Modeling currents at satellite altitudes

A mathematical formalism for modeling the poloidal magnetic field and current density at satellite altitudes is presented

Comparing hard and soft prior bounds in geophysical inverse problems

In linear inversion of a finite-dimensional data vector y to estimate a finite-dimensional prediction vector z, prior information about X sub E is essential if y is to supply useful limits for z. The one exception occurs when all the prediction functionals are linear combinations of the data functionals. Two forms of prior information are compared: a soft bound on X sub E is a probability distribution p sub x on X which describeds the observer's opinion about where X sub E is likely to be in X; a hard bound on X sub E is an inequality Q sub x(X sub E, X sub E) is equal to or less than 1, where Q sub x is a positive definite quadratic form on X. A hard bound Q sub x can be softened to many different probability distributions p sub x, but all these p sub x's carry much new information about X sub E which is absent from Q sub x, and some information which contradicts Q sub x. Both stochastic inversion (SI) and Bayesian inference (BI) estimate z from y and a soft prior bound p sub x. If that probability distribution was obtained by softening a hard prior bound Q sub x, rather than by objective statistical inference independent of y, then p sub x contains so much unsupported new information absent from Q sub x that conclusions about z obtained with SI or BI would seen to be suspect

Reverse Engineering the Yield Curve

Prices of riskfree bonds in any arbitrage-free environment are governed by a pricing kernel: given a kernel, we can compute prices of bonds of any maturity we like. We use observed prices of multi-period bonds to estimate, in a log-linear theoretical setting, the pricing kernel that gave rise to them. The high-order dynamics of our estimated kernel help to explain why first-order, one-factor models of the term structure have had difficulty reconciling the shape of the yield curve with the persistence of the short rate. We use the estimated kernel to provide a new perspective on Hansen-Jagannathan bounds, the price of risk, and the pricing of bond options and futures.

Does the geoid drift west?

In 1970 Hide and Malin noted a correlation of about 0.8 between the geoid and the geomagnetic potential at the Earth's surface when the latter is rotated eastward in longitude by about 160 degrees and the spherical harmonic expansions of both functions are truncated at degree 4. From a century of magnetic observatory data, Hide and Malin inferred an average magnetic westward drift rate of about 0.27 degrees/year. They attributed the magnetic-gravitational correlation to a core event at about 1350 A.D. which impressed the mantle's gravity pattern at long wavelengths onto the core motion and the resulting magnetic field. The impressed pattern was then carried westward 160 degrees by the nsuing magnetic westward drift. An alternative possibility is some sort of steady physical coupling between the magnetic and gravitational fields (perhaps migration of Hide's bumps on the core-mantle interface). This model predicts that the geoid will drift west at the magnetic rate. On a rigid earth, the resulting changes in sea level would be easily observed, but they could be masked by adjustment of the mantle if it has a shell with viscosity considerably less than 10 to the 21 poise. However, steady westward drift of the geoid also predicts secular changes in g, the local acceleration of gravity, at land stations. These changes are now ruled out by recent independent high-accuracy absolute measurements of g made by several workers at various locations in the Northern Hemisphere

Relative price movements in dynamic general equilibrium models of international trade

We examine the behavior of international relative prices from the perspective of dynamic general equilibrium theory, with particular emphasis on the variability of the terms of trade and the relation between the terms of trade and net exports. We highlight aspects of the theory that are critical in determining these properties, contrast our perspective with those associated with the Marshall-Lerner condition and the Harberger-Laursen-Metzler effect, and point out features of the data that have proved difficult to explain within existing dynamic general equilibrium models.International trade

Dynamics of the trade balance and the terms of trade: the S-curve

We provide a theoretical interpretation of two features of international data: the countercyclical movements in net exports and the tendency for the trade balance to be negatively correlated with current and future movements in the terms of trade, but positively correlated with past movements. We document these same properties in a two-country stochastic growth model in which trade fluctuations reflect, in large part, the dynamics of capital formation. We find that the general equilibrium perspective is essential: The relation between the trade balance and the terms of trade depends critically on the source of fluctuations.Balance of trade

The scientific case for magnetic field satellites

To make full use of modern magnetic data and the paleomagnetic record, we must greatly improve our understanding of how the geodynamo system works. It is clearly nonlinear, probably chaotic, and its dimensionless parameters cannot yet be reproduced on a laboratory scale. It is accessible only to theory and to measurements made at and above the earth's surface. These measurements include essentially all geophysical types. Gravity and seismology give evidence for undulations in the core-mantle boundary (CMB) and for temperature variations in the lower mantle which can affect core convection and hence the dynamo. VLBI measurements of the variations in the Chandler wobble and length of day are affected by, among other things, the electromagnetic and mechanical transfer of angular momentum across the CMB. Finally, measurements of the vector magnetic field, its intensity, or its direction, give the most direct access to the core dynamo and the electrical conductivity of the lower mantle. The 120 gauss coefficients of degrees up to 10 probably come from the core, with only modest interference by mantle conductivity and crustal magnetization. By contrast, only three angular accelerations enter the problem of angular momentum transfer across the CMB. Satellite measurements of the vector magnetic field are uniquely able to provide the spatial coverage required for extrapolation to the CMB, and to isolate and measure certain magnetic signals which to the student of the geodynamo represent noise, but which are of great interest elsewhere in geophysics. Here, these claims are justified and the mission parameters likely to be scientifically most useful for observing the geodynamo system are described