Optimal masks for solar g-mode detection

The detection of gravity (g) modes of solar oscillations is important for probing the physical conditions in the Sun's energy-generating core. We have developed a new method of spatial masks optimized to reveal solar g-modes of angular degree l = 1-3 and applied it to Michelson Doppler Imager data in the frequency range of 50-500 μHz. These masks take into account the horizontal component of g-mode velocity eigenfunctions and the variations in the level of noise across the solar disk and adjust for the time-dependent mode projection properties caused by the inclination of the Sun's axis of rotation. They allow us to optimize the signal-to-noise ratio in the oscillation power spectra for potential g-modes of various angular order and degree. The peaks in the resulting spectra are analyzed in terms of their instrumental origin, long-term stability, and correspondence to the theoretically predicted g-mode spectrum. As a consequence of failing to detect any g-mode candidates, new upper limits for the surface amplitude of g-modes are obtained. The lowest upper limits in the range of 5-6 mm s^-1 are found for sectorial g-modes (l = m). These limits are an order of magnitude higher than the theoretical prediction of Kumar et al. in 1996.