Beyond the horizon propagation of VHF signals, atmospheric features and earthquake

Transcription

1 Электронный научный журнал «ИССЛЕДОВАНО В РОССИИ» Beyond the horizon propagation of VHF signals, atmospheric features and earthquake Devi M. (1), Barbara A.K. (1), Ruzhin Yu.Ya. (2) Depueva A.H. (2) (1) Department of Physics, Gauhati University, India, (2) IZMIRAN (Russia) Introduction Electric and magnetic perturbations in association with earthquake have been known at the end of XIX century [1, 2]. But with emerging evidences of electromagnetic emissions (EM) prior to earthquakes [3-7], importance of utilizing the wide frequency spectra of EM radiations for uncovering earthquake precursor has gained momentum of late. The effects of such waves, emissions of which are basically founded on the physics of production of electric field and EM waves under enormous compressive and shearing forces on rocks near to focal point or electric discharges due to redistribution of electric charges, prior to earthquake during its preparatory phase [8], are examined not only at the earth s surface but also at the atmosphere and their consequent contributions in changing the lower and upper atmospheric parameters are then studied. The main aims of these analysis and studies are to identify earthquake-induced features, if any, and to realize some predictor parameters [9-13] through these variables. In this communication we present a few cases of reception of beyond horizon TV programmes through lower VHF carriers, that make their appearances at Guwahati (26.11º N, 91.45º E), remarkably bearing association with occurrence of earthquakes. Explanations to possible causes leading to this observation are attempted here by bringing relevant ionospheric and tropospheric parameters into ambit of discussions. The aim of our work is also to pay attention to the problem of anomalous atmospheric or ionospheric sources of the beyond horizon propagation in VHF band appeared before some earthquakes, which could be used for practical purposes. 1. Experiment Monitoring of VHF TV signals (50 MHz to 70 MHz frequency range) was part of a programme to realize ionospheric situations favorable for propagation of VHF waves to distances beyond horizon. For this purpose a 10 element Yagi steer-able antenna of 10 db amplification coefficient was used and it was steered for maximum signal strength whenever an anomalous signal, even if weak, was detected. In normal situations, when a transmitter location is more than 100 km away, we do not receive any signal above the rash level, but in special situations, pictures do appear on TV screen and these we identify as anomalous long distance VHF receptions. The video output was then monitored as an anomalous signal strength parameter. Now, in Gauhati University troposphere is studied by means of remote atmosphere sounding facilities in acoustic (SODAR) and optic (lidar) bands.

2 Электронный научный журнал «ИССЛЕДОВАНО В РОССИИ» SODAR stands for Sonic Detection and Ranging. In this method acoustic (sonic) signal is transmitted to the atmosphere and backscatter data are received and we have recorded these returned data in a facsimile recorder. The SODAR echo pattern is controlled by temperature and its lapse rate prevailing over the probing zone. LIDAR stands for Light Detection and Ranging. Here we have transmitted 532 nm laser pulse of 10 ns width at repetition rate of 1 KHz. The backscatter counts carry information of aerosol and clouds. Regular meteorological data on air temperature, pressure and humidity in the area of the experiment were also used. 2. Some samples of anomalous TV signal monitoring Subsequently, on examination of the quality of received picture in terms of anomalous signal strength data, in relation to approaching of an earthquake, it was found that appearance of long distance TV signals had good correspondence with the event. As an example, anomalous signals made their appearance from June (Fig. 1) and the picture quality becomes very clear from June 23 as can be seen from both: picture quality and signal strength variation pattern. An important aspect of the event is that during these days the TV screen displayed very clear picture like the flag and human features of Southeast Asia, possibly associated with programmes running in a station near the epicentre, a location in Laos very close to Luang Phabang. Signal reception quality deteriorated drastically after the earthquake event and was lost soon afterwards. On later examination it was found that on June 24, 1983 there was an earthquake (N1 in the Table). Fig. 1. Anomalous signal intensity variation prior to and during earthquake of June 24, 1983 (left panel). The epicentre position (right panel). We present here two more similar events of April 1984 related to occurrence of earthquakes. Fig. 2 shows day-to-day variations of signal strength for the month of April It is significant to note that earthquakes N2 and N3 (Table) had occurred with epicentre near Malaysia/ Thailand and location near Chongquing, China, correspondingly. Large enhancement in signals had been detected 3 4 days prior to event N2. Interesting enough to note that, during these days we received programmes with well-defined writings in Islamic characters and also people with South-East Asian features, probably associated with TV programmes transmitted from stations near to the epicentre. There was another earthquake N3 and

