Motion of the Earth's inner core and related variations of polar motion and the rotational velocity

Correlations between the geomagnetic field parameters and polar motion were explained using the hypothesis of an inner core motion. For a number of periodic constituents of both phenomena it could be proved that this model works well. Secular variations of polar motion caused by inner core dynamics are in the same order of magnitude as the secular variation of the pole derived from pole coordinates of the International Latitude Service (ILS).     

A UT1-Like Quantity from Analysis of GPS Orbit Planes

This paper discusses a UT1-like quantity, UTGPS, determined daily from Earth-referenced GPS satellite orbits from the International GPS Service (IGS). For each satellite considered, the observed relation between the satellite's IGS orbit and a model of its orbit plane in inertial space is used to estimate UT1. This modeled orbit plane is initialized using the satellite's IGS orbit and the actual UT1 at an initial time. It is then propagated using standard models of gravitational forces and an empirical model representing the orbit-normal radiation pressure observed during several years of in-flight experience with the satellite. To estimate UT1, an a-priori transformation from terrestrial to true-of-date celestial coordinates is applied to the satellite's IGS orbit. The geocentric angular deviations of points of the resulting transformed orbit from the modeled orbit plane are analyzed, giving the angle between the ascending nodes of the satellite's transformed and modeled orbit planes. To this observed angle between nodes, converted to a UT1 difference, is added the a-priori UT1 value used in the transformation. From the result is subtracted a model of the angle, again converted to a UT1 difference, between the ascending nodes of the actual and modeled orbit planes. The final result is the estimate of UT1 from this satellite, and the median of the UT1 estimates from all satellites considered is UTGPS. The root-mean-square difference between UTGPS-UT1 at the beginning and at the end of an interval of one to four weeks is approximately 30 s times the square root of the interval's duration in weeks.This revised version was published online in October 2005 with corrections to the Cover Date.     

On the “rotational angular momentum” of the oceans and the corresponding polar motion

Periodic polar motions caused by ocean tides are predicted. In the Liouville equations for rotational motion the complete excitation functions for the ocean tides have to be used. This does not depend on the fact that hydrodynamical ocean tide models do not consider the centrifugal acceleration. The observable polar motion of the Celestial Ephemeris Pole CEP (more exactly: the terrestrial location of the CEP) is tabulated for the ten ocean tides M₂, S₂, N₂, K₁, O₁, P₁, M f, M f′, M m, Ssa. Typical amplitudes for the largest ocean tides are 0.4 milliarcseconds. This is within the reach of geodetic VLBI and SLR observations.     

利用端点延拓提高LS + NN模型的UT1-UTC预报精度

现有UT1-UTC预报模式在进行周期项与残差项拟合分离时,通常没有考虑最小二乘拟合序列的端部效应,预报精度难以取得较大提高。针对最小二乘拟合存在的端部效应,首先采用灰色模型在UT1-UTC序列的两端进行数据延拓,形成一个新序列,然后对新序列进行最小二乘拟合,最后再联合最小二乘和神经网络(LS+NN)模型对UT1-UTC原始序列进行外推。结果表明,对UT1-UTC序列进行端点数据延拓再进行最小二乘拟合,能够有效地改善最小二乘拟合序列的端部效应;相对于常规LS+NN模型,端部效应改善的LS+NN模型的UT1-UTC预报精度有一定提高,尤其对中长期预报精度提高更为明显。     

Effect of ocean tides on the determination of zenith delay

GPS meteorology is a new field in GPS applications, in which precise determination of zenith delay is a key component. A strong correlation exists between the vertical displacement caused by ocean tide and the zenith delay at a given GPS station, so the effect of ocean tide on the precise determination of the zenith delay can not be neglected. We have processed and analyzed the GPS data of the Crustal Motion Observation Network of China from Day 10 to Day 40 of 2001, and the result shows that such effect varies over a range of several millimeters. For example, it is ± 7mm for the coastal station at Shanghai and ±1 mm for the inland station at Wuhan at maximum.     

