Atmospheric Sciences. Biophysical controls on canopy water bal ance, Victor C. Engel, Columbia University,. New York, Kevin Griffin, April 2002. Response of ...
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Recent Ph.D.s Atmospheric Sciences Biophysical controls on canopy water bal ance, Victor C. Engel, Columbia University, New York, Kevin Griffin, April 2002. Response of mountain glaciers to climate forcing: analysis and applications, Arthur M. Greene, Columbia University, New York, Wallace Broecker, November 2001. Effect of atmospheric C0 concentration on plant isoprene emissions, Mark J. Potosnak, Columbia University, New York, Kevin Griffin, January 2002.
The genesis of E-MORB: extensions and limi tations of the hot spot model, Kathleen E. Donnelly, Columbia University, New York, Charles Langmuir, January 2002. Arc magma genesis in the eastern Mexican volcanic belt, Alexandra B. La Gatta, Columbia University New York, Steven Goldstein, May 2002. Mid-ocean ridge basalt trace element systematics: constraints from database manage ment, ICPMS analyses, global data compilation, and petrologic modeling, YongjunSu, Columbia University New York, Charles Langmuir, January 2002.
Patterns of faulting and seismicity in the mid-ocean ridge environment, DelWayne R. Bohnenstiehl, Columbia University, New York, Christopher Scholz/Maya Tolstoy/Suzanne Carbotte, May 2002. Tidally induced cross-frontal mean circulation, Changming Dong, Columbia University, New York, Hsien Wang Ou/Dake Chen, May 2002. Non-eustatic controls on sea-level in semienclosed basins, Candace O. Major, Columbia University New York, William Ryan, February 2002. Variability in coastal upwelling regions along the western coast of the Americas from nearshore geochemical and paleo-tracers, Renee K. Takesue, Columbia University New York, Lex van Geen/Jean Lynch-Stieglitz, March 2002.
Lacustrine organic sedimentation, organic metamorphism,and thermal history of selected early Mesozoic rift basins, Eastern USA, MaryAnn L Malinconico, Columbia University, New York, May 2002. Characterizing the influence of the general circulation on subtropical marine clouds and studying their interactions in an SCM, Margaret A. Rozendaal, Columbia University, New York, Anthony Del Genio, October 2001.
The E-j paradigm, where electric field and current are taken to be the primary quantities, is the approach of single-particle calculation or kinetic analysis (with full consideration of collective effects).The single-particle approach is valid when the collective behavior of the plasma under consideration is relatively unimportant. The heart of this approach is the Boltzmann equation or the Vlasov equation in the collisionless regime. Equations used in the B-u paradigm can be derived from the Vlasov equation by adopting certain approximations, much like the classical equations in Newtonian mechanics can be obtained from the special relativity equation when the speed is small rel ative to the light speed.The E-j approach is useful for treating many macroscopic magne tospheric disturbances that cannot be explained with the B-u approach. Injection of particles in the inner magnetosphere during magnetos pheric substorms and the creation of the ring current population are prime examples. Substorm injection produces clear velocity dispersion over a large region in the inner magnetosphere, leading to the recognition of the injection boundary and inference of the location of the
particle source [Mcllwain, 1974]. Many velocity dispersion features are now found in largescale regions of the magnetosphere, such as in the low-altitude dayside cusp and the boundary of the plasma sheet.These advances are invaluable in the overall understanding of magnetospheric dynamics. Mounting evidence now suggests that mag netospheric substorms are a culmination of one or more physical processes occurring over multiple localized sites with intermittent disturbances rather than the consequence of a single large-scale disturbance as once thought. Thus, substorms are analogous to terrestrial thunderstorms. Both phenomena are basically an electric discharge of the system, involving transient and localized disturbances, and yet their effects spread over regions substantially larger than those of local discharges. The cul mination of these disturbances gives the appearance of a large-scale phenomenon. This early conceptual picture, which I pre sented at the 1988 AGU Fall Meeting and doc umented later in 1991 [Lui, 1991],fits well with the later development of the forced or self-organized criticality concept [Chang, 1992; Consolini, 1997].In fact,the magnetosphere exhibits both sudden changes of state resem bling the first-order phase transitions and more gradual ones resembling the second-order phase transitions. The system evolution in these situations calls for yet another approach,
Seismology Aseismic fault slip behavior, earthquake source parameter determination and seismicity changes, Wen-xuan Du, Columbia University, New York, Lynn Sykes, August 2002. Seismological studies of inner-core rotation, Anyi Li, Columbia University, New York, Paul Richards, February 2002. Solid Earth Geophysics
FORUM PAGE 460 A Section News item by John M. Greene, "The Parker Challenge"(2 May 2000,pp. 200-201) elicited these two responses.
