  Solar flares are generally believed to be powered by free magnetic energy
stored in the corona, but the build up of coronal energy alone may be
insufficient for the imminent flare occurrence. The flare onset mechanism is a
critical but less understood problem, insights into which could be gained from
small-scale energy releases known as precursors, which are observed as small
pre-flare brightenings in various wavelengths, and also from certain
small-scale magnetic configurations such as the opposite polarity fluxes, where
magnetic orientation of small bipoles is opposite to that of the ambient main
polarities. However, high-resolution observations of flare precursors together
with the associated photospheric magnetic field dynamics are lacking. Here we
study precursors of a flare using unprecedented spatiotemporal resolution of
the 1.6 m New Solar Telescope, complemented by novel microwave data. Two
episodes of precursor brightenings are initiated at a small-scale magnetic
channel (a form of opposite polarity fluxes) with multiple polarity inversions
and enhanced magnetic fluxes and currents, lying near the footpoints of sheared
magnetic loops. The low-atmospheric origin of these precursor emissions is
corroborated by microwave spectra. We propose that the emerging magnetic
channel field interacts with the sheared arcades to cause precursor
brightenings at the main flare core region. These high-resolution results
provide evidence of low-atmospheric small-scale energy release and possible
relationship to the onset of the main flare.
