The Solar Interior

The solar equator spins around in 28 days (pink/red) while the poles take 35 days (blue/black). The model simulates this differential rotation. (Image by Mark Miesch, NCAR, ©UCAR.) The Sun’s energy is created deep within its core. This is a place where the temperature is so high (15 million degrees Celsius) and the pressure so great (340 billion times the atmospheric pressure at sea level) that nuclear reactions take place. The nuclei of hydrogen atoms collide at incredibly high speeds, fusing together in groups of four to form a helium nucleus, which scientists call an alpha particle. This particle has slightly less mass than the four protons. The difference in mass is released as energy, which gradually works its way to the Sun’s outer surface—the luminous area known as the photosphere, or “sphere of light.”

The photosphere looks extremely bright because it is about 6,000ºC, which is the temperature at which maximum amounts of radiation in the visible region of the spectrum are emitted. But it also contains dark blemishes, known as sunspots, that are cooler (about 4,000ºC). These regions of concentrated magnetic fields tend to develop in groups, with some individual spots covering areas 20 times larger than a circle the diameter of Earth . They may last for weeks or months and develop in cycles, with the maximum number of sunspots occurring about every 11 years. The most recent solar maximum occurred in 2001.

Scientists have long wondered why the solar cycle averages 11 years and what the reasons for sunspot patterns might be. In 2004, NCAR researchers unveiled a computer model indicating the source of concentrated magnetic fields that can create sunspots. These fields are transported by a circulating current of gas, which flows on the Sun's surface, from its equator to its poles, and then sinks, returning to the equator some 200 million meters below the surface—about a third of the way to the center of the Sun—at the base of the solar convection zone. This motion, sometimes compared to a conveyor belt, would explain why sunspots, originating from the base of the convection zone, occur at certain times and in certain areas of the Sun. Scientists are beginning to use this theory to make predictions about solar activity, which could eventually help society better prepare for solar storms.

Much remains unknown about the solar interior because it is impossible to observe directly. NCAR researchers and colleagues elsewhere who study solar magnetism and activity rely in part on helioseismic observations, which are measurements of movements of the Sun’s surface. These movements, analogous to movements produced by earthquakes on our planet, are caused by pressure fluctuations from deep within the Sun’s surface. Using these observations, scientists are gaining insights into temperature, density, and movement deep within the Sun.