A note concerning the 2:1 and the 3:2 resonances in the asteroid belt

The dynamical behavior of asteroids inside the 2:1 and 3:2 commensurabilities with Jupiter presents a challenge. Indeed most of the studies, either analytical or numerical, point out that the two resonances have a very similar dynamical behavior. In spite of that, the 3:2 resonance, a little outside the main belt, hosts a family of asteroids, called the Hildas, while the 2:1, inside the main belt, is associated to a gap (the Hecuba gap) in the distribution of asteroids.In his search for a dynamical explanation for the Hecuba gap, Wisdom (1987) pointed out the existence of orbits starting with low eccentricity and inclination inside the 2:1 commensurability and going to high eccentricity, and thus to possible encounters with Mars. It has been shown later (Henrard et al.), that these orbits were following a path from the low eccentric belt of secondary resonances to the high eccentric domain of secular resonances. This path crosses a bridge, at moderate inclination and large amplitude of libration, between the two chaotic domains associated with these resonances.The 3:2 resonance being similar in many respects to the 2:1 resonance, one may wonder whether it contains also such a path. Indeed we have found that it exists and is very similar to the 2:1 one. This is the object of the present paper.     

A high speed near infrared photometer for lunar occultation studies

A recently developed near infrared high speed photometer intended for lunar oc-cultation studies is described. The primary scientific objective is to reach milli arc second levels of angular resolution so that circumstellar structure of the occulted sources can be resolved. Near infrared sky brightness close to the lunar limb is also studied. Angular diameter derived from the observed occultation of IRC +20169 is presented and system performance discussed.     

Particle-mesh simulations of the Lyα forest

We have analysed short-exposure high-resolution images obtained on the William Herschel Telescope (WHT), Big Telescope Azimuthal (BTA) and CanadaFranceHawaii Telescope (CFHT) in order to investigate mechanical vibrations that are capable of reducing the high angular resolution allowed by speckle-interferometric methods. After filtering the photon noise we have computed power spectra of the image centres of gravity. In these spectra we have found vibration features that differ from one telescope to another and that vary in particular with the zenith angle. In the case of the WHT we discuss these results in terms of possible causes and in terms of degradation of the transfer function. We present some means to improve the data.     

Influence of the Atmospheric Tides on the Earth Rotation

Spectral analysis of the components of the relative atmospheric angular momentum vector is performed based on the series of these components for the 6 h intervals within the period of 1958–2000. These series have been computed in the Subbureau of the Atmospheric Angular Momentum of the International Earth Rotation Service using the NCEP/NCAR reanalysis of atmospheric observations. The basic harmonics of diurnal tides are determined. New results on the fortnight's and week's duration oscillations of the equatorial components of the atmospheric angular momentum are obtained. The zonal tides transformation mechanisms in the atmosphere are discussed. It is shown that the main mechanism of the zonal tides effect on the atmospheric variability is the amplitude modulation of daily oscillations of the relative atmospheric angular momentum. The effects of the atmospheric tides on the Earth rotation are discussed.     

Gravity torques for surface waves

The effects of the gravity torques acting on the angular momentum of surface gravity waves are calculated theoretically. For short crested waves the gravity torque is caused by the force of gravity on the orbiting fluid particles acting down the slopes of the crests and troughs and in the direction parallel to the crests and troughs. The gravity torque tries to rotate the angular momentum vectors, and thus the waves themselves, counterclockwise in the horizontal plane, as viewed from above, in both hemispheres. The amount of rotation per unit time is computed to be significant assuming reasonable values for the along-crest and trough slopes for waves in a storm area. The gravity torque has a frequency which is double the frequency of the waves. For long crested waves the gravity torque acts in the vertical plane of the orbit and tries to decelerate the particles when they rise and accelerate them when they fall. By disrupting the horizontal cyclostrophic balance of forces on the fluid particles (centrifugal force versus pressure force) the gravity torque accounts qualitatively for the three characteristics of breaking waves: that they break at the surface, that they break at the crest, and that the crest breaks in the direction of wave propagation.     

Composition and distribution of phytoplankton with size fraction results at southwestern east/japan sea

Abundance and distribution of phytoplankton in seawater at southwestern East/Japan Sea near Gampo were investigated by HPLC analysis of photosynthetic pigments during summer of 1999. Detected photosynthetic pigments were chlorophyll a, b, c₁₊₂ (Chl a, Chl b, Chl c₁₊₂), fucoxanthin (Fuco), prasinoxanthin (Pras), zeaxanthin (Zea), 19’-butanoyloxyfucoxanthin (But-fuco) and beta-carotene (β-Car). Major carotenoid was fucoxanthin (bacillariophyte) and minor carotenoids were Pras (prasinophyte), Zea (cyanophyte) and But-fuco (chrysophyte). Chl a concentrations were in the range of 0.16-8.3/land subsurface chlorophyll maxima were observed at 0-10m at inshore and 30–50 m at offshore. Thermocline and nutricline tilted to the offshore direction showed a mild upwelling condition. Results from size-fraction showed that contribution from nano+picoplankton at Chl a maximum layer was increased from 18% at inshore to 69% at offshore on average. The maximum contribution from nano+picoplankton was found as 87% at St. E4. It was noteworthy that contribution from nano+picoplanktonic crysophytes and green algae to total biomass of phytoplankton was significant at offshore. Satellite images of sea surface temperature indicated that an extensive area of the East/Japan Sea showed lower temperature (<18 °C) but the enhanced Chl a patch was confined to a narrow coastal region in summer, 1999. Exceptionally high flux of low saline water from the Korea/Tsushima Strait seemed to make upwelling weak in summer of 1999 in the study area. Results of comparisons among Chl a from SeaWIFs, HPLC and fluorometric analysis showed that presence of Chl b cause underestimation of Chl a about 30% by fluorometric analysis but overestimation by satellite data about 30-75% compared to HPLC data.     

