## 2008 Vol. 28, No. 2

Display Method:
2008, 28(2): 97-101. doi: 10.11728/cjss2008.02.097
Abstract(2313) PDF 2128KB(1138)
Abstract:
The magnetic flux ropes observed by Cluster from 2001 to 2005 in the magnetotail are surveyed in this paper. The statistic study have been performed on the relationship between the formation of the magnetic flux ropes and Interplanetary Magnetic Field (IMF). Considering the IMF condition when the flux ropes are observed, for the 80 % of the total 73 flux rope cases, there are dominant By in IMF accordingly, while in the 78 % of all cases there are core fields with the same direction as the IMF By. It is possible that IMF can change the relative direction of the magnetic fields in the northern and southern plasma sheet to form the magnetic topology favoring the formation of the magnetic flux rope. IMF also has a decisive effect on the directions of the core field in the flux ropes. As indicated in these statistic results, the formation of the flux ropes in the magnetotail is independent of the direction of the IMF Bz.
2008, 28(2): 102-106. doi: 10.11728/cjss2008.02.102
Abstract(2262) PDF 1452KB(1103)
Abstract:
Solar Proton Events (SPE) bring a significant radiation hazard to spacecraft and astronaut within and beyond the magnetosphere. Knowing and understanding the rule and precursor characteristic, establishing accurate prediction model to carry on the forecast of solar proton events, taking protective measures are of vital significance for the safety of the spacecraft and astronaut. Massive research has indicated that, before the eruption of solar activity, part of energetic particles may escape the binding of active region magnetic field, and disseminate towards interplanetary space along magnetic field line. Therefore, before the proton events, an increase of high energy proton flux may be seen in the earth-synchronous orbit. The proton energy spectrum before solar proton events which related with the solar nearby acceleration process has obvious difference with that of the general proton flux stochastic fluctuation. Therefore we may carry on the forecast of solar proton events through analyzing the energy spectrum parameters of proton before event. According to the proton energy spectrum, the variation of energy spectrum parameters before the commence of SPE was analyzed in this paper with proton flux data from channel 4~9 MeV, 9~15 MeV, 15~40 MeV, 40~80 MeV on the GOES satellite. A new method was introduced to use the variation of energy spectrum parameters and the proton flux data with energy E>10 MeV to predict SPE. Then, the method was used for SPE forecast based on the five-years data selection in 2002---2006 by test. The result turned out to be encouraging with the accuracy rate reaches to 97.5 % and have an advance time of several tens minutes up to over a hundred hours.
2008, 28(2): 107-113. doi: 10.11728/cjss2008.02.107
Abstract(1984) PDF 1831KB(928)
Abstract:
82 boundary layer events in magnetic cloud between 1995.2---2003.8 were analyzed, using the data measured by WIND instruments. It is suggested from the results that: (1) Among the types of the directional discontinuity (DD), the distribution of RD (Rotational Discontinuity), TD (Tangential Discontinuity), ED (Either Discontinuity), ND (Neither Discontinuity) is 37:18:44:1. It is different from 51:12:35:2 in solar wind. (2) The ratio of TD at boundary of Magnetic Cloud Boundary Layers (MCBL) is high, that is 20 percents of all the DD. And the relative variety of temperature and density at either side of discontinuity is |ΔN/N| = 0.24 and |ΔT/T|= 0.19, that is higher than the average inside the MCBL. It has much more TD' characters. The number of DD in MCBL per day is 1.87 times than that in solar wind; (3) The distribution of DD' normal direction in θ-φ plane is likely a sinusoid structure, the center of which is θ = - 24.90 º, φ =217.49 º that points to opposite direction to the sun. It is rather than a simple isotropy distribution. The preliminary results suggest that discontinuities are the important structures in MCBL which has different distribution of DD' type. This paper could provide some evidences for analyzing the formation of MCBL.
2008, 28(2): 114-122. doi: 10.11728/cjss2008.02.114
Abstract(2500) PDF 4723KB(10290)
Abstract:
The effects of Hall current on the reconnection dynamics and wave behavior are numerically studied in this report. The notable reductions in the openness of outflow region and the reconnection rate are shown as Hall effect weakens. The time series of the out-of-plane magnetic field By and the velocity components vx, vz in the (x, z) plane at the given points are transferred into the power spectra by the Fast Fourier Transformation (FFT) for the cases with various intensive Hall terms. Using the Minimum Variance Analysis (MVA) on E, the propagation directions and the polarized features of waves are determined. It can be found that there exists the whistler wave with the typical features near the magnetic separatrices in the case of the stronger Hall effect and the wave signature weakens substantially as the intensity of Hall effect is reduced. These results indicate that Hall effect has an important influence on the reconnection dynamics and plays a crucial role in generating the whistler-mode wave and fast reconnection rate. Besides, there is a positive coherency between the amplitude of whistler wave and the field-aligned current Jp in the plane near the magnetic separatrices in the case with the strong Hall term. It demonstrates the closed linkage between the whistler wave and Hall effect on the other hand.
