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Terrogenic effects in the ionosphere: a review
Physics of the Earth and Planetary Interiors, 57 (1989) 115—128 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
115
Terrogenic effects in the ionosphere: a review L.N. Popov
1,
Yu.K. Krakovetzkiy
1,
M.B. Gokhberg
2
and V.A. Pilipenko
2
‘Tomsk State University, Tomsk-10, 634010 (U.S.S.R.) of the Physics of the Earth, Moscow, 123810 (USSR.)
2 Institute
(Received December 8, 1987; revision accepted May 3, 1988)
Popov, L.N., Krakovetzkiy, Yu.K., Gokhberg, M.B. and Pilipenko, V.A., 1989. Terrogenic effects in the ionosphere: a review. Phys. Earth Planet. Inter., 57: 115—128. An attempt is made to collect and to analyze critically the existing experimental evidence of terrogenic effects in the ionosphere. By terrogenic effects we mean phenomena which indicate that the ionosphere ‘feels’ the structure and the properties of the underlying surface of the Earth. The review includes discussion of coast effect in aurora and in ionospheric currents, deep faults and aurora, equatorial electrojet and tectonic features, ionospheric concentration and Moho zone, magnetic anomalies and aurora, atmospheric electricity and orographic effect and seismogenic faults and ionospheric anomalies. Special attention is paid to the consideration of experimental data provided by photometric observations at high and middle latitudes.
1. Introduction The ionosphere is a complex dynamic system controlled by many parameters, primarily by solar—geophysical factors (solar activity, magnetic storms and substorms) and additionally by ‘internal’ factors, which have been intensively investigated: (1) acoustic motions of the atmosphere of different scales, the so-called ‘meteorological control of the ionosphere’ (for a comprehensive review see Danilov et al. (1987)); (2) technogenic activity, mainly electromagnetic emission of radio-transmitters, industrial power lines, and so on. Also, rather unusual experimental evidence began to appear recently, which indicates that the ionosphere can ‘feel’ the properties of the particular region of the Earth beneath it. All such effects will be called ‘terrogenic effects in the ionosphere’. Some of the terrogenic effects result from phenomena involving acoustic action on the ionosphere, i.e. a generation of acoustic waves, which
reach the ionosphere, by large-scale mountains (Gossard and Hooke, 1971); and sporadic heating and generation of wave disturbances in the E and F layers after earthquakes and volcanic eruptions (see review by Alperovitch et al., 1985). The physical mechanisms of these phenomena have been well studied in general, and they are fully described in many review papers. However, most of the terrogenic effects are related to electromagnetic interaction in the Earth—atmosphere—ionosphere system. Experimental data on electromagnetic terrogenic effects are very scanty and not firmly proved. All of them still need further thorough verification. The observed effects are small and cannot be extracted in all circumstances in such a complex many-parameter system as the ionosphere. Descriptions of particular terrogenic effects are dispersed over various papers, which are known only to a small group of geophysicists, so it seems useful to give a brief review of the experimental facts and discuss their possible physical implications.
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2. Mutual induction in the ionosphere—Earth systern The current-carrying E layer of the ionosphere, separated from the highly conducting Earth by the atmospheric gap, which is small compared with typical horizontal dimensions of the ionospheric current system, represent a system of two inductively connected conductors. Naturally, the effects of mutual induction between ionosphere and Earth should manifest themselves in regions with intense ionospheric currents, and most of all at aurora! latitudes. 2.1 Coast effect in aurora A convenient object for investigation of electrodynamic interaction between Earth and ionosphere is an auroral arc. Its dynamics can be monitored by rather simple ground-based photometry. Real terrogenic effects in aurora are known to aurora specialists, and one of the best known is the coast effect, which was reported in 1961 by Soviet geophysicists D. Duma, and later by Nadubovitch, and Starkov (Nadubovich, 1967). A detailed description of earlier observations of coast effect and relevant references were given by Nadubovich (1967). The coast effect is the experimentally observed tendency for homogeneous auroral forms to align along the sea coastline, large rivers, bays, etc. Consequently, auroras are more frequently observed near a sea coast. This effect was statistically justified by independent groups of scientists from analysis of the ascafilms of an all-sky camera, which were obtained during several years of observations. Further research revealed other manifestations of the coast effect: fracture of an aurora! arc at a point of intersection with a coastline, and depression of auroras over islands, banks, etc. The most striking support for the coast effect was gained from the spacecraft Dynamics Explorer 1 (DE-1). Continuous global imaging of polar regions from DE-1 with auroral imaging instrumentation of the University of Iowa provided a large data set of unique information on aurora dynamics. In Fig. 1, we can see the visiblewavelength image at the 630-nm emission line of
intense auroral emissions over the western coast of Norway at 20.45 UT on 20 October 1981. An aurora! arc system is observed to transverse the Atlantic Ocean in the late evening hours of local time and to bifurcate at the Norwegian coastline. The northern branch follows the western coastline closely whereas the southern branch initially follows the southern coastline before proceeding eastward across southern Sweden. The existence of the coast effect in aurora led to the suggestion of the existence of a similar effect in the ionospheric current distribution. Indeed, this effect was found experimentally by Shpynev et al. (1977, 1987) from data of the magnetic station network in the Soviet Arctic. With the help of a specially designed algorithm for extraction of the part due to ionospheric currents from geomagnetic variations, charts of external and internal equivalent currents were constructed. The coast effect of these charts is seen usually as the increase of the current density over the coastline and as the elongation of the current lines alongside it. According to the current system cornputations (Shpynev et a!., 1987) the modulation rate of an original current as a result of the coast effect is > 50%. The coast effect has the highest probability of appearing near local midnight and at moderately disturbed auroral activity (AE 150 nT). The coincidence of morphological features shows that the coast effects in aurora and in ionospheric currents are just the two sides of the same phenomenon. It seems intuitively that the coast effect is the result of the pondermotive interaction between the west auroral electrojet and induced current in a near-coast zone of a sea. This point of view was assumed by early investigators of the coast effect and was then developed by Ponomarev (1964). In our view, the consideration of aurora! arc dynamics in the framework of a model with two currents in vacuum is an oversimplified idealization. An interesting idea was suggested by Shpynev et al. (1977): it may be that some substorms are related to the feedback instability of the magnetosphere—ionosphere current system. In local regions with small anomalies (— 1%) of ionospheric electric and magnetic fields caused by the Earth’s induction effect, the threshold for a substorm onset
117
Fig. 1. Example of geographic alignment of intense auroral emissions along the western coast of Norway (Mercator projection). The image was obtained with one of the two visible-wavelength imaging photometers carried on the NASA—GSFC spacecraft DE-1 as part of the University of Iowa’s auroral imaging instrumentation. The false-color coding presents the broad region of 630-nm emissions in red, with the more intense emissions in orange (reproduced by courtesy of Prof. L.A. Frank).
