Solar Interior

Solar interior refers to the Sun’s internal structure. It comprises three principal regions: a central core, above which is found the radiative zone, and finally the convective zone, from which material rises to the visible surface of the Photosphere.

All of the Sun’s energy is produced by nuclear reactions fusing hydrogen into helium in the dense high temperature core, which extends for about one quarter of the solar radius (174,000 km/108,000 mi) from the centre. The core accounts for only 1.6% of the Sun’s volume, but about half its mass. At its centre, the core has a temperature of 15.6 million K and a density of 148,000 kg/m³.

The radiative zone surrounds the core out to 71.3% of the Sun’s radius, 496,000 km (308,000 mi) from the solar centre. Core radiation, initially in the form of gamma rays, is continuously absorbed and re-emitted at lower temperatures (and longer wavelengths) as it travels out through the radiative zone. Recent computations indicate that it takes about 170,000 years, on average, for the radiation to work its way out from the Sun’s core through the radiative zone to the convective zone.

At the bottom of the convective zone, the temperature has become cool enough, at about one million K, to allow some heavy nuclei to capture electrons. Their light absorbing ability (opacity) obstructs the outflowing radiation and causes the plasma to become hotter than it would otherwise be. Because of its low density, the hot plasma rises, carrying energy through the convective zone from bottom to top in about 10 days. On reaching the visible solar disk, the hot material cools by radiating sunlight into space and then sinks back down to become reheated and rise again. These churning convection cells create a granulation pattern in white-light images of the photosphere, which marks the top of the convective zone.

Turbulent motions in the convective zone excite sound waves that echo and resonate through the Sun. When these sound waves strike and rebound from the photosphere, they cause the gas there to rise and fall with a period of about five minutes. Observations of these oscillations with instruments on the Solar and Heliospheric observatory (SOHO), and from the Global Oscillation Network Group (GONG), have been used to examine sound waves with different paths inside the Sun, determining its internal structure and dynamics with the techniques of Helioseismology.

Material in the Sun’s interior flows in ways other than rotation. Broad zonal bands sweep around the equatorial regions at different speeds. The velocity of the faster zonal flows is about 5 m/s (16 ft/s) higher than gases to either side, but this difference is about 400 times slower than the mean velocity of rotation. A single zonal band is more than 65,000 km (40,000 mi) wide and 20,000 km (12,000 mi) deep. These zonal bands gradually drift from high latitudes towards the equator during the 11-year solar cycle, moving in step with a similar motion of sunspots.

Both the sunspots and the zonal bands are moving against another steady flow from the equator to the poles, which has a speed of about 20 m/s (66 ft/s). This flow penetrates to a depth of at least 25,000 km (15,500 mi). Researchers suspect that a return flow towards the equator exists at deeper levels, but detailed motions have not yet been observed at this depth.

Questions to Ponder

• Why is the corona hotter than the photosphere?
• How do Astronomers estimate the temperature of various layers of the Sun?