Hydrodynamic modeling of flushing time in a small estuary of North Bay, Florida, USA

Freshwater fraction method is popular for cost-effective estimations of estuarine flushing time in response to freshwater inputs. However, due to the spatial variations of salinity, it is usually expensive to directly estimate the long-term freshwater fraction in the estuary from field observations. This paper presents the application of the 3D hydrodynamic model to estimate the distributions of salinity and thus the freshwater fractions for flushing time estimation. For a case study in a small estuary of the North Bay in Florida, USA, the hydrodynamic model was calibrated and verified using available field observations. Freshwater fractions in the estuary were determined by integrating freshwater fractions in model grids for the calculation of flushing time. The flushing time in the North Bay is calculated by the volume of freshwater fraction divided by the freshwater inflow, which is about 2.2 days under averaged flow conditions. Based on model simulations for a time series of freshwater inputs over a 2-year period, a power regression equation has been derived from model simulations to correlate estuarine flushing time to freshwater inputs. For freshwater input varying from 12 m³/s to 50 m³/s, flushing time in this small estuary of North Bay changes from 3.7 days to 1.8 days. In supporting estuarine management, the model can be used to examine the effects of upstream freshwater withdraw on estuarine salinity and flushing time.     

Numerical investigation of motions of resonant asteroids in the three-dimensional space

Motions of asteroids in mean motion resonances with Jupiter are studied in three-dimensional space. Orbital changes of fictitious asteroids in the Kirkwood gaps are calculated by numerical integrations for 10⁵ – 10⁶ years. The main results are as follows: (1) There are various motions of resonant asteroids, and some of them are very complicated and chaotic and others are regular. (2) The eccentricity of some asteroids becomes very large, and the variation of the inclination is large while the eccentricity is large. (3) In the 3:1 resonance, there is a long periodic change in the variation of the inclination, when (7 : ) is a simple ratio (7: longitude of perihelion, : longitude of node). (4) In the 7:3 resonance, the variation of the inclination of some resonant asteroids is so large that prograde motion becomes retrograde. Some asteroids in the 7:3 resonance can collide with the Sun as well as with the inner planets.     

Prediction of UT1–UTC,LOD and AAM χ<Subscript>3</Subscript> by combination of least-squares and multivariate stochastic methods

This article presents the application of a multivariate prediction technique for predicting universal time (UT1–UTC), length of day (LOD) and the axial component of atmospheric angular momentum (AAM χ ₃). The multivariate predictions of LOD and UT1–UTC are generated by means of the combination of (1) least-squares (LS) extrapolation of models for annual, semiannual, 18.6-year, 9.3-year oscillations and for the linear trend, and (2) multivariate autoregressive (MAR) stochastic prediction of LS residuals (LS + MAR). The MAR technique enables the use of the AAM χ ₃ time-series as the explanatory variable for the computation of LOD or UT1–UTC predictions. In order to evaluate the performance of this approach, two other prediction schemes are also applied: (1) LS extrapolation, (2) combination of LS extrapolation and univariate autoregressive (AR) prediction of LS residuals (LS + AR). The multivariate predictions of AAM χ ₃ data, however, are computed as a combination of the extrapolation of the LS model for annual and semiannual oscillations and the LS + MAR. The AAM χ ₃ predictions are also compared with LS extrapolation and LS + AR prediction. It is shown that the predictions of LOD and UT1–UTC based on LS + MAR taking into account the axial component of AAM are more accurate than the predictions of LOD and UT1–UTC based on LS extrapolation or on LS + AR. In particular, the UT1–UTC predictions based on LS + MAR during El Niño/La Niña events exhibit considerably smaller prediction errors than those calculated by means of LS or LS + AR. The AAM χ ₃ time-series is predicted using LS + MAR with higher accuracy than applying LS extrapolation itself in the case of medium-term predictions (up to 100 days in the future). However, the predictions of AAM χ ₃ reveal the best accuracy for LS + AR.     

改进的边缘角度直方图在遥感图像检索中的应用

针对基于形状的遥感图像检索中区域分割困难这个问题,本文提出利用边缘角度直方图作为形状特征对遥感图像进行检索;针对遥感影像分辨率和颜色反差对检索性能的影响,本文提出采用多尺度分析的思想,提取图像在多个尺度上的边缘角度直方图特征进行检索。通过单一尺度边缘角度直方图特征和多尺度边缘角度直方图特征检索的对比实验,揭示了本文所提出的改进方案使遥感图像的检索精度提高了约31个百分点。