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Anomalous Variation of VLF Signals in the Total Solar Eclipse of 22nd July 2009
CHINESE ASTRONOMY AND ASTROPHYSICS
ELSEVIER
Chinese Astrophysics35 35(2011) (2011) 54–61 ChineseAstronomy Astronomy and and Astrophysics 54–61
Anomalous Variation of VLF Signals in the Total Solar Eclipse of 22nd July 2009 ZHANG Xue-feng
DU Ruo-yin GU Sheng-min Wang You-ning
WU Tong
Nanjing Tian Jia-bing High School, Nanjing 210037
Abstract Using the sudden ionospheric disturbance (SID) monitor provided by the Stanford University, the very low frequency (VLF) signals were observed in Nanjing during the total solar eclipse on 22nd July 2009. It is found that the intensity of the VLF signal varied anomalously during the total solar eclipse, exhibiting the effect similar to the sunrise and sunset, and that the intensity of the VLF signal strongly fluctuated, exhibiting the effect similar to thunderbolts. Moreover, the signal intensity enhanced in short periods both before and after the total eclipse. Based on the discussion on the reliability of the observed data and the comparison with the previous observations, it is found that the ionosphere exhibits some variations previous to the total solar eclipse. Key words: eclipses—earth: atmosphere—methods: data analysis
1. INTRODUCTION On 22nd July 2009, a total solar eclipse happened in the southern China in the latitudes between 20◦ N and 40◦ N. With the SID monitor provided by the Solar Center of the Stanford University, USA, we have made the monitoring observation of the VLF signals during this total solar eclipse at Nanjing (118◦ 46 E, 32◦ 03 N). In the one-week periods both before and after the total solar eclipse, the instrument worked normally and the external interference was small, but in the period of the total solar eclipse, the observed signals exhibited significant variations. The SID monitor is a VLF-wave receiver (with the central frequency of 19.8 KHz). VLF signals are widely used in the radio communication, navigation, time service, the Received 2009–10–16; revised version 2009–11–04 A translation of Acta Astron. Sin. Vol. 51, No. 3, pp. 278–284, 2010 [email protected]
0275-1062/11/$-see front front mattermatter © 2011 Elsevier All rights reserved. c 2011 B.V. 0275-1062/01/$-see Elsevier Science B. V. All rights reserved. doi:10.1016/j.chinastron.2011.01.007 PII:
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
55
exploration of lower ionosphere, and other systems[1] . According to the waveguide theory of the propagation in the terrestrial ionosphere, the VLF radio waves are mainly propagated in the spherical shell-shaped space between the earth surface and the lower ionosphere[2]. Besides the effect of the earth surface, the VLF signals are mainly affected by the behavior of the lower ionosphere, and in addition to the effect of solar radiation[3], the lower ionosphere is affected also by the terrestrial magnetic field, cosmic rays, meteor showers, and other factors[4] . Because of the rather low probability of the total solar eclipse, the studies on the VLF signals during a total solar eclipse are not very many. In the available literature, what is mostly mentioned are the sunrise and sunset effects of the VLF signals during the total solar eclipse, the increase of the effective reflection height of the ionosphere, etc.[5−11] . Our analysis on the VLF data of this total solar eclipse not only confirms the above-mentioned variations, but also finds that the signal intensities in short periods both before and after the total solar eclipse exhibited certain enhancements, especially the intensity previous to the solar eclipse. So far as we know, this point has not yet been reported previously.
