2023 PDG seminars

First high-resolution coordinated observations of an active region with Solar Orbiter and DKIST

Active regions are the most dynamic structures in the solar disk. They contain numerous substructures with wide orders of sizes and lifetimes. The evolution of an active region with different time scales, the role of the small-scale structures in the active region, the mechanism triggering the transient phenomena, and the mechanism responsible for the plasma upflow at the active region border are still open issues. The main aim of the project is to focus on these open issues using high-resolution observation of the solar atmosphere obtained from different vantage points. The active region was observed during coordinated observations with Solar Orbiter and Daniel K. Inouye Solar Telescope (DKIST) on 18-21 October and 24 October 2022. Instruments onboard Solar Orbiter and DKIST provide unprecedented high-spatial and temporal simultaneous observation of the active region. Moreover, the Hinode and IRIS also focus on the same active region. We present the preliminary results of the coordinated observations from magnetic field, imaging and spectroscopy instruments.

Beyond the Standard Flare Model – Dynamics of QPPs, Fans & SADs

The standard model is able to account for many of the phenomena observed in solar flares. However, some observed features, likely relating to the higher-energy nature of flare energy release, are yet to be tied into the standard model. Quasi-Periodic Pulsations (QPPs), flare fans and Supra-Arcade Downflows (SADs) are examples of such features. We investigate dynamics of these coronal plasma phenomena across 2-3 case studies – using data from Solar Orbiter/STIX, IRIS and Hinode-EIS.

Non-Adiabatic MHD Seismology of the Solar Corona

A powerful technique for the diagnostics of physical conditions in active regions of the Sun’s corona is the method of coronal seismology, based on observations of magnetohydrodynamic (MHD) wave processes in high-resolution imaging data or indirectly as quasi-periodic pulsations in flaring light curves. Traditionally, coronal seismology is focused on the diagnostics of MHD properties of the Sun’s corona, such as the coronal magnetic field, plasma density, fine parallel and cross-field structuring, which are difficult to measure otherwise. In a series of recent works, it has been proven effective for probing not only MHD but also fundamental thermodynamic parameters of the coronal plasma through theoretical modelling and observations of MHD wave dynamics and stability in intrinsically non-adiabatic conditions and in the presence of a wave-induced thermal misbalance. In this talk, I present a few recent examples of the application of the method of non-adiabatic coronal seismology to probe such crucial parameters of the coronal plasma as energy transport coefficients, polytropic index, and heating function, regulating the delicate energy balance in the corona. More specifically, an apparent departure of the effective heat transfer coefficient from its classical Spitzer form is assessed seismologically under the assumptions of weak and full non-adiabaticity. The exact role of the effective polytropic index of the corona in the dynamics of non-adiabatic slow magnetoacoustic waves, its link with the effective thermal conduction coefficient, and shortcomings of the polytropic plasma approximation are discussed. I also show the potential of a recently developed theory of wave-induced thermal misbalance and a frequency-dependent damping of slow magnetoacoustic waves for constraining the link between the coronal magnetic field and the heating function, which is not directly available in extreme ultraviolet or soft X-ray observations traditionally used for coronal heating studies.   

Vortices as energy channels into the solar corona

From water spiralling into a sink drain to mesmerising giant storms on Jupiter, vortex motions are present throughout the universe on scales from the very small to the very large. Vortex flows have been found in the photosphere, chromosphere, and low corona in observations and simulations. It has been suggested that vortices play an important role in channeling energy and plasma into the corona. We investigate the importance of vortices for coronal heating using high-resolution simulations of coronal loops driven self-consistently by magnetoconvection. We performed 3D resistive MHD simulations with the MURaM code. Studying an isolated coronal loop in a Cartesian geometry allows us to resolve the structure of the loop interior. We find that the energy injected into the loop is generated by internal coherent motions within strong magnetic elements. A significant part of the resulting Poynting flux is channeled through the chromosphere in vortex tubes forming a magnetic connection between the photosphere and corona. Vortices have a complex relationship with the coronal emission, and I will discuss how these structures could potentially be observed in the corona.

