Spectral Diagnostics of Chromospheres—A Signature of Emergent Magnetic Flux

Abstract

The chromospheres of the Sun and solar-like stars respond to the underlying magnetic activity. The spectral lines of singly ionized calcium (e.g., Ca II K at 3933 Å) exhibit single or double reversals and serve as diagnostic tools for the magnetic activity. We present observations showing that the Ca II K spectral line may exhibit reversals in addition to the well-known K1, K2, and K3 components, when observed with high angular and spectral resolution. These Ca II K spectral line anomalies occur when small-scale, subarcsecond magnetic fields emerge through the chromosphere. The anomalous profiles originate at locations that are cospatial with Hα brightening at footpoints of dark active region fibrils, or active region filaments. The photospheric magnetic field is bipolar (small opposite polarity magnetic nodules) or has neutral lines at cospatial locations where the spectral lines are anomalous. Small-scale reconnective processes can cause these anomalous profiles. From a simultaneous time series of magnetic field data, we find that the emerging magnetic flux is associated with these profiles. The additional reversals primarily occur redward of the familiar K3 absorption trough, implying chromospheric downflow velocities that lead to the formation of shocks. The magnitude of the Doppler shift is of the order 40-50 km s-1. We present evidence to suggest that a combination of subresolution elements, each of which has regular profiles with large relative Doppler shifts could explain the formation of some of these profiles. A heuristic model that combines buoyant magnetic fields, convective collapse, gas evacuation, shock formation, heating, and a multicomponent model atmosphere can reproduce these profiles.