High resolution wave dynamics in the lower solar atmosphere

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.

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.

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.

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.

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.

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. 

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. 

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.