Plasma Dynamics Group seminars

2021 seminars

POD and DMD tutorial

With Mr Abdulrahman Albidah, 17 June 2021

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.

Do coronal loops oscillate in isolation?

With Dr Bradley W. Hindman, University of Colorado, 27 May 2021

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.

Internal gravity waves in the magnetised solar atmosphere

With Dr Vigeesh Gangadharan, Leibniz Institute for Solar Physics, 20 May 2021

Internal gravity waves (IGWs) are buoyancy-driven waves common in the Earth’s atmosphere and oceans.

IGWs have also been observed in the sun’s atmosphere and are thought to play an important role in the overall dynamics of the solar atmosphere. They supply bulk of the wave energy for the lower solar atmosphere, but their existence and role in the energy balance of the upper layer remains unclear. Using radiation-magnetohydrodynamic (R-MHD) simulations, we study naturally excited IGWs in realistic models of the solar atmosphere.

In this talk, we discuss some of our recent results on the influence of the sun's magnetic field on the propagation of IGWs and their energy transport.

Our analysis suggests that the IGWs are generated independent of the mean magnetic property of the atmosphere. However, their propagation into higher layers is strongly affected by the presence and the topology of the magnetic field.

We discuss how IGWs may play a significant role in the heating of the chromospheric layers in regions where horizontal fields are thought to be prevalent, like the internetwork region.

ParaView tutorial

With Dr Suzana de Souza e Almeida Silva, the University of Sheffield, 29 April 2021

For students doing it hands-on during the introduction, download ParaView. Also, there is this data available for the hands-on.

Zooming into the solar chromosphere

With Dr Juie Shetye, New Mexico State University, 15 April 2021

The solar chromosphere serves as a bridging layer between the photosphere and the corona. This dynamic layer 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 layer and it reveals never seen before dynamics.

In this presentation, 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 a decade long question in the solar physics, that tells us how the sun’s atmosphere is heated.

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 three times more spatial resolution than CRISP and much higher temporal resolution.

3D solar coronal loop reconstructions with machine learning

With Dr Iulia Chifu, Max Planck Institute for Solar System Research, 1 April 2021

The magnetic field plays an essential role in the initiation and evolution of different solar phenomena in the corona. The structure and evolution of the 3D coronal magnetic field are still not very well known. A way to ascertain the 3D structure of the coronal magnetic field is by performing magnetic field extrapolations from the photosphere to the corona.

In previous work, it was shown that by prescribing the 3D-reconstructed loops’ geometry, the magnetic field extrapolation produces a solution with a better agreement between the modelled field and the reconstructed loops. This also improves the quality of the field extrapolation.

Stereoscopy, which uses at least two view directions, is the traditional method for performing 3D coronal loop reconstruction. When only one vantage point of the coronal loops is available, other 3D reconstruction methods must be applied.

Within this work, we present a method for the 3D loop reconstruction based on machine learning. Our purpose for developing this method is to use as many observed coronal loops in space and time for the modelling of the coronal magnetic field.

Our results show that we can build machine-learning models that can retrieve 3D loops based only on their projection information.

Ultimately, the neural network model will be able to use only 2D information of the coronal loops, identified, traced, and extracted from the extreme-ultraviolet images, for the calculation of their 3D geometry.

Detection of small-scale chromospheric vortices and their intricate dynamics

With Dr Kostas Tziotziou, National Observatory of Athens, 18 March 2021

Small-scale vortex motions are detected at various spatial and temporal scales in the solar atmosphere, from the photosphere to the low corona. They often exhibit complex structure and dynamics and, as largely magnetic structures, can foster a variety of oscillations and wave modes.

Despite, however, recent advancements in observational and theoretical studies, as well as in simulations and modelling, their proper detection, especially in chromospheric lines such as Hα and Ca II 8542 Å is still an open issue, and their structure and dynamics remain poorly understood.

We present a novel automated method of chromospheric swirl detection based on their morphological characteristics that nicely complements previous LCT-related approaches.

We further discuss in detail the intricate dynamics of a persistent small-scale vortex flow with significant substructure, observed with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish Solar Telescope (SST), as well as oscillations and observational signatures of different types of waves within it and their propagation characteristics.

Both discussed aspects, better detection leading to a more precise estimation of their occurrence rate and wave identification and their properties, are key elements for accurately assessing the role of vortex structures in the energy budget of the solar atmosphere.

Dynamical systems approach to solar physics: from Lyapunov exponents to Lagrangian coherent structures

With Dr Érico Rempel, Institute of Aeronautical Technology (ITA), 4 March 2021

Dynamical systems, or chaos theory, has enjoyed huge success in the analysis of systems described by ordinary differential equations, such as nonlinear oscillators, chemical reactions, electronic devices, population dynamics, etc.

Usually, in the dynamical systems approach, one is concerned with the identification of the basic building blocks of the system under investigation and how they interact with each other to produce the observable dynamics, as well as how they can be manipulated to produce a desired output, in the cases where control is pursued. Examples of those building blocks are unstable equilibrium and periodic solutions, nonattracting chaotic sets and their manifolds, which are special surfaces in the phase space that basically control the dynamics, guiding solutions in preferred directions.

Despite its success in those areas, many still think that the theory has limited value when applied to fully developed turbulence, like observed in solar convection, due to the infinite dimension of the phase space.

