High resolution wave dynamics in the lower solar atmosphere

09:20-09:40 Local helioseismology of solar surface activity

Dr Hannah Schunker, Max Planck Institute for Solar System Research, Germany

Abstract

Active regions consist of strong magnetic fields on the surface of the Sun, and are the principal signature of the solar dynamo. Knowing the subsurface structure of active regions will help us understand where in the solar interior the solar dynamo is located. Dr Schunker's earlier research demonstrated that interpreting the results from helioseismology, the only tool that can probe the subsurface structure of the Sun, is difficult because the waves have been strongly perturbed by the presence of strong surface magnetic fields. In sunspots, the group believes the largest perturbations are caused by the structure of the sunspot itself and by the waves undergoing mode conversion. Different wave perturbations are observed in the surrounding plage regions, and the mechanism causing these wave perturbations is more poorly constrained. To make progress we need higher spatial resolution observations to resolve the vector magnetic field structure, and numerical simulations to understand the physical cause of the wave perturbations. Understanding the physics of wave propagation in the different regions of strong surface magnetic field will help us to constrain the transfer of energy via waves from the interior to the corona, interpret the helioseismic signature to correctly infer the subsurface structure, and could potentially be used to infer surface activity on distant Sun-like stars.

09:40-10:00 Discussion

10:00-10:20 The generation and evolution of swirls in the solar atmosphere

Oskar Steiner, Leibniz Institute for Solar Physics (KIS) and Istituto Ricerche Solari Locarno (IRSOL), Germany and Switzerland

Abstract

Vortical motions of plasma are of ubiquitous presence in photospheric and chromospheric layers of the solar atmosphere. Swirls of small scale in the chromosphere have been observed to occur co-spatially with magnetic flux concentrations of the deep photosphere, suggesting the magnetic origin of these swirls. One scenario suggests magnetic flux concentrations that root in swirling downdrafts of intergranular lanes to rotate, thereby dragging chromospheric material with them, causing the chromospheric swirls. Another scenario sees the chromospheric swirls to be a manifestation of torsional Alfvén waves, either locally generated in the chromosphere or propagating from the photosphere into the chromoshere. Looking at numerical simulations and a quantity called swirling strength, the group finds events of unidirectional torsional motion propagating with Alfvén speed from the photospheric footpoints of magnetic flux concentrations in the upward direction. Oskar Steiner concludes that such propagating, unidirectional swirls can be best interpreted in terms of a propagating torsional Alfvén-wave package that seems to arise from the deformation and strengthening of a preexisting flux concentration rather than a proper rotation of it.

10:20-10:40 Discussion

10:40-11:10 Coffee break

11:10-11:30 Waves in the Lower Solar Atmosphere: examining small-scale magnetic elements as a source of waves

Dr Peter Keys, Queen’s University Belfast, UK

Abstract

Highly dynamic small-scale features such as magnetic bright points (MBPs) are ubiquitous across the solar surface. They present an excellent opportunity to study waves in the lower solar atmosphere. Due to their ubiquity, it is essential that their contributions to energy transfer between layers is ascertained. Dr Keys will discuss how recent work highlights the complexities involved in establishing the wave properties of these dynamic features during their evolution. Dr Keys will then outline the steps that are currently underway to utilise both high resolution imaging and MHD simulations to address these issues in order to better understand wave propagation within MBPs.

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11:30-11:50 Discussion

11:50-12:10 HMI observations of MHD waves in the lower solar atmosphere

Dr Aimee Norton, Stanford University, USA

Abstract

The conversion of p-modes and other perturbations in the near-surface layers into MHD waves that propagate along and across magnetic field lines is a topic of interest for energy transport. The photospheric signatures of MHD waves are weak due to low amplitudes at the β=1 equipartion level where mode-conversion occurs. Dr Norton will briefly discuss HMI helioseismic studies that shed light on MHD wave generation in the vicinity of strong magnetic fields. She will then discuss observed oscillations and other signatures in time series analysis of HMI data. Lastly, Dr Norton will emphasis how HMI supports instruments with higher spatial and temporal resolution for the study of wave dynamics.

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12:10-12:30 Discussion

13:30-13:50 Multi-fluid effects on chromospheric waves

Dr Elena Khomenko, Instituto de Astrofisica de Canarias, Spain

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13:50-14:10 Discussion

14:10-14:30 Wave coupling and heating of expanding flux tubes in the chromosphere

Professor Tony Arber, University of Warwick, UK

Abstract

Waves travelling though the chromosphere propagate through the beta=1 surface in expanding flux tubes and may also be reflected by the temperature jump in the transition region. Determining which modes are responsible for energy transmission from photosphere to corona is essential as it determines which heating mechanisms are most likely to be effective. This talk briefly summarises recent simulations on the chromospheric resonant cavity, Alfven wave to slow wave coupling and the possibility of upper chromospheric heating from shocks.