may be reduced. As a result, in these field tubes the explosive phase of a substorm begins earlier and may hamper the growth of a similar process in neighbouring tubes, so the substorm activity over terrogenic anomalies will be statistically enhanced. Until now, however, no theory has been developed for a physical mechanism of the coast effect. The essential point that this effect definitely demonstrates is that any theory of substorm and auroral arc generation should consider the ionosphere as an active factor in the ionosphere—magnetosphere interaction, 2.2. Deep faults and aurora Another impressive manifestation of an effect similar to the coast effect is the influence of crustal deep structures of the Earth on the formation of the spatial picture of aurora. The problem of relationship between aurora and geological structure was studied by Krakovetzky et al. (1984a) with the use of all-sky camera data. In Fig. 2, taken from that paper, the isochasms of aurora appearance frequency (isoaurora) over the Norilsk
region are shown. In the same figure the system of deep faults of the transition zone from the Siberian platform to the East-Siberian plate is shown by a dashed line. We can see that, when isoauroras approach the fault, they change their normal direction relative to latitude, and after crossing the fault, resume their direction. It looks as if an aurora shows a tendency to align along a fault or even to be ‘expelled’ from a fault. The existence of the geological transition zone, revealed by auroral observations, was firmly proved by deep seismic sounding data. A detailed study by Krakovetzky et a!. (1984b) showed that these ionospheric anomalies are geographically stable and their location does not depend on season or geomagnetic activity. The stable and regular character of the inhomogeneous structure of the aurora’s statistical spatial distribution is an additional argument in favor of the reality of the terrogenic effect. 2.3. Equatorial electrojet and tectonic features The inductive coupling between Earth and the ionosphere should be effective also in another
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region of intense ionospheric currents—the equatonal electrojet. Indeed, Duhau and Osella (1985) succeeded in eliciting the mutual inductive effects at equatorial latitudes. They analyzed the data obtained at three magnetometer chains which crossed the dip equator (Fig. 3a). By separation of geomagnetic daily variation into an external and an internal part, Duhau and Osella (1985) obtamed the latitudinal profile of integrated E region current density. The mean Earth conductivity along geomagnetic chains in Nigeria and central Africa (Fig. 3a) is asymmetric relative to the dip equator—the conductivity of a continental shield is smaller than that of a continental platform. Also, latitudinal profiles of east—west ionospheric
current turned out to be asymmetric (Fig. 3b); the current intensity was anticorrelated with mean Earth conductivity. In the Peruvian chain (Fig. 3a), however, the conductivity inhomogeneity as a result of Cenozoic volcanic zone is almost centered at the dip equator. This is coincident with the fact that the profile of the ionospheric current is almost symmetrical there (Fig. 3b). These correlations between tectonic features and the ionospheric current were also confirmed by direct measurements with VHF radars (Crochet, 1977; Duhau and Osella, 1985). These effects were still not supported by theoretical estimates which should show that the variations of the upper boundary of the highly conducting region of the
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Fig. 3. (a) Tectonic map, in which the geomagnetic chains in Peru (circles), Nigeria (dots) and central Africa (crosses) are indicated. (b) Profiles of the ionospheric integrated current density obtained from external part of geomagnetic daily variations along geomagnetic chains in Peru (solid line), Nigeria (dotted line) and central Africa (dashed line).
