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Scientists listen in on the Sun to reveal new insights into its fiery dynamo

Imperial College London



Variation of the solar internal rotation relative to 1996 when the Sun's activity was at its 11-year minimum.

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 metres per second. Dotted lines indicate the base of the convection zone and the 0 degree, 30 degree and 60 degree latitudes.


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Scientists studying sound waves from deep inside the Sun have provided new insights into the solar dynamo within, which could help to explain how the Sun's colossal magnetic field behaves.

An international team of researchers report in the journal Science today (5 April issue) 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 behaviour 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.



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 colours) and slower rotation near the pole (blue colours). The deeper interior appears to rotate at a nearly uniform rate with a period of about one month.


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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 behaviour of the Sun," said Professor Thompson.

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For more information please contact:
Michael Thompson
Department of Physics
Imperial College, London, UK
Tel: +44 (0)20 7594 7660
Email: michael.thompson@ic.ac.uk

Tom Miller
Imperial College Press Office
Tel: +44 (0)20 7594 6704
Mob: +44 (0)7803 886248
Email: t.miller@ic.ac.uk

Notes to Editors:

The research is published in the 5 April 2002 issue of Science, and is embargoed for 14:00 US Eastern Standard Time / 19:00 British Summer Time on Thursday 4 April.

Title: Helioseismic Measurement of Solar Torsional Oscillations

Authors:

Sergei Vorontsov
Queen Mary, University of London, London, UK
Email: s.v.vorontsov@qmul.ac.uk

Jorgen Christensen-Dalsgaard
University of Aarhus, Aarhus, Denmark
Email: jcd@ifa.au.dk

Jesper Schou
Stanford University, USA
Email: schou@skaelv.stanford.edu

V. N. Strakhov
Institute of Physics of the Earth, Moscow, Russia
Email: strakhov@uipe-ras.scgis.ru

Michael J. Thompson
Imperial College, London, UK
(details as above)

The same issue of Science also contains a Perspectives article covering the research. Order Amidst Turbulence by Juri Toomre of the University of Colorado, Boulder, USA (jtoomre@jila.colorado.edu) provides further discussion of the solar dynamo and of other complex flows ("Solar Subsurface Weather") in which the more ordered behavior of torsional oscillations are embedded.

2. SOHO is a project of international cooperation between ESA and NASA. Web site at: http://sohowww.nascom.nasa.gov/

3. The basic data for the analyses were gathered by SOHO between May 1996 and January 2002.

4. Imperial College of Science, Technology and Medicine is the largest applied science, technology and medicine university institution in the UK. It is consistently rated in the top three UK university institutions for research quality, with one of the largest annual turnovers (UKP390 million for 2000-01) and research incomes (UKP202 million for 2000-01). In the December 2001 Research Assessment Exercise, 75 per cent of staff achieved a 5* rating, the highest proportion in any UK university. Visit: www.ic.ac.uk

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