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tokens: We are grateful to the referee for the constructive comments and substantial improvements to this work. We thank Dr. Chang Liu for providing ideas to explain the observation, and Dr. Xin Chen for providing the tracing method. The observations presented in this paper were mainly obtained with NST at the Big Bear Solar Observatory, which is operated by New Jersey Institute of Technology. The data are courtesy of the BBSO and NASA/ SDO teams. This work was partially supported by NASA grants NNX13AF76G, NNX16AF72G, NNX13AG13G, and NNX14AC12G, and by NSF grants AGS 1348513, 1408703 and 1620875. BBSO operation is supported by NJIT, US NSF AGS-1250818 and NASA NNX13AG14G, and NST operation is partly supported by the Korea Astronomy and Space Science Institute and Seoul National University, and by strategic priority research program of CAS with grant No. XDB09000000.