Study of EUV Emission and Properties of a Coronal Streamer from PROBA2/SWAP, Hinode/EIS and Mauna Loa Mk4 Observations

摘要

Wide-field EUV telescopes imaging in spectral bands sensitive to 1 MK plasmaon the Sun often observe extended ray-like coronal structures stretchingradially from active regions to distances of 1.5-2Rsun, which represent the EUVcounterparts of white-light streamers. To explain this phenomenon, weinvestigated the properties of a streamer observed on October 20-21, 2010 bythe PROBA2/SWAP EUV telescope together with the Hinode/EIS spectrometer (HOP165) and the Mauna Loa Mk4 white-light coronagraph. In the SWAP 174 A bandcomprising the Fe ix - Fe xi lines, the streamer was detected to a distance of2Rsun. We assume that the EUV emission is dominated by collisional excitationand resonant scattering of monochromatic radiation coming from the underlyingcorona. Below 1.2Rsun, the plasma density and temperature were derived from theHinode/EIS data by a line-ratio method. Plasma conditions in the streamer andin the background corona above 1.2Rsun from disk center were determined byforward-modeling the emission that best fit the observational data in both EUVand white light. It was found that plasma in the streamer above 1.2Rsun isnearly isothermal, with a temperature T=1.43+-0.08 MK. The hydrostaticscale-height temperature determined from the evaluated density distribution wassignificantly higher (1.72+-0.08 MK), which suggests the existence of outwardplasma flow along the streamer. We conclude that, inside the streamer,collisional excitation provided more than 90% of the observed EUV emission;whereas, in the background corona, the contribution of resonance scatteringbecame comparable with that of collisions at R > 2Rsun.