Left panel: example m-averaged power spectrum from filtered and tracked HMI data as a function of frequency ν and harmonic degree ℓ. To reduce random noise, an average is carried out over 30 individual daily power spectra observed in June 2010. The first visible ridge at low frequencies is the p1 modes. Right panel: histogram of mode power (blue) from the left panel as a function of ν/L, where L2 = ℓ(ℓ + 1). The corresponding radii of lower turning points from the ray approximation are indicated at the top. The dotted and dashed lines in both panels correspond to the modes that contribute most to our shallowest and deepest measurements in this work, respectively.
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