mantle is sufficient to induce the equatorial ionospheric current redistribution. 2.4. Ionospheric concentration and Moho zone Another manifestation of influence of crustal electric properties on the ionosphere was discovered by Volkova and Fedorenko (1989). They elaborated a method which enabled them to obtam a ‘pure’ N( h )-profile excluding the influence of the solar zenith angle. Processing by this method
a large number of ionograms from a middle-latitude network of stations, ranging from 40 to 700 N and 20° to 83°E, revealed that ‘pure’ electron concentration distributions showed clear and stable regional anomalies. In Fig. 4a, b, taken from Volkova and Fedorenko (1989), the meridional and latitudinal profiles of a normalized ‘pure’ electron density, averaged over a 1-month period, at the level h = 110 km are shown. Anomalies in geographical distribution of N, which can be seen in Fig. 4, diminish with height and do not reveal
120 RJ(tp)
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regularities of aurora distribution over the Yakutsk region. The reality of this effect was supported by investigations of Krakovetzky et al. (1984b). Figure 5, of Krakovetzky et al. (1984b), illustrates the relationship between the frequency of aurora appearance p over the Nonilsk area and anomalous magnetic field ~Ta. Comparison of two coincident
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Fig. 4. Mendional (a) and latitudinal (b) profiles of the monthly mean pure electron density distribution N (shown by solid line) at h = 110 km. Broken line shows the depth of Moho zone. All parameters are normalized to a value corresponding to the station with minimal latitude (*).
themselves at the F layer. It is striking that these anomalies coincide with variations of Moho zone depth. As can be seen in Fig. 4 the N is larger where the Moho border is closer to the Earth’s surface. The discovered dependence is regular and was observed at various levels of solar activity and times of day. It may be that this effect has something in common with the effect, described above, of aurora depression over land. In both effects, possibly, additional raising of the ionosphere occurs over crustal regions with enhanced conductivity. .
3. Influence of regional magnetic anomalies on aurora The influence of the largest world magnetic anomalies (for example, the South-Atlantic anomaly) on the overlying ionosphere is a known and well-understood effect. However, explorations of the fine structure of the auroral oval demonstrated that regional magnetic anomalies also took part in the formation of an aurora’s visible forms. The relationship between spatial distnibution of aurora and local structure of the Earth’s magnetic field was first indicated by Samsonov and Zaretzky (1963), who studied the statistical
profiles along the latitude 69°N in Fig. 5 shows that maxima of isoaurora fell into the regions of reduced geomagnetic field. It therefore follows, from the above investigations, that an auroral arc is ‘expelled’ from a local positive magnetic anomaly. A simple explanation cannot be given for this effect. It is doubtful whether small alterations of the loss-cone over regional magnetic anomalies are sufficient to produce noticeable change of the particle precipitation into an auroral arc. It should be noted that the result obtained in the Nonilsk area was not confirmed in another geological region (Samsonov and Vasil’eva, 1986), possibly because of differences in the geological structure and physical properties of the underlying surface or in the data-processing approaches (Degtyarev et al., 1987). Once again, this situation indicates the complexity of terrogenic effects and of the inadequacy of our knowledge of them.
4. Local distortions of the global electricity circuit We have considered above the intensive current systems of the ionospheric auroral and equatorial electrojets, generated originally by the magnetospheric plasma convection. Now we consider
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Fig. 5. Latitudinal profiles (4 = 690 N) of anomalous magnetic field ~Ta in nT (bottom) and the deviations of the aurora appearance probability lip (top) over the Norilsk region.
121
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cuit (Hays and Roble, 1979) predicted that over the largest mountain systems of the Earth areas iO~ km) with reduced ionospheric potential can emerge. The values of the horizontal electric field in the ionospheric anomalies can reach 1 mVm’. It would be difficult to extract any anomalies of the ionospheric parameters caused by such small electric field, because of the permanent meteorological factors over large mountain systems, such as rising mountain waves. The next problem of atmospheric electricity, which we now deal with, concerns the phenomena in which small change of the atmospheric electric field will result in a substantial deviation in the course of more energetic processes. As an example, we recollect the effect of the modulation of —
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Fig. 6. Schematic diagram of the global atmospheric electricity circuit. The following regions are marked: 5, storm current generator; R, enhanced output of radioactive emanations; 0, orographic features. Atmospheric current system is indicated by arrows.
another global current system—the atmospheric electricity circuit. In this circuit the near-equatorial centres of thunderstorm activity serve as a current generator. A generated current then spreads over the upper atmosphere and the ionosphere, as is shown schematically in Fig. 6, and closes at middle and high latitudes through the fair weather vertical currents. The thunderstorm generator supports the ionospheric potential at 2 x i05 V and this potential difference produces regular atmospheric electric field 2 x 102 Vm1 near the Earth’s surface. The question arises: can the regional peculiarities of the Earth’s crustal structure induce local distortions in the global electricity circuit? The answer to this question is found to be positive. The best-known effect of terrogemc distortions of the global electricity circuit is an orographic one. This effect is due to the fact that large mountains may substantially reduce a local columnar resistance between the Earth’s surface and the ionosphere. Thus the distribution of atmospheric electric fields and currents over a mountam system will be disturbed, as indicated in Fig. 6. Experiments on balloons in the stratosphere demonstrated the increase of current and electric fields over orographic anomalies (Ogawa and Tanaka, 1976). Large-scale orographic features can induce electric potential distortions, even at ionospheric heights. Detailed numerical calculations of the three-dimensional model of the electricity cir—
—
thunderstorm initiation by solar activity. Solar modulation of cosmic nay intensity results in a change of the ionization rate by 10% only. Nevertheless, even these small changes of the charge density and electric field may be a trigger for thunderstorm initiation when the meteorological conditions are favorable (Herman and Goldberg, 1978). Another terrogenic factor which modulates thunderstorm activity is the selective dependence of lightning probability on the local geological structure of the Earth’s surface (Saraev et al., 1974). Statistical analysis showed that the surface areas with more complex geological structure, with enhanced quartz in rocks, etc., were more frequently under lightning strikes than others. —
5. Influence of seismoactive faults on the ionosphere. Earlier we discussed the effects of the influence of deep geological faults and sharp inhomogeneities of the Earth’s crust on the dynamics of ionospheric processes. For seismoactive faults, however, the variety of potentially possible mechanisms to influence the ionosphere is considerably wider. As we confined ourselves in this review to consideration of only electromagnetic phenomena in the Earth—atmosphere—ionosphere system, we shall not discuss disturbances in the ionosphere as a result of acoustic impact of seismic shocks (a
122
detailed bibliography on this subject can be found in Alperovitch et al. (1985)). We shall concern ourselves with electromagnetic phenomena which arise at the final stage of earthquake preparation, several hours or days before a shock, and with their influence on the properties of the overlying ionosphere. In our view, seismo-ionospheric interactions as well as other terrogenic phenomena have a fairly weak impact on the ionosphere. On the other hand, a noticeable manifestation of such effects results from a terrogenic effect on the rate of energy dissipation and on the redistribution of energy in the course of considerably more powerful natural ionosphere—atmospheric processes.
