Solar p-mode frequencies at $\mathsf{\ell=2}$: What do analyses of unresolved observations actually measure?

Abstract

Abstract.We have studied in detail the extraction of estimates of = 2 p-mode frequencies from unresolved observations of the visible disc of the Sun. Examples of data of this type include ground-based observations made by the Birmingham SolarOscillations Network (BiSON), and space-borne observations made by the GOLF and VIRGO/SPM instruments on board the ESA/NASA SOHO satellite. The fitting of the modes is complicated in practice by the asymmetric arrangement in frequency of the three components (m = −2, 0 and 2) that are prominent in such data. In order to investigate the effect of this we used a series of 10-yr artificial datasets into which varying degrees of asymmetry were introduced. The sets were designed to mimic the characteristics of the BiSON and GOLF data, and were analyzed both with and without the BiSON window function from the period 1992 through 2001. Since reliable estimates of the asymmetry have only recently been extracted from unresolved observations (Chaplin et al. 2003a) it has for a long time been standard practice to fit the = 2 modes to a model that assumes a symmetrically arranged multiplet. We have tested the impact of this on the accuracy of the extracted frequencies. Furthermore, we demonstrate that asymmetric models can be successfully applied, provided the data are of sufficient length and quality. We also discuss the implications of our simulations for analyses of real solar data.

On the Spatial Dependence of Low-Degree Solar p -Mode Frequency Shifts from Full-Disk and Resolved-Sun Observations

“…The average monopole shift is seen to be weaker than that at the other , in each case at about the ᐉ 3 j level. The ratios agree, to within errors, with those uncovered in full-disk Birmingham Solar Oscillations Network (BiSON) data by Chaplin et al (2004b). They also confirm that given the modern quality and quantity of Sun-as-a-star data, it is now possible to uncover marginally significant -ᐉ dependence in the frequency shifts.…”

“…These ᐉ ≥ 2 FmF determine the extent to which each component influences the fitted frequency. Chaplin et al (2004c) considered in detail the case at when three components are seen. They showed ᐉ p 2 that if is the fractional contribution of the central (2, 0) S 2 component, the traditional model-fitted frequency can be represented as…”

“…Because of the differing nature of the observations, analyses of the two types yield different frequency estimates (see Chaplin et al 2004aChaplin et al , 2004c for a full discussion). While it is possible to extract from the resolved-Sun data estimates for each individual m, it is much more difficult to do the same from the full-disk observations.…”

“…An alternative approach is to use simulations of GOLF and BiSON-like data (Chaplin et al 2004a(Chaplin et al , 2004c (SFU) ᐉ p 2 2.86 ‫ע‬ 0.20 nHz (SFU) ᐉ p 3 are consistent within errors with the GOLF estimates in Table 1. GOLF and MDI are the result of the cooperative endeavors of many individuals, to whom we are deeply indebted. SOHO is a mission of international cooperation between ESA and NASA.…”

Impact of the Solar Activity Cycle on Frequency Separation Ratios in Helioseismology

“…In order to assess this we made predictions of the sizes of the underlying changes one would expect to see, based on observations of solar cycle shifts of individual m, from Michelson Doppler Imager data (Chaplin et al 2004a;Toutain & Kosovichev 2005) and information on the Sun-as-a-star fitting techniques (Chaplin et al 2004b). We also fitted power spectra made from Sun-as-a-star data, collected by the Birmingham Solar Oscillations Network (BiSON), in an attempt to uncover evidence of the predicted shifts.…”