Previous UK-SOSS Seminars
Magnetic Relaxation, Helicity and Reconnection
University of Dundee
Date:7th April 2022 10:00am (UK time)
Abstract:The relaxation of magnetic fields in plasmas with high magnetic Reynolds numbers, such as the solar corona, is constrained by the total magnetic helicity. This constraint is at the heart of Taylor’s relaxation theory, which states that the relaxed state is one that minimises the energy while keeping the global helicity fixed. We will discuss the difficulties encountered in applying this theory to the solar corona and how concepts such as the field line helicity, as well as the unsigned helicity, can be used to better explain the results of numerical relaxation experiments.
Partially ionised plasma dynamics in the solar atmosphere
University of Exeter
Date:10th March 2022 10:00am (UK time)
Abstract:The lower layers of the solar atmosphere are predominantly filled with neutral atoms with only relatively few charged particles. So, despite magnetic fields being a key driver behind many of the dynamics observed, the majority of the particles do not directly feel the Lorentz force of the magnetic field. Instead, this force is transferred through coupling processes like particle collisions and charge exchange. As all these processes happen on a finite timescale, the coupling between the neutrals and the magnetic field is not perfect. So, for example, for dynamics that can be high frequency (e.g. waves and instabilities), naturally high-frequency dynamics (e.g. shock fronts) or physical process that are fundamentally multi-scale like turbulence or magnetic reconnection, neutrals can undergo vastly different motions to those of the charged species. For these processes, it becomes important to model both the ionised plasma and the neutrals as separate fluids to properly capture the dynamics. In this talk I review recent results from my group from our studies of the role of partial ionisation in MHD dynamics looking at how instabilities, shocks and reconnection change when the charges and neutrals are modelled as separate fluids. I will also provide some thoughts on how these processes may manifest in observed dynamics in the solar atmosphere.
Bringing Balance to the Force: Equilibrium Models in Solar Physics
St. Andrews
Date:10th February 2022 10:00am (UK time)
Abstract:Equilibrium models have a variety of useful applications in solar physics, despite the fact that the Sun is highly dynamical. At a fundamental level it is often advisable to start the study of a complex system by finding its steady states. These states can then, for example, be used as a basis for investigating waves and instabilities. Furthermore, the response of the equilibria to the variation of system parameters or boundary conditions can help to explore and predict aspects of the nonlinear behaviour of the system. However, in this talk I will mainly focus on a more practical aspect, namely the use of equilibrium models in the extrapolation of photospheric magnetic field measurements into the solar corona. While potential and force-free magnetic field models are still the most widely used equilibrium solutions for this task, over the past few years some extrapolation methods have employed magnetohydrostatic solutions. I plan to discuss some of the approaches used, with an emphasis on analytical methods.
Oscillatory Reconnection: wave-generation, quasi-periodic pulsations, time-dependent reconnection and seismological tool
Northumbria University
Date:13th January 2022 (10:00 UK time)
Abstract:Oscillatory Reconnection – a relaxation mechanism with periodic changes in connectivity - has been proposed as a potential physical mechanism underpinning several periodic phenomena in the solar atmosphere, including quasi-periodic pulsations (QPPs). At its heart, Oscillatory Reconnection is a time-dependent reconnection process and the dynamic release of stored magnetic energy is central to multiple fields of study (including solar physics, astrophysics, fusion, laboratory-based plasma, computational MHD and space weather). This talk will review the state-of-the-art in this area, and we reveal a relationship between the equilibrium magnetic field strength, decay rate and period, which opens the tantalising possibility of utilising Oscillatory Reconnection as a seismological tool. Reference: McLaughlin et al. (2018, Space Science Reviews, 214, 45) + Zimovets, McLaughlin, et al. (2021, Space Science Reviews, 217, 66) + Karampelas, McLaughlin, et al. (2022, Astrophysical Journal, accepted)
New look at the Sun at radio-frequencies
University of Glasgow
Date:9th December 2021 (10:00 UK time)
Abstract: Observations of the Sun at radio-frequencies provides unique look at a variety of solar processes: from solar activity to the solar corona turbulence studies. Over the last few years, a unique set of solar radio observations with unprecedented temporal and frequency resolutions using the Low Frequency Array (LOFAR), Solar Parker Probe and RPW/Solar Orbiter opened new opportunities for detailed studies of the variety of solar processes in the outer solar corona. The new observations at sub-second scales are often intriguing and puzzling, requiring new models and re-thinking of the current models. In the talk, I will highlight these new observations and exciting opportunities with the Square Kilometre Array which is under construction.
