Earthquake precursors in magnetically conjugated ionosphere regions

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Adv. Space Res. Vol. 21, No. 3, 525-528, 1998 81998 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/98 519.00 + 0.00

Pergamon

PII: SO273-1177(97)00892-2

EARTHQUAKE PRECURSORS IN MAGNETICALLY CONJUGATED IONOSPHERE REGIONS Yu. Ya. Ruzhin, V. I. Larkina, and A. Kh. Depueva IZMIRAN, Troitsk-town, Moscow Region, 142092, Russia

ABSTRACT It is known that the highly conducting geomagnetic field flux tube and qua&rapped energetic particles provide strong electrodynamic coupling between magnetically conjugated regions in the ionosphere. Such a connection is likely to cause ionospheric disturbances before a pending earthquake not only over the epicentral zone but also in the opposite hemisphere. Data From the INTERCOSMOSand ALOUETTE satellites were analyzed from this point of view. Earthquake precursors in magneticallyconjugated ionosphere regions were found in very low frequency (VLF) emission and F2 -peak 01998 COSPAR. Published by Elsevier Science Ltd. parameters. FZprecursors appear some days before the earthquake, and manifest themselves as an Appleton-type anomaly if the epicenter of the future earthquake is situated near the magnetic equator. Estimation of the electric field magnitude necessary to generate the observed anomaly was made. It was shown that an electric field of less than one mV/m must be generated in the ionosphere. VLF precursors appearance some hours before the earthquake is localized close to the magnetic shell corresponding to the future earthquake epicenter and have a belt-like structure (longitudealigned for more than some tens thousands kilometers) in both hemispheres. VLF precursors are followed by energetic particle (electrons energy W>40 keV) precipitation. INTRODUCTION As a result of the last lo-15 years of ionospheric investigations there is sufficient evidence to support ionosphere-lithosphere coupling processes (Larkina et. al. ,1983; Larkina e2. al. ,1989; Parrot, 1990; Oraevsky et. al. ,1995). Related to lithosphere and crustal processes, ionospheric variations are found not only after strong earthquakes but some days before the main shock. These variations may be considered as earthquake precursors. It is of a great interest to systematize available information on seismoionospheric precursors and their parameters. Among a variety of known ionospheric anomalies caused by a forthcoming earthquake there are some major anomalies which to our mind could be used to develope a satellite earthquake prediction system. A comprehensive analysis of diverse seismoionospheric precursors identified three main precursor types distinguished by location and time of appearance (Oraevsky et. al. ,1995) before the earthquake, 1) Lower ionosphere reaction - some days before (up to two weeks). 2) Anomalous plasma structures throughout the ionosphere - two to three days before. 3) VLF radiation generation in the ionosphere registered on board satellites at 500-800 km altitude range - some hours before. 525

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It was shown (Oraevsky et. al. ,1995) that all the seismoionospheric precursors is occured within a radius R(M) of the epicenter of an earthquake originally defined by Dobrovolsky theory for precursor measurement by the standard seismographs situated on the earth’s surface. Moreover, disturbed zone dimensions at the ionospheric heights maintain equal to the preparation zone dimension at the ground not only over the epicentral zone but even in the magnetically-conjugated ionosphere region. Below we present some evidences which on our mind support this idea. THE EARTHQUAKE AS APOSSIBLE REASON FOR APPLETON EQUATORIAL ANOMALY It is accepted that plasma properties change in the upper ionosphere over the region of an anticipated earthquake. Thus some hours before the first shock, the electromagnetic emission bursts increase at satellite height (Larkina et. al. ,1983; Larkina et. al. ,1989), the plasma density deviates and anomalous plasma structures appear in the ionosphere some days before the main shock (Oraevsky et. al. ,1995). In our mind pending earthquakes manifest themselves at ionospheric heights in a different way at high, middle and low latitudes due to various kinds of prevailing physical processes depending on the earthquake epicenter location. In most cases there are publications concerning only ionospheric disturbances at middle latitudes. To check this idea we have analysed (Depueva and Ruzhin,1995) ALOUETTE upper ionospheric data obtained some days before and during equatorial earthquake magnitude of M=6.8 on April 13, 1963 with an epicenter 600 km N-W of Huancayo, Peru. It was noticed that the F-layer critical frequency (foF2) dependence against magnetic inclination shown in Figure 1 had a pronounced minimum close to magnetic equator and two maxima in both hemispheres one day before the earthquake although they were absent during two weeks before and after it. The regional ionospheric model doesn’t predict the existence of this kind of equatorial anomaly. It is proposed that the generation of anomalous eastward electric field at the epicenter of future earthquake together with Earth’s magnetic field at ionospheric level may cause the upward vertical ionospheric plasma drift over the epicentral zone known as ‘fountain effect’. One can conclude that the electric field of the precursor has to be less or compatible with the electric field of natural origin leading to the ‘fountain- effect’. So the electric field magnitude of the precursor which is sufficient for this aim could be less than one mV/m also at ionospheric heights. 10.0

