Asymmetric behavior of hHα footpoint emission during the early phase of an impulsive flare

We study the impulsive phase of a C9.0 solar flare using high temporal and spatial resolution Hα images from Big Bear Solar Observatory (BBSO) in conjunction with high-cadence hard X-ray (HXR) observations from Yohkoh. During the early impulsive phase, HXR emission emerged from two kernels K1 and K2 which were connected by coronal loops observed in soft X-ray (SXR) images from Yohkoh. In Hα, the initial rise was observed in one flare kernel K2, which was followed within 10 s by enhanced emission in the associated kernel K1. Such a significant asymmetry was not observed at HXR wavelengths. Our analysis shows that the asymmetric Hα footpoint emission cannot be explained by the magnetic mirroring effect in which strong field footpoints show lower precipitation rates. Instead, we study this phenomenon by investigating the atmospheric response of the lower chromosphere to nonthermal beam heating. From numerical simulations, it is suggested that a cool atmosphere does not respond rapidly to beam impact, which may explain the missing Hα emission at K1 during the early impulsive phase. At K2, the early-phase atmosphere may be preferentially heated resulting in the Hα emission rapidly following the HXR emission. This is due to the fact that K2 is a compact source which received persistent energy deposition and consequent heating in a confined area during the early phase. K1, on the other hand, is a diffused source which therefore experienced a lower heating rate per unity area. We propose a scenario in which the flare loop consists of multiple magnetic "threads" connecting the compact footpoint K2 with the diffuse footpoint K1.