TY - JOUR
AU - Kolotkov, Dmitrii
AB - Abstract Timothy Duckenfield and Dmitrii Kolotkov reflect on a meeting that looked back at 20 years of probing the Sun's corona with magneto–hydro–dynamic waves A recent online conference celebrated the field of coronal seismology, in which observations and modelling of waves in the solar atmosphere are used to infer its local properties which would otherwise be difficult or even impossible to measure. Founded about 20 years ago, this emergent field was discussed in an RAS meeting and summarized in the review articles here in A&G (Nakariakov et al. 2004, Nakariakov & Verwichte 2004). Since that time, the field has matured and expanded greatly to form a productive and sociable community that has produced many hundreds of papers, with the scope for new research only expanding. This conference, hosted remotely on 8–11 December 2020, could only offer a brief snapshot of this exciting research area, while further celebrating the career (and birthday!) of eminent scientist Valery Nakariakov (University of Warwick, UK) who has contributed much to the field. This updated review will offer a still more limited look at coronal seismology through the lens of its authors. Nonetheless it is hoped the reader will agree that even considering the myriad results already accomplished, the future for coronal seismology remains bright. The conference began with an opening talk by Leon Ofman (CUA and NASA Goddard Space Flight Center, USA), who deftly described the evolution of coronal seismology from its origins with Nobel Prize winner Hannes Alfvén. Alfvén's deduction – that the magnetic field in an ionized gas (plasma) supports further dynamics than in a neutral gas, i.e. magnetohydrodynamics (MHD; Alfvén 1942) – is essential for understanding the many oscillatory motions in the Sun and its outer layers. MHD waves may be partitioned into purely magnetic modes called Alfvén waves, and two bands of magnetoacoustic modes called the fast and slow waves. These modes are further modified in non-uniform plasma such as the solar atmosphere, which ranges from the dense bubbling photosphere at the bottom, through the partially ionized and relatively cool chromosphere, and out into its tenuous but extremely hot (>106 K) fully ionized corona. Great leaps of understanding the waves in the solar atmosphere arose from ground-based observations in the optical and radio bands, and particularly space missions, owing to the ability to observe the corona in extreme ultraviolet (EUV), which is blocked by Earth's atmosphere. Multiband observations show us that oscillatory motions are omnipresent, cover a wide range of periods from subseconds to hours, and are often trapped in the atmosphere's many non-uniformities such as plasma loops (figure 1). Models to describe these waves, such as the MHD modes of a magnetic cylinder, have proven successful in explaining many phenomena such as Doppler shift measurements from spectral line profiles, propagating and standing intensity perturbations, oscillating light curves of impulsive energy releases, and the transverse swaying of loops (Nakariakov & Kolotkov 2020). 1 Open in new tabDownload slide An EUV image of the solar corona and subsequent seismological analysis. Left: The full disk image of SDO/AIA channel 171 Å, on 26th May 2012. Top right: a coronal loop, marked by a dashed line, undergoing a decaying fast-mode kink standing oscillation. A slit of pixels is denoted by the solid line. Bottom right: the slit pixels plotted with time, clearly showing the oscillation profile of the kink wave. This observation was studied in Duckenfield et al. (2019). (SDO/AIA) 1 Open in new tabDownload slide An EUV image of the solar corona and subsequent seismological analysis. Left: The full disk image of SDO/AIA channel 171 Å, on 26th May 2012. Top right: a coronal loop, marked by a dashed line, undergoing a decaying fast-mode kink standing oscillation. A slit of pixels is denoted by the solid line. Bottom right: the slit pixels plotted with time, clearly showing the oscillation profile of the kink wave. This observation was studied in Duckenfield et al. (2019). (SDO/AIA) From decaying to decayless fast waves The successful application of coronal seismology to the transverse motions of plasma inhomogeneities, which have been shown to correspond to the propagating and decaying, fast kink mode (Pascoe et al. 2020), was demonstrated by Hui Tian (Peking University, China) using observations from the Coronal Multi-channel Polarimeter (CoMP) instrument at Mauna Loa Solar Observatory, Hawaii. These pervasive kink waves may be readily used to infer the local magnetic field strength, which is otherwise difficult to do because the coronal magnetic field is not strong enough to split spectral lines (the Zeeman effect) in comparison with their thermal broadening. The ability of continuous observations to map the global magnetic field and density in