data, these new techniques can be optimized to perform various autonomous tasks (such as classifications, regression problems, dimensionality reduction, and many others) faster and better than conventional methods. They have been used in data preprocessing, automatic solar feature segmentation, image deconvolution, acceleration of spectropolarimetric inversions, and prediction of explosive phenomena. Some years after the introduction of these modern deep learning techniques in solar physics, they are not only more mature, but new approaches combining physical constraints, statistical approaches, and the modeling capabilities of neural networks are being explored. In this contribution, I will review a selection of new applications to various problems in solar physics, discussing outstanding issues and offering a perspective for future research.
  • Monday 15.05.2023 at 14:00 CET
    • Denis Tilipman, National Solar Observatory (Boulder, USA)
    • Title: Quantifying the flux of magnetic energy through the quiet Sun photosphere: how can we do it and what can we learn?
    • Abstract: The flux of magnetic energy, or Poynting flux, is usually thought to be responsible for the heating of the solar chromosphere and corona. Quantifying it is important in order to understand the energy budget at different heights in the solar atmosphere, but doing so requires computing several intermediate quantities, particularly full magnetic and velocity vectors. This requires, among other problems, to address the issue of 180º magnetic field azimuthal ambiguity. Poynting flux has been computed for active regions and plage, but the quiet Sun (QS) is a difficult and poorly studied target due to inherently weak, compact, and transient magnetic field concentrations. In this talk, I will discuss our efforts to compute Poynting fluxes using Sunrise/IMaX magnetograms of QS and how they can inform observing campaigns, e.g. with DKIST. We find that the magnetic energy flux from the photosphere is insufficient to explain chromospheric and coronal heating. However we also find strong biases that likely lower the observable flux. Finally, we find that the choice of azimuth disambiguation method does not significantly affect Poynting flux values.
  • Thursday 13.04.2023 at 15:00CET
    • Tatiana Kaltman, Special Astrophysical Observatory (St. Petersburg, Russia)
    • Title: The structure of the solar  active regions based on microwave observations and modeling of radio emission.
    • Abstract: Radio astronomical methods are important tools for studying  the solar atmosphere. They provide information  about  plasma parameters  in a wide range of heights that are difficult to measure by other methods.  Large radio telescopes with high sensitivity and good spatial and frequency resolution are necessary for the diagnosis of physical conditions in the transition zone and the lower corona of the Sun. In this contribution I present  the structure of the solar active regions  with a different set of components, both according to microwave observations and modeling. The main components of the active region are spots, floccules, halos, and sources above the neutral line. It is proposed to diagnose the active region, accompanying the processing of observations with model calculations. A diagnostic technique based on multi-frequency microwave observations, magnetic field reconstruction and radio emission calculations is presented. The technique makes it possible to estimate the electronic temperature at different heights and in different parts of the active region. In addition,  evaluation of the contribution of various harmonics of cyclotron emission  improves the measurement of the magnetic field by the radio astronomy method.  This diagnostic technique has been tested on one-dimensional observational data of the RATAN-600 radio telescope, and the next step is to extend it to images obtained using interferometer.
  • Monday 27.03.2023 at 15:00CET
    • Ellena Ralli, Musikhochschule Freiburg
    • Title: Composition process through the sonification of scientific data from various areas. "Arp 319" for violin, viola, bass clarinet, flute and electronics
    • Note:  the title of the piece refers to the cataloge number of the galaxy group Stephan's Quintet. en.wikipedia.org/wiki/Stephan%27s_Quintet
  • Monday 17.10.2022 at 16:00CET
    • Robert Tawa, DKIST Data Center (National Solar Observatory, Boulder/US)
    • Title: DKIST Data Center -- Process and Design
    • Abstract: The DKIST Data Center was designed and built with a number of architectural qualities in mind - scalability and reliability being two obvious ones among  many others. However, one of the key qualities that was designed into the DKIST system was observability.  This talk will discuss why this quality was designed into the system from the start, and how it has allowed the DKIST Data Center to maintain the system.
  • Monday 10.10.2022 at 16:00CET
    • Prof. Dr. Manuel Collados Vera, Instituto de Astrofísica de Canarias (Spain)
    • Title: The European Solar Telescope: status and perspectives
    • Abstract: With first light expected at the end of the present decade, the European Solar Telescope (EST) represents the most important technological joint effort made by the European ground-based Solar Physics community. EST will improve considerably the present observational capabilities thanks to its 4-metre diameter. Its optical design is especially designed to study magnetic phenomena taking place in the solar atmosphere, optimising two crucial aspects. On the one hand, its polarimetrically-compensated design is conceived to cancel out the instrumental polarisation induced by the individual elements of the telescope optical train. This property is crucial to make feasible the detection of very small, spatial and temporal, fluctuations of the magnetic field. Secondly, its design includes a powerful multi-conjugate adaptive optics system (MCAO) to optimally correct the wave-front distortions introduced by the Earth's atmosphere. With its MCAO system, EST is intended to measure the Sun at diffraction limit, with a spatial resolution of 20-30 km and a cadence of few seconds. The design is complemented with the most advanced suite of instruments that will operate simultaneously, to extract the maximum information about the dynamics, thermodynamics and magnetism of the solar plasma at different layers. In this talk, the status of the project will be presented, putting emphasis on the most recent technical developments, on the instrumentation and scientific goals that will be addressable with this facility and the most recent and upcoming milestones.
  • Monday 12.09.2022 at 16:00CET
    • Dr. Andrés Asensio Ramos, Instituto de Astrofísica de Canarias (Spain)
    • Title: Methods in Machine Learning for solar spectroscopy
    • Abstract: Solar spectropolarimetry is entering the realm of big data. Current and future telescopes will produce data at a rate that will make it hard to store in a single machine and even harder to operate on the data. Thankfully, in the last decade, machine learning has experienced an enormous advance, thanks to the open possibility of training very deep and complex neural networks. In this contribution I show options to explore to deal with the big data problem and also how deep learning can be used to efficiently solve difficult problems in Solar Physics. I will focus on how differentiable programming (aka deep learning) is helping us to have access to velocity fields in the solar atmosphere, correct for the atmospheric degradation of spectropolarimetric data and carry out fast 3D inversions of the Stokes parameters to get physical information of the solar atmosphere.
  • Tuesday 05.04.2022 at 16:00
    • Dr. David Orozco Suárez. 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.
  • 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 20.06.2022
    • Dr. Ryan Campbell. Queen's University (Belfast, UK)
    • Title: Exploring pervasive small-scale internetwork magnetism with the GRIS-IFU
    • Abstract: Synthetic observations produced from MURaM outputs have predicted that higher polarization fractions in the quiet solar photosphere would be revealed by increasing the total integration time or increasing the spatial resolution of GREGOR Infrared Spectrograph Integral Field Unit (GRIS-IFU)/GREGOR observations. We take advantage of improvements to GREGOR to reveal an unprecedented abundance of weak, small-scale internetwork (IN) magnetism. We present several new GRIS-IFU datasets of the Fe I 1564.9 nm line, with two datasets in particular taken during exceptional seeing conditions showing very high polarization fractions. We uncover evidence that could suggest GREGOR's overhaul has helped achieve a higher effective spatial resolution, and modelling with degraded synthetic profiles suggests that these datasets, compared to previous observations, are closer to the diffraction limit in the near infrared. By observing with high signal-to-noise (S/N), high spatial resolution, one of the most magnetically sensitive spectral line available to solar observations, and exceptional seeing conditions, we reveal one of the highest fraction of polarization ever recorded in the (not-so) quiet Sun. We present several case studies of the zoo of magnetic features present in these data, including small-scale magnetic loops and serpentine fields, focusing on regions where the full Stokes vector has been recorded. The open-source Python 3 Inversion Explorer application will also be showcased, which has been developed to help the community to interrogate spectropolarimetric inversions with ease.
  • Monday 27.06.2022
    • Dr. Joao da Silva Santos. National Solar Observatory (Boulder, USA)
    • Title: Observations and modeling of heating in chromospheric current sheets
    • Abstract: The chromosphere is a transition layer between the cool photosphere and the million-degree hot corona. Different heating mechanisms, such as damping of magnetohydrodynamics waves and dissipation of magnetic energy in current sheets or field reconnection, may account for a significant fraction of the energy input required to sustain the observed radiative cooling. The active chromosphere, in particular, appears significantly brighter than the quiet one, suggesting magnetically mediated heating. We present co-temporal SDO ultraviolet imaging, SST spectropolarimetry in the Fe I 6173 Å and Ca II 8542Å lines, and ALMA brightness temperature maps in the continuum at 3 mm of a solar active region. Nonlocal thermodynamic equilibrium inversions of the SST+ALMA dataset constrain the atmospheric stratification and show that enhanced chromospheric temperatures and cooling rates are associated with strong and inclined magnetic fields that connect patches of opposite magnetic polarity in the photosphere. Numerical simulations show that energy dissipation in current sheets during flux emergence leads to enhanced radiative losses and a range of observational signatures in the millimeter continuum from compact, transient brightenings to warm fibril-like structures consistent with the observations. Finally, we will discuss how coordination between different observatories offers a promising albeit challenging way of addressing chromospheric heating.
  • Monday 04.07.2022
    • Dr. Juie Shetye. New Mexico State University (Las Cruces, NM, USA)
    • Title: Using chromospheric jets to connect the solar atmosphere