Cometary cryo-volcanism: Source regions and a model for the UT 2005 June 14 and other mini-outbursts on Comet 9P/Tempel 1

Data on the UT 2005 June 14 mini-outburst of Comet 9P/Tempel 1 taken from different viewpoints have been examined for morphological differences and parallax. The data were taken with the Hubble Space Telescope (HST), from the Deep Impact (DI) spacecraft, and from the Calar Alto Observatory, Spain. The mini-outburst source region was found to be located near 218 ± 6E, 6 ± 5N on the Deep Impact nucleus shape model. The mini-outburst occurred at ∼12 pm local solar time. The distribution of light in the mini-outburst is similar to that expected for an ejecta curtain. The method and software used to determine the surface location was checked using position angles of the impact ejecta plume as seen from DI and HST. The general region of impact was recovered and a downrange tilt of the ejecta curtain axis of 10.2 deg from the surface normal was found. We computed tracks of possible source regions for nine other mini-outbursts seen from DI. Five of these tracks converge on the 2005 June 14 event location. Three of the tracks converge at a second location near (60E, 20S), well separated from the first. Multiple mini-outbursts arise at each location either from a single source or from a few sources in close proximity. The mini-outbursts occur both at night and during the day indicating at most weak, if any, control by direct sunlight. The times of outburst are non-random with a preference for early afternoon, dusk and midnight. None of the mini-outbursts occurred near dawn. They occur at low latitudes (between ±40 deg) near the points where the principal axis of minimum moment of inertia cuts the surface. These regions are furthest from the center of figure and have the lowest effective surface gravity. We use these results to develop a conceptual model of the mini-outburst process and make comparisons with the theoretical calculations. We find that the tensile strength of the sub-surface material must be very low (e.g., ) and, on the basis of features imaged on the western facet of the nucleus, suggest that inflation of the sub-surface may be occurring. Our model makes specific predictions about the kind of surface morphology that should result from mini-outburst activity. We show that one of the isolated rimless depressions and the close-packed depressions found in the Deep Impact images have the properties needed and identify them as possible sites of past and current mini-outburst activity.     

Constraints on the composition of the martian south polar cap from gravity and topography

The polar caps of Mars have long been acknowledged to be composed of unknown proportions of water ice, solid CO₂ (dry ice), and dust. Gravity and topography data are here analyzed over the southern cap to place constraints on its density, and hence composition. Using a localized spectral analysis combined with a lithospheric flexure model of ice cap loading, the best fit density of the volatile-rich south polar layered deposits is found to be 1271 kg m⁻³ with 1-σ limits of 1166 and 1391 kg m⁻³. The best fit elastic thickness of this geologically young deposit is 140 km, though any value greater than 102 km can fit the observations. The best fit density implies that about 55% dry ice by volume could be sequestered in these deposits if they were completely dust free. Alternatively, if these deposits were completely free of solid CO₂, the dust content would be constrained to lie between about 14 and 28% by volume. The bulk thermal conductivity of the polar cap is not significantly affected by these maximum allowable concentrations of dust. However, even if a moderate quantity of solid CO₂ were present as horizontal layers, the bulk thermal conductivity of the polar cap would be significantly reduced. Reasonable estimates of the present day heat flow of Mars predict that dry ice beneath the thicker portions of the south polar cap would have melted. Depending on the quantity of solid CO₂ in these deposits today, it is even possible that water ice could melt where the cap is thickest. If independent estimates for either the dust or CO₂ content of the south polar cap could be obtained, and if radar sounding data could determine whether this polar cap is presently experiencing basal melting or not, it would be possible to use these observations to place tight constraints on the present day heat flow of Mars.     