3 Электронный научный журнал «ИССЛЕДОВАНО В РОССИИ» Table N Date Latitude, deg. N Longitude, deg. E Magnitude Characteristic radius of preparation zone, km, according to [14] we received strong anomalous signals from April 18 onwards (Fig. 2), significantly carrying writings in Chinese characters. Fig. 2. Beyond the horizon TV signal strength during April 13, 1984 and April 24, 1984 earthquakes (left panel). The epicentres are indicated in right panel for aforesaid events. 3. Anomalous TV signal amplitude dependence on earthquake magnitude It was noticed from ( ) observations that if the epicenter lies near to the recording site (Guwahati, in these cases), received VHF signal strength may not be as strong as in the above cases where relative distances between the two are significantly large. As an example, during the earthquake of May 6, 1984 (N4 in the Table) a position near to Guwahati, we have not received strong signals prior to this event in contrast to the cases when the epicentre locations are far away. Fig. 3 shows observed signal strength variations before the May 6 event (N4 in the Table), along with that for another earthquake of May 18, 1984 (N5 in the Table). In the former case, changes in signal strength prior to the earthquake are not significant, though in the latter case, an exponential rise in the intensity of beyond the horizon signal is detected before and during the event, with same order of earthquake intensities (scale 5.7 and 5.6 respectively); when for N5 earthquake the estimated distance from the epicentre to the observing station comes to around 1200 km; for N4 event this distance is less than even 400 km. This character is reflected in Fig.4, where anomalous signal strength with the distance between epicenter and receiving

4 Электронный научный журнал «ИССЛЕДОВАНО В РОССИИ» station is displayed. The entire analysis deals with events of similar tremor magnitudes (5.5 to 6.0) to eliminate role of other interrelated parameters with intensity. Fig. 3. TV signal strength received prior to and during earthquakes of May 6, 1984 and of May 18, 1984 In Fig. 4 anomalous signal strength dependence on the distance between the epicenter and receiving station is shown. The analysis was made for all earthquakes of approximately equal magnitude ( ) in order to exclude relations of other parameters associated with anomalous propagation. Fig. 4. Anomalous TV signal strength (maximum intensity) variation with relative distance between epicentre and receiving station. 4. Ionospheric propagation as a reason of the observed effect of extra-distance propagation In an attempt of offering an explanation to the above observations, we note that one of the possible modes of receiving such over horizon (>1000 km) VHF signal is through ionosphere, especially in the lower VHF range [15]. Signals of wavelength more than 4-6 m (75-50 MHz) are reflected intensively from the ionosphere during high solar activity. The connection is very unstable, its duration limited to some minutes to 1-2 h, depending on frequency, signal bandwidth and distance. Good results could be obtained at distances of km by receiving scattered VHF signal from F2 layer of ionosphere. For instance, 23 February 2001 the link Cyprus - New Zealand was available [ The shorter the stretch, the worse the quality of reception.