the plane of sky of the corona is promising for validating atmospheric models and magnetic field extrapolations. The success of CoMP, despite its poor sampling of the line profile, demonstrates the need for a future spectrometer with better spectral resolution, confirmed recently by approval of the JAXA/NASA EUVST mission. Its usefulness was strengthened by Richard Morton (Northumbria University, UK), who also discussed the use of CoMP as well as the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory (SDO/AIA), which is an EUV imager. The statistics of these propagating kink waves were compiled, and a comparison of the power spectra for different regions of the Sun shows that the transition region (TR) between the chromosphere and corona could be often underestimated in width. The consequent lower Alfvén speed gradient has important consequences for the reflection of upwardly propagating waves, and hence energy, at the TR. The traditional explanation of the observed damping of kink waves (even in the linear regime) is through the process of resonant absorption, explained eloquently by Marcel Goossens (KU Leuven, Belgium) as the coupling of the body kink wave to an Alfvén wave, in a resonant layer at the boundary of the non-uniformity. Parallel vorticity is a marker for Alfvén waves, and while compression is considered a marker for magnetoacoustic waves, it was shown that the MHD wave induced by the resonant absorption in fact accompanies both parallel vorticity and pressure variations. There was consequently a robust debate on the distinction (if any) between Alfvén waves and Alfvénic waves, the latter of which are argued to result from the coupling between fast magnetoacoustic branch and pure Alfvén modes. Remarkably, in the past decade a decayless regime of standing kink oscillations in coronal loops has been discovered and studied. The omnipresence of these decayless oscillations opens the prospect of routine seismology of the corona, yet they are small, with displacement amplitudes less than a typical pixel size (some 200 km). Also, the reason why they appear not to be subject to the same damping mechanisms as their rapidly decaying comrades is unresolved. Qingmin Zhang (Purple Mountain Observatory CAS, China) presented observations of two circular ribbon flares, which were clearly seen to initially induce these mysterious decayless oscillations, and subsequently develop a kink mode of the decaying regime. This observation prompted a great deal of discussion, including whether the flare could impulsively drive persistent decayless oscillations, and why different oscillation quality factors were seen in different bandpasses of SDO/AIA (171 Å, 193 Å, 211 Å) while the decayless oscillation had the same period in all. Konstantinos Karampelas (Northumbria University) presented two preliminary models for generating decayless oscillations through self-oscillations, in which the system itself sets the oscillation frequency while being supplied by a (near) constant driver (Nakariakov et al. 2016). The first of these models was a constant flow across the loop footpoints, while the second used vortex shedding from a flow across the entire loop (see figure 3). In both cases, numerical simulations proved both as proof of concepts, generating the requisite periodic signal (loop eigenfrequencies) from a non-periodic, continuous flow in 3D. 2 Open in new tabDownload slide The Siberian Radioheliograph (SRH), sited 220 km from Irkutsk, Russia, which consists of three T-shaped solar-dedicated antenna arrays. In terms of coronal seismology, radio/microwave solar observations such as those taken by SRH are invaluable because they provide an independent measure of the magnetic field and plasma density, complementing EUV observations. (Courtesy of Dr S Lesovoi) 2 Open in new tabDownload slide The Siberian Radioheliograph (SRH), sited 220 km from Irkutsk, Russia, which consists of three T-shaped solar-dedicated antenna arrays. In terms of coronal seismology, radio/microwave solar observations such as those taken by SRH are invaluable because they provide an independent measure of the magnetic field and plasma density, complementing EUV observations. (Courtesy of Dr S Lesovoi) 3 Open in new tabDownload slide A schematic representation of a 3D MHD simulation driving decayless kink oscillations of a magnetic cylinder with flows, described in Karampelas & Van Doorsselaere (2021), from which this figure was taken (with permission). 