STUDY OF EXTREME-ULTRAVIOLET EMISSION AND PROPERTIES OF A CORONAL STREAMER FROM<i>PROBA2</i>/SWAP,<i>HINODE</i>/EIS AND MAUNA LOA Mk4 OBSERVATIONS

Abstract

Wide-field EUV telescopes imaging in spectral bands sensitive to 1 MK plasma\non the Sun often observe extended ray-like coronal structures stretching\nradially from active regions to distances of 1.5-2Rsun, which represent the EUV\ncounterparts of white-light streamers. To explain this phenomenon, we\ninvestigated the properties of a streamer observed on October 20-21, 2010 by\nthe PROBA2/SWAP EUV telescope together with the Hinode/EIS spectrometer (HOP\n165) and the Mauna Loa Mk4 white-light coronagraph. In the SWAP 174 A band\ncomprising the Fe ix - Fe xi lines, the streamer was detected to a distance of\n2Rsun. We assume that the EUV emission is dominated by collisional excitation\nand resonant scattering of monochromatic radiation coming from the underlying\ncorona. Below 1.2Rsun, the plasma density and temperature were derived from the\nHinode/EIS data by a line-ratio method. Plasma conditions in the streamer and\nin the background corona above 1.2Rsun from disk center were determined by\nforward-modeling the emission that best fit the observational data in both EUV\nand white light. It was found that plasma in the streamer above 1.2Rsun is\nnearly isothermal, with a temperature T=1.43+-0.08 MK. The hydrostatic\nscale-height temperature determined from the evaluated density distribution was\nsignificantly higher (1.72+-0.08 MK), which suggests the existence of outward\nplasma flow along the streamer. We conclude that, inside the streamer,\ncollisional excitation provided more than 90% of the observed EUV emission;\nwhereas, in the background corona, the contribution of resonance scattering\nbecame comparable with that of collisions at R &gt; 2Rsun.\n