We use a mechanistically based ecosystem simulation model to describe and analyze the spatial and temporal patterns of terrestrial net primary productivity (NPP) in South America. The Terrestrial Ecosystem Model (TEM) is designed to predict major carbon and nitrogen fluxes and pool sizes in terrestrial ecosystems at continental to global scales. Information from intensively studies field sites is used in combination with continental-scale information on climate, soils, and vegetation to estimate NPP in each of 5888 non-wetland, 0.5° latitude °0.5° longitude grid cells in South America, at monthly time steps. Preliminary analyses are presented for the scenario of natural vegetation throughout the continent, as a prelude to evaluating human impacts on terrestrial NPP. The potential annual NPP of South America is estimated to be 12.5 Pg/yr of carbon (26.3 Pg/yr of organic matter) in a non-wetland area of 17.0 ° 106 km2 . More than 50% of this production occurs in the tropical and subtropical evergreen forest region. Six independent model runs, each based on an independently derived set of model parameters, generated mean annual NPP estimates for the tropical evergreen forest region ranging from 900 to 1510 g°m-2 °yr-1 of carbon, with an overall mean of 1170 g°m-2 °yr-1 . Coefficients of variation in estimated annual NPP averaged 20% for any specific location in the evergreen forests, which is probably within the confidence limits of extant NPP measurements. Predicted rates of mean annual NPP in other types of vegetation ranged from 95 g°m-2 °yr-1 in arid shrublands to 930 g°m@ ?yr-1 in savannas, and were within the ranges measured in empirical studies. The spatial distribution of predicted NPP was directly compared with estimates made using the Miami mode of Lieth (1975). Overall, TEM predictions were °10% lower than those of the Miami model, but the two models agreed closely on the spatial patterns of NPP in south America. Unlike previous models, however, TEM estimates NPP monthly, allowing for the evaluation of seasonal phenomena. This is an important step toward integration of ecosystem models with remotely sensed information, global climate models, and atmospheric transport models, all of which are evaluated at comparable spatial and temporal scales. Seasonal patterns of NPP in South America are correlated with moisture availability in most vegetation types, but are strongly influenced by seasonal differences in cloudiness in the tropical evergreen forests. On an annual basis, moisture availability was the factor that was correlated most strongly with annual NPP in South America, but differences were again observed among vegetation types. These results allow for the investigation and analysis of climatic controls over NPP at continental scales, within and among vegetation types, and within years. Further model validation is needed. Nevertheless, the ability to investigate NPP-environment interactions with a high spatial and temporal resolution at continental scales should prove useful if not essential for rigorous analysis of the potential effects of global climate changes on terrestrial ecosystems.

/pdf/potential-net-primary-productivity-in-south-america-2xjja5q7y2.pdf

Potential Net Primary Productivity in South America: Application of a Global Model

The diffuse X-ray emission from the thin disk surrounding the Galactic mid-plane (the so-called Galactic ridge) was measured with RXTE PCA in order to determine the spatial extent, spectral nature, and origin of the emission. Spatial analysis of the emission in the central 30° of the plane in Galactic longitude reveals the presence of a thin component of width ≈0°.2 and a broad component that can be approximated as a Gaussian distribution with FWHM of ≈8°. The spectrum can be interpreted as the superposition of the two spatial components with differing amount of absorption. Each spatial component can be modelled with a Raymond-Smith plasma component of temperature ∼ 2 keV and a power law component with photon index of ∼ 2.5. Based on these findings, we argue that the temperature of the hot phase of the Interstellar Medium (ISM) is less than the previously reported values of 5 – 15 keV. We find that a supernova explosion rate of ∼ 1 – 5 per century is adequate to power the thermal emission from the ridge. We discuss the origin of the emission in the hard X-ray band in terms of non-thermal bremsstrahlung of accelerated electrons and protons in supernova remnant (SNR) sites.