亚洲区域构造块体运动模型的建立及其运动学分析

利用中国地壳运动监测网、亚太地球动力学计划(APSG)联测以及IGS速度场建立亚洲构造块体运动模型,分析其板内形变,并求出各构造块体相对欧亚板块运动的欧拉矢量,进而求出各构造块体绝对运动速度,分析各构造块体绝对运动,揭示亚洲区域构造块体的运动规律和特征．     

A model for the polar transition layer and corona for November 1967

A model for the chromospheric-coronal transition layer and lower corona has been constructed for the south polar region. EUV observations acquired by the Harvard OSO-4 experiment in the fall of 1967 were used in the analysis. The observations can be explained with a simple model consisting of two types of regions. One region has a temperature-density structure similar to that in models developed for typical equatorial quiet areas. The other region has a corona in which the temperature and density are a factor of about 2 lower and the chromospheric-coronal temperature gradient is less steep by a factor of 4.     

Observations of the north polar water ice annulus on Mars using THEMIS and TES

The Martian seasonal CO₂ ice caps advance and retreat each year. In the spring, as the CO₂ cap gradually retreats, it leaves behind an extensive defrosting zone from the solid CO₂ cap to the location where all CO₂ frost has sublimated. We have been studying this phenomenon in the north polar region using data from the THermal EMission Imaging System (THEMIS), a visible and infra-red (IR) camera on the Mars Odyssey spacecraft, and the Thermal Emission Spectrometer (TES) on Mars Global Surveyor. Recently, we discovered that some THEMIS images of the CO₂ defrosting zone contain evidence for a distinct defrosting phenomenon: some areas just south of the CO₂ cap edge are too bright in visible wavelengths to be defrosted terrain, but too warm in the IR to be CO₂ ice. We hypothesize that we are seeing evidence for a seasonal annulus of water ice (frost) that recedes with the seasonal CO₂ cap, as predicted by previous workers. In this paper, we describe our observations with THEMIS and compare them to simultaneous observations by TES and OMEGA. All three instruments find that this phenomenon is distinct from the CO₂ cap and most likely composed of water ice. We also find strong evidence that the annulus widens as it recedes. Finally, we show that this annulus can be detected in the raw THEMIS data as it is collected, enabling future long-term onboard monitoring.     

A 1 Gyr climate model for Mars: new orbital statistics and the importance of seasonally resolved polar processes

New results from a 1 Gyr integration of the martian orbit are presented along with a seasonally resolved energy balance climate model employed to illuminate the gross characteristics of the long-term atmospheric pressure evolution. We present a new analysis of the statistical variation of the martian obliquity and precession prior to and subsequent to the formation of the Tharsis uplift, and explore the long term effects on the martian climate. We find that seasonal polar cycles have a critical influence on the ability for the regolith to release CO₂ at high obliquities, and find that the atmospheric CO₂ actually decreases at high obliquities due to the cooling effect of polar deposits at latitudes where seasonal caps form. At low obliquity, the formation of massive, permanent polar caps depends critically on the values of the frost albedo, A_frost, and frost emissivity, ?_frost. Using our model with values of A_frost=0.67 and ?_frost=0.55, matched to the NASA Ames General Circulation Model (GCM) results (Haberle et al., 1993, J. Geophys. Res. 98, 3093-3123, and Haberle et al., 2003, Icarus 161, 66-89), we find that permanent caps only form at low obliquities (<13°), suggesting that any permanent deposits on the surface of Mars today may be residuals left over from a period of very low obliquity, or are the result of mechanisms not represented by this model. Thus, contrary to expectations, the martian atmospheric pressure is remarkable static over time, and decreases both at high and low obliquity. Also, from our one billion year orbital model, we present new results on the fraction of time Mars is expected to experience periods of low obliquity and high obliquity.     

