4 Results

   
4.1 Space/time diagrams

Figure 2 shows the space/time diagrams for the four spectral lines. As indicated by Fig. 1, the left-hand side of the slit crossed penumbral filaments that were directed at small angles towards disk centre. Here, the Evershed signal is strongest as opposed to the right-hand side of the diagrams, where the slit crossed penumbral filaments perpendicular to the line-of-sight and the Evershed signal is virtually absent. In addition, on the left-hand side of the diagrams, the slit crosses the outer regions of the penumbra, where the Evershed effect is known to be generally stronger than for smaller radial distances from the umbra.

A close comparison between the intensitygrams and the Dopplergrams shows that the regions where the Evershed effect is strongest coincide with dark filaments. As noted in Sect. 3.3, this is a confirmation of a regularly reported observation. Note however, that the velocity signal in the bright filaments is not absent everywhere, most notably on the left side of the slit in Fig. 2 (see also Sect. 4.4). This points to a non-perfect correlation which might be the basis of a number of conflicting observations of an unclear correlation (see e.g. Wiehr & Stellmacher 1989; Lites et al. 1990; Johannesson 1993; Hirzberger & Kneer 2001; Paper I).

The velocity/intensity diagram pairs in Fig. 2 are ordered for increasing line-core formation height. It is easily recognized that the Evershed signal decreases in magnitude for increasing formation height, a commonly known property (see e.g. St. John 1913a,b; Rimmele 1995; Paper I).

For a number filaments, e.g. A, B, C and F, Evershed signal is detected during the whole period when covered by the spectrograph slit. These filaments can be characterized as Evershed channels, filaments with a persistent Evershed flow.

The Dopplergrams show that the Evershed signal is not stationary, periods with stronger signals alternate with periods with weaker signals, e.g. for filament C, a period of strong signal ends at $t \approx 40$ min and starts again at $t \approx 55$ min. On the right-hand side in the penumbra, where the Evershed signal is weak, the Doppler signal is dominated by p-mode oscillations, recognized by horizontal bright and dark bands, spatially coherent over a significant fraction of the slit.

   
4.2 Velocity signal in dark filaments

Figure 3: Velocity signals in filament C for all spectral lines. Large dots mark the selected data points from slit positions separated less than 2 slit widths from each other. The small dots mark all other data points, i.e. from slit positions with larger spatial offsets, which are omitted from further analysis. A 5.5-min boxcar smoothing is outlined by the thick solid line.

Figure 4: Velocity signals in filament H for all spectral lines.

Figure 5: Velocity signal for all filaments for Mn I 3926. The spatial ordering of the filaments can be found in Fig. 2. Filament A and B were only partly covered; image rotation moved the spectrograph slit away from these filaments after approximately 40 min.

In Fig. 3, the velocity signal in filament C is shown for the four spectral lines. Filament C is covered by the spectrograph slit during the whole sequence and hosts a strong Evershed flow. The larger dots (connected with the thin solid lines) are data points from the selected series, slit positions that are separated less than two slit widths from each other (see Sect. 2). The power spectrum of the velocity signal (not shown) has a prominent peak around 5 min for all four lines so that on short time scales, the variability in the velocity signal is dominated by p-mode oscillations. The thick solid lines show the residual signal after suppression of the short-time variability by a 5.5-min boxcar smoothing which leaves a significant variability on longer time scales. The largest amplitudes are found for the lowest formed line, Mn I 3926.

The velocity signals for filament H are shown in Fig. 4. The orientation of filament H is close to perpendicular to the line-of-sight and no significant Evershed effect is observed in this filament. The mean velocity offset of ${\sim}{-}100$ m s^-1 is well within the uncertainty of the absolute velocity calibration. Like for filament C, the variability on short time scales is dominated by p-mode oscillations, the smoothed curve does not show any significant residual variability.

The velocity signals of Mn I 3926 for all dark filaments are shown in Fig. 5. The ordering of the filaments is (see also Fig. 2) from a part where a strong Evershed signal is observed to a part where no significant Evershed signal is observed. The connected thick dots (and the power spectra) show p-mode contribution in all filaments, but the smoothed curves show only significant variability in the filaments with stronger Evershed signal. This time variation can be attributed to variability of the Evershed effect.

The comparison between filaments with strong Evershed signal showing long-time variability and filaments without Evershed signal, is a strong argument against any systematic effects (e.g. instrumental, in the velocity calibration etc.) or beats from higher frequency oscillations. Another argument supporting the existence of long-term Evershed variability in the data is that similar time behaviour is observed for all spectral lines. Finally, errors from noise, seeing and image jitter are estimated to be low enough for the observed Evershed variability to be real.