3 Open in new tabDownload slide A schematic representation of a 3D MHD simulation driving decayless kink oscillations of a magnetic cylinder with flows, described in Karampelas & Van Doorsselaere (2021), from which this figure was taken (with permission). Taking a different approach, Michael Ruderman (University of Sheffield, UK; IKI, Russia) studied analytically the driving of decayless kink oscillations in a magnetic cylinder by a stationary random function. A formula relating the power spectrum of the driver to the power spectrum of the eigenmodes of the cylinder was derived. Fascinatingly, it was found that the width of the resulting power spectrum may be related to the width of the loop boundary's transitional layer, potentially opening a new seismological method for probing the transverse loop profiles. Andrey Afanasyev (LASP, University of Colorado Boulder, USA) woke up exceptionally early to present his 3D simulations of the excitation of decayless oscillations through randomly driven footpoints. The model was shown to reproduce successfully many properties of observed decayless oscillations and, curiously, it was found that the Kelvin–Helmholtz instability (KHi) did not develop over the simulation run time. It was suggested that the instability development time may be too long because of the low amplitudes; regardless, the role of KHi in the solar atmosphere remains an open question (Pascoe et al. 2020). Omnipresent slow waves Effective thermal conduction, the misbalance of heating and cooling processes, and compressive viscosity may strongly affect another mode of magnetoacoustic waves: slow waves (Wang et al. 2021). Tongjiang Wang (CUA and NASA Goddard Space Flight Center) gave a talk on 1D MHD simulations of the standing slow mode in a flaring loop, combined with linear theory, in an effort to determine from observations (such as damping time) the transport coefficients governing efficacy of these effects. Indeed, the understanding of these transport coefficients has not seen much development since the Braginskii equations, which rely on strict assumptions such as Maxwellian distributions, laminar flows and limited mean-free paths of electrons. As an aside, Tongjiang was undoubtedly a hero of this conference, staying up the entire night beyond his own talk to lend his expertise and insight, and answer questions from junior scientists. The excellent discussion was the richer for it, and the community thanks him. Krishna Prasad Sayamanthula (KU Leuven) presented some of the most hotly discussed observations in the meeting, and thankfully was not hampered by the dramatic interruption of several earthquakes in the home region of the session chair, Sergey Anfinogentov; we were assured that earthquakes are common in Irkutsk. The observations showed a (previously studied) flaring loop hosting an oscillation in which a large bright blob of plasma bounced back and forth between the loop footpoints. A distinction was made between a standing slow wave (fundamental mode only) and a “sloshing” oscillation, in which many harmonics are present (i.e. there exists a localized blob). In this preliminary analysis, it appeared the transition from a sloshing to a standing slow wave is seen for the first time, and this transition was linked to cooling of the loop. Andrea Costa (IATE-CONICET, Argentina) analysed numerically the capability of different perturbations to excite slow- and fast-mode sausage waves within solar flaring loops. It was supposed that the fast sausage modes are far rarer (in observations) because they require thermal conduction to be dominant over radiative cooling, which happens rarely in coronal conditions. María Valeria Sieyra (KU Leuven) presented observations of the EUV corona above a sunspot, spotting the well-known upwardly propagating intensity disturbances that are referred to as three-minute oscillations. These perturbations are the propagating form of slow magnetoacoustic waves, whose phase speed depends approximately upon the square root of temperature (as with a sound wave) and damps extremely quickly. In this talk, 2D numerical simulations based on a potential magnetic field configuration in a simplified atmosphere showed that line-of-sight effects may explain apparent acceleration of these disturbances. Timothy Duckenfield (KU Leuven) presented similar observations of slow magnetoacoustic waves above a sunspot, although in this case two different temperature loops coincided along the line of sight. The fact that different loops within the fan emanating from a sunspot have distinct temperatures provides an ideal opportunity to probe the thermal equilibrium with these slow waves. Kyungsuk Cho (Korea Astronomy and Space Science Institute, South Korea) used Big Bear Solar Observatory (BBSO; California, USA) and EUV data to look at propagating slow waves, this time away from active regions in a polar coronal hole (a dark, lower-density region of open magnetic field, expelling a fast solar wind). In these regions the distinction between slow wave and background flow is not so clear, because of line-of-sight effects and dispersion from various sources changing the phase speed of slow waves, such that they disobey the dependence on temperature. Dipankar Banerjee (Aryabhatta Research Institute of Observational Sciences, India) provided further clarity on waves in the plume regions and other open coronal