Latitudinal dependence of the ionospheric ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, A06301, doi:10.1029/2010JA016305, 2011

Latitudinal dependence of the ionospheric response to solar eclipse of 15 January 2010 Gang Chen,1 Zhengyu Zhao,1 Baiqi Ning,2 Zhongxin Deng,1 Guobin Yang,1 Chen Zhou,1 Ming Yao,3 Shipeng Li,1 and Ning Li1 Received 22 November 2010; revised 9 March 2011; accepted 16 March 2011; published 1 June 2011.

[1] The ionospheric responses to the solar eclipse of 15 January 2010 in the equatorial anomaly region have been investigated by three vertical‐incidence and seven oblique‐incidence ionosondes arranged along the meridian from geomagnetic latitudes 18°N to 30°N in eastern China. Though the solar eclipse occurred later in the evening, the eclipse effect on electron density and reflection height of ionospheric F2 layer was clearly observed. The study of the eclipse lag (the time lag between the occurrence of the eclipse maximum obscuration and the occurrence of the maximum depletion of fo F2) with latitude indicates it increased with F2 layer altitude. Results suggest also that this eclipse enhanced the prereversal enhancement. An unusual peak occurred after the maximum reduction in fo F2 and this was observed by all our ionosondes. The following F2 layer plasma density increase was considered to be caused by the increased westward electric field. Citation: Chen, G., Z. Zhao, B. Ning, Z. Deng, G. Yang, C. Zhou, M. Yao, S. Li, and N. Li (2011), Latitudinal dependence of the ionospheric response to solar eclipse of 15 January 2010, J. Geophys. Res., 116, A06301, doi:10.1029/2010JA016305.

1. Introduction [2] The equatorial ionospheric anomaly (EIA) phenomena cover a region from the magnetic equator to 30° geomagnetic latitude in each hemisphere. The anomaly regions are of importance due to their connection with the equatorial ionosphere, and their location as the latitudinal transition region from low latitudes into the tropical and midlatitudes [Appleton, 1946; Liang, 1947; Balan and Bailey, 1995]. The ionospheric plasma behaviors in the EIA region are dominated by the fountain effect, which has important effects on the low‐latitude ionospheric responses to solar eclipse [Le et al., 2009]. On the other hand, the local photoionization effect, though weaker than the fountain effect, cannot be completely ignored [Yeh et al., 1997]. [3] Studies of the source‐response relationship between production and loss mechanisms and dynamics of the ionosphere in the equatorial anomaly region during solar eclipses have been carried out mostly from ground‐based observation and model investigation. These studies have found a drastic decrease in electron density of E and F1 layers with maximum eclipse occultation [Van der Laan, 1970]. The F2 region behavior may be quite different. It can show various magnitudes of decrease or even a small 1 Ionosphere Laboratory, School of Electronic Information, Wuhan University, Wuhan, China. 2 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 3 Institute of Space Science and Technology, Nanchang University, Nanchang, China.

increase in the electron concentration [Evans, 1965a, 1965b; Le et al., 2009]. The reductions in fo F2 induced by solar eclipse have been observed to have a delay varying from several minutes to more than an hour with respect to the beginning of the eclipse [Bertin et al., 1977; Cheng et al., 1992; Adeniyi et al., 2007]. Variation of electron density with height showed that the decrease in the electron density occurred throughout the E and F1 heights at about the same time of eclipse occultation while that of the F2 region began at lower heights and extended progressively toward the peak of electron density height of the layer [Salah et al., 1986; Jakowski et al., 2008]. The time delays between maximum decrease in fo F2 and eclipse maximum vary greatly from one observing station to another [Bertin et al., 1977]. Considering the plasma fountain effect along the magnetic field lines, it is interesting to study the latitudinal dependence of the ionospheric F2 layer response to solar eclipse in the EIA region. In addition, the decrease of solar ionizing radiation during solar eclipse does not necessarily lead to corresponding drop of the ionospheric electron content. On the contrary, the enhancement of sporadic E was observed during the solar eclipse of 22 July 2009 [Chen et al., 2010] and the increased electron density in F2 layer was recorded during the 20 July 1963 total solar eclipse [Evans, 1965a, 1965b]. [4] During the eclipse of 15 January 2010, vertical‐ incidence (VI) and oblique‐incidence (OI) ionosondes were arranged along the meridian of about 115° E in eastern China to study the ionospheric response to the solar eclipse. The critical frequency (fo F2) and virtual height (h’F2) of the ionospheric F2 layer were recorded. The time delay of the fo F2 responding to the solar eclipse in different locations

