Evidence of scattering effects influenced by plasma flows in C VI  1548 Å, 1550 Å spectral lines emitted from the Sun

Abstract

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 I 1548 /I 1550 , 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.

Statistical analysis of UV spectra of a quiescent prominence observed by IRIS

“…We plan to extend this kind of analysis also to our 2D fine-structure models with PCTR depending on the orientation of the prominence magnetic field (see, e.g., Gunár et al 2010Gunár et al , 2014Gunár & Mackay 2015). A possible role of radiative excitation in the Si iv line is also a subject to be investigated (see, e.g., Gontikakis & Vial 2016).…”

Effects of resonant scattering of the Si IV doublet near 140 nm in a solar active region

“…Analysing a few complex profiles of the C iv doublet at 1548.189 Å and 1550.775Å, Gontikakis & Vial (2016) found intensity ratios of 1548/1550, higher than 2, which is the ratio of the oscillator strengths, while the spectral shape of the C iv 1548.189 Å profile was different from the spectral shape of the A&A 619, A64 (2018) . Fig.…”

“…The interpretation was that these anomalous profiles are emitted by plasma volumes that are nearby and below SUMER's spatial resolution, while they have different line-of-sight (LOS) Doppler shifts and they are differently affected by resonant scattering (Gontikakis & Vial 2016). As will be explained later on in the text, when resonant scattering becomes important, the intensity ratio 1548/1550 is higher than 2.…”

“…Observations with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER; Wilhelm et al 1995) on board the SOlar and Heliospheric Observatory (SOHO) in the C iv 1548.189 Å and 1550.775 Å have drawn attention to some inconsistencies in the usual "thermal" analysis of active regions (Gontikakis & Vial 2016).…”

“…The usual spectro-imaging implies the rastering of the slit which takes (at least) a few tens of seconds (in the case of SUMER) and has a good chance of missing the location of the flaring (small) region. Moreover, as pointed out by Gontikakis & Vial (2016), in the case of SUMER the radiative diagnostic may be complicated by the fact that the plane mirror allowing for the spectral scan is actually a non-negligible polarization analyser (Hassler et al 1997). This last feature leads to a selective recording of the 1548.189 Å (polarized) line versus the 1550.775 Å line.…”