2. Observational data
From the available intensity observations, we have chosen data from four instruments, SLOT and LOI-T, both ground based, and from the space experiments IPHIR and ACRIM.
The Solar Luminosity Oscillation Telescope (SLOT) is a four channel photometer built to measure disk integrated sunlight (Andersen et al. 1988a). Solar irradiance has been measured simultaneously at four different wavelengths using interference filters, at the Observatorio del Teide (Tenerife) from August 1984 to May 1989 (Jiménez et al. 1987, 1988, 1990). The best 37 days (7.4 hours around noon), with the smallest variance during the observational period, from the 500 nm channel (10 nm full width) have been selected for the current study. For an extensive discussion on the calculation of residuals see Jiménez et al. (1988) and Rabello Soares et al. (1994). Fig. 1 shows the irradiance residual data for a typical day. The daily power spectra ( = 37.56 µHz) are computed and the average of 37 such spectra shown in Fig. 2.
The LOI (Luminosity Oscillation Imager) photometric telescope is built following an Andersen et al.'s (1988b) original idea. It is one of the instruments included in the VIRGO experiment on-board the SOHO spacecraft. The Qualification Model (LOI-T) was installed at the Observatorio del Teide (Tenerife) in May 1994, and has been in operation continuously since then (Appourchaux et al. 1995a,b). It consists of a Ritchey-Chrétien Telescope (f = 130.7 cm) imaging the Sun through a 5 nm passband interference filter at 500 nm. The image is projected on a photodiode array detector made up of 12 scientific pixels and 4 guiding pixels (see Appourchaux et al. 1995c).
As the LOI-T instrument has some spatial resolution, it is possible to reduce the effects of the atmospheric extinction. The ratio of the sum of the 4 central pixels (30 of solar disk) to the sum of all 12 pixels (c = ) is used, instead of the solar irradiance obtained by the other experiments. This ratio reduces the effects of the earth's atmosphere (see Rabello Soares et al. 1995). Due to the lack of spatial and time correlation of the surface structures causing the solar background signal, this rationing increases the solar signal relative to the noise produced by the Earth's atmosphere. Fig. 1 shows the typical daily variation of the signal; its power spectrum is calculated using 7.6 hours for each day around noon. Finally, the average of the power density spectra of the best 27 days, selected as before, is obtained and plotted in Fig. 2. Note, in both figures, the substantial reduction of the (atmospheric) noise level as compared to SLOT data.
The InterPlanetary Helioseismology by IRradiance (IPHIR) measurements, flown on the PHOBOS mission to Mars, gathered long and uninterrupted solar irradiance time series. The IPHIR instrument, built at PMOD/WRC in Davos, was a three channel sunphotometer which measured the solar irradiance at 335, 500 and 865 nm (Fröhlich et al. 1988).
The data analysed in this study were obtained in the period from July to December 1988 of the 500 nm channel (5 nm full width). All the observations were divided into series of 23.4 hours obtaining a total amount of 160 consecutive series of 2048 points each. Fig. 1 shows the typical residuals for one of the series calculated as described by Schrijver et al. (1991). Unfortunately, the data exhibited a strong influence of the pointing errors of the spacecraft, and high-pass filters had to be applied before any further meaningful analysis could be done. Therefore, spectral data below 1500 µHz have not been used in this analysis. The filtered power spectra of each series was computed and the average value of the 160 spectra is shown in Fig. 2.
The ACRIM-I (Active Cavity Radiometer Monitor) data on board the SMM (Solar Maximum Mission) satellite (e.g. Fröhlich et al. 1991) were also used in this work, providing some information at the lower frequency range (below 90 µHz) obtained from orbital averaged data (Fröhlich & Delache 1984), and at higher frequencies (higher than 480 µHz) time series of the so-called "no-shutter" mode ( 3 months of data in late 1989) of the ACRIM were used (Hudson 1993). In Fig. 1 the orbital averaged data for May 13, 1980 series are shown, while Fig. 2 presents the power spectra (smoothed over 9 points) for years 1980 and 1985. These years correspond to maximum and minimum solar activity cycle, respectively (Fröhlich et al. 1991). The smoothed power spectrum at higher frequencies by Hudson (1993) is also shown in Fig. 2.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998