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Astron. Astrophys. 333, 1053-1068 (1998)

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2. The observations

The ASP provides precise Stokes spectro-polarimetry from which it is possible to extract quantitative measures of the vector field in the photosphere. This is accomplished through the interpretation of the full state of polarization (the Stokes vector) in spectrally resolved line profiles observed simultaneously in Fe I 630.15 and 630.25 nm: members of the same multiplet which differ in both Zeeman splitting and overall line strength. A description of this instrument is given by Elmore et al. (1992); detailed descriptions of the data and its analysis may be found in Lites et al. (1993, 1994a), and discussion of the calibration of these data is presented in Skumanich et al. (1997b). We use a least-squares (hereafter LSQ) line profile fitting technique, based on the Rachkovsky analytic solution to the transfer equations of polarized radiation in a Milne-Eddington model atmosphere, to extract measures of the vector field from these polarization data, as described in prior papers for analysis of ASP data cited herein. Using analysis of both simulated and actual data, Westendorp Plaza et al. (1997) have demonstrated recently that the LSQ technique provides useful measures of field strength well below that corresponding to a splitting roughly equal to a typical Doppler width in the 630.25 nm line ([FORMULA] 600 G). Even for longitudinal fields having strengths as low as 200 G and a filling factor of about 0.2, one expects errors in the field strength typically of order 50% for the signal/noise ratio [FORMULA] typical of ASP observations. Errors in the field strength are of similar order for transverse fields of strength 300 G and a filling factor of 0.3. This is considerably better than our prior expectations founded on the belief that meaningful field strength measurements would result only when the Zeeman broadening is greater than the typical Doppler width of the lines. These tests strengthen our confidence in the results of our inversion as applied to weak field regions. The Westendorp Plaza et al. findings support the inferences of the present study of emerging flux, and of prior measurements of quiet internetwork regions (Lites et al. 1996), where often we recover field strengths of a few hundred Gauss.

The data presented herein consist of sequences of spatial maps of Stokes spectra from which we extract two-dimensional images of magnetic field and thermodynamic parameters. After each spatial map, we recorded a sequence of monochromatic images of the spectrograph slit ("slit-jaw" images) as analyzed by the Universal Birefringent Filter (UBF - Beckers et al. 1975). The wavelengths and polarization samples of each image may be selected by the observer. The default set used here includes on- and off-band [FORMULA] (0.05 nm to both red and blue of line center) and continuum at 630.1 nm. From these data we are able to construct crude Dopplergrams of chromospheric motions in [FORMULA].

Table 1 catalogs observations of the three young bipolar active regions (hereafter Regions 1-3) that are the subject of the present investigation. They are young enough to show active emergence of flux in the form of horizontal magnetic fields between the more vertically oriented leading and following polarities. An examination of the record from Solar Geophysical Data for these three regions reveals that none survived into the following rotation. Their history may be summarized as follows:


Table 1. Summary of observational data

Region 1: NOAA 7195 (1992 11 June): Our observations of this region are from its first day of appearance, as it was not visible in either magnetograms or white light on previous days.

Region 2: NOAA 7551 (1993 July 23): This region first appeared on the limb on 1993 July 18, and was visible on that day in white light, magnetograms, and in Yohkoh X-rays. It grew considerably during the 24 hours preceding the observations reported here. The sunspots of this region were gone by July 26, but the region continued to be visible in magnetograms and Yohkoh X-rays until its passage beyond the west limb. This region is somewhat older than the other two regions presented. We present certain aspects of the the vector field measurements distinguish it from the other two.

Region 3: NOAA 7781 (1994 Sept. 22-27): The region is first apparent in Mt. Wilson magnetograms on Sept. 21, one day before the start of our observations. Yohkoh SXT images show some evidence of the region on Sept. 20. We observed the development of this region during six consecutive days. This sequence covers the early stages of formation of a pore which develops over the subsequent days into a small sunspot having a penumbra. We are also able to follow the dissolution of another small sunspot in this region, and to track the history of magnetic flux within the confines of the field-of-view (FOV).

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998