2 The theoretical method

2.1 The L-function method for emission line analysis

  The method used for this calibration study is a byproduct of the temperature and density diagnostic procedure shown in Landi & Landini (1997) and has already been applied by Landi et al. (1997) for determining the sensitivity of the NIS 1 instrument. As the adopted analysis method requires to determine the Differential Emission Measure (DEM ) of the emitting plasma as a preliminary step, the DEM diagnostic method described in Landi & Landini (1997) has been adopted in the present study. The reader is referred to these two papers for details on the analysis method and on its application to calibration of EUV spectrometers.

Lines of the same ion which occur in more than one detector are used for intercalibration between these detectors and the second order lines available in the GIS 3 and 4 sections are used to evaluate the ratio of the second order to first order efficiency for those two detectors.

In the present study the DEM functions are derived using lines coming from all the four GIS detectors, and therefore the determination of these curves provides a first indication of the relative intensity calibration of the spectral detectors.

For this kind of study high quality atomic data are required. In the present study the Arcetri Spectral Code (Landi & Landini 1998a) is used; this is an update of the older version (Monsignori Fossi & Landini 1995) using the whole CHIANTI database (Dere et al. 1997) and additional data for ions not yet included in CHIANTI. The ionization equilibrium of Arnaud & Raymond (1992) has been adopted for the Iron ions, while the Arnaud & Rothenflug (1985) ionization equilibrium data are used for the other elements. Ion fractions for the less abundant elements come instead from Landini & Monsignori Fossi (1991). The element abundances are taken from Feldman (1992).