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In the case of spatially-revolved helioseismic data (such as MDI, GONG, HMI),the usual mode-fitting analysis consists of fitting the 2l+1 individualm-spectra of a given multiplet (n, l) either individually or simultaneously.Such fitting methods fail to obtain reliable estimates of the mode parameters(frequency, splitting, ...) when the signal-to-noise ratio (SNR) is low, whichmakes those methods not suitable when one wants to look at the low-amplitude,long-lived solar p modes in the low-frequency range. Instead, Salabert et al.(2007) developed a new method to extract the mode parameters by adjusting therotation- and structure-induced frequency shift for each m-spectrum to minimizethe mode width in the m-averaged spectrum (a "collapsogram"). The m-averagedspectrum technique, applied to the spatially-resolved GONG and MDI data,appeared to be a powerful tool for low SNR modes in the low-frequency range.Another possibility to increase the SNR is to combine data from differentinstruments (Garcia et al. 2004a). We present here an adaptation of bothtechniques: the "collapsograms" applied to a combination of observations from aSun-as-a-star instrument, GOLF, and a disk-imaged one, GONG.

Low-frequency solar p modes as seen by GOLF and GONG instruments