5.1. Experimental observations Most studies devoted to a search for ionospheric effects produced by seismic activity before earthquakes were based on a hypothesis concerning the emergence of intense large-scale electrostatic fields in the preparation zone. Indeed, some fragmentary information on atmospheric electric field disturbances in seismoactive areas before weak shocks can be found in earlier papers (Kondo, 1968; Tzerfas, 1971). Moreover, the emergence of seismogenic intense electric fields were indirectly indicated by observation of phenomena such as near-surface glow of the air before an earthquake, the spontaneous switching on of luminescent lamps and breakdown of cables (Tzerfas, 1971; Derr, 1973). . Analyses of numerous reports of natural luminous events in various regions led Derr and Persinger (1986) to the conclusion that one class of luminous phenomena (mostly nocturnal lights) may result from small rock fractures as tectonic strain accumulates over a large region, whereas another class of earthquake lights may result from the release of that strain by fault breakage. It follows therefore that, in addition to hypothetical electric fields which appear several hours or days before an earthquake itself, an electric field impulse, with intensity sufficient to induce light effects, may be produced when the main rupture occurs (Lockner et al., 1983). It should be taken
into account that under favorable meteorological conditions (e.g. overcooled water vapor, aerosols, mist, drizzle, etc., in the atmosphere), electric fields of considerably lower intensity than 3 x 106 Vm1 (critical breakdown voltage under normal conditions) may ignite the crown discharge (Tnibusch, 1978). The impulse of an electric field at the moment of a shock may produce a short synchronous disturbance of the ionosphere. However, direct experimental evidence which could convincingly prove the emergence at that moment of intense electric fields has not yet been obtained. As regular direct observations of electrostatic fields are extremely rare, efforts were made to collect information on the existence of such fields with the help of data obtained by vertical ionosphere sounding stations. Many publications have reported successful efforts to discover ionospheric anomalies preceding earthquakes. Some effects were identified by Sobolev and Husamiddinov (1985), and by Datchenko et al. (1972) in the course of a retrospective analysis of vertical ionospheric sounding station data in Central Asia. These authors discovered an anomalous increase (up to 20%) in the critical frequency f 0F2 of the night-time ionosphere at the periods of nearby seismicity. The plasma density in the F layer began to increase, on average, several days before a shock, and an earthquake itself occurred while the anomaly was waning. Unusual reflections and traces on ionograms which appeared before the earthquake were also seen (Sobolev and Husamiddinov, 1985). The results, presented in a number of articles in this issue, also confirm the existence of seismo-ionospheric precursors. Inhomogenities of E and D layers over the epicenters of future earthquakes were also recorded on the basis of disruptions in propagation conditions for MF and VLF radiowaves. Nestorov (1979) observed unusual fading of MF radiotraces 1 h before the Vrancha earthquake. De and Sarkar (1984) also recorded short-period fadings at long many-hop paths = 40 kHz), accompanying a large number of earthquakes. This fading often began many hours before an earthquake and lasted for hours after it. Gokhberg et al. (1987) reported the extensive statistics of clear phase anomalies at VLF radiotraces from the ‘Omega’ navigation sys(f
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Fig. 7. Variations of night airglow intensity (green oxygen line) over the Trans-Caucasus fault, recorded at Abastumam observatory. Arrow marks the moment of seismic shock.
