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Astron. Astrophys. 317, 925-928 (1997)

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2. Data analysis

Data sets from the DIFOS photometer contain full disc solar intensities in the wavelength ranges:

  • around 550 nm with a bandpass of 100 nm,
  • around 750 nm with a bandpass of 100 nm and
  • from 400 nm to 1100 nm defined by the spectral sensitivity of the silicon photodiode.

They have a time resolution of 16 s. For 550 nm the data set starts only on March 25. The raw data are influenced by different disturbances, especially by variations of the photometer sensitivity in dependence on the temperature, degradation of the silicon photodiodes and different contributions from light scattered from the Earth's atmosphere into the measuring device.

The data reduction included the following two steps:

  1. Calculation of the mean intensity during each orbit of the satellite. Therefore we used all measured values excluding 40% after leaving from and 30% before entering into the Earth's shadow (Fig.1).
  2. Elimination of the trend by use of polynomial approximations. In Fig. 2 one can find the resulting irradiance variation after a third order approximation for all channels.
Pflug et al. (1996) described this procedure in detail.

During the observational period we find a good correlation (coefficient 0.64) between the channels 400 - 1000 nm and 550 nm. The amplitudes are in both channels [FORMULA] 0.003. The channel 750 nm is less correlated which will be discussed later.

[FIGURE] Fig. 1. Example of DIFOS raw data over 10 satellite orbits from the window 400-1000 nm. The curve connects mean values of each orbit.

[FIGURE] Fig. 2. DIFOS irradiance variation. Relative intensities are represented in the channels 400-1000 nm (above), 750 nm (centre), and 550 nm (below) in dependence on the time. The trend is eliminated by use of a 3-rd order polynomial.
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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998