2. SPACE WEATHER MONITOR PROGRAM AND OPERATIONAL PRINCIPLE OF SID The SID monitor that we used is the space weather monitor developed by the VLF group of Electrical Engineering Institute of Stanford University in collaboration with local teachers. It can be used to detect the ionospheric variations caused by solar activity and other disturbances. Nowadays under the organization of Stanford’s Solar Center, there are about 100 SID instruments distributed around the world to monitor continuously the sudden ionospheric variations at different places, and all data are up-transmitted to http:sid.stanford.edu /database-browser. The SID monitor receives the VLF radio waves transmitted from distant transmitters and reflected back from the terrestrial ionosphere to monitor the effect of the solar activity on the ionosphere by recording the strength of the VLF radio signal. The sun affects the earth by two mechanisms. The first is the radiation. The sun radiates in all directions the ever lasting X-ray and extreme-ultraviolet (EUV) emissions. When solar flares occur on the sun, it suddenly emits even stronger X-ray and EUV emissions. These emissions travel with the velocity of light, and can reach the earth taking only 8 min. Another mechanism is the large quantity of plasma-state matter ejected from the sun, it is called the coronal mass ejection. These matter streams surge toward the earth with a velocity of about 2×106 km/h, and take about 72 h to reach the earth. The SID monitor can track the sudden ionospheric disturbances caused by the solar activity, their energy comes from the sun and cosmic rays, and it can always affect the ionosphere at the attitude of 60 km above the earth surface. The ionosphere is formed because electrons escape from the control of atomic nuclei under the action of solar high-energy emission or cosmic rays. The nighttime ionosphere can be divided into the E and F layers. In the daytime, the solar X-rays and ultraviolet radiation can increase the ionization degree of the ionosphere, therefore the D layer is produced, the E layer is strengthened and the F layer is divided into two portions. Generally speaking, the D layer is not dense enough to reflect radio waves. Hence the VLF signals can penetrate the D layer, are reflected by the E layer, then return
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back to the ground through the D layer. As the signals lose a part of energy when they penetrate the D layer, so in the daytime the received signals are weaker. When a solar flare happens, the D layer is ionized sufficiently, and it can reflect the signals, hence the signals become stronger[12] .
3. GEOGRAPHIC POSITIONS OF THE SID MONITOR AND LAUNCHING SITE OF VLF SIGNALS The adopted SID monitor has the central frequency 19.8 KHz and is located in the Nanjing city (118◦ 46 E, 32◦ 03 N). The data up-transmitted to the data base of Stanford’s Solar Center are named as China-10. The VLF radio waves received by the monitor come from a transmitter located at North West Cape (114◦ 02 E, 21◦ 08 S) of Australia. The eclipse zone of this total solar eclipse is a narrow and long region in China along the west-east direction between the latitudes 20◦ and 40◦ . From Fig.1, we can find that the path of the radio wave propagation and the eclipse zone are almost perpendicularly crossed, and Nanjing is just positioned at the north of the crossing point. During the total solar eclipse, the VLF radio waves received in Nanjing are mostly reflected to the ground from the ionosphere right above the eclipse zone. Hence any sudden ionospheric disturbance during the total solar eclipse can be confirmed very well, and its time ought to be equivalent to the time of the total solar eclipse which happened in the southern area of Nanjing.
Fig. 1 Total solar eclipse region and propagation path of VLF signals
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
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4. ANALYSIS OF THE DATA RELIABILITY Since all kinds of space weather phenomena have certain influences on the VLF signals, we have to make a reliability analysis on the obtained data, in order to verify whether the anomaly of received signals is really caused by the total solar eclipse. Fig.2 shows the time evolution of signal intensity from 6 o’clock in the morning to the 6 o’clock in the evening. From this figure we can find that in about 3 hours while the eclipse happened, the VLF signals exhibited rather large differences in comparison with other times. Whether is this really caused by the total solar eclipse or by other interferences? In order to make this certain, we will present an explanation from the following several aspects.
Fig. 2 Curve of the VLF signal intensity for the route from North West Cape to Nanjing on 22nd Jul., 2009
In Fig.3 we list the daytime data of the two days after the total solar eclipse (the data of the day before the eclipse are not listed, because the thunderstorm weather happened in the Nanjing area). From Fig.3 we can find that the signals in these two days are stationary. In the two days the signal intensities at corresponding times are equivalent, the two curves are highly coincident, and that no long-duration magnetoelectric interference appeared.
