空间科学学报

2008 Vol. 28, No. 3

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Deflection of Titan’s Wake Related With Asymmetry

YANG Yafen, CHEN Chuxin

2008, 28(3): 185-188. doi: 10.11728/cjss2008.03.185

Abstract(2039) PDF 1043KB(1083)

Abstract:
Both the Voyager 1 and the Cassini spacecraft had close encounters with Titan, and detected that the ions distribute asymmetrically between the Saturn-facing and the anti-Saturn facing sides of Titan's wake. In this paper the trajectories of ions born in Titan's exosphere are simulated through a simple model. The results show that the heavy ions have gyroradii comparable with Titan's radius but the light ions' gyroradii are not so large. Because of the finite gyroradius effects, most of the heavy ions on the side toward Saturn are absorbed by Titan's atmosphere. Therefore the densities of ions on the two sides of Titan's wake are asymmetrical. This asymmetry makes the wake unbalanced and deviate from the corotation direction. Through the mechanics equilibrium of the wake it is found that the wake will deflect toward Saturn by about 19o from the corotation direction. This is consistent with the observation of the Voyager 1.

2D MHD Simulation of the Lunar Wake

CUI Wei, LI Lei

2008, 28(3): 189-193. doi: 10.11728/cjss2008.03.189

Abstract(2178) PDF 1891KB(1253)

Abstract:
Two-dimentional ideal MHD simulation of the interaction between the solar wind and the Moon reveals the particle and magnetic field distribution in the lunar wake. The results show that the density of the solar wind falls ramatically and the interplanetary magnetic field increases in the umbra of the Moon. When the interplanetary magnetic field is parallel to the direction of the solar wind velocity, the wake is very long. When the magnetic field is normal to the velocity of the solar wind, the wake is shorter.

Numerical Research About Interaction Between Two Tube Models Containing Spiral Magnetic Field Lines With a Little Small Length-Scale

YE Zhanyin

2008, 28(3): 194-202. doi: 10.11728/cjss2008.03.194

Abstract(3059) PDF 4694KB(1219)

Abstract:
After a long time interaction, governed by the magnetohydrodynamic (MHD) equations, between the Parker solar wind flow and a potential magnetic field combined by a dipole and a hexapole, a special streamer background structure in the solar meridian plane is presented. With the background structure, two tube models containing spiral magnetic field lines with a small length-scale can move in the computational domain step by step, interact afterward and trigger a Coronal Mass Ejection (CME) finally. The tube model is approached by a structure made up of two-dimensional co-circular magnetic field lines, with a ratio of the plasma pressure at its center to that at its edge m=2 and with a radius a for two cases: a=0.07 Rs (Rs is the solar radius) and a=0.1 Rs respectively. Corresponding to the two cases, two typical numerical results are obtained. In the case a=0.07 Rs, the two tube models interact with each other, merge into one set of co-circular magnetic field lines in seven solar radii and propagate outward. In the other case a=0.1 Rs, the two tube models do not merge into one and keep two sets of co-circular magnetic field lines exist in the computational domain during their propagation as a whole.

Accuracy of Empirical and Paraboloid Magnetic Field Models Estimated by Cluster Magnetic Data

ZHANG Xiaofang, FU Yang, YAN Wei

2008, 28(3): 203-209. doi: 10.11728/cjss2008.03.203

Abstract(2288) PDF 3980KB(1145)

Abstract:
Since the Cluster passes through the crucial regions where the solar wind and the magnetosphere interact, this paper compares the empirical magnetic field T96, T01 models and the paraboloid A2000 model with Cluster magnetic data. The reliability of the three models for different orbit whose perigee is either on the dayside or on the nightside of the magnetosphere is examined. The results show that, (1) Magnetic filed near the perigees of Cluster orbits is mainly influenced by Birkland currents, the T01 and T96 models give a good estimation of the location of passage through the large-scale currents, while the modeled magnitude being not accurate, and due to the absence of II region field-aligned current, the A2000 model can't correctly predict the disturbance of magnetic filed near the perigee; (2) For cases of 0>Dst>-100 nT, the reliability of T96 is comparable to that of T01, while due to the absence of the partial ring current, the T96 model can't correctly reproduce the magnetic field near the dayside perigee close to the dusk region; (3) The residual RMS deviation of the T96 and T01 models is about 40 % of the average magnitude of the observed external field for nightside perigee cases, while due to the effect of currents near cusp, that increases to 50 % for dayside perigee cases. Contrastively, the residual RMS deviation of the A2000 model is apparently larger, which is 58 % of the observed field.

Analysis of the Time Lag Between Geomagnetic Ap Index and Solar Cycle

XU Tong, WU Jian, WU Zhensen, FENG Jian, WANG Maoyan

2008, 28(3): 210-214. doi: 10.11728/cjss2008.03.210

Abstract(2809) PDF 1726KB(1470)

Abstract:
The data of twelve-month running average of monthly mean Ap index from 1932 to 2006 is decomposed into two components, (Ap)R and (Ap)I. (Ap)R is a linear function of the twelve-month running average of monthly mean sunspot number R, which has the same phase with sunspot number R and is possible due to Coronal Mass Ejections and equatorial coronal holes. The latter is due to long lived solar wind sources such as polar coronal holes extending toward equator and is inconsistent with sunspot number R in phase. Analyzing of (Ap)I and polar coronal holes variation of cycle 20, in which the lag between twelve-month running average of monthly mean Ap index and twelve-month running average of monthly mean R is remarkable, it is testified that the (Ap)I component corresponds to polar coronal holes. Thus it is concluded that the variation of polar coronal holes is a very important factor, which contributes possibly to the lag time between Ap index and solar cycle.

