Steady flows in the solar transition region observed with SMM

Variability of EUV-spectra from the quiet upper solar atmosphere: Intensity and Doppler shift

“…In addition, Stucki et al (2000) also showed that in coronal holes, the sign of the correlation is reversed. Such a correlation supports the model of Hansteen (1993) who proposed that nano-flares occurring at the top of coronal loops generate MHD waves that propagate downward along the magnetic fields towards and through the transition Send offprint requests to: A. Brković, e-mail: alen@kis.uni-freiburg.de region in their footpoints, which lie in the network. The net redshifts in TR spectral lines are a result of the correlation between the intensity and velocity that occurs in downward propagating acoustic waves.…”

“…This behaviour can be shown in simple terms analytically for optically thin lines formed in an isothermal constant-pressure atmosphere. Investigating this effect Hansteen (1993) numerically modeled coronal loops including sound waves propagating along the loops in order to understand the transition region line shifts (solving the full 1D problem along the loop).…”

“…Models proposed to explain these shifts include siphon flows through loops, explosive events, waves due to nano-flares or return of spicular material (Antiochos 1984;McClymont & Craig 1987;Mariska 1988;Hansteen 1993;Spadaro et al 1996; for a discussion see Peter & Judge 1999). Early studies of the correlation between Doppler shifts and line intensities in the quiet Sun gave inconsistent results.…”

“…Athay et al (1983) and Dere et al (1984) found no correlation between intensity and velocity in the data covering the C IV 1548 Å transition region line. Gebbie et al (1981) analysed spectra of the C II 1336 Å, Si IV 1393 Å and C IV 1548 Å lines and found that redshifted regions were correlated with regions of bright network emission and blueshifted regions tend to be associated with darker areas. More recently, Stucki et al (2000) and Hansteen et al (2000) both obtained a positive correlation between network emission and redshift of transition region lines in the quiet Sun.…”

Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations

“…Using UVSP Dopplergram observations of C II, Si IV and C IV spectral lines made with a 3 × 3 entrance slit, Gebbie et al (1981) found larger downflows in brighter regions of the quiet-Sun network. Lacking a wavelength calibration, the absolute speed of the flows could not be determined, however, a bimodal behaviour of the LOS velocities as a function of the line radiance of C IV 154.8 nm emerged with a "network" branch and one with lower velocities in distinct bright points.…”