On The Interpretation Of H-alpha Filtergrams

For over 150 years the H-alpha line has been a mainstay of chromospheric studies, which have revealed many important aspects of the structure and dynamics of the upper solar atmosphere. The strong absorption of this line in the visible range has long made it possible to isolate its chromospheric contribution in the solar spectrum using spectroheliographs, and later using various filter technologies. The great bulk of observations in H-alpha, covering decades, have been obtained using relatively broad-band filters (0.5-2 Å FWHM). More recently, imaging spectroscopy in H-alpha, usually performed with Fabry-Perot interferometers, has revealed new information about the detailed spectral behavior and characteristics of this line. Using such observations, we can now simulate and better interpret observations taken through the more widely used, broad filters.

In this paper, I will evaluate the sensitivity of filter measurements to chromospheric temperatures and velocities, depending on the placement and shape of the filter passband. In particular, I will discuss how this will aid in the interpretation of observations that will be obtained from the Visual Broadband Imager (VBI) on the Daniel K. Inouye Solar Telescope (DKIST).