A new sequence of international latitude service pole coordinates and the secular polar motion

From individual ILS data in a homogeneous system, we derived a new sequence of the coordinates of the pole. This was then used in an analysis of the secular polar motion. We found: 1) At a confidence level of 95%, the linear drift of the ILS mean pole over the last 80 years is along 63° 3 longitude West, at an average speed of 0″.00305/yr. 2) The libration of the mean pole is rather regular, with a prominent term of about 30 yr, and detectable terms of 18.6 yr and 9.3 yr. 3) Station Ukiah is drifting northwards at a speed of 0″.00276/yr, while all the other stations are quite stable. Hence the ILS data cannot be taken as showing an anti-clockwise rotation of the Pacific coast at present.     

A complex analytic solution for the attitude motion of a near-symmetric rigid body under body-fixed torques

Although analytic solutions for the attitude motion of a rigid body are available for several special cases, a comprehensive theory does not exist in the literature for the more complicated problems found in spacecraft dynamics. In the present paper, analytic solutions in complex form are derived for the attitude motion of a near-symmetric rigid body under the influence of constant body-fixed torques. The solution is very compact, which enables efficient and rapid machine computation. Numerical simulations reveal that the solution is very accurate when applied to typical spinning spacecraft problems.     

Measurement of the absolute proper motion of M3 and its space motion

We have measured the absolute proper motions of 534 stars in a 1.5° × 1.5° region around the cluster M3, using 14 plates taken with a 40-cm refractor spanning 80 years. 24 stars in the ACT catalogue were used to define the reference frame and the reduction was made using the central overlapping technique. Using the new data, the membership probabilities were redetermined. The mean absolute motion of the cluster was found to be −0.3 ± 0.3 mas/yr in R.A., and −3.1 ± 0.3 mas/yr in declination. Combining the present data with the known distance and radial velocity, the three-dimensional galactic orbit of M3 was calculated for Allen's galactic potential.     

用于测量世界时的大型光纤陀螺仪的噪声分析

大型光纤陀螺仪可以精确测量地球自转角速率,进而可以运用于世界时(UT1)的解算工作中.光纤陀螺仪包含的噪声会影响测量的精确度以及稳定性,运用Allan方差可以对光纤陀螺仪的输出数据进行噪声分析,同时对陀螺仪测量数据进行功率谱分析,分析测量数据中存在的高频振动变化影响,并结合分析结果,从数据处理方法上提出改进措施.分析结果可以对光纤陀螺仪的改进以及数据处理方法的建立提供参考.     

基于混沌时间序列的Volterra自适应滤波极移预报方法

针对极移复杂的时变特性, 根据混沌相空间坐标延迟重构理论, 提出一种基于Volterra自适应滤波的极移预报方法. 首先, 利用最小二乘拟合算法分离极移序列中的线性趋势项、钱德勒项和周年项, 获得线性极移、钱德勒极移和周年极移的外推值; 其次, 通过C-C关联积分法对最小二乘拟合残差序列进行相空间重构, 并利用小数据量法计算残差序列的最大Lyapunov指数验证其混沌特性, 在此基础上, 构建Volterra自适应滤波器对残差序列进行预测; 最后, 将线性极移、钱德勒极移和周年极移的外推值以及最小二乘拟合残差的预测值相加获得极移最终预报值. 利用国际地球自转服务局(International Earth Rotation and Reference Systems Service, IERS)提供的极移数据进行1--60d跨度预报, 并将预报结果分别与国际地球定向参数预报比较竞赛(Earth Orientation Parameters Prediction Comparison Campaign, EOP PCC)结果和IERS A公报发布的极移预报产品进行对比, 结果表明: 对于1--30d的短期预报, 该方法的预报精度与EOP PCC最优预报方法相当, 当预报跨度超过30d时, 该方法的预报精度低于EOP PCC最优预报方法, 优于参与EOP PCC的其他方法; 与IERS A公报相比, 该方法的短期预报效果较好, 当预报跨度增加时预报精度低于IERS A公报. 预报结果表明该方法更适合于极移短期预报.     

