Vol50,1,2007 111 ANNALS OF GEOPHYSICS, VOL. 50, N. 1, February 2007 Key words ionosphere – E- and F-layers – earth- quakes-magnetic storm – foF2 and foEs 1. Introduction Ever since Davies and Baker (1965) reported the first seismo-ionospheric anomalies related to Alaskan earthquake, several workers have at- tempted to study this problem using ground and satellite based observations. However, some convincing evidences of this phenomenon have come rather recently (Pulinets et al., 1991, 1994; Ruzhin and Depueva, 1996) in which it has been found that the seismic effect on ionospheric F2- layer is observed in terms of reduction in morn- ing time foF2 about a week before the occurrence of the earthquakes and this phenomenon may be observed over a wide ionospheric area covered by 30° latitude and 60° longitude. More recently similar studies have been made by Liu et al. (2000, 2001), Chuo et al. (2002) and Singh et al. (2004) in which they have reported afternoon re- ductions in foF2 also apart from morning time re- ductions. Anomalies in foF2 and Total Electron Content (TEC) have also been reported by Devi et al. (2001, 2004) prior to the occurrence of earthquakes. They have shown that both positive and negative ionospheric effects develop prior to an earthquake depending on the epicenter posi- tion with respect to the observing site. Simultaneous ionospheric E- and F-layer perturbations caused by some major earthquakes in India Birbal Singh (1) and Om P. Singh (2) (1) Department of Electronics and Communication Engineering, Faculty of Engineering and Technology, R.B.S. College, Bichpuri, Agra, India (2) Department of Physics, Faculty of Engineering and Technology, R.B.S. College, Bichpuri, Agra, India Abstract The variation in nighttime ionospheric E- and F-region critical frequencies (foEs and foF2) is examined for the months of August 1988, September 1993 and May 1997 in which major earthquakes (6 < M < 6.6) occurred in In- dian seismic zones of Bihar-Nepal border, Latur in West India, and Jabalpur in Central India. The ionospheric data are obtained from Ahmedabad (latitude 23.01°N, longitude 72.36°E). The percent deviations of foF2 and foEs from their monthly median are determined for pre-midnight (18:00-00:00 h LT) and post-midnight (00:00- 06:00 h LT) periods and studied over a span of 20 days before the occurrence of earthquakes. The results show that foF2 are reduced in both the time sectors prior to the occurrence of main shocks. In pre-midnight sector the reduction is between 24 and 30%, 0 to 4 days before the main shocks and in the post-midnight sector it is be- tween 18 and 26%, 1 to 15 days before the main shocks. The foEs show enhancements by 100 to 155% during the same periods. The effects of magnetic storms (Kp and Dst variations) on the data are identified clearly but they do not vitiate the effects of earthquakes. The earthquake related anomalies are interpreted in terms of elec- tromagnetic coupling between the lithosphere and ionosphere during earthquakes preparation processes. Mailing address: Dr. Birbal Singh, Department of Electronics and Communication Engineering, Faculty of Engineering and Technology, R.B.S. College, Bichpuri, Agra-283105, India; e-mail: birbal@sancharnet.in 112 Birbal Singh and Om P. Singh Several workers have also reported the ionospheric anomalies in E-layers prior to the occurrence of earthquakes (Gokhberg et al., 1988; Liperovski et al., 1992; Pulinets et al., 1994; Ondoh, 2000; Ogawa et al., 2002). Re- cently, Ondoh and Hayakawa (1999), Ondoh (2003) and Singh and Singh (2004) have made an elaborate study of this problem and have shown anomalous foEs increases 2-20 days be- fore the occurrence of earthquakes. Although, many workers have examined the effects of earthquakes on ionospheric E- and F- layers separately as stated above there is no report of simultaneous observations of E- and F- layer perturbations related to seismic activities. In the present paper, we examine this problem in detail by considering three major earthquakes that oc- curred in India in recent years. Our results show simultaneous earthquake induced changes in the E- and F-layers which are reflected by simultane- ous reductions in foF2 and enhancements in foEs. 2. Ionospheric, earthquake and magnetic storm data The ionospheric data have been obtained from Physical Research Laboratory, Ahmedabad (latitude 23.01°N, longitude 72.36°E), India where a digital ionosonde has been in operation for a long time. The data recorded at this station includes foF2, foEs, hpF2 and median values for respective months. The earthquake data have been obtained from India Meteorological Depart- ment, New Delhi, India. Since ionospheric anom- alies appear during or after the occurrence of magnetic storms also it is worthwhile to examine the foF2 and foEs changes in relation to magnetic storms also. For this purpose, we consider Kp in- dex and Dst index data, which have been ob- tained from Indian Institute of Geomagnetism, Colaba, Mumbai, India and World Data Center C2, Kyoto University, Kyoto, Japan respectively. 