structures. Also included was an advertisement for the series of review papers on coronal seismology being published in volume 217 of Space Science Reviews at the start of 2021. Beyond linear theory The linear theory of MHD modes in a magnetic cylinder has been remarkably successful in describing phenomena in the solar atmosphere, but including nonlinear effects is also vital. Norbert Magyar (University of Warwick) reminded us that for the kink wave in thin flux tubes, linear theory is not well justified for even the displacements on the order of a loop's minor radius. Thus, even decayless oscillations could be affected by nonlinear effects, such as KHi; the coupling of the kink mode to higher azimuthal (fluting) modes, and the accumulation of mass at a loop apex through the ponderomotive force. The observations, analytics and simulations were then combined to find an elegant expression for the theoretical amplitude-dependent damping time of propagating kink waves from nonlinearity, the ramifications of which are still being explored. Patrick Antolin (Northumbria University) also discussed the effect of nonlinearity on kink waves, explaining his thoughts on why the observational signatures of KHi seem absent despite simulations and theory predicting their abundance. While magnetic twist may suppress KHi somewhat, simulations assure us that the development of KHi must be present, and may be manifested observationally through herringbone Doppler shift patterns and strand-like structures in low-lying jets and loops. Mingzhe Guo (Shandong University at Weihai, China) reminded us that current models may not only be limited by the assumption of linearity, but also by their geometry. It was shown with simulations that if the waveguide for a kink mode is elliptical, rather than circular, transverse spatial scales may form quickly and the asymmetry permits both a major and minor mode to exist, where only one mode existed before. This may result in multiperiodic signals and affects the damping of these waves. Similarly, Rajab Ismayilli (KU Leuven) described the role of uniturbulence, whereby unidirectional waves passing through inhomogeneities form turbulence and deform nonlinearly by forming small transverse spatial scales. The role of Alfvén waves in the solar atmosphere was also discussed by Soheil Vasheghani Farahani (Tafresh University, Iran). In this talk it was argued that the torsional Alfvén wave, not the plane wave (as is often used) should be considered in the solar atmosphere. Additionally, magnetic twist, rotation or shear boundary flow can affect the Alfvén wave such that it is quite different to its “standard” form, being accompanied by density perturbations as with the nonlinear regime. These differences have consequences for the collimation of jets, the formation of shocks and, crucially, imply that dissipation rates for Alfvén waves are overestimated. Igor Lopin (Institute of Applied Astronomy of RAS, Russia) also considered the effect of twist on waves analytically, focusing on the fast axisymmetric, or sausage MHD wave (Li et al. 2020). It was found that a twisted magnetic tube can trap the fundamental sausage mode with a phase speed near the external Alfvén speed, eliminating the mode leakage – a promising result for seismology. One of the research topics addressed least in this meeting was the relationship between coronal waves and the intrinsically nonlinear process of magnetic reconnection. James McLaughlin (Northumbria University) presented a talk on 2D and 3D simulations of fast MHD waves launched into a magnetic null point, finding that an aperiodic driver can still lead to periodic behaviour. Somaye Sabri (University of Tabriz, Iran) also used simulations launching waves towards the coronal magnetic null point, finding variously: plasmoid ejections, the collapse of the null point and mode conversion. Despite these advances, the interplay between reconnection and waves could and should be pursued in future research. Which came first: heated wave or wave heating? One of the most important, and persistent, questions in solar physics is the possible role of MHD waves in heating the solar (and stellar) atmosphere (e.g. Van Doorsselaere et al. 2020). Ineke De Moortel (University of St Andrews, UK) addressed this question in her talk, comprehensively covering a range of simulations and observational studies. Simulations of upwardly propagating kink waves were driven from below with the same power spectrum as observed, for example, by CoMP, and found to deposit into the plasma only a tiny fraction of the energy required to balance the atmosphere's colossal radiative losses. Further simulations which included complex, braided magnetic field structures found that, while these structures do enhance the plasma heating by waves, the effect is modest and still inconsequential compared to the heating we know must occur. Although the dissipation of waves may