5 Электронный научный журнал «ИССЛЕДОВАНО В РОССИИ» A ground based TV signal within these bands will have to traverse km to touch ionospheric height of km, if we assume the striking angle to be as low as 10º ; then it will be possible for signals to appear at thousands of kilometers away from the transmitting station by ionospheric reflection (when maximum usable frequency through such mode enhances), depending on point of striking-latitude and ionospheric height where layer density must be large enough to reflect a signal of MHz. However to explain such phenomenon, contribution of the ionosphere alone is not inclusive, role of troposphere is also to be considered taking relevant parameters in to discussions. Generation of favorable density conditions necessary for reflecting lower VHF range can be explained with earthquake associated E-field at ionospheric heights and generated through earthquake preparatory processes leading to enhancements in ionisation density [16-18]. It is also possible that frequencies of MHz may undergo reflection in the F-layer when its critical frequency goes beyond 10 MHz (at striking latitude) and signal incident angle is as low as 10º. This will be true for Es layer also when foes exceeds, say, 15 MHz [19]. Inside the region in study (Guwahati is situated at the northern crest of ionospheric equatorial anomaly) there is one more scenario of latitudinal plasma density redistribution. Now, when an electric field generated during earthquake preparatory process near to an epicentre close to dip equator enters into E-layer heights, the additional field would lead to intensification of equatorial electrojet strength and it will amplify ExB drift. The process in turn will enhance the E- and F-layer critical frequencies through density dumping process from epicentre to off epicenter zone. Augmentations (or inhibition) of the anomaly during earthquake preparatory processes have in-fact been examined [20-22] while analysing topside electron density over a dip latitude zone of ± 30º. These workers have shown that considerable changes in the development processes of the anomaly do take place prior to an earthquake and also that position of the epicentre relatively to magnetic equator is critical in controlling the magnitude of anomaly. Enhancement in ionospheric electron content at off epicentre position (at the crest region) prior to an earthquake is also observed in [23, 24] from Guwahati TEC data received through VHF RB signals. Similar results were reported in [25, 26]. Therefore, a favorable situation at ionospheric heights for reflection of VHF signals may be generated through earthquake preparatory processes, leading to anomalous VHF propagation. But for explanation of observed effects the only ionosphere impact is insufficient. We need to estimate the troposphere impact as well. 5. Observation by SODAR Propagation of VHF TV signals to distances thousand km and beyond is not a new observation, though explanation to why and how a terrestrial LOS signal will reach an ionospheric height is still a point of discussion, though beyond the horizon VHF propagations within hundreds of km through tropospheric modes are well dealt with now by many workers and such events most of which are observed a few days to weeks prior to strong earthquakes are extensively discussed. Large enhancements in FM signal about 7 days before an earthquake as observed by Fukumoto et al. [27] have been associated with earthquake induced tropospheric variabilities, when lower atmospheric parameters like temperature, water vapour in the atmosphere prior to such events undergo significant changes [26, 27]. Our limited study on temperature variations and tropospheric features over this station also indicates changes in atmospheric parameters in some earthquake events. One such case seen by SODAR is reproduced in Fig.5, for November 19, 1996 earthquake, occurring at h (N6 in the Table) which is just over Guwahati. It is significant to note from the figure that at around h the normal atmospheric boundary layer has disappeared indicating that the atmospheric situation especially temperature, had undergone sudden

6 Электронный научный журнал «ИССЛЕДОВАНО В РОССИИ» changes causing a breakdown in the prevailing inversion layer. In fact the routine record of surface temperature taken at the SODAR site (Gauhati University) had displayed an abnormal increase on 18-th night to early morning hours of 19 th November 1996 [28]. SODAR echograms also reflect generation of unstable structures at about 1 km height (coinciding with the break down of surface inversion layer), which then slowly migrated downward as seen in the alternate dense and light reflections. Perhaps explanations to our observation can be sought in the physics of interaction and exchange of heat and energy between warm air packet ejected in the earthquake preparatory processes [28] supported by another cold atmospheric front at the top, thereby creating a thermodynamically unstable situation leading to formation of vortices and eddy structures [29]. Fig. 5. Sodar echogram received on November 1996, at Gauhati University. Earthquake occurred on November 19, 1996 at h, with epicentre at 24.50º N, 92.64º E. 6. Lidar measurements As a further exercise towards addressing earthquake-meteorology association, we examine remote sensing data received by lidar at Guwahati, on the background of reports of appearance of earthquakeclouds [30]. Our attempts have revealed that thin clouds do develop prior to and during some of the earthquakes and we present here one such case, amongst many as a representative cloud feature, in Fig. 6. The picture presented here is detected prior to May 3, 2001 earthquake which occurred at h (IST) with epicentre at 12.28º N, 93.50º E and clouds of this type make their appearances in the lidar echogram from April 28, 2001 (not shown in the figure). It is a point to note that in the period of 2001 to 2004 out of recorded tremour events 50% occurs in the geographical zone of 92º E 96º E and 2º N 22º N with peak occurrence at 12º N, (93 94)º E. We in fact have recorded quite a few TV programmes bearing Islamic characters and also people with South-East Asian features as described in Fig. 2, in association with earthquake. It is possible that with epicenter about Andaman archipelago zone, earthquake clouds if generated at this active area may appear at Guwahati situated at the same longitude, as such structures are reported to be non localised in nature and their shape gets more elongated pointing towards the epicenter [30]. We have examined temporal characteristics of the cloud streaks in relation to their relative distance between epicenter and observing station. Our limited results indicate that such structures maintain a complex association with epicentral distance, earthquake magnitude and season of