Space weather climate on solar cycle time-scales
University of Warwick
Date:11th November 2021 10:00 (UK time)
Abstract: Space weather and solar terrestrial physics observations are increasingly becoming a data analytics challenge and there are common approaches with other fields such as earth climate observations. Whilst focussing on specific applications, this talk aims to present generic methodology for inhomogeneous ‘real world’ data. Over the last 5 cycles we have high-quality in-situ solar wind parameters and ground-based geomagnetic indices. Climate is the distribution of weather, and we will examine how the distributions of these parameters and indices vary within and between solar cycles. Over the last 14 cycles we have geomagnetic indices such as the aa index which are poorly resolved in amplitude but nevertheless contain information on the likelihood of occurrence of extreme space weather events, and we discuss how this can be quantified, setting the Carrington event in the context of extreme events that have occurred over the last 150 years. Each solar cycle is of unique duration, and we show how the Hilbert transform of daily sunspot number can be used to map each cycle onto a uniform time-base or ‘sun clock’. We can then use this mapping to quantify how activity in space weather relevant parameters, and in particular the likelihood of super-storms, is modulated by the solar cycle. This reveals the phases in the solar cycle where there is a clear ‘switch on’ and ‘switch off’ of activity and these are in principle predictable. Sun clocks for both the Schwabe and Hale cycles will be discussed.
From high-resolution observations of the Sun to the solar-stellar connection
Queen's University Belfast
Date:14th October 2021 10:00 (UK time)
Abstract: Our nearest star has always been a unique source for our understanding of the universe in its many forms. The solar atmosphere and interior provide a working example where structures and dynamics can be studied over an enormous range of spatial and temporal scales. The seminar will begin with some global solar properties and phenomena, placing them in the context of stellar magnetic activity. We will then highlight some of the smallest features that can be resolved in the lower solar atmosphere and the physical parameters that can be extracted from them. We will comment on the high-resolution observations of the Sun and how they can help us tackle some of the challenges of stellar astrophysics.
What can magnetic helicity tell us about the likelihood of an eruption from active regions?
UCL, Mullard Space Science Laboratory
Date:24th June 10:00 (UK time)
Abstract: Understanding the physical processes that underly the occurrence of coronal mass ejections is a key area of research, whilst being able to forecast an ejection beforehand would provide significant benefits to space weather forecasting lead-times. In this talk, these two aims will be discussed in the context of a quantity known as magnetic helicity, in particular the so-called helicity proxy that can be determined from modelled active region magnetic field and which has shown potential in being able to indicate when an active region will produce eruptive activity. Results will be presented for NOAA active region 11158, that builds on the helicity proxy analysis already presented by Thalmann et al. (2019) to incorporate the observed evolution of the coronal field and the physical processes taking place in the time leading up to an eruption.
Carrington Events
University of Glasgow and UC Berkeley
Date:13th May 10:00 (UK time)
Abstract: We often cite the Carrington flare (SOL1859-09-01) as an extreme flare/CME/geostorm event and a prototype "superflare." I discuss the original observations in the context of what we now know about solar and stellar flares. Although not an extreme event in the sense of being truly exceptional, it has become clear that the Carrington flare became a milestone in what we now call "multimessenger astronomy."
Taking stock of our understanding of Coronal Mass Ejections
Chief Scientist, RAL Space
Date: 10:00 (UK time) 15th April 2021
Abstract: This presentation will take stock of where we are with Coronal Mass Ejection (CME) research, taking a brief look at the history of CME observations and the early interpretations of the phenomenon, through to the present day where we have multi-spacecraft observations with coronagraphs and heliospheric imagers and a wide range of modelling techniques, many of which are now geared towards space weather impacts. This is a research area that has matured dramatically, since the launch of the SOHO spacecraft in particular, but especially with the increased interest in space weather and missions such as STEREO and Lagrange. It is a good time to take stock and in doing so to revisit some basic issues, including the flare-CME relationship, stealth CMEs, coronal dimming and CME-CME interactions, as well as lessons learnt from imaging and tracking CMEs in the corona and in the heliosphere. Perhaps it is also a useful time to pause and ask the questions, what else do we want to know about CMEs, and how are we going to satisfy that desire?
Solar Diagnostic Spectroscopy - a Personal Perspective
DAMTP, University of Cambridge
Date: 10:00 (UK time) 18th March 2021
Abstract: Spectroscopic diagnostics have enabled us to determine the physical parameters of plasma for different solar features (active regions, jets, flares etc). Helen started her career studying the visible coronal lines from the 1952 eclipse observations. She then studied the UV and X-ray spectrum of the Sun, working on many joint UK, NASA, ESA and Japanese solar space projects including Skylab, the SMM (Solar Maximum Mission), Yohkoh, SoHO (Solar and Heliospheric Observatory), Hinode, SDO (Solar Dynamics Observatory) and IRIS (Interface Region Imaging Spectrograph). She was a founder member of the CHIANTI team, an atomic database which has been extensively used for solar data analysis. In this talk, she will pick out a few key results as examples of the value of spectroscopic diagnostics (in the transition region and corona). She will also look towards the current opportunities for research in this field and the future prospects for spectrometers. For a recent review, see: Del Zanna and Mason, 2018, Solar UV and X-ray spectral diagnostics’, Sol. Phys. Liv. Reviews.
