Enigmatic Line Broadening During Solar Flares: Magnetic Field Broadening?
The origin of the extreme broadening observed in chromospheric metal lines during solar and stellar flares, particularly Mg II h&k and Ca II H&K, remains poorly understood. These lines often display Lorentzian like wings whose widths exceed standard Stark broadening predictions by factors of approx. 30, with no known collisional mechanism capable of producing such enhancements. We posit that magnetic fields are responsible for this additional broadening due to the increase of magnetic activity during flares. A magnetic-field distribution of the form where P(B) goes as B^-3 reproduces the observed Mg II profile wings while leaving H I Balmer lines and optically thin transitions largely unaffected. To explain the broadening using magnetic fields, the high B tail can extend up to 10^6 G with extremely low probabilities where the filling factors are less than about 10^-6. We propose that observations of flares using spectropolarimetry can verify whether the anomalous broadening is from magnetic structures in flare ribbons.