Seminar history    



Feb 25 Thu Suzana de Souza e Almeida Silva (Sheffield) European Solar Physics Online Seminars (ESPOS)
 
  Abstract:
We present the state-of-art detection method of three-dimensional vortices and apply it to realistic magneto-convections simulations performed by the MURaM code. The detected vortices extend from the photosphere to the low chromosphere, presenting similar behaviour at all height levels. The vortices concentrate the magnetic field, and thereby the plasma dynamics inside the vortex is considerably influenced by the Lorentz force. Rotational motions also perturb the magnetic field lines, but they lead to only slightly bent flux tubes as the magnetic field tension is too high for the vortex flow to significantly twist the magnetic lines. We find that twisted magnetic flux tubes are created by shear motions in regions where plasma-beta>1, regardless of the existence of flow vortices.

Feb 25 Thu Suzana de Souza e Almeida Silva (University of Sheffield, Plasma Dynamics Group (UK)) SP2RC/ESPOS
 
  Abstract:
We present the state-of-art detection method of three-dimensional vortices and apply it to realistic magneto-convections simulations performed by the MURaM code. The detected vortices extend from the photosphere to the low chromosphere, presenting similar behaviour at all height levels. The vortices concentrate the magnetic field, and thereby the plasma dynamics inside the vortex is considerably influenced by the Lorentz force. Rotational motions also perturb the magnetic field lines, but they lead to only slightly bent flux tubes as the magnetic field tension is too high for the vortex flow to significantly twist the magnetic lines. We find that twisted magnetic flux tubes are created by shear motions in regions where plasma-beta>1, regardless of the existence of flow vortices.

Feb 24 Wed Reinder Meinsma Algebraic Geometry Learning Seminar: Toric varieties

Feb 18 Thu Dr Huw Morgan (Aberystwyth University) SP2RC seminar
 
  Abstract:
Any remote measurement of the solar corona in white light (or other) wavelength is an integration of emission along an extended line of sight. Historically, most studies necessarily assumed an axi-symmetric distribution to the density, thus derived properties contained an inherent and unquantified uncertainty. From the SOHO era onwards, space-based coronagraphs (LASCO/SOHO, and COR/STEREO) make frequent, uninterrupted, and high-quality observations of the corona which allow estimates of the true density distribution using coronal rotational tomography (CRT). A recent breakthrough in CRT is revealing a new view of the corona which is gained directly from observation. For the first time, we can view long-term trends in the coronal rotation rate, find meaningul links between coronal and interplanetary density structures, and use estimated densities at a range of heights to constrain outflow velocity and acceleration. The density distributions provide a ground truth for model extrapolations of the photospheric magnetic field, and new empirical boundary conditions for solar wind models. Tentative evidence of the Parker spiral onset can be seen close to the Sun. The next step in CRT methods is the inclusion of a time-dependent density distribution: initial results show promising correlations with Parker Solar Probe measurements, and the discovery of large variations on daily timescales not associated with mass ejections.

Feb 18 Thu Mijie Shi (KU Leuven, Belgium) Plasma Dynamics Group
 
  Abstract:
In the quest to solve the long-standing coronal heating problem, it has been suggested that coronal loops could be heated by waves. Despite the accumulating observational evidence of the possible importance of coronal waves, still very few 3D MHD simulations exist that show significant heating by MHD waves. In this seminar, I will present our recent 3D coronal loop model heated by transverse waves against radiative cooling. The coronal loop is driven at the footpoint by transverse oscillations and subsequently the induced Kelvin-Helmholtz instability deforms the loop cross-section to a fully turbulent state. Wave energy is transferred to smaller scales where it is dissipated, overcoming the internal energy losses by radiation. These results open up a new avenue to address the coronal heating problem.

Feb 17 Wed Kang Li (KU Leuven) Pure Maths Colloquium
 
  Abstract:
Roughly speaking, a ghost operator is often an infinite matrix such that its matrix entries vanish at the infinity. This notion was introduced by Guoliang Yu in the study of the so-called coarse Baum-Connes conjecture. It is a very central topic in coarse geometry and operator algebras with applications to provide counterexamples to the coarse Baum–Connes conjecture, the existence of non-exact groups and the rigidity problem for Roe-type algebras. In this talk, we will visualize a class of ghost projections in terms of expanderish graphs.