The Characteristics of Coronal Streamers Over a Solar Cycle: Resolving the Line-of-Sight with Coronal Rotational Tomography
Aberystwyth University
Date: 10:00 (UK time) 18th February 2021
Abstract: Any remote measurement of the solar corona in white light (or other) wavelength is an integration of emission along an extended line of sight. Historically, most studies necessarily assumed an axi-symmetric distribution to the density, thus derived properties contained an inherent and unquantified uncertainty. From the SOHO era onwards, space-based coronagraphs (LASCO/SOHO, and COR/STEREO) make frequent, uninterrupted, and high-quality observations of the corona which allow estimates of the true density distribution using coronal rotational tomography (CRT). A recent breakthrough in CRT is revealing a new view of the corona which is gained directly from observation. For the first time, we can view long-term trends in the coronal rotation rate, find meaningul links between coronal and interplanetary density structures, and use estimated densities at a range of heights to constrain outflow velocity and acceleration. The density distributions provide a ground truth for model extrapolations of the photospheric magnetic field, and new empirical boundary conditions for solar wind models. Tentative evidence of the Parker spiral onset can be seen close to the Sun. The next step in CRT methods is the inclusion of a time-dependent density distribution: initial results show promising correlations with Parker Solar Probe measurements, and the discovery of large variations on daily timescales not associated with mass ejections.
Magnetohydrodynamic Seismology of the Solar Coronal Plasma with Kink Oscillations
University of Warwick
Date: 10:00 am (UK time), 21st January 2021
Abstract: Standing transverse oscillations of the plasma loops of the solar corona have been intensively studied for the last 20 years as a tool for the diagnostics of the coronal magnetic field. Those oscillations are confidently interpreted as standing fast magnetoacoustic kink modes of the plasma non-uniformities. Statistical analysis demonstrates that, in the majority of cases, the oscillations are excited by a mechanical displacement of the loop from an equilibrium by a low coronal eruption. Standing kink oscillations are observed to operate in two regimes: rapidly decaying large amplitude oscillations and undamped small amplitude oscillations. In both these regimes, different loops oscillate with different periods that scale with the oscillating loop length. The oscillation amplitude does not show dependence on the loop length or the oscillation period. In the decayless regime the damping should be compensated by energy supply which allows the loop to perform almost monochromatic oscillations with almost constant amplitude and phase. We developed a low-dimensional model explaining the undamped kink oscillations as a self-oscillatory process caused by the effect of negative friction, which is analogous to producing a tune by moving a bow across a violin string. The period of self-oscillations is determined by the frequency of the kink mode. The ubiquity of decayless kink oscillations makes them an excellent tool for MHD seismology, in particular, for probing free magnetic energy in preflaring active regions.
Heating and particle acceleration in twisted flux ropes in solar and stellar flares
University of Manchester
Date: 10:00 am (UK time), 10th December 2020
Abstract: Twisted magnetic flux ropes are reservoirs of free magnetic energy. I will describe some recent advances in modelling plasma heating and non-thermal particle acceleration in twisted magnetic flux ropes in the context of solar flares. After an overview of twisted magnetic flux ropes in the corona, I will show how magnetic reconnection in fragmented current structures in kink-unstable twisted loops can both heat plasma and efficiently accelerate both electrons and ions. 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. Furthermore, the reconnection activity can drive oscillations which may be observable as oscillations in the microwave emission. Then, coronal loops 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 charged particles throughout the structure. Many other stars exhibit flares, and I will briefly describe 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..
Solar Energetic Particles: origins and propagation
University of Central Lancashire
Date: 10:00 am (UK time), 12th November 2020
Abstract: Solar Energetic Particles (SEPs) are ions and electrons detected in interplanetary space in association with flare and coronal mass ejection events. By propagating through the solar wind’s magnetic field, these particles may reach near-Earth locations, where they pose a radiation risk to humans in space and satellite hardware. This talk will review our understanding of the origin and transport of SEPs, based on a large body of data gathered by spacecraft detectors and on theoretical models. It will focus on recent results of test particle simulations, which show that accurate modelling of SEP propagation requires a 3D approach, due to guiding centre drifts and magnetic field line meandering.