RXTE measurement of the diffuse X-ray emission from the galactic ridge: Implications for the energetics of the interstellar medium

We present here a new calculation of the gamma-ray spectrum from pp->pi^0 in the Galactic ridge environment. The calculation includes the diffractive pp interaction and incorporates the Feynman scaling violation for the first time. Galactic diffuse gamma-rays come, predominantly, from pi^0->gamma gamma in the sub-GeV to multi-GeV range. Hunter et al. found, however, an excess in the GeV range ("GeV Excess") in the EGRET Galactic diffuse spectrum above the prediction based on experimental pp->pi^0 cross-sections and the Feynman scaling hypothesis. We show, in this work, that the diffractive process makes the gamma-ray spectrum harder than the incident proton spectrum by ~0.05 in power-law index, and, that the scaling violation produces 30-80% more pi^0 than the scaling model for incident proton energies above 100GeV. Combination of the two can explain about a half of the "GeV Excess" with the local cosmic proton (power-law index ~2.7). The excess can be fully explained if the proton spectral index in the Galactic ridge is a little harder (~0.2 in power-law index) than the local spectrum. Given also in the paper is that the diffractive process enhances e^+ over e^- and the scaling violation gives 50-100% higher p-bar yield than without the violation, both in the multi-GeV range.

Diffractive Interaction and Scaling Violation in pp->pi^0 Interaction and GeV Excess in Galactic Diffuse Gamma-Ray Spectrum of EGRET

We present a search for high-energy gamma-ray emission from nine nearby starburst galaxies and M31 with the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory (CGRO) Although the diffuse gamma-ray emission from starburst galaxies was suspected to be detectable, we find no emission from NGC 253, from M82, or from the average of all nine galaxies The 2 σ upper limit for the EGRET flux above 100 MeV for the averaged survey observations is 18 × 10-8 photons cm-2 s-1 From a model of the expected radio and gamma-ray emission, we find that the magnetic field in the nuclei of these galaxies is greater than 25 μG, and the ratio of proton and electron densities is less than 400 The EGRET limits indicate that the rate of massive star formation in the survey galaxies is only about an order of magnitude higher than in the Milky Way The upper limit to the gamma-ray flux above 100 MeV for M31 is 16 × 10-8 photons cm-2 s-1 At the distance of M31, the Milky Way flux would be over twice this value, indicating higher gamma-ray emissivities in our Galaxy Therefore, since the supernova rate of the Milky Way is higher than that in M31, our null detection of M31 supports the theory of the supernova origin of cosmic rays in galaxies

/pdf/diffuse-gamma-ray-emission-from-starburst-galaxies-and-m31-4a6q2qn293.pdf

Diffuse Gamma-Ray Emission from Starburst Galaxies and M31

We present a search for high energy gamma-ray emission from 9 nearby starburst galaxies and M31 with the EGRET instrument aboard CGRO. Though the diffuse gamma-ray emission from starburst galaxies was suspected to be detectable, we find no emission from NGC 253, M82 nor from the average of all 9 galaxies. The 2 sigma upper limit for the EGRET flux above 100 MeV for the averaged survey observations is 1.8 x 10-8 ph cm-2 s-1. From a model of the expected radio and gamma-ray emission, we find that the magnetic field in the nuclei of these galaxies is > 25 micro Gauss, and the ratio of proton and electron densities is < 400. The EGRET limits indicate that the rate of massive star formation in the survey galaxies is only about an order of magnitude higher than in the Milky Way. The upper limit to the gamma-ray flux above 100 MeV for M31 is 1.6 x 10-8 ph cm-2 s-1. At the distance of M31, the Milky Way flux would be over twice this value, indicating higher gamma-ray emissivities in our Galaxy. Therefore, since the supernova rate of the Milky Way is higher than in M31, our null detection of M31 supports the theory of the supernova origin of cosmic rays in galaxies.

There is active interest in the extent to which unresolved γ-ray pulsars contribute to the Galactic diffuse emission, and in whether unresolved γ-ray pulsars could be responsible for the excess of diffuse Galactic emission above 1 GeV that has been observed by EGRET. The diffuse γ-ray intensity due to unresolved pulsars is directly linked to the number of objects that should be observed in the EGRET data. We can therefore use our knowledge of the unidentified EGRET sources to constrain model parameters like the pulsar birthrate and their beaming angle. This analysis is based only on the properties of the six pulsars that have been identified in the EGRET data and is independent of choice of a pulsar emission model. We find that pulsars contribute very little to the diffuse emission at lower energies, whereas above 1 GeV they can account for 18% of the observed intensity in selected regions for a reasonable number of directly observable γ-ray pulsars (~14). The latitude distribution of the diffuse emission caused by unresolved pulsars is narrower than that of the observed diffuse emission. While the excess above 1 GeV γ-ray energy is observed up to at least 6°-8° off the plane, the pulsar contribution would be small there. Thus, pulsars do significantly contribute to the diffuse Galactic γ-ray emission above 1 GeV, but they cannot be made responsible for all the discrepancies between observed intensity and model predictions in this energy range.

