Hα as a Chromospheric Diagnostic

I first illustrate with images from the Dutch Open Telescope (DOT) that Hα is the principal diagnostic of the solar chromosphere. The DOT movies at http://dot.astro.uu.nl demonstrate this fact even more vividly. I then summarize, on the basis of the recent numerical simulations by Leenaarts et al. (2007), why Hα is such an omnipresent diagnostic of the chromosphere. The ubiquity of Hα fibrils in both hot and cool gas is due to (i) – the presence of shocks everywhere, guided by the magnetic field into dynamic fibrils near the network and pushing the canopy and transition region upward in weaker-field internetwork regions, (ii) – the large rate difference between the fast hydrogen ionization/recombination balancing in hot shocks and the slow balancing in cool post-shock gas, and (iii) – the large excitation energy of Hα’s n=2 lower level, causing strong coupling to the ion population. These three facts combine to cause appreciable Hα opacity throughout the chromosphere, enormously in excess of instantaneous Saha-Boltzmann partitioning in cool post-shock gas. Thus, sluggish post-shock recombination causes Hα to be visible everywhere. Finally, I address Hα observing. Since Hinode’s Hα imaging is affected by bubbles and limited in cadence, the DOT may serve as a complementary facility furnishing profile-sampling Hα image sequences at the same 0.3 arcsec angular resolution as Hinode whenever the La Palma seeing is good. However, imminent loss of DOT funding requires outside financing of an on-site observer for DOT utilization in co-pointed joint observing