# Seminars this semester

Series:

Sep 23 Thu Rahul Yadav (Stockholm University, Sweden) SP2RC/ESPOS
10:00 Zoom link: https://zoom.us/j/165498165 (Meeting ID: 165 498 165) Multiline Spectropolarimetric Observation of a C2-Class Solar Flare

Abstract:
We present high-resolution and multiline spectropolarimetric observations of a C2-class solar flare (SOL2019-05-06T08:47). The rise, peak, and decay phases of the flare were recorded continuously and simultaneously in the Ca II K, Ca II 8542 Å, and Fe I 6173 Å lines with the CRISP and CHROMIS instruments at the Swedish Solar Telescope (SST). At the flare footpoints, a non-LTE multiline inversion code (STiC) was employed to infer the temperature, magnetic field, line-of-sight (LOS) velocity, and microturbulent velocity. The temporal analysis of the inferred temperature at the flare footpoints shows that the flaring atmosphere from log τ500 ∼ −2.5 to −3.5 is heated up to 7 kK, whereas from log τ500 ∼ −3.5 to −5 the inferred temperature ranges between ∼7.5 kK and ∼11 kK. During the flare peak time, the LOS velocity shows both upflows (evaporation) and downflows (condensation) around the flare footpoints in the upper chromosphere and lower chromosphere, respectively. Moreover, the temporal analysis of the LOS magnetic field exhibits a maximum change of ∼600 G. After the flare, the LOS magnetic field decreases to the non-flaring value, exhibiting no permanent or step-wise change. The analysis of response functions to the temperature, LOS magnetic field, and velocity shows that the Ca II lines exhibit enhanced sensitivity to the deeper layers compared to the non-flaring atmosphere. We also estimated the radiative losses from the singly ionized Ca and Mg atoms using the semi-empirical model atmosphere inferred from the inversion of the CRISP and CHROMIS dataset. The obtained radiative losses range from 50 to 180 kW/m^2 near the flare footpoints. Our observations illustrate that even a less intense C-class flare can heat the deeper layers of the solar chromosphere, mainly at the flare footpoints, without affecting the photosphere.

Sep 29 Wed Leanne Durkan (University College Dublin) Cosmology, Relativity and Gravitation
15:00 Seminar Room B19, 301 Glossop Road Modelling Extreme Mass Ratio Inspirals for LISA

Abstract:
Extreme mass-ratio inspirals (EMRIs) are a key source of gravitational waves for the future space based detector, LISA. To model EMRI waveforms to the accuracy required for parameter estimation by LISA, we require black hole perturbation theory and gravitational self-force theory, where the perturbing quantity is the small mass ratio. In my talk I will provide an overview of how we model EMRIs, using the example of a Schwarzschild background and circular orbits. I will justify why we must calculate the metric perturbation to second-order in the small mass ratio, why we use the Lorenz gauge and why, to first-order, we can treat the smaller compact body as a point-like particle. I will present some recent results of the gravitational wave energy flux at future null infinity, discussing in more detail the contribution of the slow-time derivative of the first-order metric perturbation, which appears in the second-order source to the field equations. I will also briefly mention current research being explored to extend the calculation of the Lorenz gauge metric perturbation to eccentric orbits and to Kerr, required to develop physically realistic models for LISA.

Oct 6 Wed Christiane Klein (Leipzig) Cosmology, Relativity and Gravitation
15:00 Blackboard Collaborate Charged scalar fields inside charged black holes

Abstract:
The strong cosmic censorship conjecture states that black hole spacetimes cannot be continued beyond their inner horizon due to the divergence of local observables, such as the stress-energy tensor of a classical or quantum scalar field, at that horizon. In the case of a spherically symmetric, charged black hole, numerical and analytical studies indicate, that this conjecture is violated classically, even for charged scalar fields, but that the conjecture can be restored by quantum effects in the real scalar case. Here, we present a study on the behaviour of quantum charged scalar fields in a charged, non-rotating black hole. Apart from an extension of the results for real quantum fields, we focus on the charge current induced by this field. We derive an expression for the renormalized current in the Unruh vacuum. In addition, we demonstrate numerically, that the quantum scalar field can charge, instead of discharge, the black hole near the inner horizon.

