Probing Chromospheric Temperatures and Dynamics with ALMA
Studies of the thermal structure of the solar chromosphere using spectral lines are typically hampered by the complexities of non-LTE radiative transfer. This issue can be avoided by using observations of the millimeter continuum, which directly probes the electron temperatures in the chromosphere. In recent years, the Atacama Large Millimeter/submillimeter Array (ALMA) has made it possible, for the first time, to obtain millimeter observations of sufficient spatial resolution to supplement spectral line observations and inversions. Here, we present observations of magnetic network in the 3.0 mm and 1.2 mm continua with ~2 arcsecond resolution, combined with simultaneous imaging spectroscopy observations from the Interferometric Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope. We compare the observed ALMA brightness temperatures with temperatures inferred from spectroscopic inversions using the Na D1 5896 Å and Ca II 8542 Å lines, and investigate the wide range of physical heights probed by the millimeter continuum. In addition, we examine the effects of including ALMA data in the inversions. We find that the millimeter emission arises from a range of heights both above and below the chromospheric calcium line, depending on the local temperature profile and electron densities. Furthermore, we identify several problems that arise from the technical limitations of hydrostatic 1.5-D inversion codes, particularly when inverting mm-continuum and spectroscopic diagnostics with similar heights of formation, such as ALMA Band 6 and the Ca II infrared triplet. We conclude with a brief description of current and near-future projects, including epoch analysis of time-series observations of the quiet sun with ALMA and IBIS, as well as two proposals that have been accepted as part of the DKIST OCP campaign.
Ryan received his BS in 2016 at the University of Arizona. He then moved to Boulder to pursue his graduate studies in astrophysics. In 2019, he joined the DKIST Level-2 program as a DKIST Ambassador, taking an active role in the development of tools and techniques for the analysis of data from DKIST and other solar observatories, as well as mentoring the next generation of solar physicists. He currently works with Kevin Reardon at the National Solar Observatory, where he studies temperature diagnostics in the solar chromosphere, and he hopes to defend his dissertation in the summer of 2023.