The solar chromosphere forms a crucial interface region between the solar photosphere and the heliosphere. In the chromosphere, almost all the mechanical energy flux supplied to the outer solar atmosphere by magneto-convection is converted into heat and radiation, leaving a small amount to power the solar wind and the hot corona: the chromosphere requires over 30 times more energy than the corona and heliosphere combined. Despite its importance for understanding the outer solar atmosphere, the chromosphere is poorly understood for a variety of reasons: it is a highly dynamic, finely structured environment in which magnetic fields and plasma compete for dominance, non-local radiative transfer and non-equilibrium ionization/recombination effects dominate the local energy balance, with multi- fluid effects that may play a significant role in the dissipation of magnetic energy. Despite these complexities, the potential for major advances in our understanding of how the chromosphere is heated and impacts the outer solar atmosphere are now within grasp because of the advent of novel and powerful instrumentation that captures full spectral information at high-resolution (e.g., Fabry-Perot interferometers like IBIS and CRISP or NASAʼs IRIS small explorer). These observational advances are being matched by those in massively parallellized numerical simulations that incorporate non-equilibrium ionization and non-local radiative transfer towards physically complete chromospheric models that, by design, augment and complement the detailed measurements.

Chromospheric physics covers a wide range of physical phenomena with much of the previous work focusing on the (“non-magnetic”) chromospheric internetwork regions that are dominated by waves and shock waves. Instead, we will focus on the physics of the magnetized chromosphere that is associated with the network (and plage), regions where strong magnetic fields are concentrated because of convective flows on large, supergranular scales (~20,000 km). We propose to organize three meetings that focus on what heats the magnetized chromosphere, a topic that has been relatively ignored, despite the fact that it provides most of the magnetic flux and mechanical energy to the outer solar atmosphere and heliosphere. The aim of the meetings is to critically assess and test the assumptions underlying the variety of theoretical models for what heats the magnetic chromosphere by confronting them with the highest resolution observations.

The first two meetings will focus on comparing CRISP (and IBIS) data of several chromospheric lines (including Ca II 8542Å or Hα 6563Å) with current modeling efforts to explore the observable consequences of such diverse mechanisms as magnetic reconnection (including the effects of weak granular magnetic fields), or the dissipation of currents (including effects from the interaction between neutral gases and plasma), Alfvén waves, or high frequency sound waves. The third meeting will be held after the launch of the Interface Region Imaging Spectrograph (IRIS, planned for December 2012), and will exploit the consolidated knowledge of the first two meetings to confront the models with the high-resolution chromospheric spectra in the strong Mg II h and k lines that IRIS will routinely obtain.

The proposed team includes modelers that have developed the most advanced numerical models of the solar atmosphere and observers with direct access to the highest quality chromospheric imaging spectroscopic data. With this breadth of composition we expect to make major advances in understanding the heating of the magnetized chromosphere. Given the large energy flux required to heat the chromosphere, and the magnetic connections of network/plage to the outer solar atmosphere, our results will impact our understanding of the outer solar atmosphere and, by extension, the heliospheric system.