Oct 7 Thu Artem Koval (Astronomical Institute of the CAS) SP2RC/ESPOS
10:00 Zoom link: https://zoom.us/j/165498165 (Meeting ID: 165 498 165) Shock-wave radio probing of solar wind sources in coronal magnetic fields

Abstract:
The Space Weather effects in the near-Earth environment as well as in atmospheres of other terrestrial planets arise by corpuscular radiation from the Sun, known as the solar wind. The solar magnetic fields govern the solar corona structure. Magnetic-field strength values in the solar wind sources - key information for modeling and forecasting the Space Weather climate - are derived from various solar space- and ground-based observations, but, so far not accounting for specific types of radio bursts. These are “fractured” type II radio bursts attributed to collisions of shock waves with coronal structures emitting the solar wind. Here, we report about radio observations of two “fractured” type II bursts to demonstrate a novel tool for probing of magnetic field variations in the solar wind sources. These results have direct impact on interpretations of this class of bursts and contribute to the current studies of the solar wind emitters.

Oct 7 Thu Sergiy Shelyag (School of Information Technology, Deakin University, Australia) Plasma Dynamics Group
12:00 Meet Technicalities of MURaM solar photospheric models: how to use them and what to expect from them

Abstract:
In my presentation, I will provide a brief look into the details of how MURaM photospheric models are generated. I will review the fundamentals of numerical modelling of solar plasmas and discuss particular difficulties for such modelling and implemented solutions. Then, I will demonstrate the output of MURaM, show basic IDL scripts designed for reading the models and the processing techniques. Finally, I will describe technical details on why and how the MURaM models have to be converted to observables prior to attempts to compare them with the real observational data.

Oct 8 Fri Bodan Arsovski (Sheffield) Number Theory seminar
13:00 Meet Limiting measures of supersingularities

Abstract:
In 2001, Gouvêa and Buzzard noticed several interesting phenomena in the distributions of the p-adic slopes of weight k, level Γ_0(Np) eigenforms, the two main ones being that the slopes, in almost all cases, are integers; and that the slopes, in almost all cases, are no larger than (k-1)/(p+1), a much smaller bound than expected. In trying to explain the second phenomenon, Gouvêa made a conjecture that, after normalizing the slopes by dividing by k-1 so that they lie in the interval [0,1], their distributions tend to the uniform measure on [0,1/(p+1)]∪[p/(p+1),1] as k tends to infinity. In particular, Gouvêa's conjecture implies that the slopes are concentrated away from the middle interval (1/(p+1),p/(p+1)). This conjecture can be seen as the p-adic version of an interesting twist on the Sato–Tate conjecture: while the Sato–Tate conjecture asks about the distribution of the (real) slopes of a fixed modular form for varying primes, here one is interested in the distributions of the (p-adic) slopes of varying modular forms for a fixed prime. In this talk we discuss a proof that the slopes are indeed concentrated away from the middle interval (1/(p+1),p/(p+1)) when p>3 is Γ_0(N)-regular, which uses the p-adic local Langlands correspondence.

Oct 13 Wed Alexander Smith (Saint Anselm College) Cosmology, Relativity and Gravitation
13:30 Blackboard Collaborate Relational dynamics and quantum time dilation

Abstract:
The lesson of general relativity is background independence, which results in a Hamiltonian constraint. This presents a challenge for quantum gravity because the quantisation of this constraint demands that physical states of geometry and matter are frozen, leading to one aspect of the problem of time. We must then explain how the conventional notion of time evolution emerges, which motivates the need for a relational description of quantum dynamics. Using quantum clocks and covariant time observables, I will introduce a formulation of relational quantum dynamics that allows for a probabilistic notion of relativistic time dilation and a proper time / rest mass uncertainty relation. This framework will then be used to describe a quantum time dilation effect that occurs when a clock moves in a superposition of different relativistic momenta. I will argue that this time dilation effect may be observable with present-day technology and offers a new test of relativistic quantum mechanics.