The pulsar contribution to the diffuse galactic gamma-ray emission

We present a concept for an imaging gamma-ray polarimeter operating from ~50 MeV to ~1 GeV. Such an instrument would be valuable for the study of high-energy pulsars, active galactic nuclei, supernova remnants, and gamma-ray bursts. The concept makes use of pixelized gas micro-well detectors, under development at Goddard Space Flight Center, to record the electron-positron tracks from pair-production events in a large gas volume. Pixelized micro-well detectors have the potential to form large-volume 3-D track imagers with ~100 micron (rms) position resolution at moderate cost. The combination of high spatial resolution and a continuous low-density gas medium permits many thousands of measurements per radiation length, allowing the particle tracks to be imaged accurately before multiple scattering masks their original directions. The polarization of the incoming radiation may then be determined from the azimuthal distribution of the electron-positron pairs. We have performed Geant4 simulations of these processes to estimate the polarization sensitivity of a simple telescope geometry at 100 MeV.

/pdf/a-concept-for-a-high-energy-gamma-ray-polarimeter-3o5y7tse4l.pdf

A Concept for a High-Energy Gamma-ray Polarimeter

We present a concept for an imaging gamma-ray polarimeter operating from approx. 50 MeV to approx. 1 GeV. Such an instrument would be valuable for the study of high-energy pulsars, active galactic nuclei, supernova remnants, and gamma-ray bursts. The concept makes use of pixelized gas micro-well detectors, under development at Goddard Space Flight Center, to record the electron-positron tracks from pair-production events in a large gas volume. Pixelized micro-well detectors have the potential to form large-volume 3-D track imagers with approx. 100 micron (rms) position resolution at moderate cost. The combination of high spatial resolution and a continuous low-density gas medium permits many thousands of measurements per radiation length, allowing the particle tracks to be imaged accurately before multiple scattering masks their original directions. The polarization of the incoming radiation may then be determined from the azimuthal distribution of the electron-positron pairs. We have performed Geant4 simulations of these processes to estimate the polarization sensitivity as a function of instrument parameters and event selection criteria.

/pdf/a-concept-for-a-high-energy-gamma-ray-polarimeter-4cguywjvvv.pdf

A concept for a high-energy gamma-ray polarimeter

There is active interest in the extent to which unresolved gamma-ray pulsars contribute to the galactic diffuse emission, and whether unresolved gamma-ray pulsars could be responsible for the excess of diffuse galactic emission above 1 GeV which has been observed by EGRET. The diffuse gamma-ray intensity due to unresolved pulsars is directly linked to the number of objects which should be observed in the EGRET data. We can therefore use our knowledge of the unidentified EGRET sources to constrain model parameters like the pulsar birth rate, their beaming angle, etc. This analysis is based only on the properties of the six pulsars which have been identified in the EGRET data, and is independent of choice of a pulsar emission model. We find that pulsars contribute very little to the diffuse emission at lower energies, whereas above 1 GeV they can account for 18% of the observed intensity in selected regions for a reasonable number of directly observable gamma-ray pulsars (14). The latitude distribution of the diffuse emission caused by unresolved pulsars is narrower than that of the observed diffuse emission. While the excess above 1 GeV gamma-ray energy is observed up to at least 6-8 degrees off the plane, the pulsar contribution would be small there. Thus pulsars do significantly contribute to the diffuse galactic gamma-ray emission above 1 GeV, but they can not be made responsible for all the discrepancy between observed intensity and model predictions in this energy range.

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The pulsar contribution to the diffuse galactic gamma-ray emission

A Concept for a High-Energy Gamma-ray Polarimeter

A concept for a high-energy gamma-ray polarimeter

The pulsar contribution to the diffuse galactic gamma-ray emission