5.2 Early low-degree helioseismic results

Around the early 1970s there were numerous attempts to search for global p-mode oscillations, with interest at first focusing on longer-period oscillations, the low-order, low-degree modes. Various theoretical predictions (Scuflaire et al., 1975Iben Jr, 1976Christensen-Dalsgaard and Gough, 1976) of the periods were available, offering the hope that global oscillations could be used to probe the rotation and structure deep inside the Sun. At first most of the results (Livingston et al., 1977Musman and Nye, 1977Grec and Fossat, 1977), were negative, except for the 160-minute period of Severnyi et al. (1976); Brookes et al. (1976), which was later (Elsworth et al., 1989) determined to be spurious and will not be further discussed here. The SCLERA group (Brown et al., 1978Hill and Caudell, 1979Caudell and Hill, 1980) found a variety of longer-period fluctuations in their solar-diameter data, but these results were not universally accepted; for example Fossat et al. (1981a, see also references therein) claimed that the SCLERA results were consistent with pure noise.

Low-degree helioseismology became a reality when the Birmingham group (Claverie et al., 1979) identified oscillations in the five-minute frequency band in integrated sunlight as low-degree global modes, using observations from Tenerife and Pic du Midi during the summers of 1976 – 1978; these initial data were adequate only to identify the spacing between modes of the same l and different n, without resolving separate l = 0 and l = 1 peaks.

A French–American team (Grec et al., 1980Fossat et al., 1981bJump To The Next Citation Point) obtained five days of continuous observations at the South Pole in the austral summer of 1979 – 1980, and were able to identify peaks of degree 0, 1, 2, and 3 and even a weak l = 4 peak by superposing sections of the acoustic spectrum with different radial order. These modes were identified as being of radial order around 12 – 30, as opposed to the very low-order modes that had been sought in the low-frequency spectrum; both the noise characteristics of the spectrum and the low amplitude of the lower-order modes mean that the fundamental (l = 0,n = 1) mode remains unobserved to this day, although some low-degree modes with single-digit n have been identified (Chaplin et al., 1996b).

Soon, the Birmingham team (Claverie et al., 1981Jump To The Next Citation Point), using 28 days of integrated-sunlight data from the Tenerife site and an analysis that involved “collapsing” segments of the acoustic spectrum so as to average together modes of the same degree and different radial order, reported finding three rotationally split components in the l = 1 modes and five in l = 2, with an average separation of 0.75 μHz. If correct, this would have implied a solar core rotation substantially faster than the surface. Isaak (1982) suggested that the excess component peaks (when two and three would be the expected number for l = 1 and l = 2 respectively) could be explained if the solar core were rotating on an oblique axis and had a very strong magnetic field; this idea, which was also mooted by Dicke (1983) to explain an oscillation of about half the solar rotation period seen in the oblateness data (Dicke, 1976), was rebutted in some detail by Gough (1982).

Fossat et al. (1981bJump To The Next Citation Point) reported that initial results from 5 days of low-degree observations at the South Pole suggested quite short lifetimes, about 2 days; the l = 0 peaks appeared narrower than those of l = 1 and l = 2. Grec et al. (1983) later identified about 80 normal modes in the South Pole data, but did not confirm the Claverie et al. (1981Jump To The Next Citation Point) rotational splitting result, instead reporting that the l = 1 peak seemed too narrow to accommodate the reported splitting.

Claverie et al. (1982) reported a periodicity of approximately 13 days in the radial solar velocity, as measured using the resonant-scattering technique and the potassium D-line, and interpreted this as an effect of the solar core rotation; however, this effect was quickly explained away (Durrant and Schröter, 1983Andersen and Maltby, 1983Edmunds and Gough, 1983Duvall Jr et al., 1983) as an artifact caused by the rotation of surface features – sunspots and plage – across the disk.

Meanwhile, the low-degree five-minute acoustic spectrum had also been observed using the Active Cavity Irradiance Monitor (ACRIM) aboard the Solar Maximum Mission (SMM) spacecraft (Woodard and Hudson, 1983b). Woodard and Hudson (1983a) agreed with Fossat et al. (1981b) in finding that the modes had lifetimes of about two days, too short for the rotational splitting reported by Claverie et al. (1981) to be real.

Later work (Libbrecht, 1988aElsworth et al., 1990bChaplin et al., 1997) revealed that the width of the peaks – inversely proportional to the mode lifetimes – was strongly dependent on frequency across the five-minute spectrum, with lifetimes of a few days in the middle of the five-minute band and weeks or months at low frequencies where, unfortunately, the amplitudes of the modes are also small. Reliable direct measurement of the low-degree splittings would have to wait for some years, while sufficiently long, high-quality time series of data accumulated.

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