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Fig. 1. The plasma structure in the ionosphere one day before the pending earthquake 12.04.1963 (M=6.9) (big black circles). The rest two lines are foF2 values more earlier and after the eatthquke.Thick horizontal line is the esimated earthquake preparation zone dimension. By EQ sign the epicenter position is shown. SEISMOIONOSPHERIC VLF-EFFECTS IN CONJUGATED HEMISPHERES It was shown that the onboard satellite registrations of the VLF-ELF noise in ionosphere permits detection of the space-temporary behaviour of the zone dimensions where the intensive low frequency bursts (over

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epicentrial region) are observed (Larlcina et. al. ,1983). The reliability of this observed precursor effect was estimated on the basis of experimental data to be about 85 to 90% (Larkina et. al ., 1989). The appearance of low frequency noises (emissions), so-called “noise belts” in the upper ionosphere was associated with the longitudinal drift of the energetic particle precipitation of the inner radiation belt. The relationship of VLF field intensity and associated energetic electron flux density were investigated over epicentral zone of the pending earthquake.We obtained that shape-correlated bursts were registered 2 to 2.5 hours before the earthquake and that for the specific pulses , the correlation coefficient between electron flow and electric component of the VLF field bursts are 0.60 to 0.72. For the first time , reliable evidence of sufficiently close connection of the VLF wave - precursors with radiation belt particles were obtained. It is necessary to note, that by investigation of magnetic conjugation of the low frequency noise anomaly pulses in upper ionosphere , we find the good conjugation between the bursts in the northern hemisphere (ionosphere above the future earthquake) and in the southern one. Shown in the Figure 2 in the geographic latitude-longitude coordinates are the INTERCOSMOS 19 satellite projection parth of orbits 9189-9196 in the period before the earthquake of November 23,198O (t =18:34 UT; (p==-41,l’N; X=15,34” E; M=6,7; H=33 km; LT=19:34) and after when the emission burst was detected. The L-shell and the geomagnetic meridians have been mapped. The position of the earthquake epicentre is marked by a cross (x). The moment of the main shocks indicated near the cross. Parts of the orbit projections are blackened where bursts of emission amplification are found. The emission levels of magnetic (to the right of the trajectory) and electric field components on the frequency of 4650 Hz are denoted on the map by hatched bands. Each step corresponds to an increase in the signal intensity by 10 dB with respect to the background level. The absolute time values and time differences with respect to the earthquake start time t (“+” after this moment and “-” before it) are indicated. L 2.50

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Fig.2. Two VLF ‘noise belts’ were observed in magnetically conjugated regions. They were stretched along the same L-shell (L=l.7) that cross the titure earthquake epicentre position marked by cross. As a result of our analyses, the conjugated VLF precursors are obviously seen for three known (Larkina et.. al., 1983; Larkina et. al., 1989) earthquakes. So, it is found that at magnetically conjugated hemispheres the precursors like VLF intensity bursts are observable. It is shown that epicentral and conjugated ionospheric bursts are similar or close to each other in amplitude and form.

