The Pulse of the Solar Dynamo

This page contains notes prepared by the authors of the paper, Rachel Howe, Jørgen Christensen-Dalsgaard, Frank Hill, Rudi Komm, Rasmus Larsen, Jesper Schou, Mike Thompson, and Juri Toomre with additional notes and/or images prepared by Phil Scherrer.

ILLUSTRATIONS

The following illustrations are available as small "gifs", as larger "gifs", in TWO forms: one form with an "NSF/NSO" attribution in the lower left corner and one without. Please include attribution to "NSF's National Solar Observatory" if these images are used. Postscript and/or tiff formats are present in most cases also - without the logos.

	`Rotation Profiles' (line plot): gif, no_logo gif, no_logo postscript Time-averaged rotation rates, plotted as a function of radius at different latitudes within the Sun. The tachocline -- a region where the rotatation rate changes from differential rotation in the convection zone to nearly solid-body rotation in the interior, is evident near the base of the convection zone, determined to be at radius 0.71 R (where R is the overall solar radius). (Image courtesy NSF's National Solar Observatory)
	`Variation in Time Above and Below Tachocline' (color time plots): gif, no_logo gif, no_logo postscript Variations with time of the difference of the rotation rate from the temporal mean at two radii deep within the Sun, with the site at 0.72 R located above the tachocline and that at 0.63 R below it, both sampling speeding up and slowing down in the equatorial region. Results obtained from GONG data for two different inversions are shown with black symbols, those from MDI with red symbols. (Image courtesy NSF's National Solar Observatory)
	`Sphere in Cutaway stills'gif, no_logo gif, no_logo TIFF Cutaway images of solar rotation showing a peak and a trough of the 0.72R variation, [Color table near surface has faster rotation in red, slower in green, and yellow as intermediate; color table below 0.85 R has faster rotation in red and slower in blue. The left hand side of each sphere shows the surface view. The arrow-tip indicates the position of interest. (Image courtesy NSF's National Solar Observatory)
	`Sphere in Cutaway stills' gif, no_logo gif, Cutaway images of solar rotation showing a peak and a trough of the 0.72R variation, with the same color table as above. The white line indicates 0.71R radius, the base of the convection zone. A TIFF file version ( no logo tiff ) 1440x722 pixels is also available. NB This does not include any indicator lines or arrows. (Image courtesy NSF's National Solar Observatory)
	`Sphere in Cutaway stills' (monochrome) gif, no_logo gif, no_logo TIFF Cutaway images of solar rotation showing a peak and a trough of the 0.72R variation, with black indicating slow rotation, grey intermediate, and white fast. (Image courtesy NSF's National Solar Observatory)
	`Sphere in Cutaway stills' (monochrome) gif, no_logo gif, no_logo TIFF Cutaway images of solar rotation showing a peak and a trough of the 0.72R variation, with black indicating slow rotation, grey intermediate, and white fast; the white line indicates 0.71R radius, the base of the convection zone. (Image courtesy NSF's National Solar Observatory)
	`Migrating Zonal Bands' (color panel): gif, no_logo gif, no_logo postscript, Close to the solar surface, variation of rotation rate with latitude and time from which a temporal average has been subtracted, revealing banded zonal flows migrating toward the equator. [Color table has faster rotation as red/yellow, slower as green/blue.] (Image courtesy NSF's National Solar Observatory)

Animations

	`Sphere in Cutaway Movie with Zoom-In'
	`Sphere in Cutaway Movie with Zoom-In and arrows'
	`Sphere in Cutaway Movie with Zoom-In and line indicating convection zone base, 720x486 pixels' no_logo
	`Sphere in Cutaway Movie with surface sequence, zoom-in and line indicating convection zone base, 720x486 pixels no_logo'

All images courtesy of NSF's National Solar Observatory

Shown in cutaway, animations of variations with time in the solar rotation rate, showing the `pulse' near the base of the convection zone (ie about 30% by radius below the surface. [The color table in this deeper region has light brown tones indicating fast rotation, blue tones slow rotation, and white as intermediate; this color table is used below 0.85 R in radius.] Accompanying these variations above and below the tachocline are other zonal flows called `torsional oscillations' closer to the solar surface. Shown both on the sphere and in cutaway, these appear as bands of faster and slower than average rotation rate moving equatorward, and extending about 8% by radius into the Sun. [Color table near surface has faster rotation in red, slower in green, and yellow as intermediate.]

TOUCHING THE HEART OF THE SOLAR DYNAMO

Variations in the rotation rate of the Sun have been detected deep within its interior and close to the presumed site of the solar dynamo which generates the 22-year cycles of magnetic activity. Reporting in Science, an international team has found successive speeding up and slowing down of rotation near the base of the solar convection zone, with an unexpected period of about 1.3 years near the equator and about 1.0 years at high latitudes. These results were obtained from helioseismic analysis of data obtained during the past four years with an experiment aboard the SOHO spacecraft and a network of ground-based observatories called GONG. The largest temporal changes in the rotation rate are found both above and below the `tachocline', a layer of intense rotational shear located at the interface between the convection zone and the deeper radiative interior, at a depth of about 30% by radius into the Sun. The variations near the equator are strikingly out of phase above and below the tachocline, and involve changes in rotation rate of about 6 nHz, which is a substantial fraction of the 30 nHz difference in angular velocity with radius across the tachocline.