not be important for the coronal heating, the wave is sensitive to the effects of heating and cooling processes and so it is imperative that the community continues to study the interaction between waves and thermodynamics in the solar atmosphere. Gabriel Pelouze (KU Leuven) presented the results of some 9000 simulations, exploring the interaction between an idealized 1D coronal loop and different heating parameters. It was found that asymmetry in the heating can lead to drastically different dynamical behaviour of the loops, such as catastrophic condensation or the generation of standing modes, leading to the phenomenon of coronal rain. The loop rarely exists in thermal equilibrium and these simulations add to the research topic referred to as “thermal non-equilibrium cycles”. In a similar but subtly different vein, Dmitrii Zavershinskii (Samara National Research University, Russia) discussed how the imbalance between coronal heating and cooling processes affects the properties of slow magnetoacoustic waves. This modified acoustic wave was shown to undergo dispersion and damping or amplification from this effect, even when propagating through a uniform plasma. The subsequent observable effects such as damping rate or change in the wave phase speed may allow scientists to use the slow waves in the corona as probes of coronal heating, which is undoubtedly an exciting prospect. Waves in non-coronal plasmas The solar atmosphere is home to many oscillations from which information about the local plasma may be gained, and not all these waves are confined to the coronal plasma. The application of seismology to waves in the photosphere and chromosphere presents its own challenges, often related to the different plasma composition, optical thickness, and (partial) decoupling of ions and electrons. For example, Manuel Luna (Universitat de les Illes Balears, Spain) considered how large-amplitude waves are excited in solar prominences, which are cold, dense and heavy filaments of chromospheric plasma suspended in the corona by the magnetic field. By simulating how a jet can trigger transverse oscillations of prominences, it was found that a mixture of wave modes (Alfvénic and sonic) is inevitable. Rakesh Mazumder (Indian Institute of Astrophysics, Bangalore, India) considered a similar prominence oscillation through observations, showing a novel method using seismological inferences of both the longitudinal and (simultaneous) transverse oscillation. The importance of energy and mass transport through the atmosphere means it is imperative that MHD waves are well understood below the corona too. For example, Pradeep Kumar Kayshap (VIT, Bhopal University, India) examined how waves in the photosphere relate to those in the transition region through wavelet analysis of spectral data, searching for correlations. Similarly, Andrei Chelpanov (Institute of Solar–Terrestrial Physics, Russia) presented work using the same idea but for waves which had been amplitude-modulated by a minuscule flare. Il-Hyun Cho (Kyung-Hee University, South Korea) considered the flux tubes associated with the umbrae (core) of sunspots. By considering the dispersion relation of slow magnetoacoustic waves in a stratified photosphere, combined with power spectra obtained from the light curves of umbrae, beautiful seismological inversions were able to be performed to estimate Alfvén speed, mass density and the ratio of magnetic to gas pressure in the umbrae. A more general case was illustrated by the presentation of Michaël Geeraerts (KU Leuven), who considered analytically the dispersion relation for sausage modes in photospheric magnetic pores, accounting for electrical resistivity. It was found that, unlike in the corona, resistivity plays a key role in the damping of these waves. The mathematics also revealed that the distinction between surface and body modes was far more muddled in this plasma than in the idealized case. Yuhang Gao (Peking University, China) also considered sausage modes in the photosphere, derived from their observations of photospheric “bright points”. Taking advantage of the high-resolution BBSO telescope, an in-phase relationship between area and intensity confirmed the presence of sausage mode waves. Tanmoy Samanta (NASA Marshall Space Flight Center, USA) described what they hope is a link between the lower atmosphere and the hotter corona, the transient chromospheric fine-scale jets known as spicules. The magnetic field is key in their formation and they provide some role in mass and energy transport in the lower atmosphere. However, these complex structures are difficult to observe, a difficulty compounded by their short lifetime of some hundreds of seconds, so they are poorly understood. Teimuraz Zaqarashvili (University of Graz, Austria) provided a potential explanation of spicules' brief existence, through his talk about the instability of jets. Despite the stabilizing effect of