Comment Recently John Greene took up the "Parker Challenge" [Parker, 1996]. Parker argued that the correct approach to understanding magnetospheric phenomena is to use the magnetic field and plasma flow velocity (B,u) as the primary quantities, with the electric field and current (E,j) being secondary in that E and j can be determined from B and u. Here, I propose the correct approach to be dependent on the phenomenon under investigation. There is no single paradigm that is always superior to others for treating macroscopic magnetospheric problems. Insisting on one particular approach as the only correct one is unjustified and may stifle innovative pursuit in research.Thus, I am motivated to address both the limitations of the B-u paradigm, and the merits of the E-j paradigm.
Honors Brian Tucker and Paul Wennberg have been selected as MacArthur Fellows, given and funded by the John D. and Catherine T. MacArthur Foundation.This "genius award" consists of $500,000, and is given annually to "talented individuals who have shown extraor dinary originality and dedication in their cre ative pursuits, and a marked capacity for self-direction." Tucker, a seismologist, has worked on preventing avoidable disasters in developing countries by using affordable civil engineering practices. He is founder of GeoHazards International, Palo Alto, California, a nonprofit international organ ization that seeks to reduce the consequences of earthquakes through preparedness and mitigation. Tucker has been an AGU member (Seismology) since 1970. Wennberg, an atmospheric chemist, has worked to significantly improve our understanding of natural and anthropogenic influences on the chemistry of the atmosphere. His research focuses on the upper troposphere and strato sphere. He applies traditional physical chem istry tools to study chemical transformations in the Earth's atmosphere, and to better under stand how ozone and greenhouse gases are formed. Wennberg has been an AGU member (Atmospheric Sciences) since 1989.
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Ocean Sciences
Thermal structure, magmatism, and evolution of fast-spreading mid-ocean ridges, Anjana K. Shah, Columbia University, NewYork,W Roger Buck, November 2001.
Eos, Vol. 83, No. 4 1 , 8 October 2002 such as the renormalization group [Chang, 1992] .There are many limitations of the B-u paradigm and merits of the E-j paradigm, and I shall not elaborate further about these here. Interested readers may find a more extensive discussion of these issues in Lui [2000], which also includes a discussion about the similarities and differences between current disruption and magnetic reconnection. In summary, however, the main objection to the B-u paradigm is its reliance on magnetohydrodynamics, when many of the processes occurring in the magnetosphere are inherently kinetic, requiring an approach that can better be summed up as the E-j paradigm. A longer, more detailed version of this article is available on the Eos Electronic Supplement; go to http://wwwagu.org/eos_elec/010093e.html.
Acknowledgments This work was supported by a NASA grant (NAG5-7797) to the Johns Hopkins University Applied Physics Laboratory Author A T.YLui The Johns Hopkins University Applied Physics Laboratory, Laurel, Md., USA
References Chang,T. S. C , Low dimensional behavior and sym metry breaking of stochastic systems near criticality c a n these effects be observed in s p a c e and in the laboratory? IEEE Trans. Plasma ScL,
20,691,1992.
Consolini,G.,Sandpile cellular automata and magnetospheric dynamics, Proc. Cosmic Physics in the Year 2000,58,
edited by S. Aiello et al., Societa
Italiana di Fisica, Bologna, Italy 1997. Lui, A.T.Y, Plasma transport in the Earth's magnetotail, in Modeling Magnetospheric Plasma Processes, edited by G.R.Wilson and M.O.Chandler,AGU Geophys. Monogr,Ser,
63, p. 4 1 , 1 9 9 1 .
Lui, A.T.Y, Electric current approach to magnetos pheric dynamics and the distinction between cur rent disruption and magnetic reconnection, Magnetospheric Current Systems, AGU Monogr.Ser,
Geophys.
118, p. 3 1 , 2 0 0 0 .
McIlwain,C.E.,Substorm injection boundaries, in Magnetospheric Physics, edited by B. M. McCormac, p. 143, D. Reidel, Hingham, Mass., 1974.
