空间科学学报

2009 Vol. 29, No. 4

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On the Expansion of Coronal Funnels in the Quiet Sun

Tan Bo, Tian Hui, He Jiansen

2009, 29(4): 353-358. doi: 10.11728/cjss2009.04.353

Abstract(2772) PDF 618KB(1191)

Abstract:
Coronal funnels originated from chromospheric networks are key regions of solar wind origin and mass supply to large coronal loops. However, due to the lack of observational coronal magnetic field data, the geometrical and physical properties of coronal funnels can not be obtained through direct observations. Through applying force-free magnetic field extrapolation to a rectangular quiet-Sun region based on the observational MDI magnetogram, the coronal magnetic field up to 40 Mm above the photosphere can be reconstructed. Below 20 Mm, there are over 10 small funnels originating from magnetic networks and expanding with height. The small funnels merge into two big funnels above 20 Mm. For the first time, the height variations of the funnels' cross section areas in the quiet Sun are studied using observational data. The result reveals that coronal funnels expand almost linearly with height above the photosphere in the quiet Sun region. The expansion factor is larger in the lower part of the solar atmosphere than in the upper part. This result has been discussed with regard to 2D modeling of solar wind origin and mass flows in coronal loops. It is also found that the number of closed magnetic field lines decreases exponentially with height.

Characteristics of Electron Distribution Functions in Magnetic Cloud Boundary Layers

Zhu Chunming, Wei Fengsi, Feng Xueshang, Zuo Pingbing

2009, 29(4): 359-367. doi: 10.11728/cjss2009.04.359

Abstract(2382) PDF 853KB(1002)

Abstract:
Based on the magnetic cloud events detected by WIND spacecraft from 1995 to 2006, here are studied on the patterns of electron flows, characteristics of the electron distribution functions as well as their relationships with the heating and acceleration of electrons in the Magnetic Clouds Boundary Layers (MCBL). The main results are as follows. Firstly, there are several types of electron flow patterns in the MCBL. Secondly, compared with the background solar wind and the magnetic cloud body, the thermal core parts of the electron velocity distribution functions usually increase, together with increasing of superthermal electrons in directions perpendicular to the magnetic field and either parallel or anti-parallel to the magnetic field. Furthermore, the distributions of energetic electrons increase in all directions in almost ten percent of the MCBL. Thirdly, by comparisons of the modifications to the electron distribution functions by MCBL and interplantery shocks ahead of magnetic clouds, it suggests that the two structures result from different mechanisms. Lastly, investigation on the relationships between the enhancements of wave activities and electron distribution functions as well as ion fluxes was also made. These observations and comparisons indicate further that magnetic clouds are important dynamical structures possibly associated with magnetic reconnection as previous studies on MCBL have shown.

Quasi-27-Day Variations of Ionospheric NmF₂

Ma Ruiping, Xu Jiyao, Yuan Wei, Jiang Guoying, Liu Xiao

2009, 29(4): 368-376. doi: 10.11728/cjss2009.04.368

Abstract(2722) PDF 907KB(1309)

Abstract:
The data observed at 11 ionosonde stations in the East Asia/Australia sector during 1969---1980 and the band-pass filter method have been used to analyze the characteristics of the quasi-27-day variations of daytime NmF₂ and the relativity between NmF₂ and the solar activity index F_10.7. The results are summarized as follows. In most years, the short period (2~70 days) spectra of F_10.7 and NmF₂ have obvious maxima around 27-day period. In the quasi-27-day variations, the standard deviations of F_10.7 and NmF₂ have evident variability year-to-year. The average standard deviation of F_10.7 is 10.9\%. The standard deviations of NmF₂ depend on geomagnetic latitude, the minima appear at equator and they become greater with increasing latitude. The average standard deviation of NmF₂ of 11 stations is about 8.2%. In the quasi-27-day band, F_10.7 and NmF₂ have notable correlation. The probability of the significant correlation at 0.05 significance level exceeds 90%. The average time delay between variations of the NmF₂ and F_10.7 is about 2 days. As a whole, the quasi-27-day variations of NmF₂ primarily attribute to the quasi-27-day variations of the solar EUV flux.

Twenty-four Hour Ahead Prediction of f₀F₂

Kong Qingyan, Liu Wen, Jiao Peinan, Fan Junmei, Feng Jing, Lu Zhuanxia

2009, 29(4): 377-382. doi: 10.11728/cjss2009.04.377

Abstract(2408) PDF 615KB(1245)

Abstract:
Using three-layer feed-forward back propagation neural networks, twenty-four hour ahead prediction of the critical parameter of ionospheric F₂ layer f₀F₂ is realized. The prediction model is developed based on 11 years of data (from 1976 to 1986) measured from China vertical station (Haikou, Guangzhou, Chongqing, Lhasa, Lanzhou, Beijing, Urumchi, Changchun, Manchuria). By analyzing time series correlation of f₀F₂ and solar-terrestrial activity, five input parameters are determined. The same-time training method is selected and the prediction values within 24 hour can be obtained without changing the network frame. By comparing the prediction property of Neural Network (NN) method and the autocorrelation one (named Corr), for quite data the NN method has higher accuracy except for summer data. While for the whole year data set, the Corr is better. In order to improve the applicability of the method for storm-time data, NN is corrected, and using two specified examples to explain the improvement in the article. After such modification, NN is better than Corr for the same test data as that used above.