Oct 14 Thu Professor Mihalis Mathioudakis (Queen's University Belfast) SP2RC seminar
10:00 UK-SOSS Zoom Link: https://zoom.us/j/95338171418 Meeting ID: 953 3817 1418 UK-SOSS: From high-resolution observations of the Sun to the solar-stellar connection

Abstract:
Our nearest star has always been a unique source for our understanding of the universe in its many forms. The solar atmosphere and interior provide a working example where structures and dynamics can be studied over an enormous range of spatial and temporal scales. The seminar will begin with some global solar properties and phenomena, placing them in the context of stellar magnetic activity. We will then highlight some of the smallest features that can be resolved in the lower solar atmosphere and the physical parameters that can be extracted from them. We will comment on the high-resolution observations of the Sun and how they can help us tackle some of the challenges of stellar astrophysics.

Oct 15 Fri Guoyin Chen (School of Astronomy and Space Science, Nanjing University) SP2RC seminar
13:00 Join Zoom Meeting https://zoom.us/j/92001288271?pwd=S1FGWldWNGphUUZvZC9VNFZMWWZkUT09 Meeting ID: 920 0128 8271 Passcode: zQt2Df Coronal Loop Kink Oscillation Periods Derived with Realistic Density, Magnetic Field and Loop Geometry

Abstract:
The time domain analysis of coronal loop oscillations is an effective approach to study their physical properties, such as the magnetic field and density, which can also be derived from DEM analysis and magnetic field models independently. We take advantage of the aforementioned methods to do the following three main work: (1) the time domine analysis of coronal loop oscillations; (2) DEM diagnostics about temperature and density; (3) magnetic extrapolation to determine the realistic magnetic field distribution. These three parts are combined by the governing equation of coronal loop oscillations to show the consistency between the observed period and computed one. And we will show the impact of the loop geometry on the oscillation period.

Oct 15 Fri Haluk Sengun (Sheffield) Number Theory seminar
13:00 Meet Periods of mod p Bianchi modular forms and Selmer groups

Abstract:
The relationship between special values of L-functions of modular forms and Selmer group of modular p-adic Galois representations is a major theme in number theory. Given the developing mod p Langlands program, it is natural to ask whether there is some kind of mod p analogue of the above theme. Notice that mod p modular forms do not have associated L-functions! In this talk, I will report on ongoing work with Lewis Combes in which we formulate, and computationally test, a connection between Selmer groups of mod p Galois representations and mod p Bianchi modular forms. This is inspired by a speculation of Calegari and Venkatesh.

Oct 19 Tue Robert Kurinczuk (Sheffield) Theta Correspondence Seminar
10:00 meet.google.com/bbv-xhgy-mie The local picture: Talk 1

Oct 20 Wed Nicole Mideo (Toronto) Mathematical Biology Seminar Series
13:00 https://york-ac-uk.zoom.us/j/93472504071 Understanding variation in malaria infection dynamics

Abstract:
Infection outcomes are highly variable: some individuals suffer severe illness while others seem relatively unharmed by the same infection. Underlying this variation are numerous sources of heterogeneity, including parasite genetics, host genetics, and infectious dose, among others. Yet mechanistic explanations of differential infection outcomes remain elusive. Focusing on data from experimental malaria infections in lab mice, my research has been developing and refining mathematical models to reveal those mechanistic explanations. In this talk, I will describe what we have learned about the parasite traits, host traits, and their interactions that give rise to the observed variation in malaria infection dynamics and outcomes for hosts.

