1 Introduction: Instruments

The IRIS (International Research of Interior of the Sun) operation started at Kumbel, Uzbekistan, on July 1st, 1989. The observations consist of a time series of measurements of the solar line-of-sight velocity integrated over the solar surface. The instruments employ a resonant sodium cell spectrophotometer observing the D1 589.6 nm spectral line. The full-disk integration gives access to low degree modes, with $l\leq3$. This data set has been used to constrain solar internal structure and rotation through the precise measurement of low degree frequencies (Serebryanskiy et al. 2001; Gelly et al. 1997) and frequency splittings (Gizon et al. 1997; Lazrek et al. 1996; Loudagh et al. 1993) or an accurate measurement of the solar acoustic cutoff frequency (Fossat et al. 1992a). However, poor instrument reliability in combination with logistical and manpower difficulties has made the network unable to achieve annual duty cycles above 50%. One approach to improve the duty cycle of the IRIS network is to develop several cooperations with teams running similar observational programs. Gelly et al. (1998) tested the merging of the sodium IRIS data with the potassium (769.9 nm) resonance data from the Mark-1 instrument at Tenerife first, and subsequently with the Mauna-Loa LOWL instrument. They concluded that it is reasonably possible to include "alien'' data inside the IRIS time series. This paper describes the merging of IRIS with these two potassium data sets, resulting in an IRIS⁺⁺ network with a potential of 9 observing sites (see Fig. 1). It shows the significant improvement of the duty cycle, as well as the reasonably good quality of this IRIS⁺⁺ database, which is now freely available (http://www-astro.unice.fr/iris/).

Figure 1: IRIS++ network sites.

The Mark-1 data sets have been prepared by P. Pallé and S. J. Jiménez-Reyes. Similar to IRIS, Mark-1 (Brookes et al. 1978) is a full-disk instrument using a potassium resonance cell which is part of the Birmingham BISON network. 2894 days of Mark-1 data have been merged with the IRIS sodium data. On the other hand, the LOWL instrument is a Magneto-Optical Filter (MOF) also using a potassium resonance cell but providing Doppler images with modest spatial resolution (25 arcsec) (Tomczyk et al. 1995). The LOWL operation started in February, 1994. Before merging LOWL with full-disk IRIS and Mark-1 data, each velocity image has been integrated after apodizing with a sodium-like limb darkening function in order to make the integrated velocity signals comparable as to the relative sensitivity to the various degrees. The merging of LOWL was a very important step for improving the duty cycle because of the longitude of Hawaii, located near the center of the largest mean daily gap of the network. 1615 individual days of LOWL data, obtained and prepared by T. Corbard and S. Tomczyk, have been merged with IRIS and Mark-1.