Short-term Forecasting of the Ionosphere f₀F₂ with F_10.7

Chen Chun, Wu Zhensen, Sun Shuji, Ding Zonghua, Ban Panpan, Zhao Zhenwei

2009, 29(4): 383-388. doi: 10.11728/cjss2009.04.383

Abstract(2399) PDF 540KB(1248)

Abstract:
The critical frequency of the ionospheric F layer, f₀F₂, is of importance to shortwave propagation and ground-space communications. By using regression analysis of the f₀F₂ observational data and solar radio flux F_10.7, a short-term predicting technique of the ionosphere f₀F₂ is introduced. The solar activity index fc, defined as the moving average of F_10.7, is used as the input, and the output is the ionospheric critical frequency f₀F₂ values three days in advance. Historical data from eight ionospheric stations around China is used to build the method and verify the performance respectively. The results are compared by giving their root-mean-square errors according to different solar activity, season and local time. The results show that the predicting errors at each season and local time are acceptable, which indicate that the method can forecast the f₀F₂ values effectively. The predicting errors at low latitude are higher than those at middle latitude, which conform the complexity variations of low latitude ionosphere. In addition, this method can also be applied to the short-term forecasting of other ionospheric parameters.

Study of Passive Receiving Observation Mode for Ionospheric Digital Ionosonde

Zhu Zhengping, Ning Baiqi, Sun Fenglou, Chen Kun

2009, 29(4): 389-396. doi: 10.11728/cjss2009.04.389

Abstract(2179) PDF 740KB(1500)

Abstract:
In order to study ionospheric passive sounding method and technology, this paper realizes a new ionospheric passive sounding mode on the CADI platform. That is to say, this paper improves scan frequency receiving function of CADI's receiver, receives various space signals by using the receiver, extracts ionospheric characteristic parameters f₀F₂ and high frequency channel property parameters. The proposal and realization of the new passive sounding mode based on the ionospheric digital ionosonde for the first time not only alleviates the congestion problem of the ionospheric high frequency band, but also provides an economical and effective method for the ionosphere and high frequency channel monitoring. The paper will have an extended application perspective in the space environment monitoring and forecasting.

One Analysis on the Rocket Detection of Polar Mesosphere Summer Echoes

Li Hailong, Wu Jian, Huang Jiying, Wang Maoyan

2009, 29(4): 397-401. doi: 10.11728/cjss2009.04.397

Abstract(2608) PDF 523KB(1738)

Abstract:
Polar Mesosphere Summer Echoes (PMSE) are very strong radar echoes from altitudes near the polar summer mesopause. ECT-02 is carried out when PMSE in polar mesopause was occured. Based on the experiment results of ECT-02, the electron number densities and dust charge number densities are deduced in the paper. The data will provide a good basis to study various features of PMSE. By analyzing the data, one can find that dusty plasma in polar summer mesopause is weakly ionized and weakly coupled. The variable disturbance of electron density is used to explain the generation mechanism of PMSE. The stronger disturbance of the electron density corresponds to the stronger radar echoes, and when the disturbance becomes small, the radar echoes become weak, even disappear.

Whistler Waves Excited by Loss-cone Distribution Electrons

Zhou Lihui, Lu Quanming, Guo Jun, Wang Shui

2009, 29(4): 402-408. doi: 10.11728/cjss2009.04.402

Abstract(2692) PDF 746KB(1299)

Abstract:
One-dimensional particle-in-cell simulations are performed in this paper to investigate the nonlinear evolution of whistler waves excited by the electron temperature anisotropy of loss-cone distribution, and compared with the results of bi-Maxwellian distribution. The simulation results indicate that the dominant frequencies in linear growth stage, which contain the most power, are the same in both situations, and that the electron temperature parallel to the background magnetic field is enhanced by the excitation of the waves. The whistler waves with the dominant frequency can be excited earlier with loss-cone distribution and the power is more concentrated in higher frequency (shorter wavelength). The excitations of whistler waves in different anisotropies and different magnetic fields are also studied.