3. Results and discussion Six major earhquakes have occurred in India during the last 15 years. They include Bihar- Nepal earthquake (20 August, 1988), Uttarkashi earthquake (20 October, 1991), Latur earthquake (30 September, 1993), Jabalpur earthquake (22 May, 1997), Chamoli earthquake (29 March, 1999) and Bhuj earthquake (26 January, 2001). In order to examine the effects of these earth- quakes on E- and F-regions of the overhead ion- osphere we have attempted to analyze the ionos- pheric data obtained from Ahmedabad. Unfortu- nately, foEs data are available corresponding to three earthquakes only, which are Bihar-Nepal earthquake, Latur earthquake and Jabalpur earth- quake, and hence in the present study we consid- er the F- and E-regions anomalies corresponding to these earthquakes only. The locations of these three earthquakes are shown in the map of fig. 1 by solid circles and abbreviated words as BHR- NPL, LTR and JBL respectively. The location of Ahmedabad from where the ionospheric data have been obtained is also shown in the same fig- ure by abbreviated word AHD. Other details of these earthquakes such as the date and time of occurrence, geographic location, magnitude, depth and distance from Ahmedabad are shown in table I. It may be noted that the ionospheric parame- ters show considerable variations following geo- magnetic storms. The ionospheric responses to such storms are of negative kind and large at high and middle latitudes while they are modest and of positive kind at low latitudes especially during daytime hours. During nighttime, both positive and negative responses are possible at low and equatorial latitudes (Lakshmi et al., 1983, 1997). Here, it may also be mentioned that Jain and Singh (1977) and Jain et al. (1979) have studied the low and equatorial latitude ionospheric changes during the period of magnetic storms and have shown that these ionospheric regions are influenced significantly during prolonged magnetic disturbances only and there is almost negligible variation during isolated storms. Fur- thermore, it may be pointed out here that the ionospheric station Ahmedabad is located close to the anomaly crest region and shows mixed re- sponse to magnetic storms and, sometimes, sig- nificant variations even during quiet periods. However, in the present study adequate care has been taken to see that the ionospheric departures due to magnetic storms and unknown reasons do not vitiate the main focus of our discussion. 113 Simultaneous ionospheric E- and F-layer perturbations caused by some major earthquakes in India Fig. 1. Map of India showing the locations of three major earthquakes (solid circles). Also shown is the loca- tion of ionospheric station Ahmedabad (AHD). Table I. Details of the three earthquakes considered. Date of Location Time Latitude Longitude Magnitude Depth Distance occurrence (IST)* (°N) (°E) from AHD (km) 20/08/1988 BHR-NPL 23:09 h 26.72 86.63 6.6 35 2500 border 30/09/1993 Latur 03:55 h 18.07 76.62 6.3 12 1100 22/05/1997 Jabalpur 03:25 h 23.08 80.06 6.0 35 1600 * IST = UT + 5.50 h. 114 Birbal Singh and Om P. Singh Fig. 2a. The variations of monthly median of foF2 and foEs (shown by solid curves) and hourly measured val- ues (shown by open triangles) for ten days before the occurrence of the earthquakes in the months of August, 1988 (top two panels), September, 1993 (middle two panels) and May, 1997 (bottom two panels). Monthly me- dian is repeated on each day. The inverted arrows indicate the days of the earthquakes. 115 Simultaneous ionospheric E- and F-layer perturbations caused by some major earthquakes in India Fig. 2b. Temporal variation of monthly median foF2 and foEs (solid curves) and hourly measured values (open triangles) on the days of observed anomalies corresponding to the earthquakes under consideration. Figure 2a shows the daily variations of foF2 and foEs corresponding to three earthquakes un- der consideration. The foF2 and foEs data are shown by triangles whereas monthly medians are shown by solid lines. The data are shown for 10 days before the occurrence of the respective earthquakes. The days of earthquakes are shown by inverted arrows in all the panels. From a 116 Birbal Singh and Om P. Singh glance at the top two panels which correspond to Bihar-Nepal earthquake of 20 August, 1988 we see that there is