Abstract
The aim of this paper is to investigate whether there are any 11-yr or quasi-biennial solar cycle-related variations in solar rotational splitting frequencies of low-degree solar p modes. Although no 11-yr signals were observed, variations on a shorter timescale (~2yrs) were apparent. We show that the variations arose from complications/artifacts associated with the realization noise in the data and the process by which the data were analyzed. More specifically, the realization noise was observed to have a larger effect on the rotational splittings than accounted for by the formal uncertainties. When used to infer the rotation profile of the Sun these variations are not important. The outer regions of the solar interior can be constrained using higher-degree modes. While the variations in the low-l splittings do make large differences to the inferred rotation rate of the core, the core rotation rate is so poorly constrained, even by low-l modes, that the different inferred rotation profiles still agree within their respective 1sigma uncertainties. By contrast, in asteroseismology, only low-l modes are visible and so higher-l modes cannot be used to constrain the rotation profile of stars. Furthermore, we usually only have one data set from which to measure the observed low-l splitting. In such circumstances the inferred internal rotation rate of a main sequence star could differ significantly from estimates of the surface rotation rate, hence leading to spurious conclusions. Therefore, extreme care must be taken when using only the splittings of low-l modes to draw conclusions about the average internal rotation rate of a star.
Highlights
The frequencies of the Sun’s acoustic (p-mode) oscillations vary throughout the solar cycle with the frequencies of the most prominent modes being at their highest when solar activity is at its maximum (e.g. Woodard & Noyes 1985; Palle, Regulo & Roca Cortes 1989; Elsworth et al 1990; Salabert et al 2004; Chaplin et al 2007; Jimenez-Reyes et al 2007)
The aim of this paper is to investigate whether there are any 11-yr or quasi-biennial solar cyclerelated variations in solar rotational splitting frequencies of low-degree solar p modes
We have analyzed the p-mode rotational splittings observed by the Birmingham Solar-Oscillations Network (BiSON; Elsworth et al 1995a; Chaplin et al 1996) during the last two solar cycles in their entirety i.e. from 1986 April 14 to 2010 April 7
Summary
The frequencies of the Sun’s acoustic (p-mode) oscillations vary throughout the solar cycle with the frequencies of the most prominent modes being at their highest when solar activity is at its maximum (e.g. Woodard & Noyes 1985; Palle, Regulo & Roca Cortes 1989; Elsworth et al 1990; Salabert et al 2004; Chaplin et al 2007; Jimenez-Reyes et al 2007). No noticeable 11-yr solar cycle changes in the p-mode splittings of low-l data have been observed to date Salabert et al (2011a) observed mid-term variations in the rotational splittings of low-l modes. Mixed modes have been detected in stars that have evolved off the main sequence and the splittings of mixed modes have been used to infer the rotation rate of stellar cores (Beck et al 2012; Deheuvels et al 2012, and references therein). Here we consider only the inferred average internal rotation rate of main sequence stars, like the Sun, that do not have detectible g modes and mixed modes.
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