In this talk, we show that this difficulty can be overcome by adopting a Lagrangian reference frame, where the phase space for each fluid particle becomes three-dimensional and the building blocks of the turbulence can be efficiently extracted by appropriate numerical tools.

We reveal how finite-time Lyapunov exponents, a traditional measure of chaos, can be used to detect attracting and repelling time-dependent manifolds that divide the fluid in regions with different behaviour. These manifolds are shown to accurately mark the boundaries of granules in observational data from the photosfere.

In addition, stagnation points and vortices detected as elliptical Lagrangian coherent structures complete the set of building blocks of the photospheric turbulence. Such structures are crucial for the trapping and transport of mass and energy in the solar plasma.

Coronal loop model heated by transverse waves against radiative losses

With Dr Mijie Shi, KU Leuven, 18 February 2021

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.

2020 seminars

A new seismology of the corona

With Dr Richard Morton, University of Northumbria, 10 December 2020

The Coronal Multi-channel Polarimeter (CoMP) instrument has proved itself invaluable for the study of propagating kink waves in the corona.

After making the initial discovery back in 2007, CoMP has been able to provide a number of insights into the properties of the kink waves. The kink mode is found to be present throughout the corona and appears to be continuous. The widespread and reliable presence means that the propagating kink mode can make a fantastic tool for magneto-seismology.

While CoMP shows a persistent Doppler velocity signal related to the propagating kink mode, the continuous transverse motions of the coronal structures can also be detected with Solar Dynamics Observatory (SDO/AIA). However the scale of the displacements are at the edge of the SDO's capabilities, requiring careful measurements to be able to study them and exploit them for seismology.

In this talk, I discuss the new possibilities for coronal seismology using the propagating kink mode, demonstrating how we’ve used both CoMP and SDO/AIA to measure the young solar wind, the density structure in a coronal hole and provide the first estimates for the global coronal magnetic field.

Partial ionisation of hydrogen plasma in the solar atmosphere - a non-LTE modeler's view

With Dr Petr Heinzel, Astronomical Institute, Czech Academy of Sciences, 26 November 2020

Based on our extensive experience with the non-LTE radiative-transfer modelling of different atmospheric structures (chromosphere, flares, prominences, CME-cores), I will demonstrate the importance of partial hydrogen ionisation and review the most relevant atomic processes. I will also discuss the role of non-equilibrium ionisation of hydrogen.

A code that solves the equations of MHD coupled to radiation

With Professor Francisco Guzman, Universidad Michoacana de San Nicolas de Hidalgo, 12 November 2020

Our code is based on a finite volume discretisation, and uses high-resolution shock-capturing flux formulae of the HLL class. Concerning the MHD part, we use the divergence cleaning method to preserve the non-monopoles constraint.

For radiation, at the moment, we use the M1 closure relation within the gray body approximation. The evolution equations for radiation become stiff for high opacities, for which we use an implicit-explicit evolution method, which allows the use of a standard integration time-step.

We present our code's status and mention the solar physics scenarios where we expect to produce some applications.

Bayesian coronal seismology

With Dr Inigo Arregui, Instituto de Astrofisica de Canarias, 29 October 2020

Coronal seismology is based on the remote diagnostics of physical conditions in the solar corona by comparison between model predictions and observations of wave activity.

Our lack of direct access to the physical system of interest makes information incomplete and uncertain so our conclusions are at best probabilities.

Bayesian inference is increasingly being employed in the area, following a general trend in solar and astrophysical research.

In this seminar, I first justify the use of a Bayesian probabilistic approach to seismology diagnostics and explain its philosophy and methodology. Then, I report on recent results that demonstrate its feasibility and advantage in applications to coronal loops, prominences and extended regions of the corona.

To finish, I suggest other areas of current interest where the use of Bayesian methods could contribute to improve our understanding on the structure, dynamics and heating of the corona.

Objective material barriers to the transport of momentum and vorticity

With Professor George Haller, ETH Zurich, 15 October 2020

I discuss a recent theory for material surfaces that maximally inhibit the diffusive transport of a dynamically active (ie, velocity-dependent) vector field, such as the linear momentum, the angular momentum or the vorticity, in three-dimensional unsteady flows.

These diffusion barriers provide physics-based, observer-independent boundaries of dynamically active coherent structures. Instantaneous limits of these Lagrangian diffusion barriers mark objective Eulerian barriers to short-term active transport.

I show how active diffusion barriers can be identified with active versions of Lagrangian coherent structure (LCS) diagnostics.

In comparison to their passive counterparts, however, active LCS diagnostics require no significant fluid particle separation and hence provide substantially higher-resolved Lagrangian and Eulerian coherent structure boundaries from shorter velocity data sets.

I illustrate these results on two-dimensional turbulence and three-dimensional wall-bounded turbulence.

Frequency power spectra of Alfvén waves in a solar coronal arcade: discrete or continuous?

With Dr Rekha Jain, the University of Sheffield, 1 October 2020

In this talk, I will present theoretically computed frequency power spectra for shear Alfvén waves excited in a solar coronal arcade.

I investigate two separate perturbations, a cosine-modulated Gaussian perturbation and an impulsive driver. The arcade is assumed to consist of potential magnetic field lines embedded in stratified plasma.