a maximum reduction in morn- ing time foF2 from median values on 18 August, whereas there is a maximum enhancement in foEs on the same day. Since there is no major magnetic storm in this month before the earth- quake (as shown ahead) it may be inferred that foF2 and foEs are influenced by the earthquake giving rise to peak anomalies 2 days before the occurrence of the earthquake. Further, it will be shown that foF2 and foEs indicate decreasing and increasing trends respectively starting from 03 August itself thereby showing the influence of earthquakes from 18 days before its occurrence. The middle two panels correspond to Latur earthquake of 30 September 1993. Here, the foF2 is lowest in the morning of 28 September whereas foEs is enhanced during the midday and evening hours of 27 and 28 September. There is a reduction in foF2 and corresponding enhance- ment in foEs on 22 September also which may be attributed to a magnetic storm that occurred on 20 September (discussed ahead). The bottom two panels correspond to Jabalpur earthquake of 22 May 1997. In this case the foF2 has gone lowest and foEs largest on 19 May 1997 about three days before the occurrence of the earth- quake. The figure shows enhancements in foEs on 13 and 15 May also. The enhancement on 15 May may be attributed to severe magnetic storm (Kp = 7, Dst = − 116), which occurred on the same day. The reason for the enhancement on 13 May is not known, but it may not be at- tributed to the storm of 1 May because the ef- fect of such isolated storms cannot affect the ionosphere so late. To identify the times of oc- currence of the anomalies (reduction in foF2 and enhancement in foEs) occurring on 18 Au- gust 1988, 28 September 1993 and 19 May 1997 as shown in fig. 2a we plot the temporal variation of foF2 and foEs data along with their median values in fig. 2b. The results of maxi- mum deviation and times of occurrence are pre- sented in table II. Here it may be noted that per- cent reduction in morning time foF2 and en- hancements in foEs are the largest as compared to other days. Further, the foEs data presented in the table II on 18 August during afternoon hours shows a value of 296% where as in the abstract we have mentioned the maximum en- hancement of 155%. This result should not be taken as contradictory because the data present- ed in the abstract have resulted from the run- ning mean which will be made clear ahead. In order to make the earthquake induced anomalies more clear and distinct from those arising due to storm and other unknown factors we have made a thorough analysis of the data by following a procedure in which we have calculat- ed the percent reduction in hourly foF2 and per- cent enhancement in hourly foEs relative to monthly median values. We have chosen the data for the period 18:00-06:00 h, LT only and divid- ed them into two sectors, pre-midnight (18:00- 00:00 h) and post-midnight (00:00-06:00 h). This period of the data has been chosen to avoid com- plications arising due to variations in photoioni- sation intensity during daytime (Michel Parrot, personal communication, Bhopal, 17 November 2003). Since the data are not continuous and there are some intermittent gaps, we have taken Table II. Details of times and maximum deviation of foF2 and foEs observed during the anomalies. Maximum reduction in foF2 Maximum enhancement in foEs Days of Morning hours Afternoon hours Morning hours Afternoon hours anomaly Time Reduction Time Reduction Time Enhancement Time Enhancement (h) (%) (h) (%) (h) (%) (h) (%) 18/08/1988 03:00 26.4 18:00 13.3 03:00 40.0 19:00 296.0 28/09/1993 05:00 32.0 21:00 29.6 04:00 142.8 23:00 105.0 19/05/1997 04:00 37.5 17:00 20.4 05:00 50.0 14:00 146.5 117 Simultaneous ionospheric E- and F-layer perturbations caused by some major earthquakes in India six days running mean of the peak reduction in foF2 and peak enhancement in foEs for both the pre-midnight and post-midnight data. The vari- ations of percent reduction in foF2 and en- hancement in foEs are shown for 20 days before the occurrence of each earthquake in the top two panels of fig. 3a-c. The solid circles corre- spond to pre-midnight data and solid squares Fig. 3a,b. a) Top – six days running mean of the peak percent reductions during pre-and post-midnight sectors in foF2 for 25 days in the month of August 1988. The earthquake (M=6.6, shown by downward arrow) occurred at a ) Bihar-Nepal Border on 20 August 1988. Second – same as in top panel but for enhancements in foEs. Third – Kp index variation. Bottom – Dst index variation on the days under consideration. b) The same as (a) but for Latur earthquake (M=6.3) which occurred on 30 September 1993. a b 118 Birbal Singh and Om P. Singh