The variations in the Evershed flow do not seem to behave in a regular oscillation with a unique amplitude or period. A typical time scale for the variations lies between 8-14 min, where the power spectra have a prominent peak. This is consistent with the time scales reported by Shine et al. (1994) (10 min) and Rimmele (1994) (15 min). Values that could characterize the velocity amplitudes of the variations are given in Table 2. Here, peak-to-peak velocities are given for the first four filaments, where the Evershed effect is strongest. The largest amplitudes in the variation seem to be found where the Evershed flow is strongest: in the lowest formed lines and in the strongest channel (filament A, near the outer penumbral boundary). Typically, the amplitude is on the order 40-60% of the mean Evershed flow speed.

In an attempt to search for systematic differences in the Evershed variability between the different spectral lines, the velocity plots were closely compared. No significant time delays in the variations were found so that it seems that there are is no systematic height dependence for the propagation of the velocity variations.

 

 

Table 2: Peak-to-peak velocities [km s ^-1] for four filaments. In each column, the measured Doppler velocity (line-of-sight component of the velocity) is given with the de-projected value between parentheses. For each filament, the first column is the velocity range of the smoothed (5.5 min) data, the second column is the velocity range of all (selected) data points, i.e. including p-mode contribution. De-projection is performed under the assumption that the Evershed flow is horizontal and parallel to the orientation of the hosting filament.

	A		B
Mn I 3926	1.6 (2.5)	2.2 (3.4)	0.5 (0.8)	0.9 (1.5)
Fe I 3940	1.1 (1.7)	1.4 (2.2)	0.3 (0.5)	0.9 (1.5)
Fe I 3925	0.8 (1.3)	1.0 (1.6)	0.3 (0.5)	0.5 (0.8)
Ti I 3929	0.3 (0.5)	1.0 (1.6)	0.3 (0.5)	0.8 (1.4)
	C		D
Mn I 3926	0.5 (0.9)	1.1 (1.9)	0.3 (0.6)	0.9 (1.5)
Fe I 3940	0.4 (0.7)	1.0 (1.8)	0.3 (0.6)	0.7 (1.2)
Fe I 3925	0.3 (0.5)	0.7 (1.2)	0.2 (0.3)	0.7 (1.2)
Ti I 3929	0.3 (0.5)	0.8 (1.4)	0.3 (0.6)	0.7 (1.2)

4.3 Velocity-intensity correlation

Figure 6: Comparison of intensity in the far Ca II K wing ( $\lambda = 3924$ Å) and velocity for Mn I 3926 for four filaments. Intensity is relative to the mean intensity outside the penumbra.

Shine et al. (1994) found a positive correlation between the Evershed Doppler clouds and continuum intensity: an increase in the Evershed signal was observed to be usually associated with a relative increase in the continuum intensity. Figure 6 shows a comparison for four filaments between intensity in the far Ca II K wing (formed only just above the continuum forming layers, $\log(\tau_{5000}) \approx 0.05$ in HolMul for $\lambda = 3924$ Å, see Paper I) and velocity in Mn I 3926 (Note that for these filaments in the centre-side penumbra, an increasing Evershed flow corresponds to more negative velocity values). A few cases of large amplitude enhancements of the Evershed signal are associated with temporal enhancements of the intensity: for filament A at $t \approx$ 24 min and $t \approx$ 34 min, and for filament C at $t \approx 56$ min. Any further examples of a positive correlation are not obvious from the data.

Given their large amplitude and duration, these few cases of enhanced Evershed signal would likely have been observed as Evershed clouds if a larger surface area had been covered. Though limited in number, these cases support the positive correlation between enhancements of Evershed signal and continuum intensity found by Shine et al. (1994).

   
4.4 Velocity signal in bright filaments

Figure 7: Velocity signal for Mn I 3926 in bright filaments K, L and M, plus intensity signal in filament L.

In Fig. 7, the velocity signals for Mn I 3926 in three bright filaments are shown. Like in the dark filaments, the short-time variability is dominated by p-mode oscillations. The 5.5-min smoothed data do not suggest any significant variability on longer time scales.

Note the large mean velocity offset of filament K as compared to M and L. The orientation of filament K is at low inclination angle with the observation angle so the velocity offset can be attributed to the Evershed effect. This illustrates that the brightness/velocity relationship in the penumbra, the Evershed flow being found predominantly in dark filaments, is not perfect (see also Sects. 3.3 and 4.1).

Penumbral grains are known to migrate through the penumbra, the bulk moving towards the umbra (Muller 1973; Sobotka et al. 1999; Sobotka & Sütterlin 2001). From the slit-jaw and G-band movies, one clear example of a penumbral grain crossing the slit starting was found. This event can be identified in the intensity profile for filament L in Fig. 7 as an intensity increase starting at $t \approx 30$ min. No particular change in the velocity profile is associated with this event, which is rather unexpected considering reports of systematic upflows in penumbral grains of order 100-200 m s^-1 (Rimmele 1995) and 600 m s^-1 (Schmidt & Schlichenmaier 2000).