Systematic helioseismic observations stretch back nearly 30 years, as illustrated in the schematic chart in Figure 13.
Prior to the identification of global low-degree modes by Claverie et al. (1979), observing runs were usually short and carried out at a single site. However, the advantages of more extended observations (to obtain better frequency resolution), and of observations not modulated by the day-night cycle, were soon recognized. Grec et al. (1980), Duvall Jr and Harvey (1984), and Duvall Jr et al. (1984, 1986) carried out important observations at the South Pole during the Austral summer, but for long time series it is more practical to observe either from a network of sites spaced around the world, or from space.
Some of the first long-term sets of low-degree observations came from the Active Cavity Irradiance Monitor (ACRIM) experiment (Woodard and Noyes, 1985, 1988) aboard the Solar Maximum Mission spacecraft, which took helioseismic measurements in 1980 and 1984 – 1985, the Mark I instrument in Tenerife (Pallé et al., 1989), and the precursors of the Birmingham Integrated Solar Network (BiSON) (Elsworth et al., 1990a). Meanwhile, resolved-Sun observations were carried out at the South Pole by Duvall and collaborators, and by various other observers in the USA; these observations will be discussed in more detail later.
Libbrecht and Woodard (1990) observed the medium-degree modes from Big Bear Solar Observatory (BBSO) in the 1986 – 1990 rising phase of solar cycle 22. The first observations from widely separated sites were carried out by the Birmingham/Tenerife group in 1981 (Claverie et al., 1984), and by 1992 the six-station BiSON network was complete; it has been operating ever since. Another network of integrated-sunlight instruments, the French-based IRIS (Fossat, 1995), operated from 1989 – 2003.
The Global Oscillation Network Group (GONG) (Harvey et al., 1996) has been collecting continuous, high-duty-cycle observations of the medium-degree modes since 1995, using a six-station worldwide network, and the Michelson Doppler Imager (MDI) instrument (Scherrer et al., 1995) aboard the SOHO spacecraft has been in operation since 1996, so that these two projects have essentially complete coverage of solar cycle 23. SOHO also carries instruments dedicated to the study of low-degree oscillations; LOI (Luminosity Oscillations Imager) (Fröhlich et al., 1995), and GOLF (Global Oscillations at Low Frequencies) (Gabriel et al., 1995). This wealth of high-quality data has given us the opportunity to study the solar interior rotation and its solar-cycle changes in more detail than ever before.
Also worth noting are the LOWL-ECHO project (Tomczyk et al., 1993), which made medium-degree observations from one or two sites from 1994 to 2004, and the high-degree Taiwanese Oscillations Network (Chou et al., 1995) deployed over the 1993–1996 period.
All these observations will be considered in more detail as we proceed to examine the results pertaining to the interior rotation.
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