Self-cancellation of ephemeral regions in the quiet Sun

Yang Shuhong, shuhongyang@nao.cas.cn, NAOC, China
Zhang Jun,zjun@nao.cas.cn, NAOC
Li Ting, liting@nao.cas.cn, NAOC
Liu Yang, yliu@quake.stanford.edu, Stanford University

Abstract
With the observations from the Helioseismic and Magnetic Imager aboard the \emph{Solar Dynamics Observatory}, we statistically investigate the ephemeral regions (ERs) in the quiet Sun. We find that there are two types of ERs: normal ERs (NERs) and self-cancelled ERs (SERs). Each NER emerges and grows with separation of its opposite polarity patches which will cancel or coalesce with other magnetic flux. Each SER also emerges and grows and its dipolar patches separate at first, but a part of magnetic flux of the SER will move together and cancel gradually, which is described with the term \emph{self-cancellation} by us. We identify 2988 ERs among which there are 190 SERs, about 6.4\% of the ERs. The mean value of self-cancellation fraction of SERs is 62.5\%, and the total self-cancelled flux of SERs is 9.8\% of the total ER flux. Our results also reveal that the bigger the ER is, (a) the easier the performance of ER self-cancellation is, (b) the smaller the self-cancellation fraction is, and (c) the more the self-cancelled flux is. We think that the self-cancellation of a SER is caused by the submergence of magnetic loops connecting the dipolar patches without energy release, and 9.8\% ER flux does not contribute to coronal heating.