2019 PDG seminars

Waves and seismology of pores

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

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

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

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

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

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

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

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

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

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

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

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

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

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.