5 Summary and discussion

The time variability of the Evershed effect was studied from high-spatial resolution spectrograms in the wings of the Ca II K line combined with Ca II K slit-jaw images and G-band filtergrams. Compared to observations with tunable filter instruments, a clear disadvantage of spectrograph observations is the limited total area that is covered by the spectrograph slit. In this study, this resulted in a statistically rather limited number of filaments being covered and analysed. However, notable advantages of a spectrograph are the seeing-independent and accurate line profiles that can be obtained and the direct access to a series of diagnostic spectral lines.

The results of this study can be summarized as follows:

1.

The Evershed flow is found to show irregular variations on a time scale of 8-14 min. This confirms the observation reported by Shine et al. (1990), Shine et al. (1994) and Rimmele (1994).

2.

The largest amplitudes in the variation seems to be found where the Evershed flow is strongest: in the deeper parts of the atmosphere and in the strongest channels.

3.

There does not seem to be a systematic height dependence for the propagation of the velocity variations.

4.

Several examples are found of an Evershed signal enhancement associated with an enhancement in the intensity. This supports the positive correlation between enhancements of Evershed signal and continuum intensity reported by Shine et al. (1994).

5.

No significant variability is found in the (weak) Evershed signal detected in bright filaments.

Currently, no theoretical model incorporates the observed time variability of the Evershed effect in a satisfactory way. Montesinos & Thomas (1997) remark that siphon flows on the Sun are transient in nature because of the changing conditions at the footpoints. For magnetic loops carrying siphon flows that have one footpoint located outside the penumbra, the conditions on the downstream end of the loop are likely to be dominated by the granulation. In this context, the observed time scale of variability of the Evershed effect is in agreement with the life time of the granules. If the variation in the Evershed effect is driven from the footpoints, the radially outward moving Evershed clouds, as observed by Shine et al. (1994) and Rimmele (1994), indicate that the upstream footpoint is dominant in this process. A disturbance generated at the downstream footpoint would be observed to travel in the upstream direction - contrary to what is observed.

The time scales of the flow tubes in the moving-tube model of Schlichenmaier et al. (1998b) are set by the time scale of interchange convection in their simulations, which is on the order of an hour. Interchange convection of penumbral filaments is unlikely to drive the observed variations; the observed life time of filaments is much longer than the variation time scale (see also Rimmele 1994).

A number of observations have indicated that the bulk of the Evershed flow is concentrated in the lower parts of the atmosphere, at heights below 200 km (see e.g. Maltby 1964; Schlichenmaier & Schmidt 2000, Paper I). In Paper I, the line-core Dopplershifts for a number of spectral lines, including the four lines used in this study, were computed from a penumbral atmosphere with a typical Evershed flow channel located at varying heights. This channel was found to account for a 1 km s^-1 Dopplershift increase for Mn I 3926 when being raised from 40 to 110 km height (see Fig. 18 of Paper I). The Dopplershift of the other three spectral lines increase with a lesser amount during such a rise since their contribution functions to the line-core intensity peak at greater heights. This illustrates that a slowly rising and sinking Evershed channel could be an alternative explanation of the observed variability in the Evershed effect. As long as the channel stays at heights below the peak of the Mn I 3926 contribution function, a rising channel would give a coherent increasing Dopplershift to all four lines. The flow speed does not necessarily have to increase to give rise to larger line-core Dopplershifts. Moving kinks in the Evershed channel, localized elevations in the flow channel, could explain the moving Evershed clouds. In this context it is interesting to note that in the latest simulations of the moving tube model (Schlichenmaier 2002), the flow channel is no longer horizontal (as in the earlier simulations) but develops a wave-like form with moving crests. For this scenario to work, the optical thickness of such channel has to be limited, since an optically thick rising channel would just introduce a geometrical shift of the whole line-forming part of the atmosphere. The continuum level and contribution functions are coherently shifted and no significant change in the line-core Dopplershift could be detected. Obviously, detailed hydrodynamical simulations with a realistic treatment of the radiative transfer are needed to test if such scenario is compatible with the observations. More generally, such detailed modelling is required to get a complete understanding of the Evershed flow embedded in the complex geometry of the sunspot penumbra.

Acknowledgements

The author wishes to thank D. Kiselman for comments on the manuscript. P. Sütterlin is thanked for providing the reduced G-band observations from the Dutch Open Telescope (DOT). The DOT is funded by Utrecht University, the Netherlands Graduate School for Astronomy (NOVA) and the Netherlands Organization for Scientific Research (NWO). The DOT is operated from the Swedish solar telescope building on La Palma. The Swedish Vacuum Solar Telescope (SVST) was operated on the island of La Palma by the Royal Swedish Academy of Sciences in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.