Association Between Magnetic Pressure Difference and the Movement of Solar Pores

Solar pores are closely related to the concentration, dissipation, and transportation of solar magnetic flux. Their observable characteristics can provide constraints on models and simulations of magnetic flux emergence and formation. The specific property investigated in this study is their horizontal movement. The aim is to investigate whether the movement is correlated with any observable quantities. Our statistical analysis of 61 compact pores identified from the Spaceweather HMI Active Region Patches (SHARP) from 2011 to 2018 indicates that the direction of movement is often either parallel or antiparallel to the direction of maximum magnetic pressure difference at the opposite sides of the edges of the pores. The correlation coefficients for both the parallel and antiparallel cases are higher than 0.74. Despite the high correlation, our analysis using the transfer entropy indicates no significant causal relationship between the direction of motion and the direction of maximum magnetic pressure difference.

Vortical network connectors for turbulence modification

The interaction-driven evolution of complex systems in both natural and engineering contexts offers a unique opportunity to leverage graph theory for understanding their behavior as well as for modeling and modifying their evolution. This seminar aims to introduce a network (graph) community-based framework to perform flow-modification of turbulent vortical flows. The present framework captures vortical interactions on a network, where the vortical elements are viewed as the nodes and the vortical interactions are regarded as edges weighted by induced velocity from the Biot-Savart law. The network-based community detection algorithm is used to identify a group of closely interacting vortical elements, called communities. The inter- and intra-community interactions are used to identify the communities that have the strongest and weakest interactions amongst them, referred to as the connector and peripheral communities, respectively. For isotropic turbulence, the connector and peripheral communities correspondingly resemble shear-layer and vortex-core like structures. Results show that perturbing the connector structures enhances local turbulent mixing beyond what is achieved by perturbing the strongest vortex tube and shear-layer regions.

Alfvénic motions arising from inclined acoustic wave drivers in 3D MHD simulations of coronal loops

Alfvénic motions are ubiquitous in the solar corona and their observed properties are closely linked to those of photospheric p-modes. However, it is still unclear how a predominantly acoustic wave driver is related to these transverse oscillations in the magnetically dominated solar corona. I will talk about our 3D MHD numerical simulation which models a straight, expanding coronal loop in a gravitationally stratified solar atmosphere. We implement a driver locally at one foot-point corresponding to an acoustic-gravity wave which is inclined with respect to the vertical axis of the magnetic structure and is equivalent to a vertical driver incident on an inclined loop. We show that transverse motions are produced in the magnetic loop and study the resulting modes in the theoretical framework of a magnetic cylinder model. I will also discuss some physical properties of the Alfvénic perturbations including their speeds, velocity amplitudes and periods. Finally I will present a brief discussion on the energetics of the resulting wave modes and avenues of future work to extend the current study from other interesting features developing in the simulation.

Parametric instability/decay driven by ionisation-recombination waves in the solar atmosphere

Abstract: Non-linear and non-propagating ionisation-recombination (i-r) waves can appear in partially ionised solar atmospheric plasmas leading to a time-dependent density of charged particles. This wave can parametrically couple to other MHD wave (here we discuss only Alfvén waves) leading to a various fate of the MHD waves, depending on the strength of collisional coupling between particles. Here I am going to present some mathematical progress on the quantitative description of i-r waves, as well as the method of determining the width of parametric instability.

Magnetic Tornado Properties: A Eruptivity of Magnetic Flux Ropes in the Solar Corona

In a recent study, we investigate which scalar quantity or quantities can best predict the loss of equilibrium and subsequent eruption of magnetic flux ropes in the solar corona - one of the main causes of significant space weather events. In our numerical models the flux rope is produced self-consistently by flux cancellation combined with gradual footpoint shearing of a coronal arcade that is open at the outer boundary, representing the magnetic field in decaying active regions on the Sun. We use both full MHD (magnetohydrodynamics) in cartesian coordinates and the magnetofrictional approach in cartesian and polar coordinates, and by running thousands of 2.5D simulations find that there are several scalar criteria that could theoretically be used as a proxy for eruptivity, discussing the established 'eruptivity index' alongside newly-defined diagnostic ratios that appear to be very good predictors of eruptive behaviour. 