Fig. 3 Curves of the VLF signal intensity for the route from North West Cape to Nanjing on 23nd and 24th Jul., 2009
However, the signal curves of these two days are not smooth and some sudden intensity variations still exist. What is the reason of these sudden variations? From longtime observations we find that generally the sudden variations of the SID monitor signals occur in the
58
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
following conditions: the electromagnetic interferences of unknown reasons, the sunrise and sunset, solar flares, thunderstorm weather, etc., but the effects of all these conditions have their own special features. For example, the electromagnetic interference displays signal variations in a short period of time, then returns to the normal state in a short time (see Fig.4). As the anomaly during the total solar eclipse has the feature obviously different from that of the electromagnetic interference, so the magnetoelectric interference can be ruled out. On 22nd July, Nanjing and its southern region had no thunderstorm weather. By consulting the GEOS database, we find that on 22nd July, no solar flare stronger than the M-class happened. Additionally, the time of signal anomaly is coincident with the time of the total solar eclipse. Hence we judge that this signal anomaly is caused by the total solar eclipse.
Fig. 4 Curve of the VLF signal intensity for the route from North West Cape to Nanjing on 10th Apr. 2009
5. CONCLUSIONS Based on the comparisons of the signal intensity curve during the 22nd total solar eclipse with the signal curves of several days before and after in the same time segment, we have made an analysis and obtained the following conclusions: (1) In the period of the total solar eclipse, the VLF signal enhanced anomalously, exhibiting the effect similar to sunrise and sunset. The signal curve during the total solar eclipse is similar to those of sunrise and sunset. It exhibits a peak-shaped rise, similar to the sunset shape in the Nanjing area, but it varies even violently. At 08:40 (Beijing time) the signal began to enhance, from 08:53 to 08:58 the signal was strengthened quickly from -1.113 V to the maximum value 0.586 V, the increment is 1.699 V. In the two and more hours from the sunrise to the eclipse beginning, because of sun’s illumination the signal intensity increased from -2.000 V to -1.113 V, the increment is 0.887 V. Hence, owing to the disturbance caused by the total solar eclipse, the increment of signal intensity in a short time is twice larger than that of the previous two hours. To compare again with the same time on 23rd, at 08:58 23rd the signal intensity was -1.60 V, the difference between the two is 2.15 V. This indicates that during the total solar eclipse, the variation of the VLF signal intensity is very significant. (2) During the total solar eclipse, the VLF signal fluctuated rapidly, exhibiting the effect similar to thunderbolts
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
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From 10:07 to 11:19, the VLF signal intensity fluctuated violently in a manner similar to noises, and exhibited the signal feature similar to that in the period of thunderbolts. It varied in the range from -1.25 V to 0.00 V, the amplitude of the fluctuation reached 1.25 V, but in normal cases the amplitude of the VLF signal fluctuation is only 0.05 V, the difference between the two is as large as 25 times. Fig.5 gives signal intensity curve in the early morning of 25th Sep. 2009, while a strong thunderstorm happened. In the whole period of the thunderstorm, the signal noise also obviously increased.
Fig. 5 Curve of the VLF signal intensity for the route from North West Cape to Nanjing on 25th Sep., 2009
(3) The effect of the total solar eclipse on the ionosphere exhibits the phenomena of time advance and delay. We have superposed together the daytime data of 22nd and 23rd July (see Fig.6) and found an interesting phenomenon. Early in a few hours before the total solar eclipse the VLF signal intensity has increased significantly, it kept to increase until the sudden change at the eclipse beginning, and the signal enhancement remained for 20 min after the eclipse had terminated, then the two data curves became coincident completely. This indicates that before the total solar eclipse happened, the anomalous effect on the ionosphere probably had occurred, and it remained even after the total solar eclipse had terminated.
