Jorrit Leenaarts, Mats Carlsson, Peter Gomory, Christoph Kuckein

The chromosphere is the interface between the photosphere and the corona. In the chromosphere the dynamics change from gas-pressure driving to magnetic-force driving, radiation transport changes from optically thick to optically thin, the gas state changes from neutral to ionized, and from local thermodynamic equilibrium to non-equilibrium conditions. The magnetic field is the key quantity in the physics of the chromosphere. Understanding its physics thus requires determination of the magnetic field at all locations of the chromosphere. The chromosphere is relatively dificult to study compared to the photosphere: evolution timescales are shorter, chromospheric lines are broad and deep, and thus have a relatively small photon flux, magnetic flux densities and polarization signals are weaker and fundamental spatial scales are expected to be very small. In this talk I will introduce the science that SG3 has planned for EST, some selected examples of observing plans and the requirements that we think EST should deliver in order to be able to carry out the mentioned science.