In principle, the nature of the frequency power spectra can constrain the size and the type of driver.

A new method for estimating global coronal wave properties from their interaction with solar coronal holes

With Dr Isabell Piantschitsch, Universitat de les Illes Balears, 30 July 2020

Global coronal waves (CWs) and their interaction with coronal holes (CHs) result, among other effects, in the formation of reflected and transmitted waves. Observations of such events provide us with measurements of different CW parameters, such as phase speed and intensity amplitudes.

However, several of these parameters are provided with only intermediate observational quality, other parameters, such as the phase speed of transmitted waves, can hardly be observed in general.

We present a new method to estimate crucial CW parameters, such as density and phase speed of reflected as well as transmitted waves, Mach numbers and density values of the CH's interior, by using analytical expressions in combination with basic and most accessible observational measurements.

The transmission and reflection coefficients are derived from linear theory and subsequently used to calculate estimations for phase speeds of incoming, reflected and transmitted waves. The obtained analytical expressions are validated by performing numerical simulations of CWs interacting with CHs.

This new method enables us to determine in a fast and straightforward way reliable CW and CH parameters from basic observational measurements, which provides a powerful tool to better understand the observed interaction effects between CWs and CHs.

Modal decompositions: what are they, why should we use them and how?

With Dr Jonathan Higham, University of Liverpool, 23 July 2020

The dynamics of natural systems are often complex and highly non-linear, understanding these procedures is difficult as their dynamics and complexities are usually intertwined and colluded.

Whilst we might be able to identify these systems using sets of nonlinear equations, determining the individual process is underlying a complex mechanism are non-trivial.

Over the past few decades, there has been much work to develop data driven methods to extract coherent features either in space or in time.

Two prominent methods are the proper orthogonal decomposition and the dynamic mode decomposition; in this seminar these two methods will be introduced, the underpinning mathematics and algorithms will be outlined, and variants of the algorithms and methods will also be described.

However, much of the seminar will focus on applying these methods, using them at all different scales from idealised small-scale laboratory experiments to large-scale real-world applications.

The primary aim of this seminar will be to equip you with an arsenal of spatially and temporally orthogonal tools which you can use to elucidate the complex features from your data sets.

Vortex flows in the solar atmosphere

With Dr Nitin Yadav, Max Planck Institute for Solar System Research, 2 July 2020

Vortex flows exist over various spatial and temporal scales throughout the solar atmosphere and are of great importance due to their potential in twisting the magnetic field lines and hence facilitating Poynting flux transport.

Recent advances in both observational techniques and numerical simulations have enabled us to detect a multitude of small-scale vortices in the solar atmosphere.

Smaller vortices are suggested to play an important role in the solar atmospheric heating, however, their physical properties remain poorly understood due to limited resolution in observations. Hence, it is crucial to investigate them using high-resolution simulations since they are more abundant and faster rotating flows than the larger vortices.

Using MHD simulations, we explored the the relationship between vortex flows at different spatial scales, analyse their physical properties, and investigate their contribution to Poynting flux transport from the lower to the upper layers of the solar atmosphere.

We found that a large vortex, as seen at low spatial resolution, consists of a large number of smaller vortices, when seen at high spatial resolution. Statistically, they have higher densities and higher temperatures than the average values at the same geometrical height. Their Poynting flux contribution is more than adequate to compensate for the radiative losses in the chromosphere indicating their possible role in the solar atmospheric heating.

Ubiquituous hundred-Gauss magnetic fields in solar spicules

With Dr Matheus Aguiar-Kriginsky Silva, Universitat de les Illes Balears, 18 June 2020

Even though they were observed for the first time in the 19th century, the nature of spicules is not well understood because they are are thin and elongated chromospheric jets. Therefore, their study is limited to the resolution of the instruments used.

Every time a step forward in the quality of the observations of the lower chromosphere is taken, the interest in spicules sparks. Most recently, the advent of the Hinode telescope provided high-resolution images of spicules that allowed for a better comprehension of their nature and behaviour. Studies regarding their magnetic field have been also undertaken, but most of them did not have the ideal spatial/temporal resolution needed to give definitive results.

This study is aimed to provide a step forward in this matter, with observations in the Ca II 854.2 nm line taken with the CRISP instrument at the Swedish 1-meter Solar Telescope in La Palma.

The sensitivity of the Ca II 854.2 nm line to the magnetic field is exploited and the Weak Field Approximation (WFA) is used to estimate the line-of-sight component of the magnetic field of spicules both off-limb and on the solar disk.

The WFA must be used carefully, since there are conditions that need to be met for it to be applicable. This consideration is assessed in every pixel, and a Bayesian approach is taken to infer the line-of-sight magnetic field component from the WFA equations.

It is established that magnetic fields over 100 G are abundant. The reason for the failure of previous studies to conclude this is carefully studied and is speculated to lie in the poor temporal/spatial resolution of the observations used.

Dynamics of the vortex tubes in the solar atmosphere

With Dr Suzana de Souza e Almeida Silva, the University of Sheffield, 4 June 2020

We use a state of the art vortex detection method, Instantaneous Vorticity Deviation, to define and locate three-dimensional vortices in magneto-convections simulations performed by the MURaM code.

The detected vortices extend from the photosphere to the low chromosphere. The dynamics across the vortical flows at different height levels are investigated through radial profiles.

