An international team of researchers report in Science today that they have detected variations in the speed at which different regions of the Sun's interior rotate, observations which appear to be intimately connected with the 11-year cycle of the Sun's magnetic activity.

At the peak of its magnetic activity, the Sun displays large numbers of sunspots accompanied by frequent solar flares and Earth-impacting ejections of hot plasma, events that can disrupt satellite-based communications and even terrestrial power grids.

The new findings come from analysis by Dr Sergei Vorontsov (Queen Mary, University of London) and his colleagues of data obtained over the past six years from the Michelson Doppler Imager instrument aboard the SOHO solar observatory.

The outer 30 per cent of the Sun below its surface is a turbulent cauldron of gas, known as the convection zone. To measure the behavior of the flows at various depths in and below this zone the team turned to studying solar sound waves -- akin to geologists using seismic waves from earthquakes to probe the inside of our planet.

The scientists, based at five different research institutions in the UK, Denmark, USA and Russia, used the technique to examine the magnetic bands of slower and faster rotation ("torsional oscillations") and discovered that the entire convection zone is involved in their flow.

There are clear differences in rotation across the solar surface latitudes -- equatorial regions rotating in about 25 days and polar regions in about 33 days -- while the team found the characteristic structure of the bands was retained as they probed deep into the convection zone.

Over the 11-year solar cycle, these magnetic bands migrate towards the pole at high latitudes, and towards the equator from low latitudes.

These findings fit well with the expected 11-year period of the solar cycle, providing strong evidence of dynamical changes that may accompany the operation of the solar dynamo, and could be important in building models of how the Sun behaves, say the researchers.

"At the peak of magnetic activity, the Earth is bombarded by the resulting flood of plasma from the Sun, leading to great shows of northern lights," said one of the five authors, Professor Michael Thompson of Imperial College, London.

"More hazardously to today's modern technological society, such solar outbursts are capable of disrupting satellite-based communications and even terrestrial power grids."

"We are still far from understanding, let alone predicting, such eruptions and their origin in the deep solar interior. However, results such as these are a step towards a better grasp of the complex processes in the deep solar interior and may eventually help us towards the elusive goal of predicting the violent behavior of the Sun," said Professor Thompson.

Related images:

Results at 72-day intervals are shown over the first six years of MDI measurements from SOHO. The equatorward migration of the low-latitude branch of the torsional oscillation, and the strengthening of the high-latitude branch, are visible. Red indicates the regions that have speeded up, blue those that have slowed down. The units are nHz (nanoHertz): the corresponding changes in linear speed are a few meters per second. Dotted lines indicate the base of the convection zone and the 0 degree, 30 degree and 60 degree latitudes.

Time-averaged rotation in the Sun's interior, as deduced from helioseismic measurements made by the MDI instrument on board SOHO. The Sun's equator is along the bottom of the plot, the pole at the top. The rotation in the convection zone (i.e., the outer 30 per cent) shows vivid contrast with faster rotation near the equator (red colors) and slower rotation near the pole (blue colors). The deeper interior appears to rotate at a nearly uniform rate with a period of about one month.

[Contact: Michael Thompson, Tom Miller]

05-Apr-2002