Feb 17 Wed Daniele Oriti (LMU Munich) Cosmology, Relativity and Gravitation
 
  Abstract:
We overview recent results on the extraction of an effective cosmological dynamics from fundamental quantum gravity formalisms in which spacetime is not fundamental, focusing on so called tensorial group field theories (strictly related to lattice quantum gravity and loop quantum gravity). This line of research is inspired by the idea of our universe as a quantum gravity condensate, and at the same time realizes it concretely. We emphasize how reaching the desired objective requires addressing several outstanding issues in quantum gravity: identifying quantum states in the fundamental theory with a good geometric interpretation, performing some form of coarse graining of the fundamental dynamics, defining diffeomorphism invariant observables to express the resulting coarse grained dynamics in physically transparent language. We also discuss what the theory says about the fate of the big bang singularity at the beginning of our universe.

Feb 17 Wed Evgeny Shinder Algebraic Geometry Learning Seminar: Toric varieties

Feb 11 Thu Vasco Henriques (Rosseland Centre for Solar Physics, Norway) European Solar Physics Online Seminars (ESPOS)
 
  Abstract:
The chromosphere of the umbra of sunspots is a remarkably dynamic layer featuring extremely fine sub-arcsec structure. Such structures appear dark against enveloping umbral flashes, but also bright before or after a flash, other features still are bright throughout. Only recently did we start understanding such fine features and semi-empirical modelling is converging with simulations to provide insight, not only into such fine structure, but also into the umbral flash phenomenon itself. The observational evidence weighs overwhelmingly towards a strong corrugation of the umbra where the material in short dynamic fibrils over-extends in a column of upflowing material while the adjacent areas flash. The delayed small-scale umbral brightenings at the bottom of such columns are an out-of-phase flash where the late-stage downflowing column meets the upflowing under-layers. Recent inversions using NICOLE at both umbral flashes and small-scale brightenings result in a downflow over upflow stratification perfectly bridging the transition of downflowing fibrils to upflowing fibrils as well as red-shifted absorption cores to blue-shifted absorption cores in the broader surroundings. Locally, each inverted column is remarkably similar in velocity profile to those from forward modelling, provided the formation height of the observed Ca II 8542 line is slightly lower in the Sun than in the simulations. Conspicuously, resonant cavities naturally cause the upper downflowing layers to become visible in forward modelling and the top-layer downflows to last longer than otherwise. Open questions, and how these can be addressed by future observations, are briefly discussed.

Feb 11 Thu Vasco Henriques (Rosseland Centre for Solar Physics, Norway) SP2RC/ESPOS
 
  Abstract:
The chromosphere of the umbra of sunspots is a remarkably dynamic layer featuring extremely fine sub-arcsec structure. Such structures appear dark against enveloping umbral flashes, but also bright before or after a flash, other features still are bright throughout. Only recently did we start understanding such fine features and semi-empirical modelling is converging with simulations to provide insight, not only into such fine structure, but also into the umbral flash phenomenon itself. The observational evidence weighs overwhelmingly towards a strong corrugation of the umbra where the material in short dynamic fibrils over-extends in a column of upflowing material while the adjacent areas flash. The delayed small-scale umbral brightenings at the bottom of such columns are an out-of-phase flash where the late-stage downflowing column meets the upflowing under-layers. Recent inversions using NICOLE at both umbral flashes and small-scale brightenings result in a downflow over upflow stratification perfectly bridging the transition of downflowing fibrils to upflowing fibrils as well as red-shifted absorption cores to blue-shifted absorption cores in the broader surroundings. Locally, each inverted column is remarkably similar in velocity profile to those from forward modelling, provided the formation height of the observed Ca II 8542 line is slightly lower in the Sun than in the simulations. Conspicuously, resonant cavities naturally cause the upper downflowing layers to become visible in forward modelling and the top-layer downflows to last longer than otherwise. Open questions, and how these can be addressed by future observations, are briefly discussed.

Feb 10 Wed Kevin Painter (Politecnico di Torino) Applied Mathematics Colloquium
 
  Abstract:
The formation of swarms, schools, flocks, herds, aggregates etc is a classical example of self-organisation, with the benefits of forming a high density group ranging from efficient migration to higher fecundity. Often, groups form through a mechanism of chemical signalling between population members, an evolutionary ancient communication used by both microscopic and macroscopic species. Populations in fluid environments, though, must contend with complex and turbulent flows, potentially detrimental (e.g. splitting up groups) or beneficial (e.g. coalescing individuals) to the formation and maintenance of a group. As a counter to flow, rheotaxis describes a behaviour in which individuals orient their body axis with respect to the current and is observed in both unicellular and multicellular organisms . Here we investigate the extent to which rheotaxis and flow impact on chemically-mediated aggregation, revealing these can impact both negatively and positively according to the population state and flow conditions. A hypothesised density-dependent rheotaxis appears capable of optimising group formation and maintenance, exploiting the positive benefits from each of flow and rheotaxis. The results are discussed in the context of broadcast swarming phenomena in marine invertebrates.