We found that the vortices present similar dynamics at all height levels, with nonuniform angular rotational velocity and eddy viscosity effects. The vortices intensify the magnetic field, and in turn, the vortex dynamics are affected by the magnetic field.

On the other hand, our findings hint that kinematic vortices need to present high tangential velocities at different height levels to overcome the magnetic tension and generate magnetic vortices.

Numerical studies of jet formation in the solar atmosphere

With Dr José Juan González Avilés, 21 May 2020

Using the Newtonian CAFE MHD code to perform 2.5D and 3D resistive MHD simulations in the solar atmosphere, we show that magnetic reconnection may be responsible for the formation of jets with some characteristics of Type II spicules and cool coronal jets.

We numerically model the photosphere-corona region using the C7 atmosphere model. The initial magnetic configuration in the 2.5D case consists of two symmetric neighbouring loops with opposite polarity, used to support reconnection. In the 3D case, the initial magnetic configuration is extrapolated up to the solar corona region from a dynamic realistic simulation of the solar photospheric magnetoconvection model that mimics the quiet-Sun.

In the 2.5D simulations, we include the effect of the thermal conduction along the magnetic field lines to study some properties of spicule jets. In this case, we find that thermal conductivity affects morphology, velocity, and temperature of the jets. Also, the heat flux maps indicate the head of the jet and corona interchange energy more efficiently than the body of the jet.

In the 3D simulations, we have found that the formation of the jet depends on the Lorentz force, which helps to accelerate the plasma upward. The morphology, the upward velocity covering a range up to 130 km/s, and the timescale formation of the structure between 60 and 90 s, are similar to those expected for Type II spicules.

Additionally, we analyse various properties of the jet dynamics, and find that the structure shows rotational and torsional motions which may generate torsional Alfvén waves in the corona region.

Complex 3D dynamics of solar spicule structures

With Dr Rahul Sharma, 7 May 2020

The sun’s outer atmosphere is a million degrees hotter than it’s visible surface, which is not understood with any of the known laws of thermodynamics and remains an intriguing problem for the astrophysics in general.

It is now believed that most of the energy dissipation phenomenon occurs at the interface region in between solar chromosphere and corona, which is a highly dynamic, gravitationally stratified, nonlinear, inhomogeneous environment.

Observed dynamics of thin magnetic fluxtube structures in this layer, reflects the confined magnetohydrodynamic (MHD) wave-modes (kink, sausage and torsional Alfven).

For the first time, the evolution of the resultant transverse displacement of the observed flux tube structures, estimated from perpendicular velocity components, is analysed along with cross-sectional width, photometric and azimuthal shear/torsion variations, to accurately identify the confined wave-mode(s).

In my talk, I will discuss the observational evidence of pulse-like nonlinear kink wave-mode(s), as indicated by the strong coupling in between kinematic observables, with a frequency-doubling, -tripling aspect, supported by mutual phase relations centred around 0 and +-180 (Sharma et al. 2018).

The 3D ensemble of the observed dynamic components revealed complexities pertinent to the accurate identification and interpretation of e.g. linear/nonlinear, coupled/uncoupled MHD wave-modes in the observed waveguides (spicules).

Waves in two-fluid gravitationally stratified plasmas

With Mr Abdulaziz Alharbi, 23 April 2020

The temperature in the lower part of solar atmosphere is not high enough for a complete ionisation of the plasma. Therefore, this environment region is made up of electrons, positive ions and neutrals that interact through short and long range collisions in the presence of the magnetic field. Due to the low temperature, the gravitational scale-height is also short, meaning that perturbations will be affected by gravity.

Here, we study the spatial and temporal evolution of slow magnetoacoustics waves propagating in a stratified magnetic flux tube. In the two-fluid plasma, the dynamics of neutrals and charged species has to be studied separately.

Our analysis shows that the dynamic is described by a system of coupled Klein-Gordon equations that are solved in the strongly ionised limit. For the mentioned two species we study the changes in the cut-off frequency for a range of physical parameters. Asymptotic solutions to the governing equations are obtained for a harmonic driver.

Our results reveal that ion-acoustic and neutrals-acoustic slow modes show a different damping scale.

Identifying magnetohydrodynamic vortex tubes in the sun's photosphere

With Mr Yasir Aljohani, 9 April 2020

Vortex flows in the solar photosphere are fundamentally important for the generation of magnetohydrodynamic (MHD) waves which propagate to the upper layers of the solar atmosphere. Vortex tubes are formed as coherent magnetic field structures in the solar atmosphere, eg twisted magnetic flux tubes.

In this presentation, I will discuss the method of Lagrangian Averaged Vorticity Deviation (LAVD) developed by Haller (2016) to identify vortex flows, namely the centre of circulation and their boundary. 

Then, I will present the algorithmic technique I have developed to check whether a structure detected by the LAVD method is a true vortex or not and how to determine the rotational direction (clockwise or anticlockwise).

In addition, I will apply these methods to MURaM magneto-convection simulation data to detect and track the evolution of both 2D vortices and 3D vortex tubes in the solar photosphere.