Waves and oscillations in the solar atmosphere
University of Sheffield
Date: 10:00 am (UK time), 15th October 2020
Abstract: Satellite and ground-based observations from e.g. SOHO, TRACE, STEREO, Hinode, SDO and IRIS to DST/ROSA, IBIS, CoMP, STT/CRISP have all provided a wealth of evidence of waves and oscillations present in a wide range of spatial and temporal scales of the magnetised solar atmosphere. Our understanding about localised solar structures has been considerably changed in light of these superb spatial and temporal resolution observations. However, MHD waves not only enable us to perform sub-resolution solar magneto-seismology (SMS) but are also potential candidates to carry and damp the observed non-thermal energy in localised MHD waveguides. First, we will briefly outline the basic recent developments in MHD wave theory focussing on linear MHD waves both in symmetric and asymmetric waveguides. This latter may be an important aspect for the fantastic kitty: DKIST. Next, we will concentrate on the role of the most frequently studied wave classes, including the mysterious Alfven, and magneto-acoustic sub-classes of kink and sausage waves. Finally, we will address how solar MHD waves, swirls and solar jet formation may be related. We will argue to unite MHD wave and jet theories and make efforts to develop a common modelling platform with solar applications. An example will be shown where prevalent swirls, in the form of Alfven pulses, propagate upwards through the solar atmosphere dragging with them jets and reach the chromospheric layers. We will argue why this maybe seen as an important step towards understanding better the heating problem of the solar atmosphere.
Probing the Solar Cycle with BiSON: The Solar-Stellar Connection
University of Birmingham
Date: 10:00 am (UK time), 17th September 2020
Abstract: In this talk I will review how we are utilizing helioseismic observations made by the Birmingham Solar-Oscillations Network (BiSON) to provide unique inferences on the solar activity cycle, using its unprecedented long timebase dataset that is now extending into a fifth cycle. I will also discuss how we are using these data together with asteroseismic data, collected on other Sun-like stars by the NASA Kepler Mission, to provide a broader perspective on stellar activity and dynamo action in main-sequence stars.
Magnetic helicity: decompositions and methods of localization
Date: 11:00 am, 16th of June, 2020
Abstract: Magnetic helicity is an ideal MHD invariant; it measures geometric and topological properties of a magnetic field. The talk will begin by reviewing helicity and its mathematical properties. It can be decomposed in several ways (for example, self and mutual helicity, Fourier spectra, field line helicity, linking, twist, and writhe). The talk will also review methods of measuring the helicity flux, as well as applications in solar and stellar astrophysics. I will then discuss some new developments in measuring localized concentrations of helicity in a well-defined, gauge invariant manner, using wavelets.
Where do solar eruptions come from?
Date: 11:00 am, 19th May 2020
Abstract: An oft-quoted idea in solar physics is that coronal mass ejections are, fundamentally, the Sun's way of shedding the magnetic helicity that is continually generated by its interior flows. In this talk, I will show how models are helping to give us a handle on the build up of magnetic helicity in the corona (the Sun's lower atmosphere): how much is injected, where it collects, and how it is ultimately ejected. This requires time-evolving coronal magnetic field models as well as new tools for analysing the distribution of magnetic helicity.
Probing energy release and transport in explosive events
Date: 11:00 am, 21st April 2020
Abstract: The magnetic field of the corona stores the energy that is released via magnetic reconnection during solar flares and coronal mass ejections (CMEs). Flares with CMEs are often described by the ‘standard’ eruptive flare (CSHKP) model and this offers a conceptual framework in which to investigate the global characteristics of the energy release and transport in the context of the magnetic field configuration. The low plasma beta environment of the corona means the magnetic field plays a central role in the energy transport, and different magnetic field configurations can lead to a variety of outcomes in terms of the evolution of the energy release, the efficiency of the energy transport mechanisms and the locations where the energy is deposited. Despite the often rather good agreement between observations and the ‘standard’ model, many open questions remain particularly in respect to the triggering of the energy release. In this talk I will discuss how multi-wavelength spectroscopy used in tandem with magnetic field information can help shed light on some of these open questions.
Aspects of MHD Wave Heating in the Complex Solar Atmosphere
Date: 11:00 am, 24th of March 2020
Abstract: In a series of numerical experiments, we investigate the possible role of MHD waves in the energy and mass cycle in the complex solar corona. Using 3D MHD simulations of transverse, Alfvenic waves, we look at the role of chromospheric evaporation, the complexity of the magnetic field and the power spectrum of the wave driver. We focus on the efficiency of the wave-based heating in our models, in particular whether heating provided by the waves can balance coronal losses and whether proposed wave heating mechanisms are in fact self-consistent.