7 Электронный научный журнал «ИССЛЕДОВАНО В РОССИИ» observations [31]. It is however too premature yet to couple or correlate such facts with anomalous mode of propagation, because our results are based on sounding observation taken at one location only. But this observation certainly suggests changes in tropospheric parameters possibly associated with release of water and heat during earthquake preparatory processes. Fig.6. Lidar echogram of Guwahati, (a typical case), displaying clouds associated with earthquake. The figure shows thin clouds at around 2-3 km on April 30, 2001, prior to the earthquake of May 3, 2001, which occurred at hrs (IST) with epicentre at 12.28º N, 93.50º E. 7. Regarding the possibility of tropospheric propagation The strong modification of background atmosphere layers before earthquakes, including thunderstorm activity in a whole earthquake preparation zone, is clear from noice signals earthquake precursors, and radio reflections in VHF band [12, 13]. We have put forwarded this observation here, as a part of discussion while bringing role of tropospheric variabilities on VHF propagation. In simplified tropospheric situation when n, the refractive index, monotonically decreases with height i.e., for a normal lapse rate dn/dh = - 40 km -1, where N = (n-1) x

8 Электронный научный журнал «ИССЛЕДОВАНО В РОССИИ» is the excess refractive index, VHF signal propagation has no anomalous effect in the atmosphere except for scatterings from small irregularities if present. Let s consider possibilities of the realization conditions in the ionosphere for the anomalous VHF propagation during earthquake preparation stage. For the tropospheric propagation the presence of anomalous dn/dh near the transmitter is valid in order to direct a signal along the earth surface and to put it to the receiving point. dn/dh = 77.6 [1/T x dp/dh (P/T e/T 3 )dt/dh /T 2 x de/dh] (1/m) [32 ], i.e. increased refraction depends on vertical temperature (T) gradient or/and vertical humidity (e) gradient. Therefore, quick fall of refraction index is due to increase of temperature and/or decrease of humidity. However in an atmosphere with dn/dh = - (100 km km -1 ), VHF signal may be reflected by or ducted through this medium to travel distances beyond horizon [32]. The limitation caused by tropospheric guide height (not more than 200 m usually) is that, the propagation is possible for only waves with wavelength more than λcr 8.5x10 4 ho 3/2 m. For our case the necessary waveguide height (height of inversion layer) for beyond horizon propagation is m. As a change in refractive index gradient from its normal lapse rate represents an anisotropic and inhomogeneous medium and N being a function of temperature, humidity and pressure, one can expect changes in N prior to an earthquake. The over horizon FM signal reception within a radius of 400 km reported by Fukumoto et al. [25] has been attributed to be due to effect of earthquake on troposphere and signals traveling in such a mode is expected from a distance within 500 km [15]. However in our case the VHF TV signals are received far beyond this range and signal reception quality coming from distances of thousand of km is observed to be clearer, compared to signal coming from hundreds of km (Fig. 3). The point of difference between our observation with that from Fukumoto et al., is that while the over horizon distance is limited to 400 km in their case, beyond the horizon propagation is received by us from much longer distances. Though, tropospheric mode of propagation as explained by Fukumoto cannot be fitted in to our case, we cannot eliminate the significant part played by troposphere in our observations, as changes in refractive index at tropospheric heights at earthquake time may dictate signal rays to deviate from its normal path (unlike ducting) and take a course to touch the ionosphere. Association of significant refraction effect at the troposphere, on satellite to ground propagation path for VHF range is documented as early as in 1969 by Hartman [32], when the range of receiving angles were low as 2º to 28º. He has shown that in 40% of cases when satellite signal amplitudes suffer variation greater than 6 db, radiosonde data indicate development of inversion layer with dn/dh < -50 km -1, a value approaching near to sub-refraction situation. Increase of signal strength up to 15 db, at 136 MHz satellite signal amplitudes has been reported by other workers too [15], due to changes in dn/dh profile from its normal lapse rate, a situation which can very easily be developed during earthquake preparatory process. Explanation to the reception of strong and clear signals from near-epicentre stations may possibly be coupled to such a situation with ionospheric mode of reflection. However these hypothetical explanations need further scientific experiments with high-resolution time-space analysis of propagation features of VHF (preferably with two anomalous signal reception [33, 34]) and atmospheric parameters starting from troposphere to ionosphere. Conclusion 1. Anomalous VHF propagation time coincides with the period of low solar activity. So, longlasted stable TV reception during several days do not correspond to normal ionosphere condition for the

