Low-Frequency Solar p Modes in GONG and MDI Observations using m-Averaged Spectra

Details Low-Frequency Solar p Modes in GONG and MDI Observations using m-Averaged Spectra Low-Frequency Solar p Modes in GONG and MDI Observations using m-Averaged Spectra

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsp41a..06l&link_type=abstract

American Geophysical Union, Spring Meeting 2008, abstract #SP41A-06

Physics

7522 Helioseismology

Scientific paper

The GONG and MDI global helioseismology pipelines provide solar acoustic mode parameters for 108- and 72-day time series respectively by fitting the 2 ℓ + 1 individual-m spectra of a given (n, ℓ/) multiplet either individually (GONG) or simultaneously (MDI). Our knowledge of the variable solar interior through helioseismic observations derives primarily from these two analysis pipelines. We have developed a new method to extract the mode parameters by adjusting the rotation- and structure-induced frequency shift for each m-spectrum to minimize the mode width in the m-averaged spectrum. The m-averaged spectrum appears to be a powerful tool for low signal-to-noise-ratio modes in the low-frequency range where the modes have very long lifetimes. Indeed, in the case of spatially-resolved helioseismic data (MDI, GONG, HMI), for a given multiplet (n, ℓ/), there exist 2 ℓ + 1 individual-m spectra, which can result in an average spectrum with a SNR ≫ 1 even when the individual-m spectra have a SNR < 1. We show here that the m-averaged spectrum technique, applied to the GONG 108-day and MDI 72-day time series, gives us access to a whole new range of predicted, low-SNR modes that had not been successfully fitted by the current MDI and GONG peak-fitting pipelines. We show that the modes that are measured by both techniques are extracted without bias. We apply this technique to 360-, 720-, 1080-, and 1440-day long GONG time series to infer the variability of the mode parameters with solar activity in the low-frequency range below ~ 1500 μHz.

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