Astronomers from the National Science Foundation's National Solar Observatory (NSO) have discovered a solar "heartbeat" in the motion of layers of gas circulating beneath the sun's surface. Their research shows that some layers speed up and slow down about every 16 months. This internal motion provides clues to understanding the cycle of activity observed on the surface.
Understanding the solar cycle is a fundamental objective of solar astronomy. Every 11 years, the normally quiet sun exhibits a high level of activity in the form of sunspots, solar flares and coronal mass ejections. These eruptions can affect cellular phones, power distribution systems, satellites and other sensitive technology.
Rachel Howe, Frank Hill and Rudi Komm of the NSO in Tucson, Ariz., and their colleagues analyzed more than four years of observations from the Global Oscillation Network Group (GONG), a worldwide network of solar telescopes, to detect and model motion inside the sun. They report their results in the March 31 issue of Science.
The sun is made up of layers of gas. Scientists probe these layers by using helioseismic methods to analyze sound waves traveling through the sun's interior. The techniques are similar to the seismic techniques used to study earthquakes. Howe's team examined layers extending almost halfway to the solar core and measured the speed of movement at different depths. They believe the patterns in these movements are connected to the cycle of eruptions seen on the surface.
"We listen to the sun's 'heartbeat' to understand what is happening in its core," explains Hill.
Unlike the earth, all points on the solar surface do not rotate at the same rate. The solar equator rotates once every 27 days, while the rotation rate at the sun's poles slows to once every 35 days. This "differential" rotation, long a mystery of solar physics, extends through the sun's turbulent convective layer, located about 210,000 kilometers below the surface -- nearly one-third of the distance to the solar core. Below this layer, the differential rotation vanishes.
At the edge of the convective layer, Howe and her colleagues used GONG data to determine that the rotation rate varies periodically, completing a cycle about every 15-16 months. The team used data from the NASA and European Space Agency's Solar and Heliospheric Observatory (SOHO) spacecraft to confirm the pattern of these variations.
"At first we were skeptical of the pattern. Knowing the complexity of models used to explain the solar magnetic field and its connection to observed solar activity, we were expecting nothing, or chaos, in our observations at that location," said Howe.
The GONG network (http://www.gong.noao.edu/sites/sites.html) is an international project led by the National Science Foundation. It provides continuous observations of the sun, monitoring the surface and tracking its tiny oscillations 24 hours a day. These oscillations are visual evidence of the sound waves traveling through the sun's interior.