Seminars this semester


   Series:


May 26 Wed Du Pei (University of Harvard) Pure Maths Colloquium
14:00 Meet Hidden algebraic structures in geometry from fivebranes
 
  Abstract:
The existence of quantum field theories in higher dimensions predicts many hidden algebraic structures in geometry and topology. In this talk, I will survey some recent developments where such algebraic structures lead to new insights into 1) the quantization of moduli spaces of Higgs bundles, 2) the categorification of quantum invariants of 3-manifolds, and 3) novel types of TQFTs in four dimensions.
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May 26 Wed Bruno Barros (Lisbon) Cosmology, Relativity and Gravitation
15:00 Blackboard Collaborate Dark energy interactions: phenomenology and observations
 
  Abstract:
In this talk, I will thoroughly explore the phenomenology of dark energy couplings, by exposing two distinct models where dark energy interacts with dark matter. We will journey through three different cosmological phases, so as to study the main influence of the coupling on formation of structure processes. The background cosmology is analysed by resorting to numerical and dynamical system techniques. We will follow closely the linear behaviour of the matter perturbations and test their growth against redshift space distortions data. We also shed some light on the sensitivity of future missions to constrain the dark coupling. Finally, we are going to witness the collapse of matter overdensities by inspecting the physics of the spherical collapse, tracking the evolution of perturbations along the first stages of their nonlinear regime, and compute the number of bound structures formed.
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May 27 Thu Bradley W. Hindman (University of Colorado, Boulder (USA)) Plasma Dynamics Group
16:00 Meet Do Coronal Loops Oscillate in Isolation?
 
  Abstract:
One of the most prominent features seen in images of the solar corona by EUV telescopes are the elegant arches of glowing plasma that trace magnetic field lines through the corona. Typically, these loops are preferentially illuminated segments of a larger structure comprised of an arcade of arched field lines. Such loops are often observed to undulate in response to nearby solar flares. A flurry of observational and theoretical effort has been devoted to the explanation and exploitation of these oscillations. The grand hope is that seismic techniques can be used as probes of the strength and structure of the corona’s magnetic field. The commonly accepted viewpoint is that each visible loop oscillates as an independent entity and acts as a separate wave cavity for MHD kink waves. Thus, the seismic analysis is conveniently reduced to a 1D wave problem with boundary conditions at the foot points of the loop in the photosphere. I will argue that for many events, this generally accepted model for the nature of the wave cavity is fundamentally wrong. In particular, the entire 3D magnetic arcade in which the bright loops reside participates in the oscillation. Thus, the true wave cavity is much larger than the individual loop and inherently multidimensional. I will present theoretical arguments to support this 3D viewpoint and discuss the implications and opportunities for seismology of the solar corona.
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May 28 Fri Fionnlagh Dover (SP2RC (UoS)) SP2RC seminar
13:00 Google meet link: https://meet.google.com/ciq-zovu-rzm Numerical MHD Investigation of Jets in the Solar Atmosphere
 
  Abstract:
Solar spicules are one of the dominant dynamic phenomena of the lower solar atmosphere. Here, we show our results on modelling the propagation of such localised jets driven by a momentum pulse as the exciting force near photospheric heights. Using the MPI-AMRVAC code to perform 2D MHD simulations in an idealised stratified solar atmosphere, we investigate how key parameters (e.g., driver time, equilibrium magnetic field strength, velocity amplitude of driver and tilt with respect to the magnetic field) determine the morphology of these small-scale solar jets. A parametric study is carried out and using jet tracking software we analyse the jet properties (e.g., widths, apex heights, etc). We find that jet boundary deformation occurs naturally due to speeds involved in driving these jets within the range of spicule heights that could be then a possible alternative explanation for the appearance of transverse motions (both axisymmetric and non-axisymmetric deformations). By resolving structures up to 10 km, we also find unforeseen substructures inside the spicular jet beam. We propose observers to confirm this latter finding that may be challenging due to current spatial resolution limits.
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Jun 3 Thu Azaymi Litzi Siu-Tapia (Instituto de Astrofísica de Andalucía, IAA (ES)) SP2RC/ESPOS
10:00 Zoom (Meeting ID: 165 498 165) The solar atmosphere as observed through the Mg I b2 line at high spatial resolution
 