Multi-faceted approach to decomposing and identifying individual magnetohydrodynamic (MHD) wave modes in sunspots and pores

With Mr Abdulrahman Albidah, 26 March 2020

High resolution observations of pores and sunspots show a rich and complex variety of oscillatory temporal and spatial behaviour. To decompose this data into individual magnetohydrodynamic (MHD) wave modes is non-trivial and requires a multi-faceted approach.

Here we take a three-pronged approach of combining Fourier analysis, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD).

The Fourier omega-k power spectrum provides us with a useful overall view of the particular temporal and spatial scales of interest but does not provide any cross-pixel correlation. In this regard, POD classifies modes that are orthogonal in space but places no restrictions on their frequencies. DMD has no such restrictions in space but classifies modes that are orthogonal in time, ie, identified modes cannot have the same frequency.

Each of these complementary techniques have their particular strengths which we will illustrate with synthetic data.

Vortex motions in the solar atmosphere

With Dr Viktor Fedun, 12 March 2020

Solar photosphere vortices have the potential to form coherent magnetic field structures, eg twisted magnetic flux tubes and, therefore, may play a key role in the transport of energy and momentum from the lower atmosphere into the upper solar atmosphere.

In this talk, I will review existing methods for their identification and discuss our approach, which is based on Gamma detection and LAVD of inter-granular photospheric intensity vortices.

I will also present a new mechanism for the generation of magnetic waveguide from the lower solar atmosphere to the corona. This waveguide appears as the result of interacting perturbations (initially generated by photospheric vortex motions) in neighbouring magnetic flux tubes (modelled in the framework of self-similar approach).

Mode conversion of two-fluid shocks in a partially-ionised, isothermal, stratified atmosphere

With Dr Ben Snow, 6 March 2020

The plasma of the lower solar atmosphere consists of mostly neutral particles, whereas the upper solar atmosphere is mostly ionised particles and electrons.

A shock that propagates upwards in the solar atmosphere therefore undergoes a transition where the dominant fluid is either neutral or ionised. An upwards propagating shock also passes a point where the sound and Alfven speed are equal. At this point the energy of the acoustic shock can separated into fast and slow components. How the energy is distributed between the two modes depends on the angle of magnetic field.

Two-fluid numerical simulations are performed of a wave steepening into a shock in an isothermal, partially-ionised atmosphere. The collisional coefficient is varied to investigate the regimes where the plasma and neutral species are weakly, strongly and finitely coupled.

The propagation speeds of the compressional waves hosted by neutral and ionised species vary, therefore velocity drift between the two species is produced as the plasma attempts to propagate faster than the neutrals. This is most extreme for a fast-mode shock.

We find that the collisional coefficient drastically changes the features present in the system, specifically the mode conversion height, type of shocks present, and the shock widths.

In the finitely-coupled regime fast-mode shock widths can exceed the pressure scale height leading to a new potential observable of two-fluid effects in the lower solar atmosphere.

Oscillation of coronal loops associated with flaring events

With Dr Sandra Milena Conde Cuellar, 27 February 2020

Loops are fascinating structures that bring us a lot of information about the exchange of energy in the solar atmosphere. Oscillations and waves represent one of the most fascinating events in the loops, which also plays a key role in the study of coronal seismology.

It is not clear how the disturbances are excited, however, there are several candidates, eg, flares, emerging flux, and eruptions.

In this talk, I present a summary of oscillations observed in different active regions in the presence of flares and other events.

This analysis has been done with data provided by IRIS, SDO and GOES-15 spacecraft. We have found excitation sources of some disturbances in lower heights of the solar atmosphere. This matches with oscillations found in the top and the footpoints of the coronal loops.

We used this information together with semi-empirical models to study the distribution of physical variables in the loops.

Magneto-acoustic waves in the lower solar atmosphere at high resolution

With Dr Shahin Jafarzadeh, 13 February 2020

Fibrillar structures of different appearances and/or properties have ubiquitously been observed throughout the sun's chromosphere. They are often thought to map the magnetic fields, and are likely rooted in small-scale magnetic elements in the solar photosphere.

Here, we present properties of magnetohydrodynamic-wave dynamics in various fibrillar structures as well as in small magnetic elements in the low solar atmosphere, at high-spatial resolution, from the SUNRISE balloon-borne observatory as well as the Swedish Solar Telescope.

Our analysis reveals the prevalence of kink and sausage waves in both types of magnetic structures, propagating at similar high frequencies. The estimated energy flux carried by the observed waves is marginally enough to heat the chromosphere (and perhaps the corona).

Furthermore, such waves are compared with temperature fluctuations in the fibrils from high-temporal resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph (IRIS) explorer, simultaneously observed at several millimetre and ultraviolet bands of, eg, ALMA 1.3 mm as well as IRIS Mg II h and k, Si IV, and C II spectral lines, from which, physical properties of the fibrillar structures are also discussed.

2019 seminars

Waves and seismology of pores

With Dr Tom Van Doorsselaere, 5 December 2019

In this seminar, I will discuss several aspects of waves in pores. These concentrations of magnetic field, similar to miniature sunspots, are wave guides for MHD waves. In contrast to waves in coronal loops, they are resolved across the wave guide, but it is harder to know what happens further along the magnetic field.

I will discuss mode identification by using wave amplitude ratios, calculation of their energy fluxes as could be used for coronal heating, and resonant absorption of slow waves.

An outlook to future work is also included.