Oct 21 Thu Paolo Pagano (Universita di Palermo) Plasma Dynamics Group
16:00 Meet What we know about wave heating modelling. Or maybe not.

Abstract:
In this seminar, I will cover a number of results about wave heating models that emerged from our MHD simulations. Over the last few years, we have developed several models of coronal heating by phase-mixing of Alfvènic waves and, while some issues need more thorough investigation, some results seem to remain consistent across different models. I will mostly focus on the nature of the waves that can contribute to the coronal heating, on the structure of the boundary shell where the heating could happen, and on the amount of heating itself.

Oct 27 Wed Ricardo Martinez-Garcia (South American Institute for Fundamental Research, Brazil) Mathematical Biology Seminar Series
13:00 https://york-ac-uk.zoom.us/j/93472504071 Beyond the law of mass action in ecology: the effect of range-residency on encounter statistics

Abstract:
For over 100 years, mathematical models in population ecology have relied on very strong and unrealistic assumptions about the way individuals move and get to interact with each other and with the environment. Specifically, they assume that individuals behave like the molecules of an ideal gas: following completely random trajectories through the entire area occupied by the population and only interacting with each other when their trajectories intersect. Under these assumptions, the encounter rates follow the law of mass action, and individual encounter events are uniformly distributed within the population range. However, mounting empirical evidence suggests that animals use space non-uniformly, occupy home ranges substantially smaller than the population range, and are often capable of nonlocal perception. I will discuss our recent efforts to develop a refined theory for ecological encounters grounded on empirically supported individual movement behavior [1, 2]. First, I will introduce the theoretical framework and derive novel analytical expressions for the encounter rates and the spatial distribution of encounters. Second, I will apply it to animal tracking data and discuss the ecological insights we obtain from such an analysis. I will conclude with a few remarks on future directions. References: [1] Martinez-Garcia, R., Fleming, C. H., Seppelt, R., Fagan, W. F., & Calabrese, J. M. (2020). How range residency and long-range perception change encounter rates. Journal of Theoretical Biology, 498, 110267. [2] Noonan, M. J., Martinez-Garcia, R., Davis, G. H., Crofoot, M. C., Kays, R., Hirsch, B. T., ... & Calabrese, J. M. (2021). Estimating encounter location distributions from animal tracking data, Methods in Ecology and Evolution, 12 (7), 1158-1173.

Oct 27 Wed Alessandra Caraceni (Pisa) Probability
15:00 Meet Polynomial mixing time for edge flips via growing random planar maps

Abstract:
A long-standing problem proposed by David Aldous consists in giving a sharp upper bound for the mixing time of the so-called “triangulation walk”, a Markov chain defined on the set of all possible triangulations of the regular n-gon. A single step of the chain consists in performing a random edge flip, i.e. in choosing an (internal) edge of the triangulation uniformly at random and, with probability 1/2, replacing it with the other diagonal of the quadrilateral formed by the two triangles adjacent to the edge in question (with probability 1/2, the triangulation is left unchanged). While it has been shown that the relaxation time for the triangulation walk is polynomial in n and bounded below by a multiple of n^{3/2}, the conjectured sharpness of the lower bound remains firmly out of reach in spite of the apparent simplicity of the chain. For edge flip chains on different models -- such as planar maps, quadrangulations of the sphere, lattice triangulations and other geometric graphs -- even less is known. We shall discuss results concerning the mixing time of random edge flips on rooted quadrangulations of the sphere, partly obtained in joint work with Alexandre Stauffer. A “growth scheme” for quadrangulations which generates a uniform quadrangulation of the sphere by adding faces one at a time at appropriate random locations can be combined with careful combinatorial constructions to build probabilistic canonical paths in a relatively novel way. This method has immediate implications for a range of interesting edge-manipulating Markov chains on so-called Catalan structures, from “leaf translations” on plane trees to “edge rotations” on general planar maps. Moreover, we are able to apply it to flips on 2p-angulations and simple triangulation of the sphere, via newly developed “growth schemes” to appear in an upcoming paper.