  Abstract:
The Mg I b2 line at 5173 Å forms over a large range of heights but its core, which forms under conditions of non-local thermodynamic equilibrium, is most sensitive to heights near the temperature minimum, a region of the solar atmosphere that has not been sufficiently explored. The next-generation solar observatories will have access to this spectral line and will allow for multi-line observations to study the different layers of the solar atmosphere simultaneously and with unprecedented polarimetric sensitivity. We will present a morphological classification of the intensity and circular polarization profiles of this spectral line at high-spatial-resolution, using observations from the Swedish 1-m Solar Telescope. We will also discuss the results of the weak field approximation applied to the Mg I b2 line, and their comparison with inversion results of the Fe I 6173 Å line to understand how the magnetic field changes with height in the solar atmosphere.
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Jun 9 Wed Noemi Frusciante (Lisbon) Cosmology, Relativity and Gravitation
15:00 Blackboard Collaborate Probing modified gravity with cosmology and solutions to the Hubble tension
 
  Abstract:
The late time cosmic acceleration is one of the most puzzling phenomena in modern cosmology. Its modeling within General Relativity (GR) through the cosmological constant (L) results in the LCDM scenario. Although the latter gives a precise description of the Universe, it is known that it still contains a number of unresolved problems. These lead researchers to look for modified gravity models, for example by including additional degrees of freedom. In this talk I will present the phenomenology and the cosmological bounds of theories consistent with the gravitational-wave event GW170817. In particular I will discuss models which solve the Hubble tension between Planck and local measurements and for which data show a statistically significant preference over LCDM.
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Jun 17 Thu Deborah Baker (Mullard Space Science Laboratory, University College London (UK)) SP2RC/ESPOS
10:00 Zoom (Meeting ID: 165 498 165) Magnetic Perturbations in a Sunspot Chromosphere Linked to Plasma Fractionation in the Corona
 
  Abstract:
Element abundance signatures have long been used as tracers of physical processes throughout astrophysics. Understanding the spatial and temporal variations in the composition of the solar corona provides insight into how matter and energy flow from the solar chromosphere out into the heliosphere as well as from the chromospheres of solar-type stars into their astrospheres. In this work, we investigate the spatial distribution of highly varying plasma composition around one of the largest sunspots of solar cycle 24. Observations of the photosphere, chromosphere, and corona are brought together with magnetic field modeling of the sunspot in order to probe the conditions that regulate the degree of plasma fractionation within loop populations of differing connectivities. We find that, in the coronalmagnetic field above the sunspot, variation in plasma composition is highly structured, with extremes in the level of fractionation among the distinct loop populations. Loops above the umbra contain unfractionated plasma, i.e. photospheric composition, while coronal loops rooted in the penumbra contain fractionated plasma, with the highest levels observed in the loops that connect within the active region. The distribution of the highly fractionated plasma appears to be correlated with the spatial locations at which intrinsic magnetic perturbations are identified in high spatial resolution spectropolarimetric observations of the solar chromosphere. Tracing field lines from regions of highly fractionated plasma in the corona to locations of magnetic perturbations detected in the chromosphere shows that they are magnetically linked. These results indicate a direct connection between sunspot chromospheric activity and observable changes in coronal plasma composition. We interpret our findings in the wider context of coronal heating and the ponderomotive force model of elemental fractionation.
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Jun 17 Thu Abdulrahman Albidah (Sheffield) Plasma Dynamics Group
16:00 Meet POD and DMD tutorial
 
  Abstract:
Modal decomposition, like POD and DMD, are incredible methodologies with a vast range of applicability. In our group, we have an open code for that in matlab. In today's talk, Abdulrahman will explain how those codes work.
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Jun 24 Thu Prof. Lucie Green (UCL, Mullard Space Science Laboratory) SP2RC seminar
10:00 Zoom link: https://zoom.us/j/95338171418 Meeting ID 953 3817 1418 UK-SOSS: What can magnetic helicity tell us about the likelihood of an eruption from active regions?
 
