Solar core rotation from low-degree BiSON p-mode splittings: 1981-95

Details Solar core rotation from low-degree BiSON p-mode splittings: 1981-95 Solar core rotation from low-degree BiSON p-mode splittings: 1981-95

Nov 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996mnras.283l..31c&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 283, Issue 2, pp. L31-L35.

Astronomy and Astrophysics

Astronomy

3

Sun: Interior, Sun: Oscillations, Sun: Rotation

Scientific paper

In this paper, we present an analysis of the rotational splittings of low-degree solar p modes, as measured in full-disc helioseismological data collected by the Birmingham Solar-Oscillations Network (BiSON) between 1981 and 1995. These data provide coverage over solar activity cycles 21 and 22. Rotationally split multiplets were fitted in two 4-month and seven 8-month frequency spectra with a technique that minimizes a maximum-likelihood function consistent with chi^2 2-d.o.f. statistics. The fitted modes used in the analysis map the range 1<=l<=3 and 2.0<=nu<=3.1mHz. The lower limit to the analysed frequency range was determined by the poorer quality of the pre-1990 data; the upper, very conservative, cut-off was imposed in order to avoid using data where the linewidths of the modes begin to increase substantially above 1muHz. Global splitting averages (for 1<=l<=3) were computed over the quoted mode-frequency range by: (i) weighting each datum according to the scatter observed in the sectoral splittings as a function of l and (ii) weighting each datum according to its formal uncertainty, as computed by the mode-fitting procedure. The l-weighted averages show a slight decline over the period 1981-95: however, the gradient is only significant at the ~2sigma level (-3.8+/-1.8nHz yr^-1). If one were to interpret any trends in the low-degree splittings as being indicative of a modification of the rotational behaviour of the solar core (for r/R_solar<=0.25), we would estimate that a ~60 per cent change in the core rotation rate is excluded at the 3sigma level by the l-weighted data. The `formally' weighted averages imply a very significant change with epoch. However, an analysis of the data appears to show that the formal uncertainties may somewhat underestimate the actual errors on the measured splittings, leading to an overestimation of the significance of any inferred change with epoch.

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