Copyright 2011 by the American Geophysical Union. 0148‐0227/11/2010JA016305

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CHEN ET AL.: LATITUDE EFFECTS OF ECLIPSE

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Figure 1. Map showing the path of the 15 January 2010 solar eclipse over eastern China. The locations of the ionosondes in Beijing, Wuhan, and Guangzhou are indicated by the violet antennae. The blue circles display the reflection midpoints of the oblique detection from Beijing to Wenan (M1), from Beijing to Binzhou (M2), from Xinxiang to Wenan (M3), from Xinxiang to Heze (M4), from Xinxiang to Jinyang (M5), from Suzhou to Taiyuan (M6), and from Xinxiang to Mengcheng (M7). The magnetic equator is shown as a yellow curve. are estimated and compared. The latitudinal dependence of the solar eclipse effect on the ionosphere is discussed.

2. Observations [5] On Friday, 15 January 2010, an annular eclipse of the Sun was visible from within a 300‐km‐wide track that traversed half of the Earth. The path of the Moon’s antumbral shadow began in Africa, crossed the Indian Ocean and ended in the Shangdong province of China. The observations were conducted at the tail of the eclipse path at sunset as shown in Figure 1. The times of sunset at different heights were, of course, different. At ground level the sun had set at the time of maximum obscuration, but at greater heights, the sun still illuminated the ionosphere when it went below the horizon. Therefore it is possible to observe the solar eclipse effect at the ionospheric altitude at sunset in 15 January 2010. Three VI and seven OI ionosondes

located along the meridian in eastern China were used to observe the solar eclipse effect on the ionosphere. The VI ionosondes in Beijing, Wuhan, and Guangzhou recorded the ionograms at 15, 4, and 60‐min intervals, respectively. The detection paths of the OI ionosondes were from Beijing to Wenan, from Beijing to Binzhou, from Xinxiang to Wenan, from Xinxiang to Heze, from Xinxiang to Jinyang, from Taiyuan to Suzhou, and from Xinxiang to Mengcheng. The reflection midpoints of each propagation path are labeled as M1, M2, M3, M4, M5, M6, and M7 sequentially in Figure 1. All OI ionosondes recorded the swept‐frequency ionogram every 30 min. The observed ionosphere covers a region from the EIA’s north crest to its north boundary. The magnetic equator is displayed in Figure 1 as a yellow curve. The geographical coordinates and geomagnetic latitude of Beijing, Wuhan, Guangzhou, the seven midpoints, and magnetic equator on the same meridian are shown in Table 1. The corresponding eclipse time and maximum

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Table 1. Local Time of Start, Maximum Occultation, and End of 15 January 2010 Solar Eclipse and Maximum Occultation at 300 km Altitude Over Beijing, M1, M2, M3, M4, M5, M6, M7, Wuhan, and Guangzhoua Location

Geographic Longitude/Latitude

Geomagnetic Latitude

Eclipse Start (LT)

Eclipse Maximum (LT)

Eclipse End (LT)

Maximum Occultation

Beijing M1 M2 M3 M4 M5 M6 M7 Wuhan Guangzhou Magnetic Equator

116.3°E/40.0°N 116.4°E/39.5°N 117.0°E/39.1°N 115.2°E/37.1°N 114.7°E/35.2°N 111.4°E/35.0°N 116.5°E/34.5°N 115.2°E/34.3°N 114.4°E/30.5°N 113.4°E/28.1°N 115.1°E/7.5°N

