Oscillations of the H? emission in solar prominences

Abstract

The time dependence of Doppler shift and line-center intensity is simultaneously observed for the Hc~ emission of three solar prominences, each one during about two hours. Doppler oscillations with periods near one hour and amplitudes between 1 and 2 km s ~ are conspicuously visible in the recordings of all three prominences. Fourier analysis yields periods of 50, 60, and 64 min, as well as slight indications of short periods near 3 and 5 rain. No oscillations are found in the line-center brightness.The time variation of the displacement of the Ha emission was measured with the evacuated Gregory telescope at Locarno of the Grttingen Observatory (Wiehr et al., 1980) on October 1, 3, and 4, 1983 in three prominences at the NW, SW, and NW limb, respectively. An entrance hole of 8 arc sec diameter was centered on a selected area of each prominence and guided for about two hours. Using the Zeeman polarimeter (Wiehr, 1969) three spectrograph slits covered the Ha emission profile. The central-line intensity was measured through the central slit at 20 + 50 rnA, the Doppler plate was driven by the signals from the neighbouring slits at + (50 to 250) mA. Calibration of the line shift was performed turning the grating by a known amount. Both line-of-sight velocity and intensity signals were stored on magnetic tape and then subjected to a fast Fourier transform analysis (e.g. Brault and White, 1971). Edge discontinuities were not removed. For the slowly varying intensity recordings, a quadratic trend was subtracted before.The velocity recordings in Figure 1 show for all three prominences Doppler oscillations of the Ha amplitudes between 1 and 2 km s -1. The velocity power spectra Figure 2 confirm these results yielding defined periods of 60, 64, and 50 min :for the October 1, 3, and 4 prominences, respectively. This result roughly confirms the finding by Bashkirtsev et al. (1983) of 77 and 82 min periods. Figure 2 additionally indicates short-period oscillations of lower power. If the time coherence of these oscillations is short as compared to our two-hour samples, i.e. if they last only a few periods, their maximum power should increase when studying subsamples of shorter time intervals. Such a behaviour is particularly found for the 3-min period from October 1 which exists only in the last 40-min time-interval of that sample, yielding a power maximum of 4.5 • 104 (m s-1)2 corresponding to an amplitude of

2D-spectroscopy of the Evershed flow in sunspots

“…At the outer penumbral boundary we find an abrupt drop of the flow velocities which agrees well with findings of e.g. Wiehr & Balthasar (1989), Title et al (1993), Wiehr & Degenhardt (1994), and Balthasar et al (1996). At the center side parts of the penumbrae the line-of-sight velocities become even negative in the deepest probed photospheric layers.…”

“…Wiehr & Balthasar 1989;Title et al 1993;Wiehr & Degenhardt 1994;Balthasar et al 1996) have found an abrupt drop of the flow velocity whereas others (Alissandrakis et al 1988;Börner & Kneer 1992;Rimmele 1995b) have found a continuation of the flow beyond the outer penumbral boundary. Solanki et al (1994) have found a sharp drop of the Evershed flow in the low photosphere but a continuation of it in the higher so-called superpenumbral canopy.…”

Time damping of non-adiabatic magnetohydrodynamic waves in a partially ionised prominence medium: Effect of a background flow

“…Observational evidence of the time damping of small amplitude oscillations in prominences has been reported by Landman (1977), Tsubaki & Takeuchi (1986), Tsubaki (1988), Wiehr et al (1989), Molowny-Horas et al (1999), Terradas et al (2002), Lin (2004), Berger et al (2008) and Ning et al (2009a,b). MolownyHoras et al (1999) and Terradas et al (2002) studied twodimensional time series from a quiescent prominence and found that oscillations detected in large areas of the prominence were typically damped after 2−3 periods.…”

Time damping of linear non-adiabatic magnetoacoustic waves in a slab-like quiescent prominence

“…Landman et al 1997;Tsubaki & Takeuchi 1986;Wiehr et al 1989;Molowny-Horas et al 1999;Terradas et al 2002). This is then interpreted as a signature of wave damping by some as yet unknown mechanism.…”