|Number 2||September 15, 1988|
This is the second issue of the newsletter of SEDI, an IUGG Union Committee
to Study the Earth's Deep Interior. Interest in SEDI is continuing to grow;
this issue is being mailed to 335 scientists in approximately 25 countries.
Requests for additional copies of this issue or for copies of the first
issue should be addressed to David Loper, Geophysical Fluid Dynamics Institute,
Florida State University, Tallahassee, Florida 32306-3017, U.S.A. Also
news items for the third issue should be sent to the sa me address.
These constitute an illustrative sample of the questions discussed by
almost 200 scientists at the first SEDI symposium entitled "Structure and
Dynamics of the Core and Adjacent Mantle," held on the Costa Brava of Spain,
at Blanes from 23-25 June, 1988, in conjunction with the 17th International
Conference of the Committee on Mathematical Geophysics. SEDI, an acronym
for Study of the Earth's Deep Interior, is a new committee of the International
Union of Geodesy and Geophysics, created at the Vancouver General Assembly
in August, 1987. The following is a report of the SEDI symposium, which
consisted of a thirteen oral presentations, forty eight posters, and five
open discussion periods during which the posters and general issues were
Compositional buoyancy, released at the inner core-outer core boundary by the selective partial rejection of the light alloying element of the core upon solidification, continues to hold the favored position as the energy source of the geodynamo. Noneth eless, much discussion concentrated upon possible regions or layers of gravitational instability within the outer core, the distribution of the Brunt-Väisälä frequency in the core and so forth. High-pressure studies suggest a stable compositional gradien t within the outer core. On the other hand, the dynamically allowable density differences within an unstably stratified region are far too small to be detected; H. K. Moffatt (University of Cambridge) estimates the fractional density contrast to be only 3 x 10-9. Work on observing core oscillations, through the tiny gravity signals they produce, is still in its infancy, but D. Crossley (McGill University, Montreal) and D. Smylie (York University, Downsview) have secured funding for a Canadian supercond ucting gravimeter to supplement that operated by P. Melchior (Observatoire Royal, Brussels) as well as a few others elsewhere.
The composition and temperature of the core continue to be controversial topics. High pressure experiments, reported by T. Ahrens (California Institute of Technology, Pasadena), reveal that the melting temperature of FeO exceeds that of pure Fe at core pressures. Consequently, the addition of oxygen appears to raise the melting temperature of a Fe-FeO alloy and the density of the liquid exceeds that of the solid. This appears to eliminate oxygen as the light constituent in a liquid core of binary comp osition and strongly constrains the amount of oxygen that can be present in a multicomponent core. The high core temperatures found by T. Ahrens, R. Jeanloz and colleagues, based on shock experiments and static high pressure experiments involving laser h eating, were not in agreement with the much lower temperatures reported by R. Boehler (Max Planck Institut fur Chemie, Mainz), based on electrical heating.
A number of papers reported efforts to produce maps of the fluid motions at the surface of the core, expecting that this will help constrain models of the more deeply buried geodynamo. The first generation of such models were based on the frozen-flux hy pothesis, which ignores ohmic diffusion in the core, and uses only the vertical magnetic field at the CMB together with additional dynamical assumptions such as lack of up and downwelling, geostrophy, or steadiness. These results are not in good agreemen t with each other. Preliminary work on new models which permit magnetic diffusion and utilize horizontal field components was reported by E. R. Benton (University of Colorado, Boulder) and D. Gubbins and D. Lloyd (University of Cambridge, Cambridge). T aking this model a step farther, J. L. LeMouël (Institute de Physique du Globe, Paris) reported on calculations showing that decade fluctuations in the length of day can balance changes in the estimated angular momentum of the core relative to the mantle.
A large amplitude of CMB topography is at odds with other topographic constraints. J. Wahr (University of Colorado, Boulder) noted that the ycomponent of topography is constrained to be no more than a few meters in amplitude. D. Doornbos, T. Hilton and T. Kristensen (University of Oslo) reported that the dispersion of travel-time data of phases involving bottom-side reflections (PKKP, PKKKP, etc.) requires a very smooth reflector. Also, Melchior's gravity data suggests topography of no more than 200 m .
