Scientific Rationale

Heating of the solar atmosphere (i.e., the chromosphere, transition region, and corona) has been a much-debated topic in solar physics for several decades. Magneto-hydrodynamic (MHD) waves are often presented as a principal mechanism allowing the transfer of energy and momentum between the solar interior and these elevated layers.

Current generation solar telescopes, including the Swedish 1-m Solar Telescope (SST) and the Dunn Solar Telescope (DST), are producing a plethora of high-quality imaging and spectro-polarimetric datasets. Importantly, the multi-wavelength optical and infrared observations produced contain a wealth of information related to MHD wave processes created within the Sun’s dynamic lower atmosphere. These oscillations, in the form of slow/fast magneto-acoustic and Alfvén waves, have the potential to transfer vast quantities of energy flux into the solar chromosphere and corona, where the immense radiative losses need to be balanced. Furthermore, the differing energy and plasma transmission rates as one moves through the various layers of the solar atmosphere naturally provides implications when relating the Sun’s energetics to those encountered in the heliosphere.

Recent investigations have focussed on the detection and identification of mixed-property wave modes existing across different magnetic solar features (e.g., sunspots, pores, magnetic bright points, spicules, filaments, etc.). In order to make the quantification of wave properties as accurate as possible, theoretical aspects of spectro-polarimetry, partial ionisation and radiative transfer processes need to be incorporated, especially since the lower solar atmosphere is governed by optically thick plasma conditions. Many recent ground-breaking publications have begun to include Stokes inversion processes in order to better understand the spectro-polarimetric signatures resulting from the passage of energetic wave fronts through the highly stratified solar atmosphere. However, these types of processes are not without significant challenges. Often, the captured spectro-polarimetric Stokes profiles are significantly asymmetric and evolve on timescales shorter than typical camera integration times.

It is therefore imperative to bring together leading experts in observations, instrument design, wave theory, numerical simulations, spectro-polarimetric inversions and radiative transfer processes in order to drive forward cutting-edge research that will benefit the global astrophysical community for decades to come. Importantly, this team will bring together 12 international experts from 7 leading countries to concentrate research efforts in order to yield reliable estimates of the energy transported by MHD waves into the upper solar atmosphere, and provide new insight into the dissipation mechanisms of these waves and, hence, their contribution to heating the outer layers of the solar atmosphere.

The core team members hold a wealth of experience in the handling, preparation and exploitation of high-resolution data sequences from modern ground-based instruments, including CRISP, CHROMIS, IBIS, ROSA, HARDcam, and ALMA, which can be used in conjunction with numerous space-borne facilities, such as Hinode, SDO, and IRIS. It is the goal of the assembled team to collaborate intensively together in order to investigate different aspects of wave activity in the solar atmosphere. Importantly, members of the team are international experts in spectro-polarimetric signatures and radiative transfer processes, both of which need to be harnessed in order to reliably characterise the origins of detected wave signatures.

Furthermore, we plan to complement our wave studies from high-resolution data with state-of-the-art numerical simulations and theoretical models, such as those from the Bifrost, MURaM, and CO5BOLD codes. By combining the studies of wave propagation through the lower solar atmosphere from theory and simulations, with novel high resolution observations will, therefore, vastly improve our understanding of the physics of MHD waves and their role in governing the dynamics and energetics of the solar atmosphere. Combining these new simulations and observations should finally resolve earlier discrepancies in the interpretation of the various wave modes and their con- tributions to the net energy flux.

It is worth highlighting that the timeliness for activity of such a team is of vital importance, due to the upcoming Daniel K. Inouye Solar Telescope (DKIST) and European Solar Telescope (EST) facilities, which will revolutionise our vantage points of oscillatory phenomena in the Sun’s atmosphere. It is expected that the in-depth discussions and subsequent focussed research efforts will have significant implications for upcoming DKIST and EST observations.