Dynamical consequences of helioseismological inferences from SOHO
17 May 1999Helioseismology is the study of seismic waves in the Sun; it enables us tolearn the internal structure and the rotation from the solar surfacealmost all the way to the centre. The methods by which the data areanalysed are similar to those that geoseismologists use to diagnose thestructure of the Earth.The Space Science internal seminar on 7 May, "Dynamical consequences of helioseismological inferences from SOHO', was given by Douglas Gough, Professor of Theoretical Astrophysics at the University of Cambridge, and co-investigator on all three helioseismological investigations aboard SOHO.
The seminar concentrated on two inferences that have been drawn from the observations of seismic disturbances from the spacecraft SOHO, and develops arguments to infer the existence and strength of a magnetic field that pervades the interior of the Sun. Those two inferences are that in a thin layer beneath a convection zone which occupies the outer 50 per cent of the volume of the Sun, (i) the rotation rate varies quite abruptly and (ii) the sound speed is somewhat greater (by only about two parts in a thousand) than theory predicts. By analogy with a stirred cup of tea one can deduce that there must be a circulatory motion in that layer which mixes material with the convection zone above.
It must be realized that the Sun is also somewhat like orange juice: leave it alone for a long time and the orange will settle to the bottom. Actually, the Sun is composed principally of hydrogen and helium, the hydrogen acting like the water and the heavier helium like the orange. The circulatory motion in the layer mixes the helium back into the convection zone. Try stirring a glass of orange juice that has been left a long time to settle; try to stir it very carefully so that the circular motion is purely horizontal and would therefore, not cause the settled orange to mix. You will find that it is impossible. The shear at the bottom of the glass (which is inevitable because very close to the bottom of the glass the juice hardly rotates) causes the juice at the bottom to move towards the axis of rotation and rise up in the middle of the glass, thereby diluting the initially concentrated juice at the bottom. The Sun acts similarly, and it is the dilution beneath the convection zone that is responsible for the observed augmentation of the sound speed. By a long theoretical argument one can show how the magnetic field acts like the glass containing the orange juice, and how the measured properties of the sound-speed anomaly can be used to infer the strength of the magnetic field.
Why is all this interesting? Firstly, it is an illustration of how a complicated argument combining solar data with experience of natural processes on earth can lead to conclusions about processes in the Sun that could never be detected directly. Secondly, it has indirect effects concerning observations of the chemical composition of the upper layers of the convection zone (which extend right to the solar surface) and how they relate to the chemical composition deep in the core; this is also important to astronomy generally, for interpreting observations of other stars. Thirdly, it highlights how the detection of a seemingly tiny anomaly in the sound speed can be used to draw profound conclusions; further observation from SOHO may lead to the discovery of other tiny anomalies from which yet further deductions could be made. Fourthly and, in my opinion, most important of all, it shows how we are now using the Sun as a laboratory to carry out scientific studies under conditions that can never be achieved on earth: it is an example of several ways in which we are now using helioseismology to investigate the nature of matter, which is important not just for astronomy but also for understanding more ordinary material which we all encounter every day.