Oct 29 Fri Soós Szabolcs (SP2RC (UoS)) SP2RC seminar
13:00 Google meet link: https://meet.google.com/ciq-zovu-rzm On the differences in the periodic behaviour of magnetic helicity flux in flaring active regions with and without X-class events

Abstract:
Observational pre-cursors of large solar flares and eruptions offer advancement in the scientific understanding of the underlying physics, and provide a basis for future operational systems for forecasting. In this work, we study the evolution of the normalized emergence (EM), shearing (SH) and total (T) magnetic helicity flux components for 14 flaring and 14 non-flaring active regions (ARs) using the Spaceweather Helioseismic Magnetic Imager Active Region Patches (SHARP) vector magnetic field data. Each of the selected ARs contain a $\delta$-type spot. The three helicity components of all 28 ARs were analyzed using wavelet analysis. Localised peaks of the wavelet power spectrum were identified and statistically investigated in both the flaring and non-flaring AR cases. We find that: i) the peaks of the probability density functions of the EM/SH/T flux components appear in distinct bands; ii) the correlation of the period bands shows that the periodicity of the EM helicity flux component are distinct for the flaring and non-flaring ARs, while the periodicity of SH/T remain similar; iii) for the flaring ARs the distributions of the EM/SH/T peaks change after flares; and iv) when the EM periodicity does not contain harmonics, the ARs do not host a large energetic flare. Most importantly, v) significant power at long periods ($\sim$20 hour) in the T and EM components may serve as a pre-cursor for a large energetic flare, or at least an indication that the AR is likely to host such a flare.are.

Oct 29 Fri Silvia Nagy (Queen Mary) Cosmology, Relativity and Gravitation
16:00 Seminar Room B19, 301 Glossop Road Different approaches to gravity from Yang-Mills squared

Abstract:
The idea of writing various quantities in gravity as double copies of the analogous objects in Yang-Mills gauge theory has been gaining a lot of traction in recent years - I will give an overview of the numerous different formulations that have arisen from this drive. Then I will focus on 2 particular ones: the first is based on certain double copy replacement rules in the self-dual sector, and the second links with twistor theory. These have allowed for a recent expansion of the remit of the double copy in the context of symmetries and classical solutions. Finally, I will make some comments on a more ambitious question: is it possible to unify the various different formulations into a single framework?

Nov 2 Tue Robert Kurinczuk (Sheffield) Theta Correspondence Seminar
10:00 meet.google.com/bbv-xhgy-mie The local picture: Talk 2

Nov 3 Wed Amy Hurford (Memorial University of Newfoundland and Labrador, Canada) Mathematical Biology Seminar Series
13:00 https://york-ac-uk.zoom.us/j/93472504071 TBC

Abstract:
TBC

Nov 3 Wed Robert Kurinczuk (University of Sheffield) Pure Maths Colloquium
14:00 meet.google.com/qxa-rhcg-skx Local Langlands in families for classical groups

Abstract:
The conjectural local Langlands correspondence connects representations of p-adic groups to certain representations of Galois groups of local fields called Langlands parameters. In recent joint work with Dat, Helm, and Moss, we have constructed moduli spaces of Langlands parameters over Z[1/p] and studied their geometry. We expect this geometry is reflected in the representation theory of the p-adic group. Our main conjecture “local Langlands in families” describes the GIT quotient of the moduli space of Langlands parameters in terms of the centre of the category of representations of the p-adic group generalising a theorem of Helm-Moss for GL(n). I will give an introduction to this picture and explain how after inverting the "non-banal primes" one can prove this conjecture for the local Langlands correspondence for classical groups of Arthur and others.

Nov 9 Tue Haluk Sengun (Sheffield) Theta Correspondence Seminar
10:00 meet.google.com/bbv-xhgy-mie The global picture: Talk 1.