tern, which lasted — 1 h and preceded near-path earthquakes at night. To investigate seismo-ionospheric effects, Fishkova et al. (1985) proposed that use should be made of information obtained from photometric observations of night airglow. The analysis of data accumulated over many years of observations at the Abastumani observatory showed a number of anomalous increases in emission intensity of the green oxygen line (557.7 nm) several hours before the nearby earthquakes of the Trans-Caucauses fault. An example of such events taken from Fishkova et al. (1985) is shown in Fig. 7. The intensity of green oxygen emission characterizes the dynamics of the ionized component of the ionosphere in the E layer. It is interesting to note that for the red oxygen line (630.0 nm), which has a photometric center at the altitudes of the F layer, increased emission intensity was not observed. 5.2 Possible physical mechanisms of ionospheric anomaly formation The above facts gave an impetus to studies of various aspects of quasistationary electric fields of seismic origin. Various hypotheses were developed on the physical mechanism of generation of these fields. It was assumed that disturbances of electric fields in the near-Earth atmosphere may result from charge separation at dielectric grains of the crust (Vorobijev, 1970), piezoelectrical phenomena (Finkelstein et al., 1973), triboelectrical phenomena (Parkhomenko and Balbachyan, 1981), the generation of large-scale current systems in the hypocenter as a result of coherent action of mech-
anoelectrical transformers (Gokhberg et al., 1985), or enhanced output of radioactive emanations (Pierce, 1976). In most experimental studies, the authors inexplicitly proceeded from the assumption that as typical dimensions of the earthquake preparatory zone are comparable with or larger to the height of the lower ionosphere, large-scale seismic electric fields can penetrate the ionosphere and bring about the emergence of the observable anomalies. That assumption can be substantiated by simple analytical estimates. Let us suppose that an anomalous electric field E~°(f)has emerged on the Earth’s surface in the region with finite radius i~. If the typical frequency f of a non-stationary field E0° is such that f 1 kHz, then the region between the Earth’s surface and the ionosphere proves to be a near-field zone of the source on the Earth’s surface. In this case, electric fields can be calculated by means of quasistatics equations. The solution to the similar problem of penetration into the ionosphere of the field of a point charge above the Earth (modelling a thunderstorm cloud) was obtained by Park and Dejnakarintra (1975). Their calculations need to be modified for application to the case of a source on the Earth’s surface of finite size. The following properties of electric field attenuation with height h can then be established (Gokhberg et al., 1985). At low altitude, where the atmospheric conductivity is low, the field decreases rather slowly by the law E(h) (h/r0)~ (n = 0—2). As height increases the conductivity is enhanced, by the exponential law ~ (h) a~exp(h/a). At altitudes where conductivity currents become larger than ‘~
=
124
acts as a physical agent of seismo-ionospheric relationships. Let us consider some mechanisms of possible influence on the ionospheric plasma of electric fields of seismic origin. If we suppose that an
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Fig. 8. Plot of attenuation factor 8 = 1og(E~/E°) dependence on frequency f for two radii r 0 of a region with anomalous near-Earth electric field. Typical parameters of conductivity scale a and near-Earth conductivity a~were chosen.
the displacement currents, the electric field becomes severely (exponentially) attenuated. And finally, beginning from altitudes of — 70 km, where the anisotropy of ionospheric conductivity becomes important, the transverse component of the electric field E1 is shifted with no attenuation along geomagnetic field lines, and the longitudinal component (alongside the geomagnetic field) decreases rapidly. of This E reflects the equipotentiality the behavior field linesof in the ionosphere for large-scale disturbances. Estimates of the attenuation of E between the Earth’s surface and the lower ionosphere, based on the above considerations, are summarized in Fig. 8. The value of the resultant attenuation 8 = log(Eff/E~?) for various frequencies f was estimated for optimal conditions—night-time ionosphere at a point directly above the extended anomaly, i.e. r 0 >> (r, a). In the day-time ionosphere the field’s attenuation is greater by 1—1.5 orders of magnitude. The rough estimates given in Fig. 8 show that to produce a disturbed field, say E1 10 mVm in the ionosphere it is necessary to have near the Earth’s surface a large-scale stationary anomalous electric field E0° 3 x l0~Vm which is larger by only an order of magnitude than the normal value of the atmospheric potential gradient. For non-stationary disturbances the attenuation factor in accordance with Fig. 8 is even smaller. The above simple estimates thus prove the validity of the hypothesis that the quasistationary electric field ~,
~,
anomalous electric field has a constant component, then the ionospheric layers will undergo, under the influence of such a field, a regular vertical drift with velocity W. We shall confine ourselves to the solution of a one-dimensional equation of the ionization balance in a night midlatitude F layer, in which the vertical electric drift is taken into account as a small parameter (although that equation has been examined elsewhere .
.