29.9°N 29.4°N 9.0°N 27.0°N 25.1°N 24.9°N 24.4°N 24.2°N 20.4°N 18.0°N 0°N

1539 1539 1540 1539 1538 1535 1540 1539 1538 1538 1550

1658 1659 1659 1700 1700 1659 1701 1659 1701 1700 1651

1808 1808 1809 1810 1811 1811 1811 1809 1813 1811 1744

70.9% 72.8% 74.7% 78.2% 82.1% 79.1% 82.2% 74.7% 81.2% 61.1% 16.3%

a

LT is local time. The geographic coordinates and geomagnetic latitude of all the observations are also provided. LT = UT + 8 h.

occultation percentage at 300 km altitude over these locations are also provided. All the eclipse parameters of 15 January 2010 were estimated by Javascript Eclipse Calculator V3.3 (http://www.chris.obyrne.com/Eclipses/calculator.html). [6] When the solar eclipse of 15 January 2010 moved to eastern China, it is in the dusk and no occultation could be clearly observed on ground. Though darkness was falling on the ground, the sun at large zenith angle still obliquely illuminated the atmosphere at high altitude. In the ionosphere, both photochemistry and dynamics caused changes in the refection heights and the electron concentrations. Owing to the regular variations in reflection altitude and electron density in F2 layer after sunset, the decrease of fo F2 and increase of h’F2 induced by solar eclipse may be inconspicuous in the varying ionospheric background. Therefore the fo F2 and h’F2 of the control days 14 and 16 January 2010 are compared with those of the eclipse day. [7] Beijing is a midlatitude station on the north boundary of the EIA region. As shown in Figure 2, the eclipse effects on the ionospheric F2 layer over Beijing were most pronounced. In Figure 2a, the fo F2 curve of 15 January decreased rapidly with the reduction in the intensity of solar radiation and formed a trough after the eclipse maximum. The h’F2 rose with the ionization loss as the behavior of night ionosphere and then fell when the electron density began to recover as shown in Figure 2b. The maximum eclipse effect on fo F2 is labeled with the symbol A by considering the well‐pronounced minimum of the fo F2 curve, together with the prominent peak of the h’F2 curve. There was 46 min delay before the electron density peak of the F2 layer responded to the solar eclipse maximum. After the maximum eclipse effect on F2, the fo F2 not only recovered to the normal value but also continuously increased to 3.6MHz and then decreased to the normal value. The peak following the fo F2 minimum induced by the eclipse is labeled with the symbol B. According to the geometrical relationship between the time of flight and frequencies of vertical and oblique propagation [Bamford, 2000], the ionospheric critical frequency and vertical height parameters at the midpoint of oblique propagation path can be estimated by OI ionogram. The fo F2 and h’F2 curves of M1 to M7 estimated from the OI ionograms recorded in 14, 15, and 16 January 2010 are displayed in Figures 3–9, respectively. Similar to the ionospheric response to the eclipse over Beijing, the eclipse density

Figure 2. Comparison of the (a) critical frequencies and (b) virtual height of the F2 layer over Beijing on the eclipse day 15 January with those of the control days 14 and 16 January 2010. The time of eclipse start, maximum, and end are indicated by the letters S, M, and E, respectively. A and B indicate the fo F2 minimum induced by eclipse and the following peak.

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ionosondes. The local time and fo F2 values of all the observations indicated as A and B in Figures 2–11 are listed in Table 2.

3. Discussions 3.1. Latitudinal Dependence of Eclipse Lag [8] The Earth’s ionosphere is affected by solar eclipse in a complex manner. When the ionizing radiation at shorter wavelengths stems from the solar corona is fully or partially obscured by the Moon, the photo ionizations reduced almost to the nighttime levels. While the bottomside ionosphere is mainly controlled by the local solar radiation, the F2 layer and the topside ionosphere are less prone to the radiation and rather influenced by processes of plasma redistribution [Cheng et al., 1992; Jakowski et al., 2008]. It is generally known that the vertical drift of ionization is a major controlling factor in the ionospheric F2 layer in the EIA region.

