A computer-generated image of some of the 10 million modes of acoustic waves on the Sun (red indicating receding wave fronts, and blue approaching).
It is incomprehensible to
many people that the Sun’s surfacecould
be undergoing vibrations in frequency that could be used as a diagnostic tool.
This may be due to the phenomenon of solar oscillations being a relatively new
field, mainly developed in the last forty years. It allows hitherto unknown
tools to be applied to solar observations and predictions. In this chapter we
will survey the basic physics associated with these observations.
The first reckoning of non-radial
oscillations arrived ca. 1968 with the work of Frazier who made two-dimensional
plots of wavenumber vs. frequency or (k -w) diagrams. Several peaks of amplitude were
found and it was suggested that these corresponded to the fundamental and first
overtones for the solar envelope. Interestingly the patterns of solar oscillations - namely the acoustic or "p-modes" resemble those detected on drum heads by computer holography.
The Sun is clearly not a drum head, but it seems to behave like one in terms of its oscillations.Solar physicists are particularly interested
in what are called p, g and f modes given they are resonant modes
of oscillation. The p-modes are
basically for acoustic or sound waves, the g modes are for internal gravity
waves and the f modes are for surface gravity waves.The spherical harmonic function, also
peculiar to atomic physics, but here most applicable to the p-modes in the
solar oscillations context, is given by:
ynℓm=R n (r)Y ℓm
for normalized spherical harmonics:
ℓm (q,j) =
(1)m [2 ℓ +1 (ℓ - m)!/ 4n (ℓ +m)!] ½ P ℓm
(z ) exp (i m j),
z =cos qandw = 2 pnwhere n = n
the frequency of oscillation of mode n, ℓ, m. (Note: j is measured from the
meridian of the ascending node of the Sun’s equator.)
Thus, R n (r)is
applicable to radial patterns (with n the radial quantum number) whereby for a
given value of n we elicit a pattern of radial nodes, for which the position is
determined by the exact pattern of the functionR n
(r)The rest of ygives the surface pattern as a spherical
harmonic of the oscillation. The spherical harmonic, e.g.
Y ℓm (q,j),
determines the angular dependence of the eigenfunctions and hence the surface
distribution of the oscillation amplitudes, i.e. as seen by an observer.
The letters n, m and ℓ denote numbers whose meanings should be further
clarified. The first is the radial order
or the number of nodes in the radial direction. The second is the harmonic
degree or azimuthal order which
indicates the number of nodes around the equator on the three dimensional
spherical surface. Finally we have the angular degree or the number of nodes
from pole to pole, e.g. along longitude or meridian lines. The difference(ℓ- m) is also of interest as it yields the lines corresponding to parallels
of latitude. Any given combination of the numbers n, m and ℓallows a
unique frequency n to be computed. For example, if we have n=
14, m = 16 and ℓ = 20 one gets a period of 340.61 s
n =2p/ T=2p/ (340.61 s) = 2.935 x 10 -3 /s
Radial oscillations alone have ℓ=0
and we see in this case the associated Legendre function (P ℓm (q )) has:
nℓm =1 c/sthen:Y ℓm (q,j) = 1 and ynℓm=R n (r)
degree ℓ of the spherical harmonic can assume any
integer value, i.e.:
ℓ=0, 1, 2, …….
each such ℓ the azimuthal number m assumes a 2 ℓ +
1 value, i.e.
ℓ, (-ℓ +1)....0......( ℓ - 1), + ℓ
the frequency of a particular mode is given by the azimuthal eigenvalue m, and
the meriodonal eigenfunction ℓ - together with n.
Since m= 2 ℓ + 1, then the spherical surface is split into 2 ℓ + 1regions.
We are most interested in the Lamb frequency, e.g.
L=Ö(ℓ(ℓ + 1) c s2 / r2 )
And the Brunt- Vaisala or B-V frequency:
N 2 =g [G1 (d ln po/dr) - d ln ro/dr) ]
can be simplified to:
N 2 =g2 / c s2(g- 1)
holds expressly for the isothermal case.)
