2021 PDG seminars

Recovering flows in a highly magnetic environment using deep learning

The magnetohydrodynamics (MHD) model for plasmas describes many complex features of the solar atmosphere. High-quality simulations can utilise the equations of MHD to reproduce many complex features including the totality of the birth and death of active regions (AR). Forecasting the formation of ARs still provides a challenging problem due to the lack of high quality observational data and new methods - I aim to show that machine learning (ML) approach provides a grand opportunity for observational solar physics and test the capability of ML and deep learning (DL).

Damping of the small scales due to partial ionization effects in the solar chromosphere

The chromosphere is a partially ionized layer of the solar atmosphere, the transition between the photosphere where the gas motion is determined by the gas pressure and the corona dominated by the magnetic field. Partial ionization effects are usually modeled in the single-fluid approach through the ambipolar term in  a generalized Ohm's law or in a purely two-fluid model of neutral and charged particles. We study the effect of partial ionization for 2D wave  propagation in a gravitationally stratified, magnetized atmosphere with properties similar to the solar chromosphere in the single-fluid approach with ambipolar effect. We adopt an oblique uniform magnetic field in the plane of  propagation with strength suitable for  a quiet sun region. We use numerical simulations where we continuously drive fast waves at the bottom of the atmosphere. The collisional coupling between ions and neutrals decreases with the decrease of the density and the ambipolar effect becomes important. Fast waves excited at the base of the  atmosphere reach the equipartition layer and reflect or transmit as slow waves. While the waves propagate through the atmosphere and the density drops, the waves steepen into shocks. The main effect of ambipolar diffusion is  damping of the waves. We find that for the parameters chosen, the ambipolar diffusion affects the fast wave before it is reflected, with damping being more pronounced for waves which are launched in a direction perpendicular to the magnetic field. Slow waves are less affected by ambipolar effects. The damping increases for shorter periods and larger magnetic field strengths. Small scales produced by the nonlinear effects and the superposition of different types of waves created at the equipartition height are efficiently damped by ambipolar diffusion.   Magnetic energy can be converted into internal energy through the dissipation of the electric current produced by the drift between ions and neutrals.  In the two-fluid model the damping is related to the decoupling between charges and neutrals. We observed that waves in neutrals and plasma, initially coupled at the upper photosphere, become uncoupled at higher heights in the chromosphere and the waves are damped. In simulations of the Rayleigh-Taylor instability ion-neutral collision frequency is the primary determining factor in development or damping of small-scale structures.   Kinetic energy can be converted into internal energy through the frictional heating produced by the drift between ions and neutrals.

Extraction and modelling of coherent structures in turbulent flows

Detailed measurements or numerical simulations of turbulent flows reveal the existence of coherent large-scale structures, which are relevant for turbulence properties such as wall friction, heat transfer or sound radiation. Recent years have seen the emergence of modal decomposition techniques, which, when applied to turbulent flows, allow an extraction of coherent structures in a quantitative manner. Such structures have properties related to the linearised Navier-Stokes system, and may be modelled as the most amplified responses to applied forcing; this provides a rationale for understanding coherent structures as dominant waves obtained by the linearisation of the flow equations. In this presentation we will review modal decomposition of turbulent flows focusing on spectral proper orthogonal decomposition (SPOD), and linearised models using the resolvent operator. We will explore the connection between SPOD and resolvent modes in some canonical flows such as turbulent jets and channels. Finally, we will discuss how to include non-linearity in dynamical models of coherent structures, showing recent results for turbulence estimators and reduced-order models.

What we know about wave heating modelling. Or maybe not

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.

Technicalities of MURaM solar photospheric models: how to use them and what to expect from them

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.

POD and DMD tutorial

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?

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

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

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

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

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

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

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

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.