Evidence of scattering effects influenced by plasma flows in C VI 1548 Å, 1550 Å spectral lines emitted from the Sun
1 Research Center for Astronomy and Applied Mathematics, Academy of Athens, 4 Soranou Efessiou Str., 11527 Athens, Greece
2 Université Paris Sud, Institut d’Astrophysique Spatiale, UMR 8617, 91405 Orsay, France
3 CNRS, Institut d’Astrophysique Spatiale, UMR 8617, 91405 Orsay, France
Received: 12 January 2016
Accepted: 12 April 2016
Aims. We search for, and study, individual spectral profiles where the complex shape of the C iv 1548 Å line is different from the shape of the simultaneously recorded C iv 1550 Å line. Such an asymmetry is not expected for line emission resulting from collisional excitation. Therefore we propose an explanation of these observations through the differential effect of velocity fields on resonant scattering.
Methods. We analyse spectra recorded with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on the Solar and Heliospheric Observatory (SOHO) over active region, NOAA 8541 as well as a second data set on the quiet Sun. We perform Gaussian fits on the individual profiles with two or three Gaussian functions. Moreover, we parameterize the profile asymmetries by calculating the intensity ratios I1548/I1550, from the derived Gaussian functions. We also calculate artificial spectral profiles emitted from two plasma volumes, which have different line of sight motions and where the plasma emission is influenced by resonant scattering.
Results. We locate three small regions in NOAA 8541 which have spectral asymmetries. There the profiles have two or three spectral components, with different intensity ratios. Artificial profiles show that two plasma volumes, having distinct velocities relative to the observer, may reproduce the observed profiles, under the influence of resonant scattering.
Conclusions. Asymmetric profiles, found in an active region, can be used as a diagnostic for the importance of resonant scattering in transition region plasma.
Key words: Sun: transition region / Sun: UV radiation
© ESO, 2016