This report presents scientific rationale for a national investment in a new, ground-based, large-aperture solar telescope – a key facility for a new era of research to understand the Sun. Such a telescope is required to provide high angular resolution and high sensitivity measurements that cannot be done any other way.
The main reason why such a facility is needed is that the Sun is a complicated physical system that we do not, but need to understand, because of the Sun's intrinsic importance to humanity and relevance to other areas of physics and astrophysics. This complexity requires that in the new era of solar research the Sun be studied as a system using a wide range of observational and modeling tools. No single telescope, numerical model, or theory will tell us all that we need to know. A large-aperture telescope is an essential part of the set of tools needed to understand the Sun.
The Sun's physical processes span a wide range of angular and temporal scales. Indirect observational inferences and sophisticated modeling tell us that important processes are beyond the resolution limit currently available. This is the main reason that high angular resolution is required. The scale of 0.1 arcsec (70 km) is an important scientific threshold that should be crossed by the new telescope. This is the pressure scale height and photon mean-free-path in the photosphere and is therefore impressed in phenomena such as magnetic flux tubes and granular fine structure and dynamics.
The Sun changes quickly and many measurements need to be done at high signal-to-noise ratios. This is the main reason why a large photon flux is required. Accurate vector magnetic field measurements are particularly demanding. To do such measurements at high angular resolution before solar features change, or to do lower angular resolution measurements of magnetic fields in the chromosphere or corona are additional imperatives for a large aperture.
In order to conduct true systemic studies of solar processes, either by itself or especially together with space, radio and other ground-based optical equipment, the new telescope should be able to observe over a wide wavelength range and include access to the corona. These capabilities offer the additional benefits of unique, robust diagnostics in the infrared and higher quality observations of photospheric and chromospheric targets respectively.
Our conclusion is that an aperture of about three meters is the least that will meet the need to study the Sun adequately with the scientifically required angular resolution and sensitivity. Approaching the diffraction limit of such an aperture in the near infrared and blueward into the visible is feasible using a combination of adaptive optics and image reconstruction techniques.