Phase of Orbital Charged Debris Clouds Evolution

DONG Yunfeng, XIE Song, XIAO Yelun

2008, 28(3): 215-221. doi: 10.11728/cjss2008.03.215

Abstract(2705) PDF 3560KB(1140)

Abstract:
Orbital charged debris clouds evolution phase was defined as sphere, spheroid, rope, spiral, spreading and shell, based on the geometry shape and debris density in the evolution from the initial breakup point to the final coverage of Earth. The shape character and the key drive sources of each phase are addressed. The perturbation of charged debris impacted by electromagnetic field in each phase was analyzed. The evolution different between charged debris and uncharged was compared. The separate velocity decides the shape and size during the sphere phase, and the repulsion between debris accelerates separation velocity. The charged debris and uncharged is nearly accordant from spheroid phase to sphere. The charged debris evolution speed would accelerate if debris' electric charge polarity is positive and decelerate if polarity is negative. Based on the analysis that the perturbation impacted by electric field was equivalent to the change of gravity, the time mark between the phases was given and calculation formula was listed. The uncharged debris clouds and charged debris clouds evolution phase were visualized though computer simulation, that verified the rationality of phase definition and electromagnetic field perturbation analysis.

Evaluation of the Prediction Ability of the NeQuick Model on f0F2 Over Asia-Australic Sector

LU Linling, LIU Libo

2008, 28(3): 222-229. doi: 10.11728/cjss2008.03.222

Abstract(2524) PDF 3385KB(1386)

Abstract:
In this paper, with the observed critical frequencies of F2 layer at eight ionospheric observatories in Asia-Australic sector, the prediction ability of the NeQuick model on f0F2 over Asia-Australic sector were investigated. The agreement between the prediction by the NeQuick model and the observed f0F2 data in this sector is quite satisfactory. The model prediction error is bigger in the northern hemisphere than in the southern hemisphere, bigger in the years with high solar activity than in those with low solar activity, and in spring and autumn than in winter and summer. The RMSE is bigger in the years with high solar activity than in those with low solar activity.

Analysis of the Characteristics of the Stratospheric Quasi-zero Wind Layer Over China

XIAO Cunying, HU Xiong, Gong Jiancun, LIU Jia

2008, 28(3): 230-235. doi: 10.11728/cjss2008.03.230

Abstract(2656) PDF 3688KB(1715)

Abstract:
The characteristics of the stratospheric quasi-zero wind layer and its seasonal and geographic variation features over China are obtained by using the ERA-40 reanalyzed wind data which are provided by ECMWF. Results show that quasi-zero wind layer generally exists at the height range of 18--25 km and some variation of the height of the zero wind line would happen when the time or location change. According to the latitudinal variation of the quasi-zero wind layer, the atmosphere over China could be divided into three parts to discuss its features: low-latitude area (5ºN~20ºN), transition area (20ºN~32.5ºN), middle- and high-latitude area (32.5ºN~55ºN). The quasi-zero wind layer may exist in winter and at the beginning of spring in low-latitude area. In middle-and high-latitude area, the quasi-zero wind layer may be found at the end of spring and in summer. The quasi-zero wind layer in transition area relates to the equatorial quasi-biennial oscillation (QBO). During the easterly phase of QBO, the characteristics of the transition area are similar to that of the middle-latitude area, while during the westerly phase of QBO, its characteristics are similar to that of the low-latitude area. The quasi-zero wind layer varies little with the longitude. The longitudinal variation of the height of zero wind line may not exceed 2 km. However, this variation is relatively larger in the transition area.

Design of a Moon Exploration Probe Simulation and Visualization System

ZHANG Hanqing, LI Yanjun

2008, 28(3): 236-241. doi: 10.11728/cjss2008.03.236

Abstract(2529) PDF 2032KB(1142)

Abstract:
Based on the OpenGL and Visual C++ software platform, and the theory of orbit dynamics, a moon exploration probe simulation and visualization system was developed, this system can dynamically simulate the space-time relationship between probe, earth and moon, calculate the probe position and attitude in real-time, 3D visualization technology was used to display the 3D graphics vividly. The main module design methods and the software architecture was discussed here, some problems encountered in the development progress are also discussed. Practical use of the system shows that it can help users to understand moon exploration process better.

