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

Wide-field EUV telescopes imaging in spectral bands sensitive to 1 MK plasma on the Sun often observe extended ray-like coronal structures stretching radially from active regions to distances of 1.5-2Rsun, which represent the EUV counterparts of white-light streamers. To explain this phenomenon, we investigated the properties of a streamer observed on October 20-21, 2010 by the PROBA2/SWAP EUV telescope together with the Hinode/EIS spectrometer (HOP 165) and the Mauna Loa Mk4 white-light coronagraph. In the SWAP 174 A band comprising the Fe ix - Fe xi lines, the streamer was detected to a distance of 2Rsun. We assume that the EUV emission is dominated by collisional excitation and resonant scattering of monochromatic radiation coming from the underlying corona. Below 1.2Rsun, the plasma density and temperature were derived from the Hinode/EIS data by a line-ratio method. Plasma conditions in the streamer and in the background corona above 1.2Rsun from disk center were determined by forward-modeling the emission that best fit the observational data in both EUV and white light. It was found that plasma in the streamer above 1.2Rsun is nearly isothermal, with a temperature T=1.43+-0.08 MK. The hydrostatic scale-height temperature determined from the evaluated density distribution was significantly higher (1.72+-0.08 MK), which suggests the existence of outward plasma 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 scattering became comparable with that of collisions at R > 2Rsun.