These three independent data sets together suggest that the irregularities
sensed by the tomographic studies are within the mantle rather than at
the CMB. This interpretation was strengthened by the report by Doornbos
et al that the reflector depth infe rred from PKKP is 10-15 km shallower
than that inferred from PcP, suggesting the existence of a thin boundary
layer at the base of the mantle. Also, R. Haddon (Geological Survey of
Canada, Ottawa) noted that precursors to PKP imply ubiquitous and strong
lateral variations at the base of the mantle. It would take large horizontal
temperature differences, on the order of 1000°K, to produce the observed
signals. There was some discussion of whether persistent compositionally
distinct regions within the lo wer mantle could produce the signals, but
no consensus was reached.
R. Widmer, G. Masters and F. Gilbert (University of California, San Diego), reported on their attempt to improve the existing spherically averaged earth models using spheroidal and toroidal degenerate seismic normal modes with frequencies below 8 mHz. S urprisingly, they were able to fit the data from mantle-sensitive modes quite well, but were unable to fit the core-sensitive modes. This raises the disturbing possibility that the existing core models are incorrect.
Copies of the program of the first SEDI Symposium are available from Joe Cain, Department of Geology, Florida State University, Tallahassee, FL 32312.
The first item on the agenda was a report by Tom Jordan (Massachusetts Institute of Technology) on the current status of and future plans for ISOP: the International Seismic Observation Period. (A full report of this project was given in the first editi on of the DEEP EARTH DIALOG.) A proposal that SEDI officially endorse ISOP was approved by a voice vote. It was similarly agreed that SEDI should establish a working group to coordinate ISOP with other agencies. Tom Jordan was designated as Chairman of this working group. Anyone wishing to be a member of this working group should contact Tom Jordan at the Department of Earth & Planetary Science, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
Next, A. W. Green (United States Geological Survey, Denver) described INTERMAGNET: International Real Time Geomagnetic Observatory Network, which is being developed jointly by the Geological Surveys of the United States and Britain. A global network of manned and automated observatories is being assembled that will transmit geomagnetic data in real time via satellite links to two collection points, one in the United States and one in Britain. This will allow a global picture of short-lived magnetic phe nomena to be assembled without delay. A proposal that SEDI officially endorse INTERMAGNET was approved by a voice vote.
A suggestion by Coerte Voorhies (NASA Goddard Space Flight Center, Greenbelt) that diverse geophysical data types, including magnetic, earth rotation, geoid and seismic, be simultaneously inverted to yield a unified model of the coupled core-mantle syst em was discussed, but no action was taken. The consensus appeared to be that we do not yet know how to weight the various data sets relative to one another.
Ivan Cupal (Geophysical Institute, Prague) spoke of the potential value of SEDI in bringing scientists from western and eastern countries together, and read a list of names of eastern scientists who would be interested in participating in SEDI activities . On a similar note of bringing together diverse groups, attention was turned to the proposal by Tuzo Wilson that a meeting be held bringing together scientists who study deep earth processes with those interested in plate tectonics (see report of Canadi an SEDI activities). This idea was approved by a voice vote.
The format for the SEDI Symposia, consisting of a few oral presentations, many posters and time for group discussions, was considered next. There was general agreement that the basic format worked well, although there were some problems with integratio n of the posters into the discussions. A number of suggestions were made, including the idea of having selected discussants give brief summaries of small groups of related posters at the beginning of the discussion period. These suggestions will be inco rporated in the format of the second SEDI Symposium. The business meeting ended with a discussion of possible sites for the second SEDI symposium to be held in 1990.