a magnetic field, it was shown analytically that inclined jets in a solar-like plasma will usually be kink-unstable. Yet, as Rony Keppens (KU Leuven) highlighted, we shouldn't be afraid to look beyond MHD in a general plasma. Magnetohydrodynamics applies within a cold, single fluid (i.e. fully ionized) plasma for long-wavelength and low-frequency waves, and often focuses on the “exotic” cases of perfectly parallel or perpendicular propagation direction in which the MHD solutions are degenerate. Thus, relating waves in different plasma regimes requires caution. Waves used for coronal seismology need not be confined to a thin waveguide such as a coronal loop. For example, Yuandeng Shen (Yunnan Observatories, China) spoke about low-frequency, large-scale (unconfined) coronal waves visible in EUV as globally arc-shaped fronts, which are unimaginatively named EUV waves. These fast-mode magnetoacoustic waves often refract and interact with active regions, and are excited by solar eruptions and unwinding filaments, loop expansions and flare pressure pulses. Mariana Cécere (CONICET/UNC, Argentina) used simulations to consider how solar eruptions can trigger a compressive wave below in the chromosphere (a Moreton wave), finding specific conditions are required to generate the requisite interim shock wave; a further study including the effects of reconnection is needed to capture all the system dynamics. Pulsating flares Seen in solar and stellar flaring light curves are oscillatory modulations known as quasi-periodic pulsations (QPP). These are rarely stationary or harmonic, with pronounced amplitude modulation and period drifts common, and are often associated with the dynamics of MHD waves in the solar and stellar atmospheric plasmas (Kupriyanova et al. 2020). There is clearly an immense potential for seismology with QPPs, and indeed much research in this direction. Not only might their periodicities reveal information about the flaring processes occurring, but since solar and stellar QPPs behave in similar ways, there is the potential that whatever scientists can learn from the Sun may be applied to faraway stars as explained by Abhishek Kumar Srivastava (Indian Institute of Technology, India). Unfortunately, while there are many theoretical mechanisms that may generate QPPs, the direct association of them to real flares is still unknown. One suggested mechanism for QPPs is the modulation of the flaring plasma by a fast wave train, perhaps the result of dispersive evolution in a plasma waveguide. Such propagating fast-mode waves and their associated periodicity was the topic of the talk by Ding Yuan (Harbin Institute of Technology, Shenzhen, China), speaking from his relatively new institute in southern China. Yuhu Miao (Harbin Institute of Technology) also presented observations showing bidirectional fast wave train event in a funnel of coronal loops, to narrow down their excitation mechanism. In fact, the question of what the selection criterion is for such wave trains to appear in one funnel of loops, and not its neighbouring waveguide, is still open. Elena Kupriyanova (Pulkovo Observatory of RAS, Russia) considered a rich solar event comprising a circular ribbon flare, with multiple QPPs observed in different ribbons and in different spectral ranges. It was interpreted that the main QPP culprits were three-minute sunspot oscillations, an electron acceleration process in the primary flare site, and a kink oscillation of the outer spine of the magnetic system. On the other hand, Ivan Zimovets (Space Research Institute IKI of RAS, Russia) studied a different triple-ribbon solar flare event and analysed the flare's X-ray spectrum to infer that the super-hot plasma had a double temperature. In this study it was concluded the pulsations were emitted by a “single” X-ray source which was itself shifting in time, interpreted as successive energy releases from a propagating slow wave. On yet another hand, the preliminary results of Maria Toropova (Institute of Solar–Terrestrial Physics), presented by coauthor Larisa Kashapova from the same institute, found at least three distinct types of QPPs in 7 out of all 20 considered flares originating from two solar active regions, each detected by the recently commissioned Siberian Radioheliograph in microwaves (figure 2). Notably, their results were interpreted as not being three-minute sunspot oscillations. It must be said that every author was clear to say other interpretations are not excluded – QPPs are a tricky business. It is hoped that the sheer statistics of the plentiful solar and stellar QPPs may come to the rescue. To aid in this, Anne-Marie Broomhall (University of Warwick) described a “hare and hound” exercise conducted recently in which a multitude of QPPs were simulated, some real and some fake, in a challenge for different detection mechanisms from many participants to show their worth. The results were enlightening in