The Trade Effects of Preferential Arrangements: New Evidence from the Australia Productivity Commission

This paper critically examines “new” evidence from the gravity model that indicates the majority of preferential trading arrangements (PTAs) today are predominantly trade diverting. This new evidence on trade diversion was presented in a recent Australia Productivity Commission (APC) working paper. Although no major faults are found in the methodology of the APC study, the present analysis finds the opposite conclusion—that the majority of current PTAs are predominantly trade creating—when a variant of the gravity model formulated by Andrew Rose is applied to upto- date regression data using a variety of econometric methods, including the Tobit regression method employed by the APC study.trade policy, preferential trading arrangements, free trade agreements, gravity models

Predicting Trade Expansion under FTAs and Multilateral Agreements

This paper examines the historical record of eight recent free trade agreements (FTAs). It also investigates the predictive power of two popular quantitative world trade models—the single-equation gravity model and the multiequation comput-able general equilibrium (CGE) model—as applied to three major trade liberalization agreements adopted during the 1990s: Mercosur, NAFTA, and the Uruguay Round Agreement, using the Rose gravity model and the GTAP general equilibrium model. Both models are found accurate in some instances, but intervening influences in the wake of trade liberalization episodes confound the challenge of drawing a strong conclusion in favor of one modeling approach over the other. Between the “naïve” gravity model and “naïve” CGE model predictions, we find that the former tends to overpredict intrabloc trade expansion (especially over horizons of five years and less) while the latter tends to underpredict. CGE models remain favored for ex post analysis of welfare impacts and the direct and indirect linkages between policy reforms and the numerous other economic variables of concern to policymakers and the public at large.gravity models, CGE models, regional trading arrangements

Using Coronal Loops to Reconstruct the Magnetic Field of an Active Region Before and After a Major Flare

The shapes of solar coronal loops are sensitive to the presence of electrical currents that are the carriers of the nonpotential energy available for impulsive activity. We use this information in a new method for modeling the coronal magnetic field of AR 11158 as a nonlinear force-free field (NLFFF). The observations used are coronal images around time of major flare activity on 2011/02/15, together with the surface line-of-sight magnetic field measurements. The data are from the Helioseismic and Magnetic Imager and Atmospheric Imaging Assembly (HMI and AIA, respectively) onboard the Solar Dynamics Observatory (SDO). The model fields are constrained to approximate the coronal loop configurations as closely as possible, while also subject to the force-free constraints. The method does not use transverse photospheric magnetic field components as input, and is thereby distinct from methods for modeling NLFFFs based on photospheric vector magnetograms. We validate the method using observations of AR 11158 at a time well before major flaring, and subsequently review the field evolution just prior to and following an X2.2 flare and associated eruption. The models indicate that the energy released during the instability is about $1\times10^{32}$ erg, consistent with what is needed to power such a large eruptive flare. Immediately prior to the eruption the model field contains a compact sigmoid bundle of twisted flux that is not present in the post-eruption models, which is consistent with the observations. The core of that model structure is twisted by $\approx0.9$ full turns about its axis.Comment: ApJ, in pres

Agricultural trade and rural development in the Middle East and North Africa: recent developments and prospects

Despite petroleum's prominence in the Middle East and North Africa (MENA), agriculture remains important to most to the region's economies. And more robust and more dynamic agricultural growth could significantly boost sustainable economic growth and rural development in those countries. An appropriate avenue for improving agricultural growth is to expand agricultural exports in MENA countries with appreciable-to-large agricultural sectors and comparative advantage in cereal grains, agricultural raw materials, fruits and vegetables, and many categories of livestock and dairy products. But high levels of protection in many MENA countries, especially for manufactures and some food products, contribute to overvalued exchange rates and a significant bias against agriculture. Trade liberalization and other economic reforms to promote agriculture and improve rural welfare in MENA might be pursued through regional economic cooperation, but should be guided as much as possible by the principles of"open regionalism"- under which trade concessions negotiated between regional trading partners could be extended unconditionally to all trading partners, including trading partners outside the region.Economic Theory&Research,Environmental Economics&Policies,Agricultural Knowledge&Information Systems,TF054105-DONOR FUNDED OPERATION ADMINISTRATION FEE INCOME AND EXPENSE ACCOUNT,Agricultural Research

Reconstructing the Local Twist of Coronal Magnetic Fields and the Three-Dimensional Shape of the Field Lines from Coronal Loops in EUV and X-Ray Images

Non-linear force-free fields are the most general case of force-free fields, but the hardest to model as well. There are numerous methods of computing such fields by extrapolating vector magnetograms from the photosphere, but very few attempts have so far made quantitative use of coronal morphology. We present a method to make such quantitative use of X-Ray and EUV images of coronal loops. Each individual loop is fit to a field line of a linear force-free field, allowing the estimation of the field line's twist, three-dimensional geometry and the field strength along it. We assess the validity of such a reconstruction since the actual corona is probably not a linear force-free field and that the superposition of linear force-free fields is generally not itself a force-free field. To do so, we perform a series of tests on non-linear force-free fields, described in Low & Lou (1990). For model loops we project field lines onto the photosphere. We compare several results of the method with the original field, in particular the three-dimensional loop shapes, local twist (coronal alpha), distribution of twist in the model photosphere and strength of the magnetic field. We find that, (i) for these trial fields, the method reconstructs twist with mean absolute deviation of at most 15% of the range of photospheric twist, (ii) that heights of the loops are reconstructed with mean absolute deviation of at most 5% of the range of trial heights and (iii) that the magnitude of non-potential contribution to photospheric field is reconstructed with mean absolute deviation of at most 10% of the maximal value.Comment: submitted to Ap

