Study of the Streamer Current Sheet with White-Light and UV Observations

Current sheets (CSs) are the main region where magnetic reconnections can convert magnetic energy into plasma thermal and kinetic energies. We have studied a CS in the streamer occurred on 3 January 2003 by combining the White-Light (WL) images observed by the Large Angle and Spectrometric Coronagraph (LASCO) and the EUV spectra detected by the Ultraviolet Coronagraph Spectrometer (UVCS). LASCO C2 WL data show that the speed of a blob in the CS increased from 60 km ·s-¹ to 340 km ·s-¹ with an acceleration of 60 m ·s-² in its Field-of-View (FOV). By assuming the Light-of-Sight (LOS) depth of 0.3 - 1.5 R_⊙, the average electron number density of the CS is (1.52 - 7.60) ×10⁷ cm-³ at the height of the UVCS slit. We investigate intensity distributions of the [Fe XVIII] 974 Å and Ly α lines along the UVCS slit. It is shown that the intensity of the [Fe XVIII] line at the CS is significantly higher than those of the surroundings, and the deduced electron temperature range of the CS is (2.94 - 4.04) ×10⁶ K during the studied period. However, the intensity of the Ly α line at the CS does not change much when compared with those of the surroundings, and within the CS the intensity on both sides are slightly higher than that in the center. It is possible that the plasma move faster in the center resulting in stronger Doppler dimming effect. Using the observed intensity ratio of the Ly α and [Fe XVIII] lines provided by the UVCS and the calculated electron temperature as constraints, we find that the theoretically calculated emissivity ratio of the Ly α and [Fe XVIII] lines is close to their observed intensity ratio when the plasma speed range is 237 - 254 km ·s-¹ at the position of the CS. The collisional component of the Ly α line is about 42 % - 57 % of the radiative component in the CS within the speed range above. The streamer CS we studied has a higher plasma temperature and a faster blob speed than typical values in normal situations. The possible reason is that these two CMEs at the southern side enhanced the magnetic reconnection process in the CS, and more magnetic energy is released to heat and accelerate plasma. Our results on the CS can be regarded as pre-studies of the data analyses for the future mission of the Advanced Space-based Solar Observatory (ASO-S).