Complex dynamics of stochastic chaotic flows in fluids and plasmas

Turbulence in fluids and plasmas exhibits complex dynamics and structures ubiquitous in nature, e.g., cardiovascular haemodynamics, shallow-water ocean dynamics, galactic centre plasma dynamics, and solar-terrestrial plasma dynamics. Transient coherent structures known as chaotic saddles are the building blocks of turbulence. Hyperbolic and elliptic Lagrangian coherent structures provide accurate identification of transport barriers in turbulence. We report the first observational evidence of Lagrangian chaotic saddles in plasmas, given by the intersections of finite-time unstable and stable manifolds, using a sequence of spacecraft images of the horizontal velocity field of the quiet-Sun photosphere. We show that the persistent objective vortices are formed in the gap regions of Lagrangian chaotic saddles at supergranular junctions. Next, we discuss the spatiotemporal dynamics of vorticity and magnetic field in the region of a long duration photospheric vortex at a supergranular junction. We show that in an interval during the vortex lifetime, the magnetic field, the electric current density, and the electromagnetic energy flux are intensified in a region of two merging magnetic flux tubes trapped inside the vortex boundary, which is a signature of chaotic stretching-twisting-folding acting in turbulence.

Magnetic Tornado Properties: A Substantial Contribution to the Solar Coronal Heating via Efficient Energy Transfer

Solar coronal heating models are beginning to be extrapolated to modeling the coronae of exoplanet host stars to investigate their habitabilities. Thus, understanding the details of the solar coronal heating mechanism is significant for planetary science also. Recently, magnetic tornadoes, characterized as coherent, rotating magnetic field structures extending from the photosphere to the corona, have drawn growing interest as a possible means of efficient energy transfer into the corona. Despite its acknowledged importance, the underlying physics of magnetic tornadoes remains still elusive. In this talk, I’m going to talk about our recent work Kuniyoshi et al. (2023) and the plan for our future work. In our study, we conduct a three-dimensional radiative magnetohydrodynamic simulation that encompasses the upper convective layer and extends into the corona, with a view to investigating how magnetic tornadoes are generated and efficiently transfer energy into the corona. We find that a single event of magnetic flux concentration merger on the photosphere gives rise to the formationof a single magnetic tornado. The Poynting flux transferred into the corona is found to be four times greater in the presence of the magnetic tornado, as compared to its absence. This increase is attributed to a reduction in energy loss in the chromosphere, resulting from the weakened magnetic energy cascade. Based on an evaluation of the fraction of the merging events, our results suggest that magnetic tornadoes contribute approximately 50% of the Poynting flux into the corona in regions where the coronal magnetic field strength is 10 G. 

Fast 3D non-LTE spectral synthesis with neural networks

Three-dimensional non-LTE calculations are required to forward-model the strongest spectral lines in the solar chromosphere. They are also the gold standard for detailed chemical analysis in stellar atmospheres. However, such calculations require copious amounts of computing time, on the order of millions of CPU hours, and are much more time-consuming than running the 3D simulations themselves. I will give an overview of the problem and present a new approach using neural networks to dramatically speed up 3D NLTE spectral synthesis. This approach, implemented in the open-source code SunnyNet, uses a database of previous calculations to learn the translation from LTE to non-LTE atomic populations. By feeding a new atmosphere, we can then use SunnyNet to predict its non-LTE populations and subsequently compute synthetic spectra for any viewing angle. We tested this approach with 3D simulations ran with the Bifrost code for the synthesis of Hα profiles, a line strongly affected by 3D NLTE effects. Our method leads to a speedup of about 10^5 times compared to traditional methods, when running on a single GPU. The quality of the predicted populations is best when using different timesteps of the same simulation for training and testing, and translates to typical differences of less than 4\% in the Hα spatially-averaged intensity spectra. Synthetic images at the Hα line core reproduce chromospheric fibrils very well, strongly suggesting that SunnyNet is learning 3D radiative transfer, since fibrils are absent in 1D calculations. The results are less reliable when different types of simulations are used for the training and the testing. I will discuss the current limitations and caveats, and discuss possible applications of SunnyNet and future improvements.