2008, 28(2): 123-131. doi: 10.11728/cjss2008.02.123
Abstract(2083) PDF 2852KB(1312)
Abstract:
During a medium magnetic storm on 5 April 2006, ionospheric plasma bubbles have been studied in a large area using data from the Global Positioning System (GPS) observation sites between 70ºE and 210ºE in longitude, 20ºS and 40ºN in latitude. The plasma bubbles were found between 160ºE~90ºE, and 12ºS~33ºN, appeared at about 1~1.5 hour after local sunset. The plasma bubbles occurred earlier and survived longer in the southern hemisphere than those in the northern hemisphere. And in the development process of the plasma bubbles, at about 1100 km the upward drift speed was about 300 m/s mapped onto the magnetic equator. The plasma bubbles tilted during the development. The existence of the eastward electric field was supposed to be the cause triggering the plasma bubbles.
2008, 28(2): 132-136. doi: 10.11728/cjss2008.02.132
Abstract(1987) PDF 1238KB(1002)
Abstract:
Ionospheric delay resulted from radio signals traveling in ionosphere is the largest source of errors for satellite-based positioning and navigation system. A new algorithm of ionospheric delay correction was proposed in the course of taking the task of BD-2'' subsystem, and tested using 6 dual-frequency GPS stations' observational data of China crust deformation monitoring network. Further, comparison of TEC (Total Electron Content) errors derived from the proposed algorithm to the ones from ionospheric grid algorithm was considered. The results show that the gauged error of TEC is about 0.5 meter for the users located in middle latitude, but the error increases and reaches one meter for the low latitude users. The proposed method and ionospheric grid model can provide almost performance for the view of ionospheric delay correction. It is concluded that the proposed method is valid and practicable for the future satellite-based augmentation system of China.
2008, 28(2): 137-141. doi: 10.11728/cjss2008.02.137
Abstract(2448) PDF 2037KB(1268)
Abstract:
During 2005 the Solar activity level was very low, since this year February the monthly mean F10.7<100. At August 24 a severe geomagnetic storm event (Kp=9) occurred with a sudden commencement. This is a good case for the analysis of the influence of a severe geomagnetic storm on the density of the upper atmosphere. In this paper the measured data of the satellite borne atmospheric density detector. From analyzing these measurements the following results were concluded. During this severe geomagnetic storm event, the atmospheric density showed strong disturbances. There are two responses, one is a global thermosphere density enhancement follow the variations of the geomagnetic disturbance, the responding processes were 6 hours late after the geomagnetic disturbance started, the biggest change ratio is 2.5. Another is abnormal density disturbance at high latitude areas during the peak period of geomagnetic disturbances, the biggest change ratio is 5.5, and with an asymmetry feature for the sauthern and northern hemispheres. In situ measurements also show that traveling of the atmospheric density disturbance from high latitude to lower latitude region very fast, the velocity is about 15 º(latitude)/h at middle latitude region.
2008, 28(2): 142-151. doi: 10.11728/cjss2008.02.142
Abstract(2040) PDF 4716KB(1160)
Abstract:
Data obtained from Wuhan MF radar in winter of 2001 are used to study the quadratic nonlinear interactions between tides in lower thermosphere. It is observed that diurnal, semidiurnal and terdiurnal tides are the prominent perturbations in the meridional wind component near mid-latitude winter mesopause region, and the 6 h tide is clear. By bicoherence spectrum analysis, it is revealed that most prominent bicoherence peaks stand for phase correlation between tidal harmonics or self-coherence of a single tidal wave. By examining the vertical wavelength time variations, a significant correlation is found between the vertical wavelength of the observed 8 h tide and that of the supposed nonlinearly generated one by the interaction between 12 and 24 h tides. In the interval of 94.0~98.0 km, there exist not only a certain phase correlation and vertical wavenumber correlation but also a strong amplitude correlation of the oscillatory amplitudes equivalent and oscillatory phases synchronous or reversed between the prominent tides, indicating a wave-wave quadratic interaction has occurred. However, below 94.0 km, the various correlations between tides grow weaker and weaker with descending height and hence the tidal quadratic interaction is more likely a local and temporary phenomenon.
2008, 28(2): 152-158. doi: 10.11728/cjss2008.02.152
Abstract(1961) PDF 1849KB(1023)
Abstract:
Debris population estimation is an important task in statistical characterization of space debris environment, and is of great significance for validation of space debris environment models, assessment of impact risk to spacecrafts and prediction of the long-term growth potential of the number of debris. A method of estimating the confidence interval for debris population using radar in beam-park mode with the beam pointed due east and due south with any elevation is given. This method is also applied to the vertical staring radar, which is a special case when the beam elevation is 90 º. In the beam-park mode, the antenna beam is parked at fixed azimuth and elevation angles for a long time, usually longer than 24 hours. Debris objects crossing the beam randomly would be detected, and measurements for detected target would be obtained. No attempt is made to track debris objects passing through the observation volume. In this operating mode, the random event that a debris object in a given altitude bin, which may cross the beam, i.e. has large enough radar cross section and inclination relative to the latitude of the radar site, is actually observed is modeled as a (0-1) distribution. Using debris altitude and inclination data collected, the average probability of crossing the beam for all debris objects in the given altitude bin is obtained. The confidence interval for space debris population is estimated by the Central Limitation Theorem. Simulation results show the effectiveness of the method.