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14:30-14:50 Discussion

14:50-15:20 Coffee break

15:20-15:40 Acoustic-gravity wave propagation characteristics in 3D radiation hydrodynamic simulations of the solar atmosphere

Dr Bernhard Fleck, European Space Agency (ESA)

Abstract

There has been tremendous progress in developing 'realistic' 3D radiation hydrodynamic simulations of the solar atmosphere in the past decade. Four of the most frequently used numerical codes are Bifrost (Gudiksen et al 2011, Carlsson et al 2016), CO5BOLD (Freytag et al 2012), MANCHA3D (Khomenko et al 2018), and MURaM (Vögler et al 2005, Rempel 2014). Some of these models were benchmarked for their average properties in the near-surface layers, by comparing their average stratifications as well as their temporal and spatial fluctuations (eg RMS of granular contrast and vertical velocities; Beeck et al 2012). Here Dr Fleck tests and compares the wave propagation characteristics of these four models by measuring the dispersion relation of acoustic-gravity waves at various heights. The group finds considerable differences between the various model runs.

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15:40-16:00 Discussion

16:00-16:20 Oscillations observed with ALMA

Dr Shahin Jafarzadeh, University of Oslo, Norway

Abstract

Dr Jafarzadeh reports characterisations of temperature fluctuations observed with the Atacama Large Millimeter/submillimeter Array (ALMA) and highlights how they can improve our knowledge on, eg, heating dissipation in the solar chromosphere. In particular, he discusses the observed frequencies in comparison with those obtained from oscillations in temperature, line-of-sight velocity, and gas density from the Bifrost numerical simulations. In addition, Dr Jafarzadeh presents direct observations of excess temperature (by larger than 1000 K) in a small quiet-Sun region. Such excess temperature/brightness is not observed in the upper layers of the solar atmosphere (ie, the transition region), suggesting energy release at the chromospheric heights.

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16:20-17:00 Discussion

17:00-18:00 Poster session

09:20-09:40 Properties of local oscillations in lower sunspot atmospheres

Dr Robert Sych, Institute of Solar-Terrestrial Physics, Russia

Abstract

Dr Sych will present a study of wave processes in the sunspot region observed by the Goode Solar Telescope in the TiO 7057 Å and Hα 6563 Å spectral lines.

To study the distribution of power oscillations and their dynamics, the pixelized wavelet filtering technique was applied. For the first time, the group obtained the spatial structure of oscillation sources as the footpoints of fine magnetic tubes, anchored in the sunspot umbra. At the chromosphere level, the variation of emission is a combination of numerous independent oscillations located in the sources with small angular size. Their spatial shape varies from dots and cells in the umbra to filaments in the penumbra. Each narrow spectral harmonic corresponds to its source, without global correlation among themselves. At the photosphere level the group found regions with co-phased broadband oscillations of the whole umbra. Their spectrum includes the ∼3 minutes harmonic, whose maximal value is localized in umbral dots. The oscillation sources are displaced at different heights with increasing angular size. Dr Sych assumes that the observed spatial distribution of wave sources indicates the existence of a slow subphotospheric resonator with a vertical magnetic field in the umbra and a wave cutoff frequency due to inclination of the magnetic field line in the penumbra.

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09:40-10:00 Discussion

10:00-10:20 Comprehensive MHD simulations of the solar atmosphere from quiet to active Sun

Dr Matthias Rempel, High Altitude Observatory, National Center for Atmospheric Research, USA

Abstract

Coupling the photosphere and corona requires bridging a large separation of length- and time-scales. While typical photospheric time-scales of interest range from minutes (granulation) to days (active region flux emergence), numerical time-step constraints in the corona can be very severe due to Alfven velocities exceeding 100,000 km/s and very efficient heat conduction. Dr Rempel presents a recently developed version of the MURaM (Max Planck University of Chicago Radiative MHD) code that includes coronal physics in terms of optically thin radiative losses and field aligned heat conduction. The code employs the "Boris correction" (semi-relativistic MHD with a reduced speed of light) and a hyperbolic treatment of heat conduction, which allow for efficient simulations of the photosphere/corona system by avoiding the severe time-step constraints arising from Alfven wave propagation and heat conduction. He discusses applications of the code that include studies of quiet sun magnetism, flux emergence and active region formation, and a simulation of a solar flare in response to active region scale flux emergence.

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10:20-10:40 Discussion

10:40-11:10 Coffee break

11:10-11:30 The modelling and interpretation of magnetohydrodynamic wave modes in sunspots and pores

Dr Gary Verth, University of Sheffield, UK

Abstract

In this talk Dr Verth will present some interesting case studies where current theoretical understanding explains reasonably well the MHD oscillations observed in eg, the umbral regions of sunspots. In these cases he will explain what the theory predicts in terms of intensity, Doppler velocity and LOS magnetic field oscillations with regards to magnetic waveguides of both circular and elliptical cross-sectional shape. Dr Verth will then go on to describe what MHD models need to be developed to interpret oscillations detected in eg, pores of more irregular cross-sectional shape.

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11:30-11:50 Discussion

11:50-12:10 Can p-modes play a role in powering the corona?