Feb 10 Wed Luke Hart (Manchester) Cosmology, Relativity and Gravitation
 
  Abstract:
Cosmological recombination has been widely regarded a solid pillar of understanding the cosmic microwave background (CMB) and its anisotropies. For many years, the questions have been answered over the accuracy of these calculations due to exceptional codes as CosmoRec and HyRec as well as numerous publications on the intricate atomic processes. However, the era that dawned the formation of hydrogen and helium atoms has still given us brilliant insights into exotic physics as well as tribalistic disputes in the various pockets of modern cosmology. In this talk, we will briefly recap the physics of recombination before highlighting extensions to the standard model (parametric and non-parametric) that affect the surface of last scattering. Finally, we will look to the future probes that provide a direct, spectral handprint of the atomic transitions in hydrogen and helium: the recombination radiation. Here we will conclude with the feasibility of studying these lines with prospective missions such as SuperPIXIE, Voyage 2050 and what happens when the exotic physics modifications that we can test with the CMB anisotropies are propagated through to the SEDs from recombination.

Feb 10 Wed Paul Johnson Algebraic Geometry Learning Seminar: Toric varieties

Feb 5 Fri Dr. Sijie Yu (Center for Solar-Terrestrial Research, New Jersey Institute of Technology) SP2RC seminar
 
  Abstract:
Thanks to recent advances in radio interferometric instrumentation, we've entered a new era of solar radio observations---broadband dynamic imaging spectroscopy. In this talk, I will first introduce the history of solar radio observations based on either total-power (integrated over the Sun) dynamic spectral measurements or imaging at a few discrete frequencies, then review some recent progress based on dynamic imaging spectroscopy over a wide frequency range that has placed us in a strong position to make revolutionary breakthroughs in understanding high-energy processes in the solar corona. Future perspectives will also be briefly discussed.

Jan 28 Thu (University of St Andrews, Solar and Magnetospheric Theory Group (UK)) SP2RC/ESPOS

Jan 28 Thu Ben Snow (University of Exeter) European Solar Physics Online Seminars (ESPOS)
 
  Abstract:
Shocks are a universal feature of warm plasma environments, such as the lower solar atmosphere and molecular clouds, which consist of both ionised and neutral species. Including partial ionisation leads to the existence of a finite width for shocks, where the ionised and neutral species decouple and recouple. As such, drift velocities exist within the shock that lead to frictional heating between the two species, in addition to adiabatic temperature changes across the shock. The local temperature enhancements within the shock alter the recombination and ionisation rates and hence change the composition of the plasma. We study the role of collisional ionisation and recombination in slow-mode partially ionised shocks. In particular, we incorporate the ionisation potential energy loss and analyse the consequences of having a non-conservative energy equation. A semi-analytical approach is used to determine the possible equilibrium shock jumps for a two-fluid model with ionisation, recombination, ionisation potential, and arbitrary heating. Two-fluid numerical simulations are performed using the (PIP) code. Results are compared to the magnetohydrodynamic (MHD) model and the semi-analytic solution. Accounting for ionisation, recombination, and ionisation potential significantly alters the behaviour of shocks in both substructure and post-shock regions. In particular, for a given temperature, equilibrium can only exist for specific densities due to the radiative losses needing to be balanced by the heating function. A consequence of the ionisation potential is that a compressional shock will lead to a reduction in temperature in the post-shock region, rather than the increase seen for MHD. The numerical simulations pair well with the derived analytic model for shock velocities.

Jan 27 Wed Theo Torres Vicente (Nottingham) Cosmology, Relativity and Gravitation
 
  Abstract:
In this talk, we consider the electromagnetic radiation-reaction/self-force process for a charged particle orbiting a rotating black hole. We will present and complement the existing results for the scalar and gravitational cases, to give a full picture of integer spins in the Kerr spacetime. We restrict ourselves to the case of circular orbits and we will compute the dissipative and conservative components of the electromagnetic self-force numerically, by solving the inhomogeneous Teukolsky equations using the BHperturbation toolkit. The results will be compared to the scalar and gravitational cases found in the literature.

