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Optical and thermal characteristics of discharge channels during dark periods are observed by a high-sensitivity experimental platform. A narrow thermal channel is found to act as a bridge to connect the residual discharge channel to the anode. Compared to the residual hot channel, the narrow thermal channel exhibits marked differences in thermal evolutions, maintaining temperature of about 800 K and thermal diameter of 0.8 mm. The narrow channel is expected to correspond to the weak luminosity with ionization activity on optical images, supporting the hypothesis that the reactivation of discharge channels in restrike originates from the channel tip.