Dr Richard Morton, Northumbria University, UK

Abstract

Alfvénic waves are thought to play a crucial role in the heating of the Sun’s atmosphere and acceleration of the solar wind, and it is now established that they are present throughout the corona. However, there are crucial questions remaining about the journey of the waves through the lower solar atmosphere. Recent results suggest that p-modes may make an unexpected contribution to the coronal energy flux. In this talk Dr Morton will summarise what we already know about Alfvenic waves in the chromosphere and corona. He will also focus on the recent observations of the low corona (<1.3 R_sun), mainly from ground-based observations with the Coronal Multi-channel Polarimeter (CoMP), and what they reveal about the waves journey. Finally he will discuss the implications for wave heating and wind acceleration if p-modes do play an important role and avenues for probing this new pathway for energy transfer.

12:10-12:30 Discussion

13:30-13:50 The polarization profiles of Ca II 854.2 nm in a Quiet Sun simulation

Rebeca Centeno, High Altitude Observatory (NCAR), USA

Abstract

Ca II 854.2 nm has been deemed one of the most promising spectral line diagnostics for the chromospheric magnetic field, due to its accessibility from ground-based observations and the relative ease of its interpretation. The standard tools used for the interpretation of this spectral line do not account for the physics of scattering polarization nor its modification due to the Hanle effect. However, scattering polarization signatures typically dominate the linear polarization profiles of Ca.

II 854.2 nm in weak field areas (Manso Sainz & Trujillo Bueno 2010, 2003), particularly close to the limb. When combined with the enhancing effect of shocks, these linear polarization signals can reach amplitudes of up to 1% of the continuum intensity (Carlin et al 2012), which would drown Zeeman-polarization signatures induced by magnetic fields in the low hecto-gauss range.

In this work Rebecca Centeno evaluates the temporal evolution of the polarization profiles of Ca II 854.2 nm that emerge from a Quiet Sun simulation from MuRAM. The effects of the magnetic field and the line-of-sight velocity are analyzed separately in order to quantify their individual contributions to the linear polarization.

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13:50-12:10 Discussion

14:10-14:30 Waves captured by spectropolarimetric IBIS obversations

Dr Marco Stangalini, ASI, Italian Space Agency, Italy

Abstract

Spectropolarimetry has become a mature tool for the analysis of the dynamics of the lower solar atmosphere. Next generation solar instrumentation, characterized by unmatched polarimetric accuracy and sensitivity, will dramatically improve our observational capabilities, extending the use of spectropolarimetric diagnostics down to very small spatial scales (< 100 km) and higher atmospheric heights. 

In this contribution Dr Stangalini will show how simultaneous spectropolarimetric observations at multiple heights is a powerful tool to reveal the details about the plasma and magnetic field wave dynamics in the solar atmosphere, and for the identification of Alfvénic disturbances.

More in detail, he will show the capabilities of tomographic spectropolarimetric imaging in the investigation of MHD waves, by presenting recent results from high resolution state-of-the-art observations of the solar atmosphere and, in particular, from IBIS. 

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14:30-14:50 Discussion

14:50-15:20 Coffee break

15:20-15:40 Two-fluid shocks in an isothermal stratified atmosphere

Dr Ben Snow, University of Exeter, Uk

Abstract

A compressional wave propagating upwards in the solar atmosphere naturally steepens due to the stratification of the atmosphere and can readily develop nonlinearities and shock. If the magnetic field is inclined, a shock can separate into fast- and slow- mode components as it passes through the point where the sound and Alfven speed are equal. This point can occur in the lower solar atmosphere, where the plasma is partially-ionised and two-fluid effects become important. In this talk, Dr Snow will present results from two-fluid numerical simulations demonstrating the mode conversion and interplay between the ionised and neutral species for a shock wave propagating through an isothermal stratified atmosphere. 

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15:40-16:00 Discussion

16:00-16:20 Waves in the lower solar atmosphere: setting the scene for the next generation of solar telescopes

Dr David Jess, Queen's University Belfast, UK

Abstract

The development of cutting-edge three-dimensional simulations has highlighted a vast assortment of predicted solar phenomena that reside below the spatial, temporal, and spectral resolutions of current telescope facilities. Thankfully, next-generation solar telescopes (including DKIST, SUNRISE, NLST, EST, and Solar-C) will improve the spatial resolutions achievable. However, it is the role of the instruments commissioned on these revolutionary facilities that will pave the way for robust comparisons to be made to the cutting-edge numerical simulations pioneered by the likes of Mancha, Bifrost, MuRAM, and LareXd. Here, Dr Jess will present overarching requirements of such new-age instrumentation, including a description of a hyper-spectropolarimetric imager currently under construction for the Indian National Large Solar Telescope. Lastly, Dr Jess will highlight the scientific challenges discussed during the Royal Society’s High Resolution Wave Dynamics in the Lower Solar Atmosphere Theo Murphy meeting, and suggest ways how combined novel instrumentation, alongside collaborative efforts within the solar physics community, will provide rapid developments in the understanding of wave phenomena in the lower solar atmosphere.

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16:20-17:00 Meeting overview and future directions (discussion)