The activities of the workshop fall into seven categories. (1) Fast dynamos are defined as kinematic dynamos whose growth rate is proportional to the strain rate in the limit of large magnetic Reynolds number. Studies of these dynamos serve to elucidat e the inductive processes which can occur in MHD, but are not directly relevant to the geomagnetic dynamo. (2) Mean-field electrodynamics is a consistent method of obtaining a simple set of coupled equations describing the evolution of the magnetic fiel d in which the motions are parameterized by scalar or tensor coefficients. Recent work of the Potsdam and Helsinki groups has concentrated on models in which the a coefficient is a function of the magnetic energy density or the total magnetic energy. Th e aim of these parameterizations is to provide for a feedback mechanism which will allow the field strength to equilibrate to a finite value. (3) Core-mantle interactions and secular variation deal with the structure of the core-mantle boundary and of th e fluid flows and magnetic field at the top of the core. Two current issues are whether core-mantle coupling is principally topographic or magnetic and what information can be gleaned from the electromagnetic signals observed at the surface about the vel ocity field at the top of the core and the underlying dynamo process. (4) The study of core dynamics and the MHD geodynamo has increasingly turned from kinematic dynamos, in which the velocity field is prescribed, to the full MHD problem in which the vel ocity field is one of the unknowns. F. Busse and K. Zhang reported on their efforts to develop a full dynamo model by following a sequence of bifurcations of the thermally driven dynamo equations. It is anticipated that we will see in the coming years s erious efforts to model the geodynamo. (5) An important unresolved question is whether the dynamo is of Taylor type or model-z type. Earlier contentions that the dynamo could be of the weak-field variety, with toroidal field strength comparable to the p oloidal have been discounted. The current debate is whether core-mantle coupling is important, giving a model-z dynamo, or irrelevant, giving a Taylor type dynamo. M. Proctor reported on studies that show the dynamo may be in a state of isorotation in w hich contours of equal toroidal shear and meridional field lines are nearly coincident except near the core-mantle boundary. A number of studies of plane-layer models are being conducted to help elucidate the crucial mechanisms and thus resolve this ques tion. (6) The dynamo mechanism undoubtedly involves waves and instabilities within the Earth's core. This remains a difficult and complicated subject, in which many results appear to be counterintuitive. (7) The energy budget and the long-term behavior of the geodynamo are governed by the thermal interactions between the core and mantle. The favored energy supply remains compositional convection. It was pointed out that this mechanism can be parameterized by a buoyancy source at the bottom of the out er core and a uniform buoyancy sink distributed throughout the outer core.
The goals of the project are twofold. Firstly, to carry out computations to attempt to resolve the difficult question of whether the geodynamo is of the model-z type proposed by Braginsky, i.e., that core-mantle coupling has an important controlling eff ect on the dynamics of the core, or alternatively that the prescription proposed by J. B. Taylor is correct, i.e., that the mantle has no effect in the limit of vanishing viscosity. Secondly, it is planned to develop a time-stepping program to solve the full magnetohydrodynamic dynamo problem with the driving force being derived from unstable entropy gradients in the presence of strong rotation and magnetic field. This is a regime likely to be met in the Earth and leads to motion of global scale.
The AGU Committee for SEDI is charged with the responsibility of providing a focus within AGU for fostering and coordinating activities related to understanding the Earth's deep interior. Included in these activities will be: A. Definition of appropria te research programs ranging from those for individuals to those for large organizations: B. Communications with and among AGU sections; C. Building interactions with other appropriate scientific organizations, both national and international; D. Educa tion of the AGU membership and public about the nature and importance of the problems; E. Organizing of multidisciplinary meetings, workshops, and sessions. The activities of this Committee will be reviewed yearly by its Executive Committee to determine its future role, and will report to the president of AGU. The initial membership of the Executive Committee is T. Ahrens (Chairman), T. Lay (Secretary), M. Drake, R. Hemley, T. Jordan, D. Loper, P. Olson, J. Wahr, and J. Woodhouse.
Also it is hoped that we will be able to have a short (half- or
one-day) SEDI workshop at Exeter on Saturday, July 29. The purpose of this
workshop is to develop a plan of coordinated scientific activities among
the various national groups active in SED I. This workshop will be open
to all who are interested. If this workshop can be arranged, more specific
information will be distributed at a later date.
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