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This study investigates the unexplained extreme broadening of chromospheric metal lines (Mg II and Ca II) observed during solar and stellar flares, which exceeds theoretical predictions by approximately 30 times. The researchers propose that magnetic fields with a specific distribution (P(B) proportional to B^-3) extending up to 10^6 Gauss can explain this phenomenon, while leaving hydrogen Balmer lines unaffected. The model suggests these extreme magnetic fields occupy very small filling factors (less than 10^-6) and could be verified through spectropolarimetric observations of flare ribbons.
Why it matters
Understanding the physical mechanisms behind flare spectral line broadening is crucial for accurately diagnosing conditions in solar and stellar flares, which can impact space weather predictions and our ability to characterize stellar activity on other stars. The proposed magnetic field explanation could resolve a long-standing discrepancy between observations and collisional broadening theory.
arXiv:2606.14681v1 Announce Type: new
Abstract: 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.
Source: Enigmatic Line Broadening During Solar Flares: Magnetic Field Broadening?