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 1.5–2 R _ { \odot } , 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 Å band comprising the Fe ix – Fe xi lines , the streamer was detected to a distance of 2 R _ { \odot } .
We assume that the EUV emission is dominated by collisional excitation and resonant scattering of monochromatic radiation coming from the underlying corona .
Below 1.2 R _ { \odot } , 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 1.2 R _ { \odot } 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 1.2 R _ { \odot } is nearly isothermal , with a temperature T = 1.43 \pm 0.08 MK .
The hydrostatic scale-height temperature determined from the evaluated density distribution was significantly higher ( 1.72 \pm 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 \gtrsim 2 R _ { \odot } .