Solar modes of oscillation are standing sound waves, and are specified by three parameters, l, m and n. l gives the total number of surface node circles, m the number of surface node circles intersecting at the north (or south) pole, and n the number of nodes from the Sun's surface to its core. The velocity distribution at any one time is a combination of millions of these modes, with l values ranging from zero to over one thousand. They are detected from the tiny variations of the surface velocity field over the Sun's disk.
BiSON specifically observes low-l modes, i.e. l<=4. Our instruments use integrated sunlight, and so they measure the average surface velocity over the solar disk. The higher l modes have nearly equal areas of the disk which are receding and approaching, effectively cancelling out any velocity signal.
This can be seen by comparing the images above which show how the surface velocity varies over the solar disk for a pair of modes. The left image shows the l=36 m=24 mode, and the right image the l=1 m=1 mode. The redder a region, the faster it is receding, and the bluer, the faster it is approaching the observer. Node lines, where there is no net velocity, show up purple.
Over the course of a period of about five minutes, the blue areas will go through purple through to red and then back again. The red areas will likewise go through purple to blue and back again.
BiSON is sensitive only to low degree solar oscillation modes. Here we provide the modal patterns for four normal modes of degree l=3 and order m=0, 1, 2 and 3. Modes with m=0 are known as zonal or axisymmetric modes and all the nodal lines are lines of latitude. Modes with m=l are called sectorial modes, where nodal lines form lines of longitude. The amplitude of the oscillating modes have been greatly exaggerated so that the patterns are easily visible.
To download the videos please right click on the links below and choose "Save Target As". The videos use Indeo5 compression which should be available in most media players.