Page 32 - 2017Winter

Acoustics and Astronomy
The story of how the “mystery of the solar oscillations” was solved is en- tertaining but again somewhat long and involved. In the interest of space, I refer the reader to the literature. My favorite popular book is the ex- cellent Sunquakes: Probing the Inte- rior of the Sun by J. B. Zirker (2003). It has both history and science and is a wonderful introduction to the modern field of “helioseismometry.”
So, cutting to the answer, What were
the oscillations? It is found that the
sound, which is produced by the
turbulent motions in the convec-
tive zone, is trapped in the convec-
tive zone! Just below the top of the
convective zone (Figure 2), the gas
temperature drops off quickly, which
causes the upward moving sound to
be reflected back down into the con-
vective zone. At depth below, the speed of sound and tem- perature increase drastically, and this refracts downward- going sound back upward into the convective zone. The sound is thus trapped above and below. This creates verti- cal (radial) normal modes, a phenomenon well-known in earthly acoustics and seismics. However, the sun is a sphere, and any such modes also need to be quantized, positively re- inforcing modes in the north-south and east-west directions as well. These modes are well known from physics and are the so-called “spherical harmonics.” In terms of the ray pic- ture in Figure 2, this corresponds to rays that circle around the sun an integer number of times.
There are a few interesting factoids to add to this picture. First, because the sun is a fluid, it cannot support shear waves, and so this is a pure acoustics problem (the p-mode in Figure 2 is the pressure mode). Second, underneath the convection zone, in the radiative zone and core, the restor- ing force for wave motion is gravity (the buoyancy force; g- mode), not pressure, and so these are not acoustic waves but are instead like the internal wave motions seen in the earth’s ocean. These g-mode waves are damped out in the convec- tive zone and so are not seen at the surface, making it hard to get information about the core. However, recently, these faint waves may have been observed, suggesting that the core rotates every seven days, which is much faster than the radiative and convective zones. Finally, it seems that there
30 | Acoustics Today | Winter 2017
Figure 2. A picture of the sun showing its layering structure and the paths that sound takes traveling through it. Paths with an integer number of loops (which) reinforce constructively and create the spherical harmonic modes seen on the surface. p-modes, Pressure modes; g-modes, gravity modes. Courtesy of the Solar and Heliospheric Obser- vatory (SOHO; collaboration between the European Space Agency [ESA] and NASA).
is indeed an acoustic signal from these solar modes that is detectable from Earth! Seismic records on Earth contained a mysterious “hum” that recently has been identified with the solar p-modes and that is likely transmitted from the sun to Earth by the interplanetary magnetic field and the solar wind. Thus, we do hear the sound of the sun on Earth (Thompson et al., 2007; Fossat et al., 2017)!
From the Sun to the Stars: Asteroseismology
If helioseismology works well on the sun, why not use the same technique on the stars? Well, just like transplanting our earthly acoustics instruments elsewhere, it’s not quite the same problem or conditions. The stars don’t present a finite disc like the sun, and so the signals we get are inte- grated (averaged) over the surface of the star, reducing them. The intensity changes seen are on the order of one part per million. For Earth-based systems, atmospheric turbulence wreaks havoc with such low-level signals, and more over any time series made are interrupted every day and every season by the diurnal and seasonal sky changes. This means going into space if you want to pursue asteroseismology.
Luckily for the star researchers, the “exoplanet revolution,” which also needed to detect very tiny fluctuations in star’s intensities, was also in full swing (and still is!). A very sym- biotic relationship between the asteroseismologists and the