Strong Blue Asymmetry in H α Line as a Preflare Activity

Chromospheric activities before solar flares provide important clues to the mechanisms that initiate solar flares, but are as yet poorly understood. We report a significant and rapid Hα line broadening before the solar flare SOL2011-09-29T18:08 that was detected using the unprecedented high-resolution Hα imaging spectroscopy with the Fast Imaging Solar Spectrograph (FISS) installed on the 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory. The strong Hα broadening extends as a blue excursion up to −4.5 Å and as a red excursion up to 2.0 Å, which implies a mixture of velocities in the range of −130kms−1 to 38 km s⁻¹ derived by applying the cloud model, comparable to the highest chromospheric motions reported before. The Hα blueshifted broadening lasts for about six minutes and is temporally and spatially correlated with the start of a rising filament, which is later associated with the main phase of the flare as detected by the Atmosphere Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). The potential importance of this Hα blueshifted broadening as a preflare chromospheric activity is briefly discussed within the context of the two-step eruption model.