Lagrangian chaotic saddles and Lagrangian coherent structures in solar supergranular turbulence

First, we report observational evidence of Lagrangian chaotic saddles in plasmas, given by the intersections of finite-time unstable and stable manifolds, using an ≈ 22 h sequence of spacecraft images of the horizontal velocity field of the quiet-Sun photosphere. A set of 29 persistent objective vortices with lifetimes varying from 28.5 to 298.3 min are detected by computing the Lagrangian averaged vorticity deviation. The unstable manifold of the Lagrangian chaotic saddles computed for ≈ 11 h exhibits twisted folding motions indicative of recurring vortices in a magnetic mixed-polarity region. We show that the persistent objective vortices are formed in the gap regions of Lagrangian chaotic saddles at supergranular junctions. Next, we discuss the spatiotemporal dynamics of vorticity and magnetic field in the region of a long-duration photospheric vortex at a supergranular junction. We show that in a 30-min interval during the vortex lifetime, the magnetic field is intensified at the centres of two merging magnetic flux tubes trapped inside the vortex boundary. Moreover, we show that the electric current density is intensified at the interface boundary layers of merging tubes, resulting from strong vortical downflows in a supergranular vertex. Evidence of Lagrangian chaos and vortex stretching in the photospheric plasma turbulence responsible for driving the intensification of magnetic fields is analysed. In particular, we report the first solar observation of the intensification of electromagnetic energy flux resulting from the merger of magnetic flux tubes.

Rossby Waves in the Sun

Recent observations of Rossby waves and other more exotic forms of inertial oscillations in the Sun’s convective zone have kindled the hope that such waves might be used as a seismic probe of the Sun’s interior. Their presence also raises questions about their potential role in the solar cycle. Here we present a suite of 3D numerical simulations in full spherical geometry that model the Sun’s convective zone and upper radiative interior. With these models, we demonstrate that Rossby waves are ubiquitous within the radiative interior as well as within the convective zone, potentially explaining the large horizontal velocities that have been previously seen in the radiative interior of numerical models. Their presence in the radiative zone implies that the Sun’s radiative interior is not quiescent, but instead may play a role in the dynamics of the tachocline. We also demonstrate the presence of gravity waves and thermal Rossby waves. 

Optimisation problems in transitional turbulence and MHD

A typical optimisation problem might involve the minimisation/maximisation of a cost/output by the variation of a few discrete parameters, e.g. sale price or a production rate.  But what if we want to optimise a whole field, e.g. optimise a velocity field to maximise mixing in a given container?  Now our 'parameter' u(x) is actually a huge parameter set to be optimised, measured by the number of finite difference points or spectral coefficients required to represent u(x).  Until recently, methods to optimise such states were considered too computationally expensive. However, it has been recognised these problems can be formulated in an iterative Lagrangian framework, where each iteration 'only' costs a few simulations (rather than thousands of simulations, attempting to vary the parameters one by one).

Using this framework, we look at some fundamental questions in fluid mechanics and MHD:

- At the same flow rates in pipe, channel and Couette flows, either smooth laminar flow or turbulence can be observed.  What is the minimal flow disturbance required to switch from laminar to turbulent flow (e.g. to enhance mixing)?

- What is the minimal body force required to cause a switch from turbulence to laminar flow (e.g. to reduce turbulent drag)?

- In MHD, what is the minimal flow required to generate a magnetic field (kinematic dynamo)?

- How do we find configurations where the flow and magnetic field determine each other's structure (subcritical self-consistent dynamo)?

Oscillatory Reconnection: How waves emitted from merging magnetic flux ropes can be used as a seismological tool for the solar corona

Oscillatory reconnection is a time-dependent relaxation process that takes place in highly conducting, resistive plasmas, such as those found in the solar corona. Oscillatory reconnection is characterised by the emission of magnetohydrodynamic waves away from the reconnection site. Numerous studies have found a connection between the properties of these emitted waves and the physical parameters of the background plasma, opening up the possibility of using oscillatory reconnection as an observational seismological tool for the solar corona. In this presentation, I will introduce the current state of oscillatory reconnection simulations and present ongoing work by the Manchester Solar Plasma Group that aims to link this mechanism to solar flare data.

Tutorial on bisector LOS-velocity measurements

Link to the code: https://github.com/shahin-jafarzadeh/LOS-velocity