2. Observational data
The observations described here were taken during June to August 1989 with the UVSP instrument on board the SMM satellite. During this time a number of different active regions were observed. The particular active regions which are of interest to us in this work, their UVSP experiment numbers and the time and date at which the rasters were taken, are given in Table 1.
The UVSP instrument (Woodgate et al., 1980) consisted of a Gregorian telescope, a polarimeter consisting of two retarders and a linear polarizer, an Ebert-Fastie spectrograph and five photomultiplier detectors (four used in second order, 1170-1780Å and one in first order, 1780-3600Å). Unfortunately from July 1985 onwards the wavelength drive of UVSP was inoperable and all 2nd order UVSP observations were restricted to a wavelength in the 1373.5-1375.8Å range (Henze, 1993). The observations discussed in this paper were obtained using a slit with an exit width of 2.3Å. The above spectral window does not contain strong ultraviolet lines. According to the quiet sun model of Vernazza et al. (1981), the continuum in this region is due to Si I which is found in the lower chromosphere at a height above the photosphere of 600-800 kilometers ( ; we return to this point later). Unfortunately, there was no simultaneous ground-based coverage, although for some of the days involved, either magnetograms or He I 10830Å images are available for times within a few hours of the UVSP data. An example of this is given in Fig. 1 where a Kitt Peak magnetogram (taken on 19 July '89 17:54 UT) is over-plotted with the UVSP data which started at 13:12 UT July 19.
For all the UVSP experiments analyzed here, the field of view used was , rastered in steps of 5 arcsec. The individual pixel size was which means an overlap of 5 arcsec from one pixel to the next in the raster. The time taken for a typical raster of pixels was 22 seconds, with each pixel having an integration time of 0.112 seconds. Including overheads, the typical gate time was 0.128 sec. A typical experiment involved repeated rastering of the region of interest, which implied for a 129 point raster a total duration of 45 minutes. Normally, the last few rasters could not be used.
This work is a followup to an earlier analysis by Drake et al. (1989) who studied several datasets taken in July 1988. In that work, Drake et al. looked at three types of variability; flares, bursts and oscillations. It is the oscillation-type that is the main subject of the present work. This earlier work noted the presence of oscillations in the 3-5 min. range and concluded that these were of solar origin as opposed to artifacts due to instruments drifts. There are essential two main reasons why these can not be instrumental; (i) the entrance slit used was , thus only pointing drifts approaching 10 arcsec could produce such oscillations and these were very rare in the pointing, (ii) in the pixel raster not all the pixels showed this type of periodicity, normally a few adjacent pixels would show an oscillation while the next few pixels would have no periodic variability.
The different datasets given in Table 1 and the resulting 169 individual light curves (from the pixels) in each dataset were reduced by the Fourier analysis technique as outlined in Sect. 3.
In order to interpret the observed emission it is also useful to consider the emission expected from an optically thin plasma in collisional equilibrium. Using the spectral code SPEX (Kaastra et al., 1996) we determined the emissivity in the UVSP bandpass (1373.5Å-1375.8Å) under the assumption of solar photospheric abundances (Anders and Grevesse, 1989). The resulting emissivity (photons/sec.) as a function of temperature is shown in Fig. 2 for an assumed emission measure of . In order to get the observed flux we divide by with d the distance of the source. The observed emission consists only of continuum emission (the nearest line is that from OV at 1371.3Å). At temperatures below the main contribution to the UVSP continuum is by two-photon emission. Above free-free emission starts to dominate. Note that the vertical axis has a logarithmic scale. The emissivity has a strong peak at a temperature of and the main contribution, say 0.1 maximum, is in the temperature range . In the optically thin interpretation the photon flux scales with the density squared times the emissivity. The shape of the emissivity curves indicates that at temperatures lower than the photon flux is more sensitive to changes in the temperature while at higher temperature it is more sensitive to density variations.
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998