2 Observations

Figure 1: Two Ca II K slit-jaw images, one from the beginning (top) and one from the end of the sequence, and one G-band image (DOT). The black lines drawn in the slit-jaw images mark the location of the spectrograph slit. Note the change in slit angle due to image rotation. The approximate location of the slit in the G-band image is marked with the dashed line. The white arrow in the central image points towards disk centre.

The leader sunspot in NOAA Active Region 8704 was observed on 20 September 1999 (S19.2$^\circ$, E31.5$^\circ$, $\mu=0.77$) with the Swedish Vacuum Solar Telescope (SVST) on La Palma (Scharmer et al. 1985). Spectra of the Ca II K line (3933.66 Å) were obtained with the short Littrow spectrograph (Scharmer et al. 1985), spanning a wavelength interval of 16.7 Å (starting at 3923.7 Å) which covers a significant fraction of the extended Ca II K damping wings. Operating with a 25 $\mu$m (0 $.\!\!^{\prime\prime}$23) wide slit, the spectral resolution was slit-limited corresponding to a bandpass of 30 mÅ at the wavelength of Ca II K. The spectrograph camera recorded spectrograms with an exposure time of 0.35 s and operated at a pixel scale that corresponded to 11 mÅ in the spectral direction and 0 $.\!\!^{\prime\prime}$083 in the spatial direction. The exact location of the spectrograph slit on the Sun was determined from slit-jaw images recorded with a narrow (3 Å bandpass) interference filter centered on the Ca II K line core. In order to assure an accurate correspondence between the spectrogram and the associated slit-jaw images, the exposures were recorded strictly simultaneously using synchronized CCD cameras. Photospheric reference images were recorded independently with a 12 Å bandpass interference filter centered on the G-band (4305 Å).

The time series discussed in this article span almost 64 min, starting at 13:47 UT. The spectrograph slit covered the penumbra on the disk-centre side of the sunspot, as illustrated in the slit-jaw images in Fig. 1. The penumbral filaments that were crossed by the spectrograph slit around spatial location = 20 $^{\prime\prime}$, were oriented perpendicularly to the line-of-sight. No Evershed effect was observed in this particular part of the penumbra, since the direction of the flow is along the penumbral filaments.

On-line frame selection (Scharmer & Löfdahl 1991) was applied to select the best quality spectrogram from a series of images taken in a 18.5 s interval, resulting in a total of 207 spectrogram/slit-jaw image pairs. The correlation tracker (Shand et al. 1995) stabilized the images to a residual image jitter of less than 0 $.\!\!^{\prime\prime}$5 rms, with a maximum of less than 1 $^{\prime\prime}$. In order to keep the spectrograph slit covering the same penumbral features during the time series, the telescope was operating in a 3-step scanning mode. Between frame selection intervals in a scan cycle, the image was moved one slit width (0 $.\!\!^{\prime\prime}$23) perpendicularly to the slit direction. After selecting the third spectrogram, a new cycle was begun at the starting position.

The altaz mount of the telescope turret introduced an image rotation of 13$^\circ$ between the first and last frame, with the rotation axis centered in the umbra (see Fig. 1). This resulted in a drift with respect to the spectrograph slit, with a magnitude depending on the distance from the rotation axis. In the penumbra, the maximum drift was less than 3 $^{\prime\prime}$ across the slit, and about 2 $.\!\!^{\prime\prime}$5 along the slit.

The seeing conditions at La Palma on 20 September 1999 were very favourable and part of the SVST observations obtained that day are discussed in Rouppe van der Voort (2002, hereafter Paper I) to which the reader is referred for more details on the observations and instrumentation.

The wings of the Ca II K line were selected for its broad depth-range in temperature sensitivity and the presence of a large number of isolated spectral line blends with varying line strengths that can be used as velocity diagnostics. Like described in Paper I, inversions of the Ca II K wings yield information on the temperature stratification of the atmosphere under the spectrograph slit, while the combined measurements of line-core Doppler shifts provide information on velocity gradients. However, this paper focuses on the velocity measurements of the line blends only.

During part of the time interval (starting from 14:10 UT), the Dutch Open Telescope (DOT, Hammerschlag & Bettonvil 1998) observed the same sunspot with a G-band filter. This frame-selected filtergram series has a cadence of 20.3 s, which matches the spectrogram cadence more closely than the SVST G-band series (frame rate was 1 per 60 s) and was included in this work to study the photospheric penumbral scene around the location covered by the spectrograph slit. More details about the DOT observations can be found in Sobotka & Sütterlin (2001) and Balthasar et al. (2001).