Sigmoid Formation through Slippage of a Single J-shaped Coronal Loop

A well-known precursor of an imminent solar eruption is the appearance a hot S-shaped loop, also known as sigmoid, in active region (AR). Classically, formation such loop envisaged to be implemented by magnetic reconnection two oppositely oriented J-shaped loops. However, details are elusive due weak emission and subtle evolution during pre-eruptive phase. In this paper, we investigate how single transforms into one through slippage its footpoints NOAA AR 11719 on 2013 April 11. During interval about 16 min, slips low-corona strong electric current density bursty fashion, reaching peak apparent speed fast over 1000 km s$^{-1}$, at slipping footpoint. The enhancement density, suggested non-linear force-free field modeling, indicates that "non-idealness" coronal plasma becomes locally important, which may facilitate reconnection. segment undergoing motions heated; meanwhile, above fixed footpoint dims combination effect lengthening heating latter manifested temporal variation dimming slope measure. These features together support asymmetric scenario sigmoid differs from standard tether-cutting involving double system.