Since the 17th century, the Sun's poles and equator are known not to rotate at the same speed. In 2004, the European SOHO satellite discovered that its nucleus rotates like a solid body.
In order to better understand the inner working of the stars, a primordial step is to better understand how the Sun's inner layers move. The seismology from space revealed that the Sun's outer layers, which form the convection zone, accompany the visible surface in its rotation whereas the inner part - the Sun's nucleus - rotates like a solid body. The limit between the two rotation patterns is located at two-third of the Sun's radius. This frontier was named the tachocline.
Below, the patterns predicted that the rotation would keep increasing when moving towards the centre. The GOLF and SOHO instruments demonstrated that up to 0.15 solar radius (i.e. up to 100.000 km from the centre), the Sun acts like a solid body with a constant rotation speed.
The astonishing discovery allows a strong stabilising influence of the magnetic field on the inside of this huge gas bubble - the Sun - to be considered.
The method consists in precisely measuring the effects of rotation on the Sun's vibrations the speed of which is only a few millimetres per second over periods of several tens of minutes at the surface. These vibrations are named acoustic vibrations because of the low pressure variations which are similar to the ones produced by a speaker in the air.
The Sun's innermost region (i.e. less than 100,000 km) was not explored yet. One cannot exclude the possibility of the rotation speed to be higher in this region, like a remnant of the Sun's formation.