2008, 28(2): 159-163. doi: 10.11728/cjss2008.02.159
Abstract(2494) PDF 914KB(1168)
Abstract:
The lateral motion of bubbles attached to the heater surface during nucleate pool boiling of subcooled liquids is studied in the present paper. It is assumed that there exists the Marangoni convection surrounding a discrete vapor bubble due to the temperature difference acting on the interface between vapor and liquid. The mutual entrainment of adjacent bubbles in the Marangoni convection might drive bubbles toward each other during nucleate boiling of a subcooled liquid. This kind of motion promotes coalescence of adjacent bubbles. A scaling analysis on such flow is presented. The characteristic velocity of the lateral motion is obtained approximately. Furthermore, an approximate equation for estimating the observability of such motion is also presented. The predictions are consistent with the experimental observations. Moreover, it is predicted that the intensity of the Marangoni convection may be very high for fine bubbles initiated at some active cavities on the heater surface, which will result in the micro-jet at the top of the bubble. The effect would be especially important in cases where the bubble release rate is low such as boiling on horizontal down-facing surfaces and boiling in microgravity.
2008, 28(2): 164-168. doi: 10.11728/cjss2008.02.164
Abstract(2589) PDF 616KB(1038)
Abstract:
Early in the 1960s, there had been two geocentric equatorial coordinate systems to deal with the vibration of earth's equator, namely the Geocentric Equatorial Inertial Coordinate Systems (Mean Equator Mean Equinox, MEME) and the Geocentric True Equator Mean Equinox Coordinate Systems (TEME, also referred to as the Orbital Coordinate System). The adoption of the latter definition could avoid the trouble caused by the additional perturbation due to the change of coordinate system, while bring in disagreement and extra coordinate transformation with the MEME. This paper provides a solution to eliminate redundant transformation by giving analytical perturbation formulae directly in the MEME. In that case, the TEME is no longer necessary in orbit determination and forecast for all related work could be done in a uniform MEME coordinate system (now it's of Epoch J2000.0).
2008, 28(2): 169-172. doi: 10.11728/cjss2008.02.169
Abstract(1974) PDF 630KB(1048)
Abstract:
To make the best of information source for spacecraft initial orbit determination and to continuously inprove the ground Tracking, Telemetry and Command (TTC) orbit determination means and method, this paper proposes a spacecraft initial orbit determination algorithm based on the launch vehicle apparent velocity. Firstly, the principium of measuring trajectory by use of the launch apparent position and apparent velocity is analyzed. Secondly, using the launch vehicle second node computer word (i.e. apparent velocity) and the initial trajectory of a certain point (initial position vector and velocity vector), the position vector and velocity vector of each second are acquired by means of performing the second-by-second integration of the launch vehicle motion equation, and then converted into the ballistic data to directly determine the spacecraft initial orbit. Finally, simulation result on the real-measure data of a sun-synchronous orbit satellite proves the correctness and reliability of this algorithm.
2008, 28(2): 173-179. doi: 10.11728/cjss2008.02.173
Abstract(2163) PDF 1523KB(1186)
Abstract:
The control problem of coordinated motion of free-floating space manipulator with an attitude controlled base is
discussed. Firstly, with the Lagrangian formulation and the relationship of the system linear momentum conversation, the dynamic equation of system is derived. Based on the above results and the RBF neural network technique, the Ge-Lee (GL) matrix and its product operator, the dynamics of free-floating space manipulator is approximated. And then with all unknown inertial parameters of the manipulator, the adaptive neural network control scheme of coordinated motion between the base's attitude and the manipulator's joints of free-floating space manipulator is developed. It need neither linearly parameterize the dynamic equation of system, nor know any actual inertial parameters. And the neural network need not do training and learning online too. Therefore, the control scheme is prone to real-time application. Lastly, a planar free-floating space manipulator is simulated to verify the proposed control scheme.
2008, 28(2): 180-184. doi: 10.11728/cjss2008.02.180
Abstract(2248) PDF 1181KB(1155)
Abstract:
Satellite and deep space communication channels are regarded approximately as white Gaussian noise channels approximately, so it's significant to study on channel capacities using usual modulation schemes under this channel. Starting from Shannon's information theory, this paper develops the channel capacities of one-dimensional and two-dimensional modulation schemes. Based on above, it explains that special modulation schemes ought to be adopted according to different signal to noise ratio. For example, BPSK modulation schemes is used in deep space communication because of very low signal to noise ratio. It also explains the essential of coding gain with the same spectral efficiency applying coded modulation. For instance, a rate R=2/3 convolutional code can be combined with 8-PSK modulation which can be compared with uncoded QPSK modulation, since they both have the same spectral efficiency of η=2 bit/symbol. The Shannon's limit of AM and PSK without bandwidth constraint and the asymptotic capacity of PSK with high signal to ratio are discussed at the end of this paper.