correspond to post-midnight data. The days of the earthquakes are shown by inverted arrows with the magnitude of the earthquakes indicated nearby. In the bottom two panels of these fig- ures we show the variation of the magnetic storms by three hour Kp indices and correspon- ding Dst indices. Here, it may be noted that Kp ≤ 4, 4 ≤ Kp ≤ 6 and Kp≥6 indicate quiet, mod- erate and severe magnetic storms respectively. The magnetic storm condition is also denoted in terms of Dst index and the value of Dst ≤ − 50 nT represents minor storms. In general, the low lati- tude ionosphere is influenced by large reductions in Dst (Dst≤ − 100) only (B.M. Reddy, pers. comm., November 2003). From fig. 3a it may be seen that there are en- hancements in percent reduction in foF2 and en- hancements in foEs from 3 August itself about 18 days before the occurrence of the earthquake and the effect is reflected in both the pre and post- midnight data of foF2 and in pre-midnight data of foEs. The largest enhancement in pre-mid- night foF2 reduction amounts to 24%, one day before the earthquake whereas the same in post- midnight hours amounts to 26%, four days be- fore the earthquake. The maximum enhance- ments in foEs of 155% on 15 August and 102% on 7 August are observed in pre and post-mid- night data respectively. During the periods of these enhancements the magnetic condition was quiet. Hence, the anomalies may be attributed to the earthquake only. The magnetic storms oc- curred on 20 and 21 August, after the occur- rence of the earthquake, which may not influ- ence the ionosphere in advance. It may be men- tioned here that the days of maximum enhance- ment in reduction of foF2 and maximum en- hancement in foEs as seen in this as well as in the next two figures do not match exactly with those shown in fig. 2. This discrepancy has been produced by taking the six days running mean of the data, otherwise the days of anom- alies remain unchanged. Figure 3b presents the results corresponding to Latur earthquake. The top two panels show that there are large enhance- ments in reduction in foF2 between 17 and 23 September and large enhancements in foEs on 15 September (which appears on 13 September be- cause of six days running mean), which may be attributed to the severe magnetic storm of 13 Sep- tember (Kp = 8, ΣKp = 48). However, the en- hancements in reduction in foF2 and enhance- ments in foEs during 25-30 September may be attributed to the earthquake of 30 September only. Maximum enhancements in foF2 reduc- tions are observed on 28 September by 18% in post-midnight data and by 30% on 30 Septem- Fig. 3c. The same as fig. 3a but for Jabalpur earth- quake (M = 6.0) which occurred on 22 May 1997. 119 Simultaneous ionospheric E- and F-layer perturbations caused by some major earthquakes in India ber in the pre-midnight data. In case of foEs, maximum enhancements of 140% on 25 Sep- tember and of 117% on 27 September are ob- served in post and pre-midnight data respective- ly. The moderate magnetic storms (Kp = 5) on 20, 23 and 30 September are unlikely to influ- ence the data as stated before that only severe and prolonged magnetic storms are found to in- fluence the low latitude ionosphere. Figure 3c shows the results corresponding to the Jabalpur earthquake of 22 May 1997. In this month two magnetic storms (Kp = 6.7) occurred on 1 and 15 May 1997. The Dst variation shows the val- ues −63 and −116 on these two days respective- ly. As shown in the top panel of the figure, there are increases in foF2 reductions during 4-9 May as well as on 16 and 18 May. These increases in reductions may be attributed to the storms of 1 and 15 May respectively. However, the maxi- mum enhancement in reduction in the pre-mid- night data by 25% on 20 May and by 18% on 22 May may possibly be due to the earthquake that occurred on 22 May. The second panel shows that there is a gradual increase in foEs in both the pre and post-midnight data starting from 2 May to 21 May, up to one day before the occurrence of the earthquake. The maximum enhancement in post-midnight data on 21 May is 114% whereas in pre-midnight data it is 100%. This gradual increase in foEs from near- ly 20 days before may be due to the earthquake of 22 May. The transient increase in foEs during 16-17 May may be attributed to the storm of 15 May. From the results presented in fig. 3a-c it may be seen that the effects of earthquakes on foF2 and foEs data corresponding to Latur and Ja- balpur earthquakes are not as much convincing as that related to Bihar-Nepal earthquake pre- sented in fig. 3a. One reason for this is that there occurred magnetic storms during the period of analysis in the later two cases, which complicat- ed