Nov 10 Wed Rachel Warnock (Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany) Mathematical Biology Seminar Series
13:00 https://york-ac-uk.zoom.us/j/93472504071 TBC

Abstract:
TBC

Nov 10 Wed Ananyo Dan (University of Sheffield) Pure Maths Colloquium
14:00 meet.google.com/qxa-rhcg-skx McKay correspondence for isolated Q-Gorenstein singularities

Abstract:
The McKay correspondence is a (natural) correspondence between the (non-trivial) irreducible representations of a finite subgroup G of $SL(2,\mathbb{C})$ and the irreducible components of the exceptional divisor of a minimal resolution of the associated quotient singularity $\mathbb{C}^2//G$. A geometric construction for this correspondence was given by González-Sprinberg and Verdier, who showed that the two sets also correspond bijectively to the set of indecomposable reflexive modules on the quotient singularity. This was generalized to higher dimensional quotient singularities (i.e., quotient of $\mathbb{C}^n$ by a finite subgroup of $SL(n,\mathbb{C})$) by Ito-Reid, where the above sets were substituted by certain smaller subsets. It was further generalized to more general quotient singularities by Bridgeland-King-Reid, Iyama-Wemyss and others, using the language of derived categories. In this talk, I will survey past results and discuss what happens for the isolated Q-Gorenstein singularities case (not necessarily a quotient singularity). If time permits, I will discuss applications to Matrix factorization. This is joint work in progress with J. F. de Bobadilla and A. Romano-Velazquez.

Nov 10 Wed Eugene Lim (King's College) Cosmology, Relativity and Gravitation
15:00 Hicks LT9 TBA

Nov 12 Fri Hiraku Atobe (Hokkaido University) Number Theory seminar
13:00 Meet Local newforms for GL(n)

Abstract:
In 1981, Jacquet--Piatetskii-Shapiro--Shalika established the theory of local newforms for irreducible generic representations of general linear groups over p-adic fields. In this talk, we extend their results to all irreducible representations. To do this, we introduce a new family of compact open subgroups indexed by certain tuples of non-negative integers. For the proof, we define local Rankin--Selberg integrals for Speh representations. This is a joint work with Satoshi Kondo and Seidai Yasuda.

Nov 16 Tue Haluk Sengun (Sheffield) Theta Correspondence Seminar
10:00 meet.google.com/bbv-xhgy-mie The global picture: Talk 2.

Nov 17 Wed Yamir Moreno (Universidad de Zaragoza, Spain) Mathematical Biology Seminar Series

Abstract:
TBC

Nov 17 Wed Silvia Pla García (Valencia) Cosmology, Relativity and Gravitation
15:00 Hicks LT9 TBA

Nov 19 Fri Vaidehee Thatte (King's College London) Number Theory seminar
13:00 Meet

Nov 24 Wed Oscar Randal-Williams (University of Cambridge) Pure Maths Colloquium

Nov 24 Wed Antonio Ferreiro (University College Dublin) Cosmology, Relativity and Gravitation
15:00 Blackboard Collaborate TBA

Nov 26 Fri Narasimha Kumar (Indian Institute of Technology Hyderabad) Number Theory seminar
13:00 Meet

Dec 1 Wed Vishwesha Guttal (Indian Institute of Science, India) Mathematical Biology Seminar Series
13:00 https://york-ac-uk.zoom.us/j/93472504071 TBC

Abstract:
TBC

Dec 1 Wed Sven Raum (University of Stockholm) Pure Maths Colloquium

Dec 1 Wed Julio Arrechea (IAA, Granada) Cosmology, Relativity and Gravitation
15:00 Blackboard Collaborate TBA

Dec 3 Fri Hanneke Wiersema (Cambridge) Number Theory seminar
13:00 Meet

Dec 8 Wed Jan Grabowski (University of Lancaster) Pure Maths Colloquium