Parker, E.N.,The alternative paradigm for magnetos pheric physics,./ Geophys. Res.,
101,10,587,1996.
Comment Fundamental to considering the relative merits of a (B, V) versus an (E,j) description of a plasma, one must first define the bulk fluid velocity and characterize the dissipation mechanism responsible for fluid cohesion. While the equations of magnetohydrodynamics (MHD) owe a heritage to studies of Coulombcollisionally dominated plasma, many geospace regions of interest are, in fact, Coulomb-collisionless.Yet, the fluid equations of collisional MHD give a remarkably good and predictive description of this plasma. Still, collisionless plasma have qualities not captured by the col lisional MHD paradigm, and their recognition requires an interpretation of the bulk fluid velocity that distinguishes the frozen-in frame and the flux-defined fluid velocity of conven tional MHD. The fluid cohesiveness of collisionless plasma is maintained by hydromagnetic fluctuations, and modern turbulence theory recognizes that the fluid and field variables can be resolved into fluctuations and averages. When we speak of the Parker spiral or acceleration of the solar wind, we are speaking of average quantities. For a collisionless fluid, the fluctuations drive the dissipation in the average fluid variables. Anal ogously one can define an average particle dis tribution function. Resolving theVlasov equation into averages and fluctuations relates the evo lution of the average of the particle distribution to its fluctuations.The full Vlasov equation describes microprocesses that in concert affect the bulk flow If the bulk flow only is of interest, a reasonable parameterization of the microprocesses, such as scaling with gyro-radius, can simplify calcu lation of the average distribution and its inte grals while still capturing the essential magnitude and momentum dependence of the dissipative mechanisms. Just as the Boltzmann collision term can be modeled as a relaxation to Maxwellian,the fluctuations driving the evolution of the average distribution can be modeled as relaxation to isotropy. Relaxation to isotropy is generally a dominant dissipation mode for collisionless plasma, and it is all that is necessary to describe the generic non-thermal collisionless heating
ABOUT A G U PAGE 460
B.Randall Tufts (1948-2002) Randy Tufts, an explorer who made major discoveries on Earth and beyond, died on 1 April.The date surprised no one who knew him. Nationally known for his model of envi ronmental stewardship, which stemmed from
his co-discovery of Kartchner Caverns in Arizona, Tufts more recently had turned to planetary exploration and conducted path-breaking research into the geology and geophysics of Jupiter's moon Europa. Tufts began spelunking as a high school stu dent, and he vowed to friends that one day he would discover a cave. As with everything he did, he committed himself completely to reaching that goal. He majored in geology at the Uni versity of Arizona and combed the region for
that is at work all the way to cosmic-ray energies. Any shearing or accelerating collisionless plasma will dissipate bulk kinetic energy as non-thermal heat that will back-react on the bulk plasma.The momentum scaling of the hydromagnetic relaxation time scale and non thermal particle distributions produce transport coefficient integrals that are quite different from what one would expect for a collisional plasma. Our description of dissipation sets the fluid frame, and relaxation to isotropy of the average distribution must occur in the frame in which the average electric field vanishes.This must define the frame for the fluid velocity of a col lisionless MHD description. It is the natural frame from which to characterize the hydromagnetic dissipation. Defining the average fluid velocity U as the frame in which the electric field vanishes is an alternate premise from which one recovers a set of equations similar, but not identical to, the standard equations of MHD. Defining the fluid velocity in such a way is at odds with the traditional definition of bulk velocity V implied by standard MHD: mW = ffPcfP such that P = p + mV. But if the electric field must vanish in the fluid frame, then /fpcfp = F 5 * 0. One then introduces modifi cations to the momentum equation of the form: 5
dfmlP + F ) ~ V ( F * + F U + PlP +m^U U 1
6
a
b
}
The extra term F corresponds to a fluid-frame mass flux. It describes diffusion through a col lisionless co-moving fluid flow and plays an essential role in models of cosmic-ray hydro dynamics. It includes contributions from diffusive flux and fluid-frame acceleration. Augmenting the transport equation for the average distri bution in terms of the fluid flow, one employs the momentum equation above to describe the fluid flow in terms of integrals of the distri bution. One can then uniquely specify the col lisionless fluid with the isotropic part of the distribution and the fluid velocity Moments of the distribution yield a fully fluid description involving hydromagnetic transport coefficients. For further discussion, see Williams, L. L., Nonclassical theory of turbulent collisionless p\
