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Using the bow shock crossing events from four spacecraft: IMP 8, Geotail, Magion‐4, and Cluster 1, a new three‐dimensional asymmetric bow shock model is constructed. The model is parameterized by the solar wind dynamic pressure, the interplanetary magnetic field, magnetosonic Mach number, solar wind β, and the Earth's dipole tilt angle. It is shown that the shape and size of bow shock are both affected by the dipole tilt angle. The dipole tilt angle causes asymmetries in the meridional plane: (1) the bow shock subsolar standoff distance and the north‐south asymmetry increase with the dipole tilt angle; (2) as the dipole tilt angle increases, the shock flaring angle in the equatorial plane is slightly reduced, while in the meridional plane the flaring angle obviously decreases in Southern Hemisphere and keeps almost unchanged in the Northern Hemisphere. The flaring angle in the Northern Hemisphere is larger than in the Southern Hemisphere; (3) the effects of negative dipole tilt angle on shock flaring are just the opposite of those for positive tilt, and the effects of dipole tilt angle on the shape of the bow shock are north‐south symmetric. The model results are also validated by comparing with one previous empirical model and with observational crossings, and it is demonstrated that the new model is able to predict the observed crossings more accurately and can better describe the rotational asymmetry and north‐south asymmetry of the Earth's bow shock. Key Points: We construct an asymmetric three‐dimensional bow shock model with dipole tilt effectsShape and size of bow shock are both affected by the dipole tilt angleThe dipole tilt angle causes asymmetries of bow shock in the meridional plane [ABSTRACT FROM AUTHOR]

Abstract: In this study we use the bow shock crossings contained in the Space Physics Data Facility database, collected by four spacecraft (IMP 8, Geotail, Magion‐4, and Cluster1) to analyze the effect of the interplanetary magnetic field (IMF) By component on the bow shock position and shape. Although the IMF Bz component is usually considered much more geoeffective than By, we find that the dayside bow shock is more responsive to the eastward component of the IMF than the north‐south one. We believe that the explanation lies in the changes that the Bz component induces on the magnetopause location and shape, which largely compensate the corresponding changes in the dayside bow shock location. In the tail, we find that the bow shock cross section is elongated roughly in the direction perpendicular to the IMF direction, which agrees with earlier modeling studies. [ABSTRACT FROM AUTHOR]

Changes in land use are likely to affect the abundance and functioning of microorganisms in the soil. In China many paddy fields are being converted for vegetable growing. We wished to determine how these changes affected the microbial populations in one particular region in Hunan province, where the climate is subtropical monsoon, as an example. We sampled the soil down to 1 m in several fields: three that were still growing paddy rice, three that had been converted for vegetable growing 2 years earlier and three that had been growing vegetables for 25 years. Quantitative polymerase chain reaction ( qPCR) and terminal-restriction fragment length polymorphism ( T-RFLP) were used to determine the abundance and community composition of nxrA-containing nitrifiers and narG-containing denitrifiers in the soil. The abundances of these organisms depended largely on the amount of organic matter in the soil and decreased with increasing depth, as did the potential nitrification rate ( PNR) and nitrate reductase activity ( NRA). Enzyme activity was significantly correlated with the abundance of nitrogen-cycling bacteria. The change from rice to vegetable growing resulted in more residual nitrate- N in the soil, which correlated more strongly with the abundance of nxrA-containing nitrifiers in the topsoil (0-20 cm) and narG-containing denitrifiers in the deeper soil (80-100 cm). In general, the numbers of nitrogen-cycling microorganisms decreased markedly with increasing depth, but were less affected by the change from rice to vegetable cultivation in the fields investigated. Our results suggest that the abundances of nitrogen-cycling microbial communities are affected more by depth in the soil than by change of land use in these circumstances. Highlights The abundance and function of soil microorganisms change when paddy fields are converted to vegetable growing., Residual nitrate- N in the soil is affected by nitrifiers in topsoil and denitrifiers in the deeper soil., Their abundances depend largely on the amount of organic matter in soil and decrease with increasing depth., The genetic structures of the microorganisms depend more on depth in the soil than on change from rice to vegetable growing. [ABSTRACT FROM AUTHOR]