The main mission of EST is to observe the Sun. This star is a fundamental model for understanding the rest of the universe (all of the stars are suns). The Sun is the only star that can be studied in high resolution. EST will look at the fundamental solar processes at their smallest scales, allowing us to analyse physical phenomena in the greatest possible detail.
A consensus exists among solar astronomers worldwide that a significant increase in observing capability is needed to understand the fundamental processes that control plasma physics in the Sun's outer atmosphere, approaching the following key questions as a priority goal:
- What can the Sun teach us about fundamental astrophysical processes? Observations of the Sun reveal intricate patterns of magnetic fields and the complex dynamics of a stellar atmosphere at the physically relevant spatial scales.
- What drives solar variability on all scales? The Sun varies on a wide range of spatial and temporal scales, displaying important energetic phenomena over the whole range. We do not fully understand and cannot accurately predict basic aspects of solar variability.
- What is the impact of solar activity on life on Earth? Solar magnetic activity variations induce terrestrial changes which can affect millions of humans on short and long time scales. We need to predict disturbances of the space environment which are induced by the Sun and to understand the links between the solar output and the Earth’s climate.
EST will incorporate adaptive optics and integral-field spectropolarimeters, with a precision of one part in 104, to resolve scales of the of order 10 km in the photosphere, to observe astrophysical processes at their intrinsic scales, and thereby observe the interaction of magnetic fields and plasma motions in the solar atmosphere.
The optical design and instruments of EST are optimised for observation of the coupling of the atmospheric layers. Suitable techniques have been developed to determine the thermal, dynamic and magnetic properties of the plasma over many scales heights using imaging, spectroscopy and spectropolarimetry instrumentation. EST will be equipped with a suite of instruments to observe simultaneously in various wavelengths, so that the solar photon flux can be exploited more efficiently than at other current or future ground-based or space telescopes. In order to meet its scientific goals, EST will also require high spatial and temporal resolution.
The aperture of a telescope determines its resolving power. But, until recently, ground-based solar telescopes have been limited by wavefront distortion in the Earth's atmosphere. The advent of powerful adaptive optics techniques now allow for correction of most of this distortion. This makes it possible to capture the first glimpses of the solar surface fine structure, demonstrating that the technique is sufficiently mature and well developed. The resolving capability is now limited by telescope size only and not by atmospheric distortion. We can now look for answers to the fundamental questions of the physics of solar activity and its variability.
In addition to spatial resolution, the light collecting power of a large aperture is crucial for solar investigations. Magnetic fields are detected and characterized by analysing their imprint on the polarisation of light in suitably chosen spectral lines. The fraction of light that is polarised is very small (sometimes below 10-3). The accuracy achieved in these polarimetric measurements, and therefore the ability to observe magnetic fields, is limited by the number of photons collected. With a large aperture, a greater number of photons can be detected in any given zone on the surface of the sun. This is vital to achieve the required accuracy for polarimetric measurements of 10-4. The time scales determining changes in solar structures are related to the speed of sound (variable through the atmosphere, but typically around 7 km/s in the lower photosphere), in such a way that smaller structures evolve more quickly in time. The temporal resolution required is then just a few seconds, which can only accomplished by a large aperture telescope.
Together with the American DKIST, EST will fill a gap not covered by any other instrument, either ground-based or space-borne, currently existing or in the immediate future: it will be able to examine magnetic coupling in the solar atmosphere from the deepest layers of the photosphere to the highest layers of the chromosphere, to uncover the thermal, dynamic and magnetic properties of the sun's plasma at high spatial and temporal resolution. To do this, EST will need to specialise in high-precision polarimetry at many simultaneous wavelengths.