Previous UK-SOSS Seminars
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.