Observational upper limits to low-degree solar g-modes

Observations made by the Michelson Doppler Imager (MDI) and Variability of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and Heliosph eric Observatory (SOHO) and by the groundbased Birmingham Solar Oscillation s Network (BiSON) and Global Oscillations Network Group (GONG) have been us ed in a concerted effort to search for solar gravity oscillations. All spec tra are dominated by solar noise in the frequency region from 100 to 1000 m u Hz, where g-modes are expected to be found. Several methods have been use d in an effort to extract any g-mode signal present. These include (1) the correlation of data-both full-disk and imaged (with different spatial-mask properties)-collected over different time intervals from the same instrumen t, (2) the correlation of near-contemporaneous data from different instrume nts, and (3) the extraction-through the application of complex filtering te chniques-of the coherent part of data collected at different heights in the solar atmosphere. The detection limit is set by the loss of coherence caus ed by the temporal evolution and the motion (e.g., rotation) of superficial structures. Although we cannot identify any g-mode signature, we have neve rtheless set a firm upper limit to the amplitudes of the modes: at 200 mu H z, they are below 10 mm s(-1) in velocity, and below 0.5 parts per million in intensity. The velocity limit corresponds very approximately to a peak-t o-peak vertical displacement of delta R/R. = 2.3 x 10(-8) at the solar surf ace. These levels which are much lower than prior claims, are consistent wi th theoretical predictions.