A new analysis procedure for detecting periodicities within complex solar coronal arcades

With Mr Farhad Allian, 21 November 2019

Coronal loop arcades form the building blocks of the hot and dynamic solar atmosphere. In particular, their oscillations serve as an indispensable tool in estimating the physical properties of the local environment by means of seismology. However, due to the nature of the arcade's complexity, these oscillations can be difficult to analyse.

In this talk, I will present a novel image-analysis procedure based on the spatio-temporal autocorrelation function that can be utilised to reveal 'hidden' periodicities within EUV imagery of complex coronal loop systems.

Simulations of MHD waves in structured plasmas

With Dr Norbert Magyar, 7 November 2019

It is well known that in an infinite and homogeneous plasma, there are three types of waves: fast, slow, and Alfven. However, richer dynamics appear in MHD once inhomogeneities are considered.

The solar corona and solar wind is often seen to be highly structured, most probably even way below the current resolving capabilities of imaging instruments. The structuring of the plasma gives rise to some well-known phenomena such as surface and body modes, reflection/refraction of waves, phase mixing, resonant absorption and so on. The nonlinear implications of structuring are less well-known, though.

In a series of numerical simulations, we will review the basic dynamics of waves supported by structures, and will connect these findings to the generation of turbulence in a structured plasma.

The solar spicule tracking code

With Mr Yuyang Yuan, 24 October 2019

In this talk, I will explain and demonstrate the Solar Spicule Tracking Code (SSTC) that I have developed. This code has the ability to automatically detect and track the motion spicules in imaging data.

I will specifically demonstrate the code working with images obtained using the H alpha line from the CRisp Imaging SpectroPolarimeter (CRISP) based at the Swedish Solar Telescope.

Solar atmospheric magnetohydrodynamic wave modes in magnetic flux tubes of elliptical cross-sectional shape

With Ms Anwar Aldhafeeri, 10 October 2019

The approach to understanding and analysing the behaviour of MHD we observed in the solar atmosphere is to find a relevant wave solution for the MHD equations. Therefore, many previous studies focused on deriving a dispersion relation equation and solving this equation for a cylindrical tube.

We know perfectly well that sunspots and pores do not have an ideal circular cross-section. Therefore, any imbalance in waveguide’s diameters, even if very small, will move the study of the problem from the cylindrical coordinates to elliptical coordinates.

Thus the emphasis on knowing the properties and what type of wave modes exist in elliptical waveguides are much more critical than studying them in cylindrical coordinates.

In this talk, I will start by deriving the dispersion relation in a compressible flux tube with elliptical cross-sectional shape. I will then solve the dispersion equation and discuss the solution of dispersion equation and how the ellipticity of tube effects the solutions with applications to coronal and photospheric conditions.

However, the information we get from the dispersion diagram does not give the full picture of how we can observe a wave, and how much the wave mode changes when the cross-sectional shape of waveguide changes. Therefore, I will present some visualisations of eigenfunctions of MHD wave modes and explain how the eccentricity effects each MHD wave mode.

Resonance cavities: a wave amplification mechanism above highly magnetic sunspots

With Dr Dave Jess, 30 May 2019

The solar atmosphere provides a unique astrophysical laboratory to study the formation, propagation, and subsequent dissipation of magnetohydrodynamic (MHD) waves across a diverse range of spatial scales.

The concentrated magnetic fields synonymous with sunspots allow the examination of guided magneto-acoustic modes as they propagate upwards into the solar corona, where they exist as ubiquitous three-minute waves readily observed along loops, plumes and fan structures.

While cutting-edge observations and simulations are providing insights into the underlying wave generation and damping mechanisms, the in-situ amplification of magneto-acoustic waves as they propagate through the solar chromosphere has proved difficult to explain.

Here we provide observational evidence of a resonance cavity existing above a magnetic sunspot, where the intrinsic temperature stratification provides the necessary atmospheric boundaries responsible for the resonant amplification of these waves.

Through comparisons with high-resolution numerical MHD simulations, the geometry of the resonance cavity is mapped across the diameter of the underlying sunspot, with the upper boundaries of the chromosphere ranging between 1300–2300 km.

This brings forth important implications for next-generation ground-based observing facilities, and provides an unprecedented insight into the MHD wave modelling requirements for laboratory and astrophysical plasmas.

Reconnection, topology and solar eruptions

With Dr Peter Wyper, 16 May 2019

The majority of free energy in the solar corona is stored within sheared magnetic field structures known as filament channels. Filament channels spend most of their life in force balance before violently erupting.

The largest produce powerful solar flares and coronal mass ejections (CMEs), whereby the filament channel is bodily ejected from the sun. However, a whole range of smaller eruptions and flares also occur throughout the corona. Some are ejective, whilst others are confined.

Recently it has been established that coronal jets are also typically the result of a filament channel eruption. The filament channels involved in jets are orders of magnitude smaller than the ones which produce CMEs.

In this talk I will start by considering these tiny, jet producing eruptions. I will introduce our MHD simulation model that well describes them and then discuss what jets can tell us about solar eruptions in general.

Specifically, I will argue that many different types of eruption can be understood by considering two defining features: the scale of the filament channel and its interaction via reconnection with its surrounding magnetic topology.