Fig. 6 Superposition of the VLF signal intensity curves of 22nd and 23rd July, 2009
In order to confirm this suggestion, we have collected the data during the total solar eclipse happened on 9th Mar. 1997 at Xinxiang of Henan province[13], as shown in Fig.7. From Fig.7, we can find that on 9th March, soon after the sunrise, the total solar eclipse which happened, and that previous to the eclipse beginning, namely previous to the local sunrise, the VLF signal intensity was higher than the intensity at the corresponding time on 8th March. Hence these two observations have actually revealed the same phenomenon. This implies that probably the ionosphere may provide precursors for the occurrence of some
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natural phenomena. Besides, after the Indonesian tsunami, a research group has discovered that the sunset signal was delayed for 9 min[14] .
Fig. 7 Amplitude curve at 11.9 KHz for the Henan Xinxiang—Alpha station route on 9th Mar., 1997
6. A SUGGESTION FOR THE MECHANISM OF INFLUENCE According to the waveguide theory of VLF propagation, the VLF radio waves are propagated in the waveguide shaped by the concentric spherical shells composed of the lower boundary of the lower ionosphere and the ground, and called as the earth-ionosphere waveguide propagation. When there occurs a sudden change in the lower boundary of the ionosphere, the VLF signal will have a significant response, this is also the reason why the VLF curve exhibits large fluctuations during the sunrise and sunset. When a total solar eclipse happens, the sunlight projecting on the earth is blocked by the moon, the recombination of the ions and electrons in the lower ionosphere is gradually accelerated, and on the path of VLF signal propagation, the effective reflection height of the lower ionosphere increases gradually, and the D layer produced in the daytime is suddenly weakened. When the signal penetrates through the D layer, its attenuation becomes small, hence the signal is enhanced when it reaches the ground. And at the third contact after the maximum of the total solar eclipse, the atmospheric ionization caused by the solar radiation becomes gradually intense. This makes the effective reflection height reduce gradually, and makes the D layer enhance gradually, thus the VLF signal gradually recovers to its normal state. At present, we can hardly give a reasonable explanation for the variation of the VLF signal previous to the total solar eclipse, but we know that during the total solar eclipse, because that the sun, moon and earth are co-linear, it will produce a strong gravitational effect on the earth, even cause the deformation of rocks and other effects[15] . Then would’t it have an influence on the ionosphere? Now, there have been some studies about the effect of crust movement on the ionosphere, and a certain progress has been achieved. For studying the effect of the ionosphere previous to the total solar eclipse, we still need more practical examples. ACKNOWLEDGEMENTS We thank the Referee for valuable opinion, Dr. Sharon Murrel and Dr. Zhao Jun-wei of Stanford University for providing the SID monitor, Dr.
