The Formation of Neutral Current Sheets and the Associated Loop Coronal Transients

Under a bipolar background magnetic field, a neutral current sheet is liable to form in between the new emerging magnetic field and the background owing to an eruption of opposite magnetic flux. Based on the ideal magnetohydrodynamic equations, this paper has made a numerical study of the above Formation of current sheets, considering the interaction between the magnetic field and the coronal plasma. The result shows that a subsonic eruption gives rise to a four-region structure from inside to outside: (1) a low-temperature, high-density prominence, consisting of the erupted material and located at the most-interior-region, (2) a low-temperature rarefaction region right next to the erupted prominence, (3) a high-temperature, high-density loop formed by the concentration of both erupted and coronal material into the neutral current sheet, and (4) a forerunner around the loop with the density slightly higher than the coronal background, due to the fast magnetosonic wave. The above mentioned structure is conformed with the observed features of typical loop coronal transients. Consequently, the eruption of opposite magnetic flaxes under the bipolar background field is most likely an important mechanism for triggering such kind of transients.