Lagrangian coherent structures: overview and applications in solar physics

With Dr Suzana de Souza e Almeida Silva, 13 May 2019

Lagrangian coherent structures (LCS) is a newly developed theory which describes the skeleton of turbulent flows.

LCS act as barriers in the flow, separating regions with different dynamics and organising the flow into coherent patterns.

This talk will introduce some concepts of LCT techniques as well as recent application to solar physics problems.

Amplification of magnetic twists during prominence formation

With Dr Youra Taroyan, 2 May 2019

Solar prominences are dense magnetic structures that are anchored to the visible surface known as the photosphere. They extend outwards into the sun’s upper atmosphere known as the corona.

Twists in prominence field lines are believed to play an important role in supporting the dense plasma against gravity as well as in prominence eruptions and coronal mass ejections (CMEs), which may have severe impact on the Earth and its near environment.

We will use a simple model to mimic the formation of a prominence thread by plasma condensation. The process of coupling between the inflows and the twists will be discussed.

We show that arbitrarily small magnetic twists should be amplified in time during the mass accumulation process. The growth rate of the twists is proportional to the mass condensation rate.

Plasma heating and particle acceleration by magnetic reconnection in solar and stellar flares

With Professor Philippa Browning, 18 April 2019

In this talk, I will describe recent models of plasma heating and non-thermal particle acceleration in flares, focussing on the role of twisted magnetic flux ropes as reservoirs of free magnetic energy.

First, using 2D magnetohydrodynamic simulations coupled with a guiding-centre test-particle code, I will describe magnetic reconnection and particle acceleration in a large-scale flaring current sheet, triggered by an external perturbation – the “forced reconnection” scenario.

I will show how reconnection is involved both in creating twisted flux ropes, and in their merger, how this depends on the nature of the driving disturbance, and how particles are accelerated by the different modes of reconnection.

Moving to 3D models, I will show how fragmented current structures in kink-unstable twisted loops can both heat plasma and accelerate charged particles. Forward modelling of the observational signatures of this process in EUV, hard X-rays and microwaves will be described, and the potential for observational identification of twisted magnetic fields in the solar corona discussed.

Then, coronal structure with multiple twisted threads will be considered, showing how instability in a single unstable twisted thread may trigger reconnection with stable neighbours, releasing their stored energy and causing an "avalanche" of heating events, with important implications for solar coronal heating. This avalanche can also accelerate electrons and ions throughout the structure.

Many other stars exhibit flares, and I will briefly discuss recent work on modelling radio emission in flares in young stars (T Tauri stars). In particular, the enhanced radio luminosity of these stars relative to scaling laws for the sun and other main sequence stars will be discussed.

Small-scale magnetic field evolution with high resolution observations

With Dr Peter Keys, 21 March 2019

Small-scale magnetic fields, ubiquitous across the solar surface, manifest as intensity enhancements in intergranular lanes and, thus, often receive the moniker of magnetic bright point (MBP). MBPs are frequently studied as they are considered as a fundamental building block of magnetism in the solar atmosphere.

The theory of convective collapse developed in the late 70s and early 80s is often used to explain how kilogauss fields form in MBPs. The dynamic nature of MBPs coupled with these kilogauss fields means that they are frequently posited as a source of wave phenomena in the solar atmosphere.

Here, with high resolution observations of the quiet sun with full Stokes spectropolarimetry, we investigate the magnetic properties of MBPs.

By analysing the temporal evolution of various physical properties obtained from inversions, we show that kilogauss fields in MBPs can appear due to a variety of reasons, and is not limited to the process of convective collapse.

Analysis of MURaM simulations confirms the processes we observe in our data. Also, magnetic field amplification happens on rapid timescales, which has significant implications for many wave studies.

Transverse MHD waves and associated dynamic instabilities in the solar atmosphere

With Dr Patrick Antolin, 7 March 2019

A large amount of recent simulations and analytical work indicate that standing transverse MHD waves in loops should easily lead to the generation of dynamic instabilities at their edges, and in particular of the Kelvin-Helmholtz kind.

While a direct observation of these transverse wave-induced Kelvin-Helmholtz rolls (or TWIH rolls) is still lacking, the forward modelling of these simulations give us an indication of what to look for in next generation instrumentation, and which currently observed features could actually be the result of TWIKH rolls.

In this talk, I will go through some of these results, comparing observations with various instruments with simulations of coronal loops, prominences and spicules.

1201 alarm project

With Dr Mark Wrigley, 28 February 2019

The 1201 alarm project is the restoration, exhibition and sharing of materials recorded in 1969 of the Apollo moon landings from a domestic television.

The talk will review the Apollo flight plan, the recording technologies of the day and the impact that it had on the speaker.

The materials will form the basis for an exhibition celebrating the 50th anniversary of moon landings to be held at the National Science and Media Museum in Bradford, Yorkshire.

The effect of thermal misbalance on compressive oscillations in solar coronal loops

With Professor Valery Nakariakov, 31 January 2019

Fast and slow magneto-acoustic waves are a promising tool for the seismological diagnostics of physical parameters of various plasma structures in the corona of the sun.

In particular, compressive waves can provide us with information about the thermodynamic equilibrium in the coronal plasma, and hence the heating function.