    • Abstract: The solar chromosphere serves as a bridging region between the photosphere, which is the surface of the Sun, and the corona, its outer region. This dynamic region is filled with a plethora of features that vary in time and space. With the advent of high-resolution ground-based observations we can discover new features.  We use some of the World’s biggest solar telescopes to zoom into this region and it reveals never seen before dynamics.

      Here, I present detailed observations of two science topics that are guided by observations.  I show a statistical study of spicules, which are long-thin grass-like features observed on the sun.  These events wiggle-jiggle and sway around their axes or along a common centre of mass to create wave-like motions on the Sun. These waves can travel with speeds on 100s of km per second to energise the solar chromosphere.  The second example I show are swirling-whirling events, that look like Tornadoes on the Earth. These churn the matter from the Lowe photosphere to the chromosphere. Studying the behaviour of such events is vital in understanding the workings of mass and energy cycles of the solar atmosphere. 
      In addition, the current work presented already tests the limits of current telescopes in terms of the temporal and spatial resolution. The answer to exploring the depth of chromosphere lies in building next-generation solar physics observatories such as DKIST that have 3 times more spatial resolution than CRISP/SST.

  • Monday 18.07.2022
    • Dr. Maria Weber. Delta State University (Cleveland, MS, USA)
    • Title: How Suns Get Their Spots: What we know and what we don’t
    • Abstract: Starspots are manifestations of a star’s internal magnetic processes.  However, even for our own Sun, there are still many uncertainties about the complex mechanisms that give the Sun its spots. Within the context of magnetic flux emergence in the Sun, I will discuss what we know and what we don’t about how active region magnetism is generated and rises through the solar interior to produce sunspot observables. I will also discuss our work to establish links between dynamo-generated magnetic fields, shearing and fluid motions, and observations of starspots in solar-like stars down to fully convective M dwarfs.