in detail by numerical methods). In this case, simple estimates can be obtained for the distortion of general parameters of the F layer under the influence of an applied electric field: the height hm of maximum F2 layer T —~
—~
(la)
a density A~of the ionized component ~N, T/3/TW 1~2T /T h >
—
tion in the ionosphere is as follows. Diffusional flow of plasma from the plasmasphere into the night-time ionosphere becomes slower when moving downward. As a result, the density N~increases. At low altitudes, however, the plasma density rapidly diminishes because of recombination. As a consequence, the concentration maximum2, ofwhere the F alayer is formed at a certainbalance height diffusion—recombination h mF T~) takes place. The influence of electric (TD fields on the plasma density is typical of the physics of terrogenic effects. The applied electric field does not produce additional ionization of the ionosphere but merely shifts the ionospheric layers
125
slightly and redistributes concentration with altitude. As is evident from eqns. (1), hm increases under the influence of the upward drift. At the same time N1 increases at the maximum of the
However, the typical size of microcracks is many orders of magnitude less than the length of VLF radiowaves, so it is hardly to be expected that
layer and above and decreases below it. To produce anomalies that can be reliably registered (z~ N1/N1 10%; Sobolev and Husamiddinov, 1985; Gokhberg et a!., 1983) it is sufficient, in accordance with eqn. (1), to have an electric field — 1 mVm’. Thus, the parameters of the night F layer may serve as sensitive indicators of rather weak electric fields. In view of the fact that a permanent component of a near-Earth electric field penetrates to ionospheric altitudes with great attenuation, as it is evident from Fig. 8, it is worthwhile to consider the possibility of non-stationary disturbances influencing the ionosphere. The impact of a low-frequency electric field on the ionosphere plasma will result in an ion Joule heating. The rate of increase in ion temperature T1 can be estimated on the basis of the equations of dynamics and heat balance for ions
microcracks may be an effective source of EME and generate considerable signals at great distances in a far-zone. Nevertheless, there are events where seismogenic EME in the VLF range was recorded at distances up to l0~ km from the epicentre (Gokhberg et a!., 1979; Sobolev and Husamiddinov, 1985), so it is reasonable to assume that the emission from a seismic source will be noticeable at ionospheric altitudes as well. The power of a seismic emission is clearly insufficient to serve as a direct source of any substantial disturbance in the ionosphere. None the less, there exists a possibility in principle which makes it feasible for a low-power ground-based radioemitter (0.1—1 kW) to produce a noticeable disturbance in the ionosphere. This possibility is implemented through the resonant interaction between EME and energetic trapped electrons of the inner magnetosphere and is revealed as a stimu-
L~T
lated precipitation (Helliwell et al., 1973). of particles, An additional the ‘trimpi flux of effect’ en-
—
f ~Ej/Epj~p \2
The value of E~1,the ‘ion plasma field’, is determined by 3T M = .L (~o~ + e where w~is an ion gyrofrequency, p1 is collision frequency, 1~is temperature of neutrals, M1 and are ion and neutral masses. According to estimates, the value of ~ is minimal for E layer of the night-time ionosphere 50 mVm ‘. However, for a noticeable contribution from ion heating to the heat balance of the night-time ionosphere (say aT~/at 10 deg h l) it is necessary to have electric fields with intensity no less than several hundred mVm ~. __!~
.
—
5.3. Seismic trimpi effect Now we shall discuss impulsive electromagnetic emission (EME) as another possible agent of seismo-ionospheric relationships. The sources of this emission are the processes of shifting and cracking which take place alongside active faults and which are intensified before earthquakes.
ergetic precipitated electrons may produce an additiona! ionization of D and E layers, stimulate increase of airglow intensity in the lower ionosphere, etc. Numerical estimates (Inan et al., 1978) show that the energy density of precipitated partides may exceed by several orders the energy density in a wave. It is essential that the effectiveness of pitch-angle scattering by both coherent and non-coherent emission is practically the same (man, 1987). Thus, in this mechanism, in the spirit of the physics of terrogenic effects, the energy of fast particles accumulated in the radiation belt can be partly released under the influence of seismic radio emission. It is possible that a ‘seismic Trimpi effect’ is partly responsible for enhancement of airgiow intensity over a seismogenic fault (Fishkova et al., 1985) and for phase disturbances at VLF radiopaths passing over seismoactive regions (Gokhberg et al., 1987). 5.4. Quasi-orographic seismic effects The process, which is similar to the orographic effect in atmospheric electricity, may be an ad-
126
ditional mechanism of seismo-atmospheric relationships. Enhanced emanations of radioactive elements along faults, which become more intense before earthquakes, will lead to the decrease of an atmospheric near-surface potential gradient (Kondo, 1969; Pierce, 1976). Moreover, the reduced columnar resistance between the ionosphere and the Earth’s surface results, in a similar way to the orographic effect, in the increase of the fair weather vertical current and electric field at higher atmospheric layers. In a qualitative way,. this phenomenon is shown in Fig. 6. In accordance with the experimental facts on solar-thunderstorm relations mentioned in section 4, we may expect that even a small change of the atmospheric electric field may influence meteorological processes (creation of cloud, thunderstorm generation, etc.) in favorable conditions. These processes may then be responsible for the more substantial distortion of atmospheric electric field before an earthquake, and even for its reversal, as was observed by Tzerfas (1971). Moreover, it has been suggested that seismic processes may have an effect even on weather. Milkis (1986) analysed a long-term set of data from a wide network of meteorological stations in Middle Asia. Before strong earthquakes a gradual trend to atmospheric warming, and decrease of humidity and precipitation in comparison with normal values was observed. A typical feature of the periods preceding earthquakes was also the lower number of overcast days and reduced cloudness. Meteorological anomalies gave local spatial zones, which extended along deep faults. Besides a hypothetical ‘quasi-orographic effect’ caused by enhanced radioactive emanations, the additional heating along seismogenic faults as a result of crustal block movement may influence meteorological processes (Milkis, 1986).
6. Geographically stable anomalies in lower atmosphere and ionosphere Physical interpretation of the above-described terrogenic effects in the ionosphere is based on guesswork, rather than on a real theory. However, there are some indications of terrogenic anomalies
in the ionosphere and lower atmosphere for which there is no physical explanation. Other interesting facts are therefore also awaiting interpretation; for instance, it was noticed by Shaftan et al. (1986) that radioaurora at middle latitudes are spatially correlated with projections on the ionosphere of the borders of tectonic features. We should like to mention that there are mdications of geographically stable ionospheric anomalies, which may result from terrogenic effects but for which we cannot find unambiguous relationships with any peculiarities of properties of the Earth’s crust. Balev et al. (1976) discovered during ‘Cosmos-469’ observations at 260 km altitude local zones (102_103 km) of ion concentration diminished by one or two orders in the night ionosphere at low latitudes. These quasi-stationary holes of the night F layer were found to coincide with local regions of enhanced ionization in the D layer (Givishvily, 1986). The anomalies in the night ionosphere were revealed for the same geographical zones at different heights even over long timeintervals (up to 9 years) (Balev et al., 1976; Givishvily, 1986; Givishvily et al., 1987). These facts are awaiting not only interpretation, but also experimental verification.