Figure 3. Comparison of the (a) critical frequencies and (b) virtual height of the F2 layer over the midpoint between Beijing and Wenan (M1) on the eclipse day 15 January with those of the control days 14 and 16 January 2010. depletion in the peak of F2 layer over M1 to M7 appeared tens of minutes after the eclipse maximum and then increased before it formed an obvious peak, after which it finally fell to the normal value. When the fo F2 dropped to its minimum value, the h’F2 rose and reached its maximum altitude. While the ionization peak in fo F2 following the eclipse density depletion appeared, there was no significant regular variation on the corresponding h’F2 curves. The fo F2 parameters recorded by VI ionosondes of Wuhan and Guangzhou are shown in Figures 10 and 11. The h’F2 values cannot be scaled due to the very weak echo trace. The time delay of fo F2 depletion with respect to the eclipse maximum recorded in Wuhan and Guangzhou exceeded one and half an hour, which is much more than that of Beijing. The obvious electron density enhancement in ionospheric F2 layer following the ionization decrease was observed by all our

Figure 4. Comparison of the (a) critical frequencies and (b) virtual height of the F2 layer over the midpoint between Beijing and Binzhou (M2) on the eclipse day 15 January with those of the control days 14 and 16 January 2010.

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through the fountain effect. After analyzing the fo F2 plots for four ionosonde stations during the solar eclipse of 30 June 1973, Bertin et al. [1977] assumed that the thermospheric neutral winds would be subject to considerable modification by the localized cooling accompanying the passage of the Moon’s shadow. Neglecting the ambient wind, it would probably be expected that the eclipse‐ associated winds would be directed toward the region of totality. The plasma drifts, under the constraint of an inclined magnetic field, would have vertical component in opposite directions to the north and south of the eclipse path with consequences in terms of the ionization loss. Rishbeth [1963] used the computer model of Briggs and Rishbeth [1961] to solve the continuity equation for electron density in the ionospheric F region. Account was taken of production, loss, and vertical diffusion of electrons. The study among other things investigated the speed with which

Figure 5. Comparison of the (a) critical frequencies and (b) virtual height of the F2 layer over the midpoint between Xinxiang and Wenan (M3) on the eclipse day 15 January with those of the control days 14 and 16 January 2010. Therefore the decrease in the electron density occurred throughout the E and F1 heights at about the same time of the solar eclipse maximum occultation, while that of the F2 region began at lower heights and extended progressively upward to the peak of electron density height of the layer [Adeniyi et al., 2007]. The time interval between the occurrence of the eclipse maximum obscuration and the occurrence of the maximum decrease in fo F2 is called eclipse lag, which was often observed during solar eclipse in the EIA region. The eclipse lag observed at different locations was different [Bertin et al., 1977; Walker et al., 1991] and a number of explanations have been offered to account for this time delay. Cheng et al. [1992] found the fo F2 of Chungli decreased steadily to a minimum value a little before the maximum occultation at the magnetic equator and then increased again and so concluded the variation of the F2 layer around the EIA region is controlled not by the local solar eclipse but by solar radiation at the magnetic equator

Figure 6. Comparison of the (a) critical frequencies and (b) virtual height of the F2 layer over the midpoint between Xinxiang and Heze (M4) on the eclipse day 15 January with those of the control days 14 and 16 January 2010.

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fountain transports the eclipse effect from the magnetic equator to north. If there was the eclipse associated winds directed toward the region of eclipse totality from north and south to induce the vertical drifting plasma under the constraint of magnetic field, the vertical components would have opposite directions to the north and south of the eclipse path. However, the observed F2 layer plasma on the north and south of the eclipse path all drifted upward and displayed different eclipse lag. Is the eclipse lag in direct proportion to the height of F2 layer? The variations of h’F2 and the mean value of the virtual height at time A of reference days of Beijing and M1 to M7 are displayed in Figure 13c. In Figure 13d, the latitudinal dependence of the virtual height values at time A is plotted. The violet fitting line illustrates the obvious downward trend and indicates that the height of F2 layer decreases with latitudinal increase. From the combined results shown in Figures 12 and 13, it can be concluded that the observed increase of