the data whereas there was no magnetic storm in the first case. However, this ambiguity can be eliminated if we look at the diurnal variation in foF2 and foEs of fig. 2 where enhancements and reductions are visible very clearly. In order to give further support to our results mentioned above we have carried out more rig- orous statistical analysis. We have employed the past 10 years of data corresponding to the months in which foF2 and foEs data have been analyzed in fig. 3a-c. Out of the bulk of 10 years data we have picked up only those, which corresponded to the same months and the years in which the Sun spot numbers were almost similar. For example, in the case of Bihar-Nepal earthquake the lowest foF2 was found to be on 18 August 1988 two days before the occurrence of the earthquake. Hence, we have considered the same day data in the years 1978, 1984, 1993, 1998 and 1999, which corresponded ap- proximately to similar Sun spot numbers. Then we determine the deviation of foF2 from monthly median (∆foF2) for each hour of the days. From these data, mean (m) and standard deviation (σ) were calculated. Similar calcula- tions have been made for foEs also. The varia- tions of m and m ± σ along with ∆foF2 and ∆foEs corresponding to each earthquake are shown by different notations in fig. 4. From this figure it is seen that ∆foF2 is deviated out of m + σ in the morning (04:00-07:00 h) in all the cases. The deviation is also seen in the after- noon (17:00-19:00 h) and in the late evening hours (21:00-23:00 h). The foEs deviations are mostly in the pre-midnight hours in all the cases except in Jabalpur case in which it is deviated during afternoon also. These results are in agree- ment with the results presented in fig. 3a-c. The anomalous state of the ionosphere dur- ing seismic activities as presented above has been interpreted by several workers in terms of lithosphere-ionosphere coupling produced by earthquake-induced electric fields, which are generated from internal gravity waves. These electric fields can penetrate the ionospheric F- and E-region heights and bring out changes in the electron density profile due to E ×B drift (Kim and Hegai, 1999; Sorokin and Chmyrev, 1999). The enhancement or depression in foF2 values may be attributed to positive or negative direction of electric fields. The effect of distant earthquakes on ionospheric data over Ahmed- abad may be interpreted in terms of E ×B drift of an electron stream upward over the epicenter of earthquakes which may then move towards or away from Ahmedabad depending upon pos- itive or negative direction of electric fields or neutral winds causing enhancement or depres- 120 Birbal Singh and Om P. Singh Fig. 4. Statistical results showing the deviation of foF2 and foEs from the monthly median on the day it was lowest in the month of the earthquake (∆foF2 and ∆foEs). The mean (m) and standard deviation (σ) around mean (m ± σ) are computed for the same days in the past ten years of data on which Sun spot numbers were nearly identical. 121 Simultaneous ionospheric E- and F-layer perturbations caused by some major earthquakes in India sion in foF2 data as discussed by other workers (Yoshimatsu, 1938; Ondoh, 1999). In case of anomalies at the E-layer heights, the upward E ×B drift will be amplified thereby augmenting the density pumping process from the epicen- tral zone to off epicenter positions in the iono- sphere (Ruzhin and Depueva, 1996). The other possibility of E-layer anomaly is quasi-static heating and production of additional ionization in the sporadic E-layers caused by cloud dis- charge in the atmosphere over seismic zones in- duced by seismic electric fields (Ondoh and Hayakawa, 2002; Ondoh, 2003). The foEs en- hancements prior to the occurrence of large earthquakes have also been suggested due to radon emanations from seismic sources, which are carried upward by the electric fields and create additional ionization at the heights of E- layer (Ondoh and Hayakawa, 2002; Ondoh, 2003). Over all we expect that the reduction in foF2 and enhancements in foEs during the peri- ods of earthquakes may probably be interpreted in terms of lifting of F- and E-layers due to E ×B drift caused by a unidirectional electric field. In this mechanism the F2 peak will be lifted to a region of lower electron density whereas Es-layer will be lifted upward to a re- gion of higher electron density producing re- duction in foF2 and enhancement in foEs re- spectively. Acknowledgements The authors are grateful to their research colleagues Mr. Manoj Tomar, Mr. Vinod Kush- wah and Mr. Vikram Singh for discussions and help in preparations of the diagrams. 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