Helioseismology: what is it?

L’helioseismology is a young discipline of theastrophysics which arose from a discovery concerning the Sun in 1960. Its full importance was not understood until the 1970s, when astrophysicists trying to unlock the secrets of our star showed that they could play a game comparable to those of their geophysicist colleagues on Earth, with the resolution inverse problems with measurements of the field of gravityfrom magnetic field but precisely in this case with seismic waves.

The common idea is simple to understand. It is equivalent to reconstructing the size, shape and composition of a musical instrument by analyzing more and more precisely and completely all the frequencies and amplitudes contained in the sounds that this instrument can emit. Like any inverse problem, it involves tracing signal data back to its source and therefore doing the inverse of the signal prediction from the characteristics of this source. The technique is particularly effective with earthquakes which occur on Earth and is obviously more difficult to implement in the case of the Sun.

It is however possible to do it and it is necessary to measure spectral shifts produced by thedoppler effect coming from matter on the surface of the Sun, which vibrates like the membrane of a drum. It was in 1960 that extremely weak pulsations of the Sun with a period of five minutes were demonstrated in this way for the first time by Robert B. Leighton (whose name is associated with the famous course of his colleague Richard Feynman at Caltech). More generally, the vibes that can be detected are the manifestation of the modes of two types of waves, which are themselves produced, in particular, by the movements turbulent solar plasma in its convective zone, as Roger Ulrich, Robert Stein and John Leibacher began to understand a decade later.

To better understand it, it is necessary to know that the Sun has a structure which can be divided into two and which corresponds to two modes of transfer of the heat in the solar furnace. There are :

• a so-called “radiative” zone, which occupies about 70% of the radius of the Sun from its center;
• a so-called “convective” zone, which envelops the first.

Gravity waves (not to be confused with gravitational waves which are vibrations of thespace-time) propagate essentially in the radiative zone whereas the acoustic waves propagate in both areas of the Sun.

Gravity waves, key to the internal structure of the Sun

As their name suggests, gravity waves correspond to the movements of matter which oscillates while resuming its equilibrium position under the effect of the force of gravity (via the Archimedes thrust), whereas for acoustic waves, the restoring force is pressure. Acoustic waves have been better and better measured and studied over the past few decades, but progress has been slower with respect to gravity waves.

However, the results obtained were spectacular. Indeed, by placing serious constraints on the models of internal structure of the Sun (in particular those explaining the genesis of its energy thanks to thermonuclear reactions in its heart), it was possible to confirm them. In doing so, we could no longer explain the famous enigma of the deficit of solar neutrinos by a misunderstanding of what was going on in that heart. The use of new physics, in this case the mechanism ofneutrino oscillation, became inevitable and the research programs that would lead to the discovery of this mechanism became more and more imperative. The Sun was therefore much more than a laboratory of nuclear physics and plasmas, but also a laboratory ofastroparticles.

In France, one of the famous pioneers of helioseismology is Eric Fossat, astronomer emeritus at the Lagrange Laboratory, of the Côte d’Azur Observatory (OCA). Helioseismology has made great progress thanks to the patient observations of the Sun made possible by the Golf instrument (Global Oscillations at Low Frequencies) fitted to the satellite Soho (Solar and Heliospheric Observatory), a joint space mission of ESA and the Nasa launched on orbit in 1995.

The methods of helioseismology have been extended to stars giving rise to what is now called asteroseismology.