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
61
Huang Guang-li and Dr. Liu Yan of Purple Mountain Observatory for the instruction to the teachers and students of our school. References 1
Tian Yu-shu, Pan Wei-yan, Yu Yun-cao, et al., National Military Standard of People’s Republic China
2
Wait J. R., Spies K. P., NBS Tech. Note, 1964, 300, 1
3
Schoute-Vanneck C. A., JATP, 1974, 36, 489
4
Niu You-tian, Hao Hao-zhen, Chen Jian-feng, et al., Journal of Henan Normal University (Natural
5
Rycroft M. J., Reeve C. D., Nature, 1970, 226, 1126
6
Schaal R. E., Mendes A. M., Ananthakrishnan S., et al., Nature, 1970, 226, 1127
7
Jain S. L., Subrahmanyam C. V., IJRSP, 1979, 8, 348
(GJB/Z 70-95), 1996
Science Edition), 2008, 36, 54
8
Pent P., Mahra H. S., BASI, 1985, 13, 80
9
De B. K., Sarkar S. K., KodOB, 1997, 13, 205
10
Kozlov V. I., Karimov R. R., Mullayarov V. A., RuPhJ, 2007, 50, 617
11
Karimov R. R., Kozlov V. I., Mullayarov V. A., Ge&Ae, 2008, 48, 240
12
Deborah S., Research with Space Weather Monitor Data, Stanford: Stanford Solar Center, 2007, 5-6
13
Niu You-tian, Chen Jian-feng, Hao Hao-zhen et al., Journal of Henan Normal University (Natural
14
Chakrabarti S. K., Saha M., Khan R., et al., Unusual Sunset Terminator Behavior of VLF signals at
15
Freund F., The Economist Newspaper and the Economist Group, 2005-12-14
Science Edition), 2009, 37, 2 17 KHz During the Earthquake Episode of Dec. 2004, URSI General Assembly, 2005
ELSEVIER
Chinese Astrophysics35 35(2011) (2011) 54–61 ChineseAstronomy Astronomy and and Astrophysics 54–61
Anomalous Variation of VLF Signals in the Total Solar Eclipse of 22nd July 2009 ZHANG Xue-feng
DU Ruo-yin GU Sheng-min Wang You-ning
WU Tong
Nanjing Tian Jia-bing High School, Nanjing 210037
Abstract Using the sudden ionospheric disturbance (SID) monitor provided by the Stanford University, the very low frequency (VLF) signals were observed in Nanjing during the total solar eclipse on 22nd July 2009. It is found that the intensity of the VLF signal varied anomalously during the total solar eclipse, exhibiting the effect similar to the sunrise and sunset, and that the intensity of the VLF signal strongly fluctuated, exhibiting the effect similar to thunderbolts. Moreover, the signal intensity enhanced in short periods both before and after the total eclipse. Based on the discussion on the reliability of the observed data and the comparison with the previous observations, it is found that the ionosphere exhibits some variations previous to the total solar eclipse. Key words: eclipses—earth: atmosphere—methods: data analysis
1. INTRODUCTION On 22nd July 2009, a total solar eclipse happened in the southern China in the latitudes between 20◦ N and 40◦ N. With the SID monitor provided by the Solar Center of the Stanford University, USA, we have made the monitoring observation of the VLF signals during this total solar eclipse at Nanjing (118◦ 46 E, 32◦ 03 N). In the one-week periods both before and after the total solar eclipse, the instrument worked normally and the external interference was small, but in the period of the total solar eclipse, the observed signals exhibited significant variations. The SID monitor is a VLF-wave receiver (with the central frequency of 19.8 KHz). VLF signals are widely used in the radio communication, navigation, time service, the Received 2009–10–16; revised version 2009–11–04 A translation of Acta Astron. Sin. Vol. 51, No. 3, pp. 278–284, 2010 [email protected]
0275-1062/11/$-see front front mattermatter © 2011 Elsevier All rights reserved. c 2011 B.V. 0275-1062/01/$-see Elsevier Science B. V. All rights reserved. doi:10.1016/j.chinastron.2011.01.007 PII:
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
55
exploration of lower ionosphere, and other systems[1] . According to the waveguide theory of the propagation in the terrestrial ionosphere, the VLF radio waves are mainly propagated in the spherical shell-shaped space between the earth surface and the lower ionosphere[2]. Besides the effect of the earth surface, the VLF signals are mainly affected by the behavior of the lower ionosphere, and in addition to the effect of solar radiation[3], the lower ionosphere is affected also by the terrestrial magnetic field, cosmic rays, meteor showers, and other factors[4] . Because of the rather low probability of the total solar eclipse, the studies on the VLF signals during a total solar eclipse are not very many. In the available literature, what is mostly mentioned are the sunrise and sunset effects of the VLF signals during the total solar eclipse, the increase of the effective reflection height of the ionosphere, etc.[5−11] . Our analysis on the VLF data of this total solar eclipse not only confirms the above-mentioned variations, but also finds that the signal intensities in short periods both before and after the total solar eclipse exhibited certain enhancements, especially the intensity previous to the solar eclipse. So far as we know, this point has not yet been reported previously.