Figure 7. Comparison of the (a) critical frequencies and (b) virtual height of the F2 layer over the midpoint between Xinxiang and Jingyang (M5) on the eclipse day 15 January with those of the control days 14 and 16 January 2010. the F layer responses to changes in ionization radiation. From the results of the study, presented as height versus eclipse lag, the eclipse lag increased with height. [9] During the solar eclipse of 15 January 2010, ten ionosondes arranged on the meridian from north magnetic latitude 18° to 30° in eastern China all have recorded the time delay of the maximum reduction in peak electron density of F2 layer responding to the eclipse maximum. The latitudinal dependence of the eclipse lags of the ten observing locations is displayed as blue circles in Figure 12 and the green line is the linear fitting of the lag circles. It is discovered that all recorded maximum decay in fo F2 occurred more than half an hour later than the eclipse maximum time at the magnetic equator of the same longitude and the eclipse lag decreases with increasing latitude. If the eclipse effect on F2 layer around the EIA region was not controlled by the local eclipse occultation but by that at the magnetic equator through the fountain effect, the eclipse lag should increase with the increase of latitude, when the

Figure 8. Comparison of the (a) critical frequencies and (b) virtual height of the F2 layer over the midpoint between Taiyuan and Suzhou (M6) on the eclipse day 15 January with those of the control days 14 and 16 January 2010.

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Figure 10. Comparison of the critical frequencies of the F2 layer over Wuhan on the eclipse day 15 January with those of the control days 14 and 16 January 2010.

by the atmospheric contraction during solar eclipse, and the reduced [O]/[N2] ratio after the eclipse [Müller‐Wodarg et al., 1998]. In contrast to the previous observations that an increase in fo F2 followed by a decrease [Evans, 1965b; Cheng et al., 1992], during the solar eclipse of 15 January 2010, all our ionosonde stations have observed the enhancement of F2 layer treading on the heels of the eclipse density depletion and the peaks are labeled with the symbol B in all the fo F2 plots.

Figure 9. Comparison of the (a) critical frequencies and (b) virtual height of the F2 layer over the midpoint between Xinxiang and Mengcheng (M7) on the eclipse day 15 January with those of the control days 14 and 16 January 2010.

eclipse lag with height is in agreement with the computer model research of Rishbeth [1963]. 3.2. F2 Enhancement [10] Observations of F2 layer during the time of solar eclipse have provided contradictory results; sometimes the density has decreased and other times it has actually increased. The ionization loss can be considered as the result of the reduced solar radiation, but the increased electron density of F2 during solar eclipse is confusing. This behavior can be explained by assuming a rapid downward diffusion of ionization toward the F2 peak and the maximum of the layer under the competing influence of diffusion and loss with eclipse occultation [Evans, 1965a; Anastassiades and Moraitis, 1968]. A modeling study explained the enhancement of NmF2

Figure 11. Comparison of the critical frequencies of the F2 layer over Guangzhou on the eclipse day 15 January with those of the control days 14 and 16 January 2010.

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Table 2. Local Time and fo F2 Values of Ionization Minimum, A, and the Following Peak, B, of All the Observationsa A

B

Location

Local Time

fo F2 (MHz)

Local Time

fo F2 (MHz)

Beijing M1 M2 M3 M4 M5 M6 M7 Wuhan Guangzhou

1744 1758 1758 1756 1755 1825 1825 1825 1833 1900

2.8 3.5 3.3 3.6 3.3 3.3 3.3 3.2 2.6 3.6

1915 1928 1927 1955 1956 1955 2024 1956 2000 2100

3.6 4.2 4.1 4.6 4.3 4.1 4.7 4.2 3.6 4.5

a

LT = UT + 8 h.

[11] The solar eclipse took place under low geomagnetic and magnetospheric activity, since the AE index never exceed the 100 nT and the Dst index values ranged above −10 nT as shown in Figures 13a and 13b [Mayaud, 1980]. Therefore the impact of the solar eclipse on the ionosphere was not unduly complicated by geomagnetic or magnetospheric disturbances. After the maximum depletion at time A in fo F2, the reduced electron density began to recover. The increase of the electron density has not ceased, when it increased to the normal value of the reference days but continued to rise for more than half an hour and reached its maximum value at

Figure 12. Latitudinal dependence of the time lag between time of A and eclipse maximum of all the 10 locations. The green line is the linear fitting of the time lag.