multifaceted ways, proving that more sophisticated techniques than a straight Fourier transform work better for detecting the non-stationary, complex QPP signals. Armed with these sophisticated techniques, Andrew Inglis (NASA Goddard Space Flight Center) discussed the statistics compiled so far on significant QPPs seen in solar and stellar light curves. While detection rates in stellar curves are less than in the solar case, it appears that the periods of QPPs are independent of the flare magnitude for both cases, and longer periods are correlated with longer flares. Novel approaches Novel processing techniques applied to observational data are also impactful for the purposes of seismology. Taking some examples from this meeting, Andrei Chelpanov (Institute of Solar–Terrestrial Physics) used phase-spectrum analysis on many different observed passbands to study how a miniscule power flare can modulate chromospheric oscillations, while David Pascoe (KU Leuven) used the robust and comprehensive Bayesian statistical framework (with MCMC sampling instead of direct integration) to great effect in probing many seismological questions. These included tracking the evolution of multiple coronal loops simultaneously to search for KHi, forward modelling EUV emission, and categorizing the signals in QPPs in terms of non-stationarity etc. Gary Verth (University of Sheffield) presented some fascinating work calculating the eigenmodes of sunspots which have a non-uniform cross-sectional shape. Attempting this devilishly tricky problem was compared to the madness of French mathematician Émile Léonard Mathieu, who once calculated the eigenmodes of a bell with just pen and paper. Thankfully, the modern civil engineering techniques of proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) can now help to show how these eigenmodes are affected by irregular boundaries, particularly at higher orders. Observations are not the only area of coronal seismology to benefit from the development of novel techniques. Niels Claes (KU Leuven) advertised new open-source code LEGOLAS (Large Eigensystem Generator for One-dimensional pLASmas; github.com/n-claes/legolas), which can calculate the full MHD spectrum of 1D equilibria with realistic set-ups and dissipative effects such as flows, resistivity (with values far exceeding general nonlinear MHD codes), radiative losses and thermal conduction. In this way new unexplored MHD modes may be revealed, or previously studied modes better understood in more general situations such as oblique angles. In another advance, Vladimir V Annenkov (Budker Institute of Nuclear Physics, Russia) demonstrated the benefits of a multidisciplinary approach in his talk on calculating plasma dispersion relations. Drawing on expertise in high-energy physics, he showed a new way to calculate the dielectric tensor (and hence the dispersion relation) accounting for a relativistic electron beam. This has applications for studying plasma-beam instabilities and radio bursts, which is especially timely for solar physics with the influx of a new generation of radio telescopes, such as the Low Frequency Array (LOFAR), the Square Kilometre Array (SKA) and Siberian Radioheliograph (SRH). For the field to continue evolving, novel ways of approaching the coronal seismology problem are key. An excellent example was given by Rekha Jain (University of Sheffield), in which the fast-mode oscillations observed in the corona are not waveguided by a 1D cylinder, but rather a 2D arcade. Then the oscillation from a quick, localized source may be considered as a pattern of interference fringes arising from different length paths in the arcade. While such an interpretation does explain several observed properties, how it may be tested with statistical observables still needs to be fleshed out – further work in this direction would undoubtedly be fruitful, as she pointed out. Another novel paradigm was developed by Martin Laming (Naval Research Laboratory, USA) who described the first ionization potential (FIP) effect and its inverse at the interface between the chromosphere and the corona, where nonlinear MHD waves can change the ions present in the plasma via the ponderomotive force. This means radiating ions, such as Fe and Si, can be depleted or increased (fractionation) wherever waves are refracted, with a variety of effects. The focus of this talk – other than promoting excellent discussion in the chat box – was the elemental fractionation induced by flares, which is analogous to laser beam particle-trapping (the subject of two Nobel Prizes) and the consequences of which also apply to stellar observations. 