7. Conclusion The main purpose of this report is to draw attention to a new class of geophysical phenomena, in many aspects still scarcely studied, namely terrogenic effects in the ionosphere. None of these effects yet have reliable physical interpretation, and their investigation may be of interest from various points of view. First, these effects represent a striking reflection of the fact that different geophysical environments—the Earth’s crust, atmosphere, ionosphere and magnetosphere—are in fact parts of a single system interacting with one another. Therefore an isolated examination of processes in one of the environments alone proves to be insufficient. Second, as we have shown, terrogenic phenomena also include seismo-ionospheric interactions, so their study by modern geophysical and radiophysical techniques, including satellite observa-
127
lions, will enable an understanding of the nature of seismo-electromagnetic phenomena at the final stage of earthquake preparation. The prospects for obtaining, by ionospheric methods, information on the existence of active faults, deep geological mnhomogeneities, increasing seismic activity, etc., are very attractive, and at the same time very realistic, Finally, it should be noted that, according to estimates, the direct influence on the ionosphere of the factors which bring about the manifestation of terrogenic effects is not strong. Nevertheless, the existence of the effects observed is indicative of the fact that even a slight outside influence can affect, under certain conditions, the dynamics of ionospheric processes. A profound understanding of the physical mechanisms of terrogenic influence on the ionosphere may help in the development of methods of active modification of ionospheric processes. Although, in this review, apparently quite different geophysical phenomena are collected, we think that for their investigation the development of the new common approach is needed. We imagine that terrogenic effects are mainly the manifestation of various aspects of the general physical situation—a small external influence on a system which is near the threshold of its internal instability. It seems to us, that this is a way to understand the physical mechanism of the appearance of terrogenic effects in the ionosphere. Because particular conditions, which are often beyond the control of an observer, are required for the manifestation of terrogenic effects, these effects cannot be monitored in every case, everywhere or in identical form. Standard statistical analysis therefore cannot be applied to study the reality of these effects.
Acknowledgements Helpful discussions with V. Vasil’eva, K. Davies, N. Gershenzon are gratefully acknowledged. We also thank L. Frank for kind permission to publish the DE-1 picture, and the referee for useful comments.
References Alperovitch, L.S., Ponomarev, E.A. and Fedorovitch, G.V., 1985. Geophysical phenomena modelled by an explosion: a review. Izv. Acad. Sci. USSR Phys. Solid Earth (U.S.A.), 21: 816—825. Balev, IV., Goldin, A.A., Greenwald, E.G., Pavlenko, V.A., Savin, A.!., Serov, M.K. and Talroze, V.K., 1976. Global research of ion concentration distribution of the ionosphere. Doki. Akad. Nauk SSSR, 230: 1062—1065 (in Russian). Crochet, M., 1977. Radar studies of longitudinal differences in the equatorial electrojet: a review. J. Atmos. Terr. Phys., 39: 1103—1117. Danilov, AD., Kazimirovsky, E.S., Vergasova, G.V. and Khachikyan, G.Yu., 1987. Meteorological Effects in the Ionosphere. Gidrometeoi.zdat, Leningrad, 271 pp. (in Russian). Datchenko, E.A., Ulomov, V.1. and Chernysheva, S.P., 1972. Anomalies of the ionospheric electron density as a possible precursor of Tashkent earthquake. Dok!. Akad. Nauk. Uzb. SSR, 12: 30—31 (in Russian). De, BK. and Sarkar, S.K., 1984. Fluctuations in ionospheric low-frequency signal strength accompanying seismic waves. Ann. Geophys., 2(4): 505—510. Degtyarev, V.1., Mishurinsky, B.E. and Nadubovich, U.A., 1987. About the character of correlation of geographical distribution of aurora and anomal geomagnetic field. Geomagn. Aeron., 27(3): 428—432. Derr, J.S., 1973. Earthquakes lights: a review of observations and present theories. Bull. Seismol. Soc. Am., 63(6): 2177—2187. Derr, iS. and Persinger, M.A., 1986. Luminous phenomena and earthquakes in southern Washington. Experientia, 42: 991—999. Duhau, S. and Osella, A.M., 1985. Evidence of mutual induction between the ionosphere and the Earth at equatorial latitudes. J. Geophys. Res., A, 90: 4434—4438. Finkeistein, D., Hill, R.D. and Powell, J., 1973. The piezoelectric theory of earthquake lightning. J. Geophys. Res., 78: 992. Fishkova, L.M., Gokhberg, M.B. and Pilipenko, V.A., 1985. Relationship between night airgiow and seismic activity. Ann. Geophys., 3: 689—694. Givishvily, G.V., Ivanov-Kholodnyi, G.S., Kochenova, NA., Kuslmerevskyi, Yu.V., Leschenko, L.N., Migulin, V.V., Nepomnjaschaja, E.V. and Fligel, M.D., 1987. About large-scale anomalous zones of F-region of the ionosphere. Dokl. Akad. Nauk SSSR, 295(6): 1330—1332 (in Russian). Givishvily, G.V., 1986. Quasi-stationary inhomogeneities of ionosphere. In: Ionosphernye issledovaniya, No. 41, pp. 60—69 (in Russian). Gokhberg, MB., Morgunov, V.A. and Aronov, EL., 1979. About high-frequency electromagnetic emission during seismic activity. Dokl. Akad. Nauk SSSR, 248: 1077—1081. Gokhberg, MB., Pilipenko, V.A. and Pokhotelov, O.A., 1983. About seismic precursors in the ionosphere. Izv. Acad. Sci. USSR Phys. Solid Earth (U.S.A.), 19: 762—765.