2. SPACE WEATHER MONITOR PROGRAM AND OPERATIONAL PRINCIPLE OF SID The SID monitor that we used is the space weather monitor developed by the VLF group of Electrical Engineering Institute of Stanford University in collaboration with local teachers. It can be used to detect the ionospheric variations caused by solar activity and other disturbances. Nowadays under the organization of Stanford’s Solar Center, there are about 100 SID instruments distributed around the world to monitor continuously the sudden ionospheric variations at different places, and all data are up-transmitted to http:sid.stanford.edu /database-browser. The SID monitor receives the VLF radio waves transmitted from distant transmitters and reflected back from the terrestrial ionosphere to monitor the effect of the solar activity on the ionosphere by recording the strength of the VLF radio signal. The sun affects the earth by two mechanisms. The first is the radiation. The sun radiates in all directions the ever lasting X-ray and extreme-ultraviolet (EUV) emissions. When solar flares occur on the sun, it suddenly emits even stronger X-ray and EUV emissions. These emissions travel with the velocity of light, and can reach the earth taking only 8 min. Another mechanism is the large quantity of plasma-state matter ejected from the sun, it is called the coronal mass ejection. These matter streams surge toward the earth with a velocity of about 2×106 km/h, and take about 72 h to reach the earth. The SID monitor can track the sudden ionospheric disturbances caused by the solar activity, their energy comes from the sun and cosmic rays, and it can always affect the ionosphere at the attitude of 60 km above the earth surface. The ionosphere is formed because electrons escape from the control of atomic nuclei under the action of solar high-energy emission or cosmic rays. The nighttime ionosphere can be divided into the E and F layers. In the daytime, the solar X-rays and ultraviolet radiation can increase the ionization degree of the ionosphere, therefore the D layer is produced, the E layer is strengthened and the F layer is divided into two portions. Generally speaking, the D layer is not dense enough to reflect radio waves. Hence the VLF signals can penetrate the D layer, are reflected by the E layer, then return
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ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
back to the ground through the D layer. As the signals lose a part of energy when they penetrate the D layer, so in the daytime the received signals are weaker. When a solar flare happens, the D layer is ionized sufficiently, and it can reflect the signals, hence the signals become stronger[12] .
3. GEOGRAPHIC POSITIONS OF THE SID MONITOR AND LAUNCHING SITE OF VLF SIGNALS The adopted SID monitor has the central frequency 19.8 KHz and is located in the Nanjing city (118◦ 46 E, 32◦ 03 N). The data up-transmitted to the data base of Stanford’s Solar Center are named as China-10. The VLF radio waves received by the monitor come from a transmitter located at North West Cape (114◦ 02 E, 21◦ 08 S) of Australia. The eclipse zone of this total solar eclipse is a narrow and long region in China along the west-east direction between the latitudes 20◦ and 40◦ . From Fig.1, we can find that the path of the radio wave propagation and the eclipse zone are almost perpendicularly crossed, and Nanjing is just positioned at the north of the crossing point. During the total solar eclipse, the VLF radio waves received in Nanjing are mostly reflected to the ground from the ionosphere right above the eclipse zone. Hence any sudden ionospheric disturbance during the total solar eclipse can be confirmed very well, and its time ought to be equivalent to the time of the total solar eclipse which happened in the southern area of Nanjing.
Fig. 1 Total solar eclipse region and propagation path of VLF signals
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
57
4. ANALYSIS OF THE DATA RELIABILITY Since all kinds of space weather phenomena have certain influences on the VLF signals, we have to make a reliability analysis on the obtained data, in order to verify whether the anomaly of received signals is really caused by the total solar eclipse. Fig.2 shows the time evolution of signal intensity from 6 o’clock in the morning to the 6 o’clock in the evening. From this figure we can find that in about 3 hours while the eclipse happened, the VLF signals exhibited rather large differences in comparison with other times. Whether is this really caused by the total solar eclipse or by other interferences? In order to make this certain, we will present an explanation from the following several aspects.