The solar magnetic dynamo is thought to operate within the tachocline, with the differential rotation there having a crucial role in generating the strong magnetic fields involved in the cycles of solar activity. There should be feedbacks from the strengthening magnetic fields upon the rotational shear in which they are embedded. Although the magnetic fields are difficult to detect directly at such great depths within the Sun, helioseismology is able to probe the rotation profile at those depths, including how it might change with time. The reported discovery of periodic changes in the rotation rate near the tachocline as the Sun's magnetic cycle progresses provides the first indications of dynamical changes that may accompany the operation of the solar dynamo.

The intense turbulence within the solar convection zone excites millions of acoustic oscillation modes of the interior that are observable at the solar surface. Precise measurement of the frequencies of these global modes, and of their frequency splittings by large-scale flows, permits inversions of such data to deduce how the Sun's rotation varies with radius and latitude throughout much of its interior. An early finding from helioseismology has been that the pronounced differential rotation observed near the solar surface, where equatorial regions rotate in about 25 days and the poles in about 33 days, extends through much of the convection zone with little radial variation. Further, at the base of this convective envelope is the tachocline of strong shear in which the differential rotation with latitude is forced to adjust to the nearly uniform rotation of the deeper radiative interior.

Detailed models for how the solar dynamo functions deep within the Sun are still at early stages. A key ingredient is that the tachocline affords ways to stretch and organize magnetic fields into toroidal bands of intense fields that can intermittently become unstable. Thus large magnetic loops can break off and rise through the convection zone, erupting through the surface to form sunspot pairs with remarkably well-defined rules for emergence latitudes and field polarity as the cycle progresses. The Sun is now close to the peak of such magnetic activity, displaying large numbers of sunspots accompanied by frequent solar flares and large coronal mass ejections. The Earth is bathed in the resulting flood of particles from the Sun, leading to great shows of northern lights. More importantly, such solar outbursts are capable of disrupting satellite-based communications and even terrestrial power grids.

The helioseismic observations have been carried out independently with the Michelson Doppler Imager (MDI) instrument aboard the SOHO spacecraft operated jointly by ESA and NASA and with the Global Oscillation Network Group (GONG) project. GONG is managed by the National Solar Observatory, a Division of the National Optical Astronomy Observatories, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofísico de Canarias, and Cerro Tololo Interamerican Observatory. The basic data for the analyses are frequency splittings resulting from solar rotation for a broad range of f- and p-mode oscillations derived from an overall 4.5 year time span (from May 1995 to November 1999). Two different inversion procedures have been applied to these frequency splittings to deduce the rotation profiles, and their results are quite comparable. In addition to discovery of variations in rotation rates near the tachocline, analyses of such data have also confirmed the presence near the surface of bands of slightly faster and slower than average rotation, called torsional oscillations. Such bands of weak zonal flows, involving speeds of order 5 m/s superposed on the smooth decrease in rotation rate from equator to pole, propagate equatorward in a manner similar to the emergence latitudes of sunspots as the cycle progresses. These helioseismic studies reveal that such zonal banded flows are not superficial features, but rather extend about 8% in radius below the solar surface. They provide evidence of another class of ordered rotational responses as the solar cycle is progressing.

CONTACT INFORMATION

The research findings are described in two papers in press, both with same author list (e-addresses for all, phone/fax numbers also provided for primary contacts Howe and Toomre):

Rachel HOWE, National Solar Observatory (NSO), Tucson

Jørgen CHRISTENSEN-DALSGAARD, Aarhus University, Aarhus, Denmark

Frank HILL, NSO

Rudi KOMM, NSO

Rasmus Munk LARSEN, Stanford University

Jesper SCHOU, Stanford University

Michael THOMPSON, Queen Mary & Westfield College, London, UK

Juri TOOMRE, University of Colorado, Boulder

PAPERS IN SCIENCE AND ASTROPHYSICAL JOURNAL LETTERS:

The first announcement of variations in rotation rate above and below the tachocline is in:

SCIENCE, `Dynamic Variations at the Base of the Solar Convection Zone', scheduled for 31 Mar 2000 issue. tachocline_paper

The banded zonal flows propagating toward the equator that are detected in the upper portion of the convection zone are discussed in:

ASTROPHYSICAL JOURNAL LETTERS, `Deeply Penetrating Banded Zonal Flows in the Solar Convection Zone', probably scheduled for 20 Apr 2000 issue. zonal_flow_Howe_paper

RELATED PRIOR PAPERS AND PRESS RELEASES

The NASA Space Science Update of 28 August 1997 described the basic results of interior solar rotation and showed the surface "bands". See "Plasma Rivers Discovered inside Sun"

The paper by Jesper Schou published in Ap.J. Letters on 1 October 1999 described the latitude "Migration of Zonal Flows Detected Using Michelson Doppler Imager f-Mode Frequency Splittings"

A more recent NASA Space Science Update (9 March 2000) described the use of sound waves to see through to the far side of the sun:

NSF press release

NASA press release