4 Open in new tabDownload slide Devasthal Campus of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. This mountain peak (“Abode of God”) hosts three – soon to be four – telescopes, and is an inspiring base for the in-person conferences that are invaluable for scientific communities. (Courtesy of Prof. D Banerjee) 4 Open in new tabDownload slide Devasthal Campus of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. This mountain peak (“Abode of God”) hosts three – soon to be four – telescopes, and is an inspiring base for the in-person conferences that are invaluable for scientific communities. (Courtesy of Prof. D Banerjee) Concluding remarks The meeting was brought to a close with a summary talk by Valery Nakariakov, who offered the following future perspectives for the field of coronal seismology: Seismology of the quiet Sun for prolonged periods of time may soon become routine, with decayless kink oscillations. The properties of the heating of the corona are intrinsically in the properties of the waves within it (particularly the slow magnetoacoustic wave). Thus, we may use observations of waves to probe the thermal equilibrium. The amplitude of the wave in the corona does matter! These nonlinear effects such as KHi, self-oscillation and amplitude-dependant damping take place in the corona. For the e-folding length of kink-mode oscillations longer than the distance between loops in an arcade, there is a need to move beyond the standard magnetic cylinder model. The compilation of event catalogues to establish empirical scalings, and test our beloved theories, is essential. The use of new terminology can be misleading, delivered as the wonderful advice: “We must not invent new names for old processes.” We are only beginning to unlock the seismological potential of QPPs. An urgent step is to classify QPPs, and search for statistical relationships between the different parameters of QPPs within the same class only. Further, there is a need for dedicated data analysis tools that address the intrinsic non-stationarity and low oscillation quality of QPPs in flares. Credit was universally heaped onto the organizing committee for accomplishing such a smooth and productive event. This was in the face of several challenges, such as ∼100 participants from across 17 time zones, and the meeting was held entirely remotely because of the coronavirus pandemic. Nonetheless, this event also delivers a warning. Despite the use of virtual breakout rooms for informal chats, in science there can be no substitute for the organic interactions that occur on the periphery of a conference timetable. While virtual events have a clear benefit in accessibility by allowing many participants from distant places to participate without prohibitive time/expense, there is an untold cost to the unexpected scientific collaborations that only arise from meeting face to face. One sees peers exchanging ideas and perspectives on the back of an envelope while in the coffee queue, or young researchers learning from and challenging the experienced academics. It is often these uniquely human and unpredictable conversations that can lead to the most provocative of ideas, the diffusion of expertise or the changes in perspective that help drive the entire community forward. Furthermore, the informal networks forged over lunches or excursions play an important role in promoting collaborations and aiding career development in the breathtaking landscapes of the Siberian Radioheliograph (figure 2), and Devasthal Campus of the Aryabhatta Research Institute of Observational Sciences, India, figure 3). It is imperative that in-person conferences and bilateral real-life visits resume as soon as it is safe to do so. AUTHORS Open in new tabDownload slide Open in new tabDownload slide Dr Timothy Duckenfield is a postdoc at the Centre for Mathematics and Plasma Astrophysics, KU Leuven, Belgium Open in new tabDownload slide Open in new tabDownload slide Dr Dmitrii Kolotkov is a postdoctoral research fellow, Centre for Fusion, Space and Astrophysics, University of Warwick, UK ORGANIZERS The meeting was organized by: Dmitrii Kolotkov, Bo Li (Univ. of Weihai, China), Sergey Anfinogentov (ISTP, Russia), Kris Murawski (UMCS, Poland), Guiseppe Nisticò (Calabria, Italy), David Tsiklauri (QMUL, UK) and Tom van Doorsselaere (KU Leuven, Belgium) REFERENCES Alfvén H 1942 Nature 150 405 Kupriyanova E G et al. 2020 Solar-Terrestrial Phys. 6 3 Li B et al. 2020 Space Sci. Rev. 216 136 Nakariakov V M et al. 2016 Astron. Astrophys. 591 1 Nakariakov V M & Kolotkov D Y 2020 Ann. Rev. Astron. Astrophys. 58 441 Nakariakov V M et al. 2004 Astron. & Geophys. 45 5.32 Nakariakov V M & Verwichte E 2004 Astron. & Geophys. 45 4.26 Pascoe D J et al. 2020 Frontiers in Astronomy and Space Sciences 7 61 Van Doorsselaere T et al. 2020 Space Sci. Rev. 216 140 Wang T et al. 2021 Space Sci. Rev. 217 34 © 2021 Royal Astronomical Society This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - Coronal seismology at 20
JF - Astronomy & Geophysics
DO - 10.1093/astrogeo/atab068
DA - 2021-06-01
UR - https://www.deepdyve.com/lp/oxford-university-press/coronal-seismology-at-20-QskmkrzD9a
SP - 3.28
EP - 3.33
VL - 62
IS - 3
DP - DeepDyve
ER -