Characteristics of Tropopause OveryWuhan and Haikou

Li Wei, Fan Chao, Yi Fan

2009, 29(4): 409-416. doi: 10.11728/cjss2009.04.409

Abstract(3114) PDF 734KB(1531)

Abstract:
The temporal variation characteristics of local tropopause are derived from the routine Radiosonde data obtained at Wuhan (30.5$^{\circ}$N, 114$^{\circ}$E) and Haikou (20$^{\circ}$N, 114$^{\circ}$E) from 2000 to 2004. The annual variations of temperature, altitude and potential temperature of tropopause are compared with those over a tropical site, Truk. A similar annual trend between Haikou and Truk was found that maximum and minimum values were reached in summer and winter, separately, while an inverse annual variation over Wuhan was shown. By discussing the temperature disturbance, lapse rate at certain pressure levels and tropospheric thickness, it's concluded that tropopause was more determined by the stratospheric behavior. And this can be further confirmed by an intensive radiosonde observation over Wuhan from Jan 11 to 15 in 2006, conducted with a 3 hours interval. By that radiosonde observation, the diurnal variation of tropopause over Wuhan was studied and an obviously regular variation similar to that of Gandanki was found, although little difference exists between them

Solar Extreme Ultraviolet Multichannel Imager

Peng Jilong, Zhu Guang Wu, Wei Fei, Li Baoquan

2009, 29(4): 417-421. doi: 10.11728/cjss2009.04.417

Abstract(2628) PDF 549KB(1273)

Abstract:
Solar extreme ultraviolet imaging plays an important role in space weather research and forecasting. Many nations have developed such kind of instruments for satellites and obtained valuable data. China has paid more attention to such research in recent years. This solar extreme ultraviolet multichannel imager that this paper presented can photograph corona and chromo sphere at four different radiation lines simultaneously. Comparison and inter-calibration with other similar instruments and more experiments can be done in the future. It takes a few resource of satellites while contributes much observation. Its application in orbit will raise up the level of space weather research and forecasting greatly.

Explicit Expression and Influencing Factor Analysis of Collision Probability Between Space Objects

Bai Xianzong, Chen Lei

2009, 29(4): 422-431. doi: 10.11728/cjss2009.04.422

Abstract(2694) PDF 762KB(1278)

Abstract:
The explicit expression of the collision probability is deduced under the assumption that the orbit is a circle. The collision probability is expressed as an explicit function of the encounter geometry (crossing altitude difference and time difference of the line of intersection of the two orbital planes, orbital planes included angle, etc) and position error variance in RSW coordinates. With the help of the explicit expression, the influencing factors of the collision probability were analyzed. Effects of factors such as altitude difference, time difference, orbital planes included angle, position predicted error variances and the equivalent radius of two objects were discussed. Some significant conclusions were obtained.

Numerical analysis of thermally induced vibration for deployable truss antenna

Yang Yulong, Guan Fuling

2009, 29(4): 432-437. doi: 10.11728/cjss2009.04.432

Abstract(2270) PDF 695KB(1261)

Abstract:
The thermal equation of a deployable truss parabolic antenna was derived by finite element method, the convergence criteria of real root and thermal steady state root about thermal equation were built according to the features of space deployable truss antenna. The thermal dynamic forces were simulated by model nodes temperature. Typical model node displacement and root mean square of antenna induced by thermal dynamic forces were discussed. The results show that thermally induced vibration occurs at the beginning of thermal dynamic forces great change and stops by damping and right structure type. The dynamic response is mainly structural basic frequency. At the same time, value of root mean square of antenna induced by thermal drastically changes at the beginning.

Application of parameter self-tuning fuzzy PI control in nano-satellite's thermal control system

Yang Juan, Li Yunze

2009, 29(4): 438-443. doi: 10.11728/cjss2009.04.438

Abstract(2868) PDF 600KB(1128)

Abstract:
On the basis of sustaining the satellite function in a good condition, when the satellite volume, size and mass keeps decreasing, a large amount of heat will be transferred from its inner space, and the conventional thermal control technology cannot satisfy the controlling needs. To keep the stability of load temperature within permissible range, new reasonable thermal control methods are needed. Based on the model of Nano-satellite's temperature system, by combining the on-line self-tuning of PI parameter controller with spacecraft thermal control technology, a new intelligent control system was established to realize the radiator adjustment. Simulation results indicate that the fuzzy self-tuning PI controller can obtain quick response, good adaptability and robustness, and eliminate the steady-state error as well. The new controller is superior to the fuzzy controller and traditional PI controller in that the former works with higher dynamic and static performance.

A class of quasi-cyclic LDPC codes and encoding

Xia Guojiang, Qi Jianzhong, Sun Huixian, Xiong Weiming

2009, 29(4): 443-447. doi: 10.11728/cjss2009.04.443

Abstract(2131) PDF 502KB(1156)

Abstract:
Firstly, based on the characters of space communication, this study improves a class of regular quasi-cyclic LDPC codes which is based on circulant matrices and obtains a kind of irregular quasi-cyclic LDPC codes. Compared with original codes, the parity check matrix called H of this irregular LDPC ensemble has three characters: 1. $\pmb H$ is row full rank; 2. H is lower triangulation; 3. $\pmb H$ contains degree one variable nodes. With the first two characters, the encoding complexity of computation and architecture of this kind of LDPC are proportion to the length of check symbols, so encoders implemented with software and hardware are quite simple. This feature is very important to deep space communication because the resource on board is constrained. The third character makes the iterative decoding threshold lower than the original codes. Moreover, the computer simulation has proved this result. Secondly, the proof for the condition of girth not shorter than 6 is simplifed compared to the original ones. Last, the computing formula is derived for parity check symbols of systemic codes. Based on this formula, the encoding circuit has been investigated using shift registers.