  We present an adaptation of the rotation-corrected, m-averaged spectrum
technique designed to observe low signal-to-noise-ratio, low-frequency solar p
modes. The frequency shift of each of the 2l+1 m spectra of a given (n,l)
multiplet is chosen that maximizes the likelihood of the m-averaged spectrum. A
high signal-to-noise ratio can result from combining individual low
signal-to-noise-ratio, individual-m spectra, none of which would yield a strong
enough peak to measure. We apply the technique to GONG and MDI data and show
that it allows us to measure modes with lower frequencies than those obtained
with classic peak-fitting analysis of the individual-m spectra. We measure
their central frequencies, splittings, asymmetries, lifetimes, and amplitudes.
The low-frequency, low- and intermediate-angular degrees rendered accessible by
this new method correspond to modes that are sensitive to the deep solar
interior down to the core and to the radiative interior. Moreover, the
low-frequency modes have deeper upper turning points, and are thus less
sensitive to the turbulence and magnetic fields of the outer layers, as well as
uncertainties in the nature of the external boundary condition. As a result of
their longer lifetimes (narrower linewidths) at the same signal-to-noise ratio
the determination of the frequencies of lower-frequency modes is more accurate,
and the resulting inversions should be more precise.
