Unlike the more easily detectable p-modes that use pressure as the restoring force, g-modes are waves of dense material that use gravity as its restoring force. The power these g-mode waves require to oscillate comes from adiabatic expansion in the stellar interior. A star that could produce a gravity mode would have a core with a very poor temperature gradient. When a hot gas packet begins to move upwards away from the hot center it starts to cool and when a gas cools it becomes denser. This cooling gas becomes dense enough that the buoyant force that caused it to rise is now too small to overcome the gravitational force of the stellar core and the gas begins falling back towards the denser center. This makes gravity the restoring force that restores the density wave back to its starting place. These oscillations happen deep in the stellar interior and are extremely difficult to detect. This initial adiabatic expansion and the restoring gravitational pull from the core result in this oscillating movement of large amounts of gas in the interior, and this is what drives g-modes. Because this process happens deep in the stars interior, the oscillations weaken dramatically as they pass through the outer convective region which acts as a dampening cushion in a similar way to how grabbing a tunning fork quickly dampens out the fork’s vibrating oscillations. Unlike p-modes, which often have much larger amplitudes at much higher frequencies making them more easily detectable, these types of g-modes will only produce small amplitudes given they are strong enough to reach the surface.

One of the big challenges faced with detecting g-modes is lowering the upper limit for g-mode amplitudes. The upper limit defines the current possible range of a potential g-mode amplitude for a given a mode frequency. The current predicted theoretical amplitude for a low order high frequency solar g-mode is estimated to be \(~{}{10}^{-2}\)cm/s and \(~{}{10}^{-3}\)cm/s for moderate to high order modes \cite{Kumar_1996}. Current upper limits for g-mode amplitude are under 10 mm/s and vary based on the instrument and length of the time series. The GOLF instrument on board the SOHO spacecraft is roughly 6.5 mm/s because its data set was taken over six years. BiSON has calculated an even lower upper limit of 3.5 mm/s from a ground based instrument using a nine year time series. The upper limit comes from a probability limit calculated in \cite{Appourchaux_2000}. The upper limit is important because instrumental observations return large amounts of noise and to hastily lower the the probability limit and thus the upper limit would mean accepting more peak values that could just be solar noise and potentially missing real g-mode peaks \cite{Appourchaux_2003}. Lowering the upper limit is extremely advantageous to helping find an actual g-mode as long as it is done carefully. A lower upper limit means a smaller range of data that must be processed and a higher likelihood of finding a g-mode. However, lowering the upper limit has been a very slow process. Alternatives to drastically lowering the upper limit will be discussed in the Future Observational Prospects section.