Wherec s2= G1po/roand:
G1= d (n lnp) / ln r
Example Problem (1):
Find the Brunt- Vaisala
frequency for the isothermal case if the sound speed in the Sun is c s = 900
m/s. (Take g =5/3 and g =273
N 2 =g2 / c s2(g- 1)
=Ö(273 ms -2 /900 m/s) [ 5/3 – 1] ) = 0.372 /s
Example Problem (2):
Find the Lamb frequency
for the same sound speed, if ℓ = 100 and r
= 2650 km.
use:L=Ö (ℓ(ℓ + 1) c 2 / r2 )
Where r = 2650 km= 2.65 x10 6m
L=Ö100(100 + 1)
(900 m/s) 2
/ (2.65 x10 6m ) 2
L= 3.4 x10 -2
/ s= 340 microhertz
It is of interest to note that only waves
with the longest horizontal wavelength ℓreach the core of the Sun while high ℓ-modes do not penetrate the
convective zone. Givenℓ = 100 we can
expect the example chosen will be near the solar surface. Also of interest in this context is the acoustic cutoff frequency, defined:
w ac=g g/ 2c
This should not be
confused with the plasma cutoff frequency, e.g. w cfor EM waves in plasmas. But there is one
common attribute for both: a cutoff frequency is for any frequency for which
the wave number k ® 0.
In typical k -w diagrams, with frequencywalong the ordinate and k
along the abscissa. One would see the p-modes in the upper left lying
abovew acand the g-modes (gravity
modes) at lower right below a dotted line for N. Another line given is for ckh which represents the Lamb waves or f-modes.How many total modes, with n, ℓ and m
distinct operate in the Sun? This is not difficult to estimate. Let’s take n
first. According to diagnostic diagrams showing “ridges" for oscillatory power at each frequency at least 20 have been observed. In the diagram shown below, the spikes or
ridges for the p-mode represent the first harmonic and the baseline smooth
curve from which they project is the fundamental.
This leads to a maximum radial order of n = 20 for the p-mode
associated ridges.. Now, for each of these n values, at least 500 angular
degrees ℓ have been observed. We also know that for each such ℓ there are at
least 2 ℓ values (actually 2 ℓ + 1). So in this case: 2 ℓ = 2(500) = 1000. Then
the total estimated modes at any given time works out to:
x 500 x 1000 =10 7
Or, ten million modes, all overlapping in
time and space. Because of this extreme multiplicity of modes, photographs of
the solar surface appear featureless or more accurately like a disk of coarse
sandpaper. Bear in mind also what we’d observe at the solar surface are
reflections of the multitude of standing sound waves (p-waves) that fill the
Sun’s interior. Not surprisingly, these match up quite well with the coarse
solar granulation, e.g
However, it has been Dopplergrams making use of the Michelson
Doppler Imager (MDI) that have given us the best observation portal on the rising
and falling super cells known as supergranules.
It was Robert Leighton – of Babcock and Leighton solar dynamo
fame- who first observationally isolated the motions of solar granules and tied
their delay time for replacement with an average lifetime of five minutes. He
found a given granule’s velocity was equal and opposite tothe velocity at the same location some 2.5 minutes later. He deduced that
something on the Sun must be oscillating. The convectivecells don’t normally do so thus it had to be explained in an
Ultimately the problem of the solar 5-minute oscillations was resolved by
treating the Sun as a resonant cavity.In 1981, Leibacher and Stein showed that if one treated the Sun as a
resonant cavity one could expect the relationship:
= (n + ½)p / w
other words for the condition at which the sound speed equals the horizontal
phase velocity (w/k
h ) one expects acoustic wave reflection.
Duvall and Harveyreinforced this work by measuring the
frequency spectrum of this »300soscillation and found it applicable for ℓ-modes
less than 140, and radial modes R with order n = 2 to 26. Posing the degree ℓ-
in terms ofthe reflection radius r:
ℓ =-1/2+[ ¼ +4p2g2 r2 /c s2]
modes were thus established as being deep in the solar interior by matching all
the modes in a series of data using the above equation.
the acoustic cutoff frequency below which all waves
would be reflected if a sound speed of 900 m/s applies.
2) Sketch the spherical surface with the mode values m = 0 and ℓ = 2, and justify your sketch. Find the
associated Legendre function:(P ℓm (q )).
3) Let a period T= 340.61 s be associated with the Lamb frequency. What
would be the associated depth of reflection in the Sun, assuming ℓ = 20 and c=
4) For the
p-mode pattern shown find the associated Legendre function:(P ℓm (q )) if If q = p/4.
Duvall, T.L. and Harvey, J.W.: 1984, Nature, 310, 19.