9 Электронный научный журнал «ИССЛЕДОВАНО В РОССИИ» beyond horizon VHF propagation. Nevertheless, the unusual ionosphere equatorial anomaly development at the earthquake preparation zone could cause the observed phenomena. 2. The existence of a suitable dn/dh (<-50 km -1 ) close to a transmitter location is important in order to direct a TV signal along the earth s surface and than to put it to the receiver. The explanation of the observed clear signals far from the transmitters located close to the earthquake epicenter, could be related to the ionospheric sort of propagation. Unfortunately, we do not know the exact position of the transmitter. 3. Our explanatory hypothesis needs further experiments including spatial-temporal analysis with high resolution for distinguishing of VHF propagation peculiarities and their relations to atmospheric parameters including ionosphere and troposphere. We want to pay attention to the problem of the source of the observed anomalous VHF propagation patterns before the earthquakes. Acknowledgement. The authors acknowledge with thanks the Ministry of Information Technology, ISRO and UGC, for the financial support received at various stages of the present work. References 1. Rikitake T Earthquake Prediction. (Published by Elsevier, Amsterdam) Mogi K Earthquake prediction. ( Published by Academic San Diego, Calif). 3. Gokhberg M. B. Morgounov V.A., Yoshino T. and Tomizawa I Experimental measurements of electromagnetic emission possibly related to earthquakes in Japan. J. Geophys. Res. V Larkina V.I., Nalivayko A.V., Gershenzon N.I., Gokhberg M.B., Liperovsky V.A., and Shalimov S.I Observations of VLF emissions related with seismic activity on the Interkosmos-19 satellite. Geomagn. Aeron. V Parrot M., Mogilevskii M.M VLF emissions associated with earthquakes and observed in the ionosphere and magnetosphere. Phys. Earth Planet. Inter Hayakawa M., Fujinawa Y Electromagnetic Phenomena Related to Earthquake Prediction. (Published by Terra Sci. Pub. Comp. Tokyo). 7. Hayakawa M Atmospheric and Ionospheric Electromagnetic phenomena Associated with Earthquake. (Published by Terra Sci. Pub. Comp, Tokyo) Honkura Y Electric and magnetic approach to earthquake prediction. In: Current research in earthquake prediction, edited by Rikitake T. (Published by I.D. Reidel, Dordrecht) Larkina V.I., Migulin V.V., Molchanov O.A., Kharkov I.P., Inchin A.S., and Schvetsova V.B Some statistical results of very low frequency radio wave emission in the upper ionosphere over earthquake zones. Phys. Earth Planet. Int Blaunstein N Remote sensing of short term ionospheric phenomena, indicators of precursors of earthquakes by use of radiophysical methods. In: Progress in Electromagnetics. Research Symposium Proceedings. Taipei. V Ruzhin Yu.Ya., Oraevsky V.N., Shagimuratov I.I., and Sinelnikov V.M Ionospheric precursors of earthquakes revealed from GPS data and their connection with sea-land boundary. In: Proceed. 16th Wroclaw EMC Symposium Ruzhin Y., Nomicos C., and Valianatos F High frequency seismoprecursor emissions. In: Proceed.15th Wroclaw EMC Symposium Ruzhin Yu., Nomicos C. Radio VHF precursors of earthquakes Natural Hazards. V DOI /s

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