Ohm's Law for Plasma in General Relativity and Cowling's Theorem

The general-relativistic Ohm's law for a two-component plasma which includes the gravitomagnetic force terms even in the case of quasi-neutrality has been derived. The equations that describe the electromagnetic processes in a plasma surrounding a neutron star are obtained by using the general relativistic form of Maxwell equations in a geometry of slow rotating gravitational object. In addition to the general-relativistic effect first discussed by Khanna \& Camenzind (1996) we predict a mechanism of the generation of azimuthal current under the general relativistic effect of dragging of inertial frames on radial current in a plasma around neutron star. The azimuthal current being proportional to the angular velocity $\omega$ of the dragging of inertial frames can give valuable contribution on the evolution of the stellar magnetic field if $\omega$ exceeds $2.7\times 10^{17} (n/\sigma) \textrm{s}^{-1}$ ($n$ is the number density of the charged particles, $\sigma$ is the conductivity of plasma). Thus in general relativity a rotating neutron star, embedded in plasma, can in principle generate axial-symmetric magnetic fields even in axisymmetry. However, classical Cowling's antidynamo theorem, according to which a stationary axial-symmetric magnetic field can not be sustained against ohmic diffusion, has to be hold in the general-relativistic case for the typical plasma being responsible for the rotating neutron star.Comment: Accepted for publication in Astrophysics & Space Scienc

Risco climático do feijão da seca no Estado de Goiás, sob preparo de solo convencional e plantio direto Climatic risk of rainfed beans in the State of Goiás, Brazil, under conventional and no-till systems

Na região dos Cerrados, os agricultores têm adotado o sistema plantio direto (SPD) como forma de proteger o ambiente e dar sustentabilidade à exploração agrícola. Neste sistema, a cobertura do solo pela palhada pode minimizar o risco climático das culturas de sequeiro, pela redução do déficit hídrico. O objetivo deste trabalho foi comparar o risco climático do feijão da "seca" no Estado de Goiás, sob preparo de solo convencional e plantio direto. Foram utilizados o modelo BIPZON para o cálculo do balanço hídrico e um sistema de informações geográficas para a espacialização dos resultados, além de identificadas as áreas de baixo, médio e alto risco climático para esta cultura, para o SPD, com 50 e 100% de cobertura do solo pela palhada e para o sistema convencional de preparo de solo. Verificou-se que a utilização do SPD causou aumento das áreas de menor risco e prolongamento do período favorável de semeadura a esta cultura, em relação ao sistema de preparo convencional. O efeito foi mais pronunciado com 100% de cobertura do solo.<br>In the Cerrado Region, farmers have been adopting the no-till system, to protect the soil and give sustainability to agriculture. In that system, soil covered by crop residues can minimize climatic risks by reducing water deficit of crops sowed in late season. The objective of this study was to compare the climatic risk of dry season common beans grown in the State of Goiás, under conventional and no-till systems. The BIPZON model was used to calculate the water balance. Geographic Information System was used for the characterization of the results in time and space. Areas of low, medium and high climatic risks were identified for the no-till system, with 50 and 100% of soil coverage by crop residues, as well as for the conventional till system. It was verified that the use of no-till system increased the areas with lower risks and extended the period of sowing for the bean crop in relation to conventional till system. The effect was higher with 100% soil covered with crop residues