The solar chromosphere and the million-degree corona are not in radiative equilibrium with the photosphere. This is most evident in the temperature stratification of empirical models of the outer solar atmosphere, where a steep temperature increase outward is required in order to explain the observed intensities. Furthermore, the chromosphere is radiating on average 4 kW/m^2 in the quiet Sun and 20 kW/m^2 on active regions, which cannot be explained by radiative transport alone. Therefore, additional energy transport and dissipation mechanisms (besides radiation) must be at work in order to explain the derived temperature gradient and the non-zero radiative losses predicted 1D empirical models: how are the outer layers of the Sun heated from a few thousand to multi-million degrees. What are the physical processes responsible for energy transport and deposition?
Chromospheric heating is very intense in magnetically active regions. Large-scale magnetic fields emerge on the solar surface as an integral part of the 11-year-solar-cycle, and they greatly affect the outer layers of the Sun. Transient magnetic activity is ultimately responsible for the coronal mass ejections and flares which cause space weather events, and it plays a fundamental role in heating the outer layers of the Sun. The importance of the chromosphere and its magnetic field is highlighted by the scientific focus of a
new class of forthcoming 4m solar telescopes that have been designed with the prime objective of studying the chromosphere. These telescopes include dedicated instrumentation to study chromospheric dynamics and magnetic fields: DKIST in USA and the European Solar Telescope.
The main scientific questions that MAGHEAT will address are:
1. How is the energy transported and released into the chromosphere?
2. What is the role of currents, waves, turbulence and particle acceleration and kinetic energy deposition in the chromospheric heating problem?
3. How much magnetic energy is stored in a pre-flare system and how much energy is released during the flare by electron beams in the chromosphere? What are the conditions that lead to trigger a flare?