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DISCUSSION It was shown on the basis of ALOUETTE data that at the equatorial region natural processes such as “fountain-effect” could be modified by an electric field related to pre-earthquake activity within the earth’s crust. The magnitude of such an electric field in the ionosphere was estimated to be less than one mV/m. The INTERCOSMOSdata analysis have shown that in upper ionosphere above the epicentre the VLF noise amplification due to seismic activity and electron flux burst zones coincide in time and spatially. Besides, the same phenomenon is observable at opposite hemisphere, the location of both above-said zones being situated very closely to each other. It means that disturbance is transported along the magnetic field line to conjugated region without suffisient distortions, In other words the seismo-precursor image in opposite hemisphere is a projection of disturbances generated over seismoactive area. On the basis of this picture the model of precursor development over seismoactive region and conjugate area is constructed. Stable pitch-angle distribution of charged energetic particles (the population of inner radiation belt) could be disturbed by the electric field fluctuations over the earthquake epicentre. These particles have a small Larmour radius in comparison with the size of the area, where the VLF forerunner is observed. This is a main factor, which keep stable the cross size of “noise belts”.The anomalous flux generated over the epicentre transfers its image to the conjugate area (“save” it) and drifts east -west along the initial magnetic L-shell. In this case VLF emissions, confidently registered in southern and northern hemispheres can be presented as a result of interaction of these flows (beams) with ionospheric plasma along the drift paths in vicinity of the satellite orbit. Thus the spatial distribution of VLF emissions (at conjugated hemispheres) is dictated by drift of anomalous electron flux and will occur along the magnetic field shell ( L-1.75, Figure 2 ), which crosses the zone of the pending earthquake. CONCLUSION Thus, our data analysis supports the existence of ionospheric anomalies as precursors to earthquake activity in both hemispheres. FZprecursors appeare some days before the earthquake and manifest themselves as an Appleton-type anomaly if the epicenter of the fbture earthquake is situated near the magnetic equator. Estimation of the electric field magnitude necessary to generate observed anomaly was made. It was shown that an electric field of less than one mV/m must be generated in the ionosphere.VLF precursors the earthquake and are localized close to the magnetic shell appear some hours before corresponding to the Wure earthquake epicenter. They have a belt-like structure (longitude-aligned for more than some tens thousands kilometers) in both hemispheres. VLF precursors are followed by energetic particle (W>40 keV) precipitation. The “noise belt” dimension in the upper ionosphere is determined by the radiation belt energetic particle precipitation area, but such precipitation in turn is “triggered “ by pre-earthquake processes in the Earth’s lithosphere. It is found that at opposite hemisphere precursors like VLF intensity bursts are also observable. REFERENCES Depueva, AKh., and YuYa Ruzhin, Seismoionospheric fountain-effect as analogue of active space experiment, A& Space Res., 15, 12, ( 1995). Larkina, V.I., V.V. Migulin, and A.V. Nalivaiko, Observation Onboard the “Intercosmos 19” satellite of VLF Emissions assosiated with Seismic Activity, Geomagn. i Aeronom., 23, 5, (1983). Larkina,V.I., V.V. Migulin, and O.A.Molchanov, Some Statistical Results on very Low Frequency Radiowave Emissions in the upper Ionosphere over Earthquake zones, Physics of the Earth and Planetary Interiors., 57, 5, (1989). Oraevsky,V.N., YuYa. Ruzhin, and A.Kh. Depueva, Anomalous Global Plasma Structures as Seismoionospheric Precursors, Ah. Space Res., 15, 11, ( 1995). Parrot, M. Electromagnetic Disturbances Associated with Earthquakes: an Analysis of Ground-based and Satellite data, J. Scient#c Exploration , 4, (1990).