Fig. 2 Curve of the VLF signal intensity for the route from North West Cape to Nanjing on 22nd Jul., 2009
In Fig.3 we list the daytime data of the two days after the total solar eclipse (the data of the day before the eclipse are not listed, because the thunderstorm weather happened in the Nanjing area). From Fig.3 we can find that the signals in these two days are stationary. In the two days the signal intensities at corresponding times are equivalent, the two curves are highly coincident, and that no long-duration magnetoelectric interference appeared.
Fig. 3 Curves of the VLF signal intensity for the route from North West Cape to Nanjing on 23nd and 24th Jul., 2009
However, the signal curves of these two days are not smooth and some sudden intensity variations still exist. What is the reason of these sudden variations? From longtime observations we find that generally the sudden variations of the SID monitor signals occur in the
58
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
following conditions: the electromagnetic interferences of unknown reasons, the sunrise and sunset, solar flares, thunderstorm weather, etc., but the effects of all these conditions have their own special features. For example, the electromagnetic interference displays signal variations in a short period of time, then returns to the normal state in a short time (see Fig.4). As the anomaly during the total solar eclipse has the feature obviously different from that of the electromagnetic interference, so the magnetoelectric interference can be ruled out. On 22nd July, Nanjing and its southern region had no thunderstorm weather. By consulting the GEOS database, we find that on 22nd July, no solar flare stronger than the M-class happened. Additionally, the time of signal anomaly is coincident with the time of the total solar eclipse. Hence we judge that this signal anomaly is caused by the total solar eclipse.
Fig. 4 Curve of the VLF signal intensity for the route from North West Cape to Nanjing on 10th Apr. 2009
5. CONCLUSIONS Based on the comparisons of the signal intensity curve during the 22nd total solar eclipse with the signal curves of several days before and after in the same time segment, we have made an analysis and obtained the following conclusions: (1) In the period of the total solar eclipse, the VLF signal enhanced anomalously, exhibiting the effect similar to sunrise and sunset. The signal curve during the total solar eclipse is similar to those of sunrise and sunset. It exhibits a peak-shaped rise, similar to the sunset shape in the Nanjing area, but it varies even violently. At 08:40 (Beijing time) the signal began to enhance, from 08:53 to 08:58 the signal was strengthened quickly from -1.113 V to the maximum value 0.586 V, the increment is 1.699 V. In the two and more hours from the sunrise to the eclipse beginning, because of sun’s illumination the signal intensity increased from -2.000 V to -1.113 V, the increment is 0.887 V. Hence, owing to the disturbance caused by the total solar eclipse, the increment of signal intensity in a short time is twice larger than that of the previous two hours. To compare again with the same time on 23rd, at 08:58 23rd the signal intensity was -1.60 V, the difference between the two is 2.15 V. This indicates that during the total solar eclipse, the variation of the VLF signal intensity is very significant. (2) During the total solar eclipse, the VLF signal fluctuated rapidly, exhibiting the effect similar to thunderbolts
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
59
From 10:07 to 11:19, the VLF signal intensity fluctuated violently in a manner similar to noises, and exhibited the signal feature similar to that in the period of thunderbolts. It varied in the range from -1.25 V to 0.00 V, the amplitude of the fluctuation reached 1.25 V, but in normal cases the amplitude of the VLF signal fluctuation is only 0.05 V, the difference between the two is as large as 25 times. Fig.5 gives signal intensity curve in the early morning of 25th Sep. 2009, while a strong thunderstorm happened. In the whole period of the thunderstorm, the signal noise also obviously increased.