Constraining 3D Magnetic Field Extrapolations Using The Twin Perspectives of STEREO

The 3D magnetic topology of a solar active region (NOAA 10956) was reconstructed using a linear force-free field extrapolation constrained using the twin perspectives of \emph{STEREO}. A set of coronal field configurations was initially generated from extrapolations of the photospheric magnetic field observed by the Michelson Doppler Imager (MDI) on \emph{SOHO}. Using an EUV intensity-based cost function, the extrapolated field lines that were most consistent with 171\AA\ passband images from the Extreme UltraViolet Imager (EUVI) on \emph{STEREO} were identified. This facilitated quantitative constraints to be placed on the twist ($\alpha$) of the extrapolated field lines, where $\nabla \times {\bf B} = \alpha {\bf B}$. Using the constrained values of $\alpha$, the evolution in time of twist, connectivity, and magnetic energy were then studied. A flux emergence event was found to result in significant changes in the magnetic topology and total magnetic energy of the region

Nonlinear force-free field modeling of a solar active region using SDO/HMI and SOLIS/VSM data

We use SDO/HMI and SOLIS/VSM photospheric magnetic field measurements to model the force-free coronal field above a solar active region, assuming magnetic forces to dominate. We take measurement uncertainties caused by, e.g., noise and the particular inversion technique into account. After searching for the optimum modeling parameters for the particular data sets, we compare the resulting nonlinear force-free model fields. We show the degree of agreement of the coronal field reconstructions from the different data sources by comparing the relative free energy content, the vertical distribution of the magnetic pressure and the vertically integrated current density. Though the longitudinal and transverse magnetic flux measured by the VSM and HMI is clearly different, we find considerable similarities in the modeled fields. This indicates the robustness of the algorithm we use to calculate the nonlinear force-free fields against differences and deficiencies of the photospheric vector maps used as an input. We also depict how much the absolute values of the total force-free, virial and the free magnetic energy differ and how the orientation of the longitudinal and transverse components of the HMI- and VSM-based model volumes compares to each other.Comment: 9 pages, 5 figure

Coronal radiation belts

The magnetic field of the solar corona has a large-scale dipole character, which maps into the bipolar field in the solar wind. Using standard representations of the coronal field, we show that high-energy ions can be trapped stably in these large-scale closed fields. The drift shells that describe the conservation of the third adiabatic invariant may have complicated geometries. Particles trapped in these zones would resemble the Van Allen Belts and could have detectable consequences. We discuss potential sources of trapped particles

The Influence of Spatial Resolution on Nonlinear Force-Free Modeling

The nonlinear force-free field (NLFFF) model is often used to describe the solar coronal magnetic field, however a series of earlier studies revealed difficulties in the numerical solution of the model in application to photospheric boundary data. We investigate the sensitivity of the modeling to the spatial resolution of the boundary data, by applying multiple codes that numerically solve the NLFFF model to a sequence of vector magnetogram data at different resolutions, prepared from a single Hinode/SOT-SP scan of NOAA Active Region 10978 on 2007 December 13. We analyze the resulting energies and relative magnetic helicities, employ a Helmholtz decomposition to characterize divergence errors, and quantify changes made by the codes to the vector magnetogram boundary data in order to be compatible with the force-free model. This study shows that NLFFF modeling results depend quantitatively on the spatial resolution of the input boundary data, and that using more highly resolved boundary data yields more self-consistent results. The free energies of the resulting solutions generally trend higher with increasing resolution, while relative magnetic helicity values vary significantly between resolutions for all methods. All methods require changing the horizontal components, and for some methods also the vertical components, of the vector magnetogram boundary field in excess of nominal uncertainties in the data. The solutions produced by the various methods are significantly different at each resolution level. We continue to recommend verifying agreement between the modeled field lines and corresponding coronal loop images before any NLFFF model is used in a scientific setting.Comment: Accepted to ApJ; comments/corrections to this article are welcome via e-mail, even after publicatio

Solar Multi-Scale Convection and Rotation Gradients Studied in Shallow Spherical Shells

The differential rotation of the sun, as deduced from helioseismology, exhibits a prominent radial shear layer near the top of the convection zone wherein negative radial gradients of angular velocity are evident in the low- and mid-latitude regions spanning the outer 5% of the solar radius. Supergranulation and related scales of turbulent convection are likely to play a significant role in the maintenance of such radial gradients, and may influence dynamics on a global scale in ways that are not yet understood. To investigate such dynamics, we have constructed a series of three-dimensional numerical simulations of turbulent compressible convection within spherical shells, dealing with shallow domains to make such modeling computationally tractable. These simulations are the first models of solar convection in a spherical geometry that can explicitly resolve both the largest dynamical scales of the system (of order the solar radius) as well as smaller-scale convective overturning motions comparable in size to solar supergranulation (20--40 Mm). We find that convection within these simulations spans a large range of horizontal scales, and that the radial angular velocity gradient in these models is typically negative, especially in low- and mid-latitude regions. Analyses of the angular momentum transport indicates that such gradients are maintained by Reynolds stresses associated with the convection, transporting angular momentum inward to balance the outward transport achieved by viscous diffusion and large-scale flows in the meridional plane. We suggest that similar mechanisms associated with smaller-scale convection in the sun may contribute to the maintenance of the observed radial shear layer located immediately below the solar photosphere.Comment: 45 pages, 17 figures, ApJ in press. A preprint of paper with hi-res figures can be found at http://www-lcd.colorado.edu/~derosa/modelling/modelling.htm