  Abstract:
Understanding the physical processes that underly the occurrence of coronal mass ejections is a key area of research, whilst being able to forecast an ejection beforehand would provide significant benefits to space weather forecasting lead-times. In this talk, these two aims will be discussed in the context of a quantity known as magnetic helicity, in particular the so-called helicity proxy that can be determined from modelled active region magnetic field and which has shown potential in being able to indicate when an active region will produce eruptive activity. Results will be presented for NOAA active region 11158, that builds on the helicity proxy analysis already presented by Thalmann et al. (2019) to incorporate the observed evolution of the coronal field and the physical processes taking place in the time leading up to an eruption.
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Jun 25 Fri Dr Marco Stangalini (Italian Space Agency (ASI,Italy)) SP2RC seminar
13:00 Google meet link: https://meet.google.com/ciq-zovu-rzm Waves in the lower solar atmosphere through spectropolarimetry: first detection of torsional oscillations in the solar photosphere
 
  Abstract:
MHD waves in the lower solar atmosphere were generally investigated through intensity and Doppler velocity data. However, modern high spatial and temporal resolution spectropolarimetry gives access to additional diagnostics, which can be helpful in the identification and the correct interpretation of the wave modes. Very recently, thanks to high-resolution spectropolarimetric observations in the lower solar atmosphere it was possible to detect for the first time anti-phase incompressible torsional oscillations in a magnetic pore. These waves have proven to be important in a range of physical systems due to their ability to transport non-thermal energy over long distances in a magnetised plasma. This property is of specific interest in solar physics where the extreme heating of the atmosphere of the Sun remains unexplained. Despite evidences in the upper atmosphere, they were not directly observed in the photosphere. The excitation of torsional waves in photospheric magnetic structures can significantly contribute to the energy transport in the solar atmosphere and the acceleration of the solar wind, especially if such signatures will be ubiquitously detected in even smaller structures with the forthcoming next generation of solar telescopes, like DKIST, EST and Solar Orbiter, which will significantly improve the spectropolarimetric accuracy and sensitivity. In this seminar, I will show and discuss these recent results and the new interesting possibilities enabled by high-resolution spectropolarimetry in the detection and identification of wave modes in solar magnetic structures.
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Jul 2 Fri Manolis K. Georgoulis (RCAAM of the Academy of Athens ) SP2RC seminar
13:00 Meet The Science of FLARECAST: Results, Open-Access Resources and Envisioned Future Trends in Solar Flare Forecasting
 
  Abstract:
As an encompassing overview of the European Union FLARECAST project will be published shortly, I will attempt a critical description of the project’s key science results, involving the motivation, the methodology, the metadata and the performance verification tasks performed in its framework. The science of FLARECAST pointed to three objectives, all falling under its main goal to better understand the drivers of solar flare forecasting and to improve prediction. These objectives were, first, to find out which preflare properties of solar active regions, treated as flare predictors, work best for forecasting purposes; second, to identify promising new predictors; and third, to perform an explorative research on flare and eruption prediction that utilized numerical simulations and investigated the physical and statistical connection between flares and coronal mass ejections. Interesting, and potentially important, leads were found, that may well help spearhead future efforts. Given the project’s limited duration, some further desired tasks were ultimately left for the future. On a top level, we concluded that the stochastic nature of solar flares’ occurrence is so deeply rooted in the Sun’s fundamental magnetic activity that even a massive Big Data and machine learning effort such as FLARECAST could not decisively lift this barrier of stochasticity. Furthermore, FLARECAST adhered to the European Union’s open access policy and consistently planned to help avoid effort duplication in future forecast attempts by making openly available all data, codes, and infrastructure it created. Detailed guidance in retrieving these resources will also be presented.
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Jul 23 Fri Bodan Arsovski (Sheffield) Number Theory seminar
14:00 Meet p-adic representations and p-adic Hodge theory
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