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
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
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.