Diagnostics of homologous solar-surge plasma as observed by IRIS and SDO

Abstract

Surges have regularly been observed in mostly H α 6563 Å and Ca II 8542 Å. However, surge responses to other prominent lines of the interface region (Mg II k 2796.35 Å and h 2803.52 Å, O IV 1401.15 Å, Si IV 1402.77 Å) are not well studied. Here, the evolution and kinematics of six homologous surges are analysed using IRIS and AIA observations. These surges were observed on 2014 July 7 and were located very close to the limb. A differential emission measure analysis is performed on these surges where the coexistence of cool (log T/K = 6.35) and relatively hot (log T/K = 6.95) components has been found at the base. This demonstrates that the bases of surges undergo substantial heating. During the emission of these surges in the above-mentioned interface-region lines, reported here for the first time, two peaks have been observed in the initial phase of emission, where one peak is found to be constant while other one varies; i.e. is non-constant (observed red- to blueshifts across the surge evolution) in nature. This suggests the rotational motion of surge plasma. The heated base and rotating plasma suggest the occurrence of magnetic reconnection as the most likely trigger for homologous surges. During the emission of these surges, it is found that, despite them being optically thick (i.e. R_kh < 2.0), central reversal was not observed for the Mg II k and h lines. Further, R_kh increases with surge emission in time and it is found to have a positive correlation with Doppler velocity and negative with Gaussian width.