Helioseismology - Types of Solar Oscillations

Types of Solar Oscillations

Individual oscillations in the Sun are damped so that they die out within a few periods. However, interference between these localised waves produces global standing waves, also known as normal modes. Analysis of these overlapping modes constitutes the discipline of global helioseismology.

Solar oscillation modes are essentially divided up into three categories, based on the restoring force that drives them: acoustic, gravity, and surface-gravity wave modes.

• p-mode or acoustic waves have pressure as their restoring force, hence the name "p-mode". Their dynamics are determined by the variation of the speed of sound inside the sun. P-mode oscillations have frequencies > 1 mHz and are very strong in the 2-4 mHz range, where they are often referred to as "5-minute oscillations". (Note: 5 minutes per cycle is 1/300 cycles per second = 3.33 mHz.) P-modes at the solar surface have amplitudes of hundreds of kilometers and are readily detectable with Doppler imaging or sensitive spectral line intensity imaging. Thousands of p-modes of high and intermediate degree l (see below for the wavenumber degree l) have been detected by the Michelson Doppler Imager (MDI) instrument aboard the SOHO spacecraft, with those of degree l below 200 clearly separated and higher degree modes ridged together. About 10 p-modes below 1.5mHz have been detected by the GOLF instrument aboard the SOHO spacecraft.

• g-mode or gravity waves are density waves which have gravity (negative buoyancy of displaced material) as their restoring force, hence the name "g-mode". The g-mode oscillations are low frequency waves (0-0.4 mHz). They are confined to the interior of the sun below the convection zone (which extends from 0.7-1.0 solar radius), and are practically inobservable at the surface. The restoring force is caused by adiabatic expansion: in the deep interior of the Sun, the temperature gradient is weak, and a small packet of gas that moves (for example) upward will be cooler and denser than the surrounding gas, and will therefore be pulled back to its original position; this restoring force drives g-modes. In the solar convection zone, the temperature gradient is slightly greater than the adiabatic lapse rate, so that there is an anti-restoring force (that drives convection) and g-modes cannot propagate. The g modes are evanescent through the entire convection zone, and are thought to have residual amplitudes of only millimeters at the photosphere, though more prominent as temperature perturbations. Since the '80s, there have been several claims of g-mode detection, none of which have been confirmed. In 2007, another g-mode detection was claimed using the GOLF data. At the GONG2008 / SOHO XXI conference held in Boulder, the Phoebus group reported that it could not confirm these findings, putting an upper limit on the g-mode amplitude to 3 mm/s, right at the detection limit of the GOLF instrument. Finally, the Phoebus group has just published a review over the current state of knowledge on the solar g modes.

• f-mode or surface gravity waves are also gravity waves, but occur at or near the photosphere, where the temperature gradient again drops below the adiabatic lapse rate. Some f-modes of moderate and high degree, between l = 117 and l = 300, (see below for the wavenumber degree l) have been observed by MDI.