The Evershed effect (Evershed 1909) is the dominant process that gives penumbral spectral lines their characteristic line profiles and plays a central role in studies of the sunspot penumbra. It is typically observed as Doppler-shifted and asymmetric spectral line profiles and is commonly interpreted as an outflow of gas directed along penumbral filaments (for recent reviews see Muller 1992; Thomas 1994; Maltby 1997; Wiehr 1999). The observational characteristics are still not fully understood and with the advancement of solar telescopes and observational techniques, new aspects are being found.
From high-spatial resolution observations with tunable filter instruments, it was observed that the Evershed flow is not a steady flow (Shine et al. 1990), but displays an irregular repetitive behaviour (Shine et al. 1994; Rimmele 1994). Movies of Dopplergrams revealed moving structures of enhanced Doppler signal moving radially outwards in the penumbra. These velocity packages, also called "Evershed clouds'', have a radial extent of about 1000 km and are radially separated by 2000-3000 km. The pattern of moving velocity packages was found to have a quasi-periodic variability on a time scale of 10-15 min. The modulation can have a peak-to-peak amplitude of 1 km s-1, superimposed on an average steady flow of 3-4 km s-1 (Shine et al. 1994).
Two models of the Evershed effect are most successful in explaining a number of observational properties: the siphon-flow model as proposed by Montesinos & Thomas (1997), and the moving-tube model by Schlichenmaier et al. (1998a,b). In the siphon-flow model, magnetic loops act as siphons where the gas flow is driven by an unbalanced gas pressure due to a difference in magnetic field strength between the foot-points. In the numerical simulations of the moving-tube model, penumbral filaments are modelled as thin magnetic flux tubes in a superadiabatic and magnetized penumbral atmosphere. As a tube convectively rises to the surface, an outflow is built up which is identified as the Evershed flow. Neither of the two models explains the observed time variability of the Evershed effect, although recent progress has been made (Schlichenmaier 2002).
In this paper, I present the first study on the time variability of the Evershed flow that employs a spectrograph, so that complete spectral line profiles are measured directly. This excludes the possibility of the Doppler measurements being contaminated by seeing effects.
Copyright ESO 2003