Compressive perturbations of the thermodynamic equilibrium by magneto-acoustic waves can cause the misbalance of the radiative cooling and unspecified heating. The effect of the misbalance is determined by the derivatives of the combined heating/cooling function with respect to the plasma density and temperature, and can lead to either enhanced damping of the compressive oscillations or their magnification.

Moreover, in the regime of strong misbalance, compressive MHD waves are subject to wave dispersion that can slow down the formation of shocks and can cause the formation of quasi-periodic wave trains.

2018 seminars

Fast MHD modes of a two (and three) shell semi-cylindrical waveguide

With Ms Hope Thackray, 29 November 2018

The modelling of coronal loop structures has long been pursued as a means of determining physical properties of the sun's corona.

Here, a 3D semi-cylindrical waveguide is proposed, representing a coronal loop arcade anchored in the photosphere.

By considering the eigenfunctions formed at the interface of a sharp density discontinuity (represented by "two-shell" and subsequently "three-shell" density structures), we show that waves are elliptically polarised, and that small changes in density contrast between shells can drastically affect the presence of eigenmodes.

Since observational information has restrictions on resolution, the implication is that two similarly determined density structures may produce vastly different estimations of potential eigenmodes.

Introduction to multiple scaling methods to solve differential equations with applications to plasma physics. Part II: Nonlinear partial differential equations

With Dr Istvan Ballai, 22 November 2018

In the second part of my seminar, I will focus on nonlinear partial differential equations that can be obtained from the MHD equations.

Using the multiple scale technique I will present a method to obtain the Korteweg-de Vries-Burgers equation in a non-ideal plasma in the presence of Hall currents.

Using simple methods, I will find solutions to the limiting cases of shock waves and solitons.

Introduction to multiple scaling methods to solve differential equations with applications to plasma physics. Part I: Ordinary linear differential equations

With Dr Istvan Ballai, 15 November 2018

Many of the equations we encounter in our research on solar and space plasma physics dynamics contain essential physical constraints (nonlinearity, singularities, complex domains of interest, complex boundary conditions, etc) that makes it difficult to find exact solutions.

Therefore, in order to obtain information about solutions of equations, we are forced to use approximative methods, numerical solutions, or both. The most important approximation methods are the perturbation methods, where the solutions are represented by the first few terms of an expansion.

In this seminar I will review the perturbation methods used to solve ordinary differential equations, highlighting their advantages and shortcomings. The presentation will revolve around simple examples of differential equations, presenting the method of finding approximative solutions of a differential equation we can derive in plasma physics.

Properties of Alfvénic waves in the solar chromosphere

With Mr Samuel Skirvin, 1 November 2018

In the first part of my talk, I will discuss the results of investigation of the properties of transverse waves existing in spicules using the automated wave tracking code NUWT.

Analysing a distance-time diagram at an altitude of 7 Mm relative to the solar limb produces the measured distribution of properties such as wave amplitude, period and velocity amplitude.

In the second part of the talk I will provide an overview of the recent studies on the effect of initial flow profiles on the dynamics of solar jets.

Introduction to the sun

With Dr Gary Verth, 18 October 2018

This talk will be an introduction to the science required to understand the sun and its atmosphere.

It is primarily intended for students starting their postgraduate research in plasma, solar, or magnetospheric physics.

Due to the introductory nature of the talk, it would also be suitable for any interested non-specialists.

Participation in external seminars and conferences

Detection and dynamics of the vortex tubes in the solar atmosphere

Dr Suzana de Souza e Almeida Silva at ESPOS, 25 February 2021

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-β>1, regardless of the existence of flow vortices.

MHD wave modes in the solar magnetic flux tubes with elliptical cross-section

Ms Anwar Aldhafeeri at ESPOS, 7 November 2019

Many previous studies of MHD modes in the magnetic flux tubes were focussed on deriving a dispersion relation for cylindrical waveguides.

However, from observations it is well known that, for example, the cross-sectional shape of sunspots and pores are not perfect circles and can often be much better approximated by ellipses. From a theoretical point of view, any imbalance in a waveguide’s diameters, even if very small, will move the study of the problem from cylindrical to elliptical coordinates.

In this talk, I will therefore describe a model that predicts the MHD wave modes that can be trapped and propagate in a compressible magnetic flux tube with an elliptical cross-section embedded in a magnetic environment.

I will discuss the resultant dispersion relations for body and surface modes, then I will show how the ellipticity of a magnetic flux tube effects these solutions (with specific applications to the coronal and photospheric conditions).

From a practical point of view the information from these dispersion diagrams does not show how these MHD modes will manifest themselves in observational data. Therefore, I will also present several visualisations of the eigenfunctions of these MHD wave modes and explain how the eccentricity effects each wave mode.

Surface waves and instabilities in the presence of an inclined magnetic field

Eleanor Vickers at ESPOS, 4 October 2018.

While surface waves propagating at tangential discontinuities have been studied in great detail, few studies have been dedicated to the investigation of the nature of waves at contact discontinuities, ie, plasma discontinuity, where the background magnetic field crosses the interface between two media.

In this talk, I will show that by introducing magnetic field inclination, the frequency of waves is rendered complex, where the imaginary part describes wave attenuation, due to lateral energy leakage.

We investigate the eigen-value and initial value problem and determine the conditions of transition from contact to tangential discontinuity.

Finally, I will present an investigation into the effect of magnetic field inclination on magnetic Rayleigh-Taylor instability.