Jan 21 Thu Professor Valery M Nakariakov (Centre for Fusion, Space & Astrophysics, University of Warwick, United Kingdom) SP2RC seminar
 
  Abstract:
Standing transverse oscillations of the plasma loops of the solar corona have been intensively studied for the last 20 years as a tool for the diagnostics of the coronal magnetic field. Those oscillations are confidently interpreted as standing fast magnetoacoustic kink modes of the plasma non-uniformities. Statistical analysis demonstrates that, in the majority of cases, the oscillations are excited by a mechanical displacement of the loop from an equilibrium by a low coronal eruption. Standing kink oscillations are observed to operate in two regimes: rapidly decaying large amplitude oscillations and undamped small amplitude oscillations. In both these regimes, different loops oscillate with different periods that scale with the oscillating loop length. The oscillation amplitude does not show dependence on the loop length or the oscillation period. In the decayless regime the damping should be compensated by energy supply which allows the loop to perform almost monochromatic oscillations with almost constant amplitude and phase. We developed a low-dimensional model explaining the undamped kink oscillations as a self-oscillatory process caused by the effect of negative friction, which is analogous to producing a tune by moving a bow across a violin string. The period of self-oscillations is determined by the frequency of the kink mode. The ubiquity of decayless kink oscillations makes them an excellent tool for MHD seismology, in particular, for probing free magnetic energy in preflaring active regions.

Jan 14 Thu Sudheer K. Mishra (Indian Institute of Technology, BHU, India) European Solar Physics Online Seminars (ESPOS)
 
  Abstract:
Using multi-wavelength imaging observation obtained from the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO), we study the evolution of Kelvin-Helmholtz (K-H) instability in a fan-spine magnetic topology. This fan-spine configuration is situated near the Active Region 12297 and is rooted in a nearby sunspot. The two layers of the cool plasma flows lift up from the fan plane in parallel and interact with each other at the maximum height of the elongated spine in the lower corona. The first layer of the plasma flow (F1) moving with a slow velocity (5 km/s) reflected from the spine’s field lines. Subsequently second layer of plasma flow (F2) with impulsive velocity (114-144 km/s) interacts with the first layer at the maximum height and generating the shear motion , which is responsible for the evolution of the Kelvin-Helmholtz instability inside the elongated spine. The amplitude and characteristics wavelength of the K-H unstable vortices increases, which satisfy the linear growing mode of this instability. Using linear stability theory of the K-H instability, we calculate the Alfvén velocity in the lower layer. We conjecture that the estimated shearing velocity is higher than the estimated the Alfvén velocity in the second denser layer, which also satisfies the classical criterion of K-H instability. The fan-spine configuration possesses magnetic field and sheared velocity component, we estimate the parametric constant [Λ≥1] which confirms that the velocity shear dominates and the linear phase of the K-H instability is evolved. The present observation indicate that in the presence of complex magnetic field structuring and plasma flows, the K-H instability evolve in the fan-spine configuration may evolve the rapid heating, and connectivity changes may occur due to the fragmentation via the K-H instability. It also act as a rapid mechanism to transfer the mass and energy release between two distinct regions separated by the fan-spine configuration.

Jan 14 Thu Sudheer K. Mishra (Indian Institute of Technology, BHU, India) SP2RC/ESPOS
 
  Abstract:
Using multi-wavelength imaging observation obtained from the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO), we study the evolution of Kelvin-Helmholtz (K-H) instability in a fan-spine magnetic topology. This fan-spine configuration is situated near the Active Region 12297 and is rooted in a nearby sunspot. The two layers of the cool plasma flows lift up from the fan plane in parallel and interact with each other at the maximum height of the elongated spine in the lower corona. The first layer of the plasma flow (F1) moving with a slow velocity (5 km/s) reflected from the spine’s field lines. Subsequently second layer of plasma flow (F2) with impulsive velocity (114-144 km/s) interacts with the first layer at the maximum height and generating the shear motion , which is responsible for the evolution of the Kelvin-Helmholtz instability inside the elongated spine. The amplitude and characteristics wavelength of the K-H unstable vortices increases, which satisfy the linear growing mode of this instability. Using linear stability theory of the K-H instability, we calculate the Alfvén velocity in the lower layer. We conjecture that the estimated shearing velocity is higher than the estimated the Alfvén velocity in the second denser layer, which also satisfies the classical criterion of K-H instability. The fan-spine configuration possesses magnetic field and sheared velocity component, we estimate the parametric constant [Λ≥1] which confirms that the velocity shear dominates and the linear phase of the K-H instability is evolved. The present observation indicate that in the presence of complex magnetic field structuring and plasma flows, the K-H instability evolve in the fan-spine configuration may evolve the rapid heating, and connectivity changes may occur due to the fragmentation via the K-H instability. It also act as a rapid mechanism to transfer the mass and energy release between two distinct regions separated by the fan-spine configuration.