time B. As shown in Table 2, the electron density enhancement observed by all the stations occurred in the night, which has made us consider the event of prereversal enhancement of the equatorial vertical upward E × B drift that causes the

Figure 13. (a) AE index. (b) Dst index. (c) The virtual height variation (curve) of eight locations during the eclipse day and the virtual height value (dot) at time A of reference days. (d) Latitudinal dependence of the reference virtual height values and their violet linear fitting line. 8 of 10

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prereversal strengthening of the forward fountain and the sudden redevelopment of the EIA [Fejer et al., 1991; Horvath and Essex, 2003]. The equatorial E × B drift is upward when the electric field is eastward and downward when the electric field is westward. The upward E × B drift is enhanced at sunset when the pre‐reversal enhancement takes place [Lin et al., 2005]. The sudden increase of h’F2 in the eclipse day shows there was a rapid upward drift of the ionospheric F2 layer in the EIA region indicating that the equatorial eastward electric field was unusually strong. The solar eclipse occurring at sunset actually increased the eastward electric field, which strengthened the prereversal enhancement. After the prereversal enhancement, the eastward electric field tuned westward and so the E × B drift became downward directed. At midlatitudes, this downward E × B drift will have a downward and a poleward component. It is the downward component that pulled down the F2 layer and decreased the enhanced h’F2 value. In a balanced model ionosphere that is assumed to consist of O+, H+, and electrons, when the downward drift component pulled the F2 layer to lower altitudes, at a given altitude O+ has been reduced. To restore the balance of the ionospheric compositions, the protons that diffuse down into the chemical equilibrium region go through the charge exchange reaction (H+ + O → H + O+) to increase O+ toward the predisturbance level. Therefore a lowering of the F2 layer has resulted in the plasma density enhancement [Park, 1971]. The ionospheric altitude decrease and the resultant plasma density increase were stronger at the eclipse time than the day before and after, indicating that the westward electric field and therefore the event of reversal, following the prereversal enhancement, was also more powerful at the eclipse time.

4. Conclusion [12] The observations were carried out at the tail of the eclipse path and darkness was falling at the time of eclipse maximum. Fortunately, the sun could still shine upon the ionosphere at large zenith angle. The ionization loss and production processes and virtual height variation of ionospheric F2 layer during the eclipse could be observed clearly by our three VI and seven OI ionosondes. Similar to the previous observations during solar eclipse in the EIA region, the eclipse density depletion traveled from the low or middle atmosphere to the peak of the ionospheric F layer and the time delay of the fo F2 response to the eclipse maximum recorded at different locations varied from 40 to 120 min. The latitudinal dependence of the eclipse lag and F2 altitude was analyzed respectively and from the results of study, the eclipse lag increased with height. A peak on fo F2 curve following the eclipse depletion was recorded by all our ionosondes. Finally, our results suggest that the solar eclipse occurring at sunset increased the eastward electric field and therefore significantly enhanced the prereversal enhancement, and consequently the westward electric field after reversal increased as well. The effects of increased westward drifts were seen at midlatitudes as sudden height decrease followed by plasma density increase. [13] Acknowledgments. This research is supported by the National Natural Science Foundation (40804042 and 41074115), the Post Doctor Foundation of China (200902445), and the Fundamental Research Funds

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for the Central Universities (4081004). The authors would like to thank the two reviewers for quite helpful comments. [14] Robert Lysak thanks Ildiko Horvath and another reviewer for their assistance in evaluating this paper.

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Yeh, K. C., et al. (1997), Ionospheric response to a solar eclipse in the equatorial anomaly region, Terr. Atmos. Oceanic Sci., 8(2), 165–178. G. Chen, Z. Deng, N. Li, S. Li, G. Yang, Z. Zhao, and C. Zhou, Ionosphere Laboratory, School of Electronic Information, Wuhan University, Wuhan 430072, China. ([email protected])

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B. Ning, Institute of Geology and Geophysics, Chinese Academy of Sciences, 19 Beitucheng Western Rd., Beijing 100864, China. M. Yao, Institute of Space Science and Technology, Nanchang University, Nanchang 330031, China.

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