Research on Simulation of Deep Charging for Spacecraft Dielectrics

WANG Ji, QIU Jiawen, QIN Xiaogang, MA Yali

2008, 28(3): 242-247. doi: 10.11728/cjss2008.03.242

Abstract(2402) PDF 2143KB(1527)

Abstract:
Based on Monte Carlo simulation and Radiation-Induced Conductivity (RIC) model of charging dynamics, a new predicting method aiming at spacecraft dielectrics deep charging is brought forward to calculate charging density and internal electric field and is also validated by ground tests. The charging model of dielectrics is simplified as a compound structure consisting of shielding aluminum and multiple-layer plates in Geant 4. The injected current density and dose rate profiles of Teflon along depth are statistically accumulated and normalized with practical flux density. Then substituting these results into RIC model, the distributions of charging density and electric field in Teflon under the condition of backside grounded are numerically calculated. Finally, the profiles of space charging density in Teflon under various injecting electron fluxes are measured by Pulse Electro-Acoustic method (PEA). Numerical and ground tests show that charging density and electric field increase while the injected electron flux is enhanced for Teflon radiated by 100 keV electron. The peak of charging density is about 0.042 mm and the maximum electric field is closed to the grounded side. With universal characteristics of Geant 4 particles transfer simulation and RIC model, the predicting method is suitable for various spacecraft dielectrics.

HXMT Quick-look Imaging Method

SHEN Zongjun, ZHOU Jianfeng

2008, 28(3): 248-253. doi: 10.11728/cjss2008.03.248

Abstract(2630) PDF 1909KB(1035)

Abstract:
The most important working mode of HXMT (Hard X-ray Modulation Telescope) is scanning mode, which includes all-sky imaging survey mode and small region scanning mode. The standard imaging analysis requires complete data of two dimensional observations in the region of interest. By using the information of space modulation and rotating modulation contained in the observed data simultaneously, quick-look imaging can obtain the intensity distribution with data of one dimensional observation. Thus quick-look imaging can obtain the image of a region soon after HXMT scan through it. The technique of HXMT quick-look imaging is described in this paper in detail. And later in this paper the distribution of sensitivity and locating accuracy of HXMT quick-look are analyzed based on numeric simulation. The quick-look imaging can improve the sensitivity of HXMT for transient sources, discover and locate high energy bursts in the FOV of HXMT, reduce the disturbance of transient sources and bursts to the standard imaging. It will be an important supplement to the standard imaging analysis.

Effect Analysis of Orbital and Attitude Parameter Error on TDI-CCD’s Line-shift Frequency and Veer Angle

LI Xinfeng

2008, 28(3): 254-259. doi: 10.11728/cjss2008.03.254

Abstract(2569) PDF 1367KB(1301)

Abstract:
Unlike other references which usually concentrate on geometric methods, kinetic approach is adopted to analyze the motion of space-born TDI-CCD camera's target, for a convenient analysis of kinetics, a series of coordinates is chosen. The velocity vector of the nadir is expressed in the sensor coordinate after some transformations which are functions of orbital and attitude parameters. For a nadir-pointing satellite, the first-order approximation of attitude parameters in the velocity vector is obtained with the tiny alignment error. From the relations between target's motion and orbital and attitude parameters, it takes little effort to derivate equations of line-shift frequency and veer angle with respect to orbit and attitude parameters, these explicit formulas of line-shift frequency and veer angle facilitate the procedure of analytical decision of error budget, especially in the primary design phase of a space mission. This application is illustrated by some typical LEO mission examples following the analysis.

Structure Design and Modes Analysis for the Truss of Light High Resolution Space Camera

ZHANG Lei, JIN Guang

2008, 28(3): 260-263. doi: 10.11728/cjss2008.03.260

Abstract(2230) PDF 943KB(1058)

Abstract:
To design the light precision truss, which is fitted to characteristic of the light high resolution space camera, the method of mode analysis and the principles of designing the main truss are studied. The natural frequency and the first two vibration modes of different types supporting structure for the second mirror are analyzed using FEA software UG and Nastran．Then, a three vanes with triangular hub structure for supporting secondary mirror is proposed, and the design of the main truss is done. The analysis results indicate that the truss design has a higher resonance frequency and a light mass. So it can be applied to the light high resolution space telescope and it can also be a special reference to the design of other large aperture space telescope.

Overview of the Advance for Mars Exploration Using Microwave Remote Sensing

JIN Yaqiu, FA Wenzhe, XU Feng

2008, 28(3): 264-272. doi: 10.11728/cjss2008.03.264

Abstract(2768) PDF 1260KB(1392)

Abstract:
Exploration of Mars and other planets is a great mission in development of space remote sensing and deep space exploration. A brief review of the Mars exploration, e.g. available research results such as the distribution, structure and dielectric properties of Martian subsurface layers and the possibility of water existence etc., is presented. Microwave remote sensing technology shows great potential in future Mars exploration mission. Based on the advancement of both active and passive remote sensing in Earth science, some approaches to explore Martian regolith layers and possible detection of water existence in Martian subsurface layers are proposed. Scattering model of inhomogeneous media with rough interfaces for Mars exploration, imaging mechanism and numerical simulation are developed. The Mueller matrix solution of inhomogeneous media under pulse incidence are numerically simulated to illustrate funtional dependence on the planet characteristic parameters. Microwave radiometry and multi-data fusion technolonogy for Mars exploration are also presented. Some examples and technical key points are discussed.