A mapping for the study of the 1/1 resonance in a galactic type Hamiltonian

We derive an algebraic mapping for an autonomous, two-dimensional galactic type Hamiltonian in the 1/1 resonance case. We use the mapping to study the stability of the periodic orbits. Using the x — p _x Poincaré surface section, we compare the results of the mapping with those found by the numerical integration of the full equations of motion. For small values of the perturbation the results of the two methods are in very good agreement while satisfactory agreement is obtained for larger perturbations.     

The effect of eccentricity and inclination on the motion near the Lagrangian points L4 and L5

We study the effect of eccentricity and inclination on small amplitude librations around the equilibrium points L ₄ and L ₅ in the circular restricted three-body problem. We show that the effective libration centres can be displaced appreciably from the equilateral configuration. The secular extrema of the eccentricity as a function of the argument of pericentre are shifted by ∼25 ° This revised version was published online in July 2006 with corrections to the Cover Date.     

A symplectic mapping model for the 3/4 exterior

We present a symplectic mapping model to study the evolution of a small body at the 3/4 exterior resonance with Neptune, for planar and for three dimensional motion. The mapping is based on the averaged Hamiltonian close to this resonance and is constructed in such a way that the topology of its phase space is similar to that of the Poincaré map of the elliptic restricted three-body problem. Using this model we study the evolution of a small object near the 3/4 resonance. Both chaotic and regular motions are found, and it is shown that the initial phase of the object plays an important role on the appearance of chaos. In the planar case, objects that are phase-protected from close encounters with Neptune have regular orbits even at eccentricities up to 0.44. On the other hand objects that are not phase protected show chaotic behaviour even at low eccentricities. The introduction of the inclination to our model affects the stable areas around the 3/4 mean motion resonance, which now become thinner and thinner and finally at is=10° the whole resonant region becomes chaotic. This may justify the absence of a large population of objects at this resonance.     

Water vapor variability in the north polar region of Mars from Viking MAWD and MGS TES datasets

Atmospheric water vapor abundances in Mars’ north polar region (NPR, from 60° to 90°N) are mapped as function of latitude and longitude for spring and summer seasons, and their spatial, seasonal, and interannual variability is discussed. Water vapor data are from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) and the Viking Orbiter (VO) Mars Atmospheric Water Detector (MAWD). The data cover three complete northern spring-summer seasons in 1977-1978, 2000-2001 and 2002-2003, and shorter periods of spring-summer seasons during 1975, 1999 and 2004. Long term interannual variability in the averaged NPR abundances may exist, with Viking MAWD observations showing twice as much water vapor during summer as the MGS TES observations more than 10 martian years (MY) later. While the averaged abundances are very similar in TES observations for the same season in different years, the spatial distributions in the early summer season do vary significantly year over year. Spatial and temporal variabilities increase between L_s ∼ 80-140°, which may be related to vapor sublimation from the North Polar Residual Cap (NPRC), or to changes in circulation. Spatial variability is observed on scales of ∼100 km and temporal variability is observed on scales of <10 sols during summer. During late spring the TES water vapor spatial distribution is seen to correlate with the low topography/low albedo region of northern Acidalia Planitia (270-360°E), and with the dust spatial distribution across the NPR during late spring-early summer. Non-uniform vertical distribution of water vapor, a regolith source or atmospheric circulation ‘pooling’ of water vapor from the NPRC into the topographic depression may be behind the correlation with low topography/low albedo. Sublimation winds carrying water vapor off the NPRC and lifting surface dust in the areas surrounding the NPRC may explain the correlation between the water vapor and dust spatial distributions. Correlation between water vapor and dust in MAWD data are only observed over low topography/low albedo area. Maximum water vapor abundances are observed at L_s = 105-115° and outside of the NPRC at 75-80°N; the TES data, however, do not extend over the NPRC and thus, this conclusion may be biased. Some water vapor appears to be released in plumes or ‘outbursts’ in the MAWD and TES datasets during late spring and early summer. We propose that the sublimation rate of ice varies across the NPRC with varying surface winds, giving rise to the observed ‘outbursts’ at some seasons.