Fig. 5 Curve of the VLF signal intensity for the route from North West Cape to Nanjing on 25th Sep., 2009
(3) The effect of the total solar eclipse on the ionosphere exhibits the phenomena of time advance and delay. We have superposed together the daytime data of 22nd and 23rd July (see Fig.6) and found an interesting phenomenon. Early in a few hours before the total solar eclipse the VLF signal intensity has increased significantly, it kept to increase until the sudden change at the eclipse beginning, and the signal enhancement remained for 20 min after the eclipse had terminated, then the two data curves became coincident completely. This indicates that before the total solar eclipse happened, the anomalous effect on the ionosphere probably had occurred, and it remained even after the total solar eclipse had terminated.
Fig. 6 Superposition of the VLF signal intensity curves of 22nd and 23rd July, 2009
In order to confirm this suggestion, we have collected the data during the total solar eclipse happened on 9th Mar. 1997 at Xinxiang of Henan province[13], as shown in Fig.7. From Fig.7, we can find that on 9th March, soon after the sunrise, the total solar eclipse which happened, and that previous to the eclipse beginning, namely previous to the local sunrise, the VLF signal intensity was higher than the intensity at the corresponding time on 8th March. Hence these two observations have actually revealed the same phenomenon. This implies that probably the ionosphere may provide precursors for the occurrence of some
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natural phenomena. Besides, after the Indonesian tsunami, a research group has discovered that the sunset signal was delayed for 9 min[14] .
Fig. 7 Amplitude curve at 11.9 KHz for the Henan Xinxiang—Alpha station route on 9th Mar., 1997
6. A SUGGESTION FOR THE MECHANISM OF INFLUENCE According to the waveguide theory of VLF propagation, the VLF radio waves are propagated in the waveguide shaped by the concentric spherical shells composed of the lower boundary of the lower ionosphere and the ground, and called as the earth-ionosphere waveguide propagation. When there occurs a sudden change in the lower boundary of the ionosphere, the VLF signal will have a significant response, this is also the reason why the VLF curve exhibits large fluctuations during the sunrise and sunset. When a total solar eclipse happens, the sunlight projecting on the earth is blocked by the moon, the recombination of the ions and electrons in the lower ionosphere is gradually accelerated, and on the path of VLF signal propagation, the effective reflection height of the lower ionosphere increases gradually, and the D layer produced in the daytime is suddenly weakened. When the signal penetrates through the D layer, its attenuation becomes small, hence the signal is enhanced when it reaches the ground. And at the third contact after the maximum of the total solar eclipse, the atmospheric ionization caused by the solar radiation becomes gradually intense. This makes the effective reflection height reduce gradually, and makes the D layer enhance gradually, thus the VLF signal gradually recovers to its normal state. At present, we can hardly give a reasonable explanation for the variation of the VLF signal previous to the total solar eclipse, but we know that during the total solar eclipse, because that the sun, moon and earth are co-linear, it will produce a strong gravitational effect on the earth, even cause the deformation of rocks and other effects[15] . Then would’t it have an influence on the ionosphere? Now, there have been some studies about the effect of crust movement on the ionosphere, and a certain progress has been achieved. For studying the effect of the ionosphere previous to the total solar eclipse, we still need more practical examples. ACKNOWLEDGEMENTS We thank the Referee for valuable opinion, Dr. Sharon Murrel and Dr. Zhao Jun-wei of Stanford University for providing the SID monitor, Dr.
ZHANG Xue-feng et al. / Chinese Astronomy and Astrophysics 35 (2011) 54–61
61
Huang Guang-li and Dr. Liu Yan of Purple Mountain Observatory for the instruction to the teachers and students of our school. References 1
Tian Yu-shu, Pan Wei-yan, Yu Yun-cao, et al., National Military Standard of People’s Republic China
2
Wait J. R., Spies K. P., NBS Tech. Note, 1964, 300, 1
3
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