The pulse of the Sun's dynamo

Click on the images for larger versions
Rotation rates near the bottom of the convection zone (white line), the level of the suspected dynamo, change markedly over 6 months. Faster/slower rates are shown in red/blue. Meanwhile near the surface (seen plainly on the left of each cutaway) bands of faster (red) and slower (green) rotation move towards the equator. Also: Colour TIFF (no line), black/white (slower/faster rotation) GIF and TIFF versions. Movie: MPEG (720x486), MPEG (720x486, long version).
Time variation of rotation rates at 0.72 (upper) and 0.63 (lower) solar radii, on each side of the tachocline. Results from GONG data using two different methods are shown in black, results from MDI are shown in red. PostScript version available.	Near-surface bands of faster (yellow/red) and slower (blue/green) rotation migrating towards the equator. PostScript version available.

Caption: Currents of gas deep inside the Sun pulsate like the blood in human arteries, speeding and slackening every 16 months. Solar scientists are astonished by this discovery. It comes from an international team pooling observations from the MDI instrument on the ESA-NASA SOHO spacecraft and from a worldwide chain of ground stations called GONG. Rachel Howe and her colleagues announce their results in the 31 March issue of the journal Science.

An urgent need to understand the Sun's magnetic behaviour, which produces space storms affecting the Earth, gives practical as well as theoretical importance to the discovery. To explain the sunspot cycle, in which the count of dark sunspots and solar storminess peak at intervals of about 11 years, theorists visualize a dynamo inside the Sun. Relative motions between neighbouring layers of electrified gas supposedly drive the dynamo. As the years pass, so the theory goes, the magnetic field becomes too strong for the gas to hold, and it breaks out to the solar surface, causing sunspots and magnetic explosions. The changes now observed are at the right depth for a dynamo.

"We are excited to see the first evidence of changes close to the location of the solar dynamo," says the lead author, Rachel Howe of the National Solar Observatory in Tucson, Arizona. "It's very surprising to find that the changes have such a short period -- 16 months or so rather than the 11 years of the solar cycle."

The news from SOHO's MDI and from GONG is that the contrast in speed between layers above and below the supposed dynamo region can change by 20 per cent in six months. When the lower gas speeds up, the upper gas slows down, and vice versa. In observations spanning 4.5 years, from May 1995 to November 1999, these alternations in speed occurred three times. They indicate a heartbeat of the Sun at one pulse per 15-16 months in equatorial regions, and perhaps faster at higher latitudes.

More information is available on these pages:

• ESA Release
• Full text of NASA Release
• NSF Press Release
• Solar Oscillations Investigation Release
• National Optical Astronomy Observatories Release
• NSF's National Solar Observatory
• GONG Home Page

Scientific papers:

• R. Howe et al., "Dynamic Variations at the Base of the Solar Convection Zone",
  Science, 31 March 2000
• R. Howe et al., "Deeply penetrating banded zonal flows in the solar convection zone",
  Astrophysical Journal Letters, 20 April 2000. Preprint available at the arXiv.org e-Print archive.

Picture/movie Credits: GONG (NSO/NSF) and SOHO/MDI (ESA & NASA)
Instrument/observatories: MDI (Michelson Doppler Imager); GONG (Global Oscillation Network Group);
Taken: 1995-1999