128 Gokhberg, MB., Gufeld, IL., Gershenson, N.!. and Pilipenko, V.A., 1985. Electromagnetic effects during rupture of the Earth’s crust. Izv. Acad. Sci. USSR Phys. Solid Earth (U.S.A.), 21: 52—63. Gokhberg, M.B., Gufeld, IL., Marenko, V.A., Roznoy, A.A., Ponomarev, E.A. and Yampolskiy, IV., 1987. Investigation of disturbances of natural and artificial electromagnetic fields by a source of seismic origin. !zv. Acad. Sci. USSR Phys. Solid Earth (U.S.A.), 2: 17—24. Gossard, E.E. and Hooke, W.H., 1975. Waves in the Atmosphere. Elsevier, New York. Hays, P.B. and Roble, R.G. 1979. A quasi-static model of global atmospheric electricity. 1. The lower atmosphere. J. Geophys. Res., 84: 3291—3305. Helliwell, R.A., Katsufrakis, J.P. and Trimpi, M.L., 1973. Whistler-induced amplitude perturbation in VLF propagation. J. Geophys. Res., 78: 4679. Herman, J.R. and Goldberg, R.A., 1978. Initiation of nontropical thunderstorms by solar activity. J. Atmos. Ten. Phys., 40: 121—134. Inan, U.S., 1987. Gyroresonant pitch angle scattering by coherent and incoherent whistler mode waves in the magnetosphere. J. Geophys. Res., 92: 127—142. man, VS., Bell, T.F. and Helliwell, R.A., 1978. Nonlinear pitch-angle scattering of energetic electrons by coherent VLF waves in the ionosphere. J. Geophys. Res., 83: 3235—3253. Kondo, G., 1968. The variation of the atmospheric field at the time of earthquake. Mem. Kakioka Mag. Obs., 13: 11—23 (in Japanese). Krakovetzsky, Yu.K., Nadubovich, Y.A., Popov, L.N. and Shumilova, A.B., 1984a. Study of spatial distribution of polar aurora. DokI. Akad. Nauk SSSR, 279(3): 580—582 (in Russian). Krakovetzky, Yu.K., Degtyarov, V.!., Platonov, 0.1., Popov, L.N. and Chagarov, L.M., 1984b. Spatial distribution of aurorae near Norilsk. Geomagn. Aeron., 24: 362—365. Lockner, D.A., Johnston, M.J.S. and Byerlee, J.D., 1983. A mechanism to explain the generation of earthquake lights. Nature (London), 302: 28—33. Millds, M.P., 1986. Meteorological precursors of strong earthquakes. Izv. Acad. Sci. USSR Phys. Solid Earth (U.S.A.), No. 3: 36—47. Nadubovich, U.A., 1967. Coast Effect in Aurora. Nauka, Moscow, 79 pp. (in Russian). Nestorov, G.T., 1979. A possible ionospheric presage of the Vrancha earthquake of March 4, 1977. C. R. Acad. Buig. Sd., 32(4): 443—446. Ogawa, T. and Tanaka, T., 1976. Land effect on the stratospheric vertical electric field and current. J. Atmos. Ten. Phys., 38: 599—604.
Park, C.G. and Dejnakarintra, M., 1975. Penetration of the thundercloud electric fields into the ionosphere and magnetosphere. J. Geophys. Res., 78: 6623—6650. Parkhomenko, El. and Balbatchan, M.Ya., 1981. Triboelectric effects of rocks. Dokl. Akad. Nauk SSSR, 261(2): 325—328 (in Russian). Pierce, E.T., 1976. Atmospheric electricity and earthquake prediction. Geophys. Res. Lett., 3: 185—188. Ponomarev, E.A., 1964. Influence of sea currents on the dynamics of auroras. In: Geo- i Geliofizicheskije Effekty v Kosmitcheskikh Luchakh i Polarnikh Sijanijakh. Nauka, Moscow, pp. 107—112 (in Russian). Samsonov, V.G. and Vasil’eva, V.G., 1986. Local inhomogeneity of distribution of aurora. Geomagn. Aeron., 26(4): 691—694. Samsonov, V.P. and Zaretzky, N.S., 1963. The azimuthal and geographic distribution of aurora revealed by the data for Yakutsk network stations. Geomagn. Aeron., 2: 198—202. Saraev, V.A., Alekhina, N.M., Valaeva, E.A., Duizin, A.L. and Domashenko, V.G., 1974. About relationship of thunderstorm phenomena with geological elements. In: Geologija i Polezniye Iskopaemyje Sibiri. Vol. 1, 143—144, Tomsk (in Russian). Shaftan, B.A., Zlotnikov, M.F., Vinogradov, U.I. and Pigin, E.G., 1986. Discovery of spatial zones of generation of atmospheric inhomogeneities by seismic phenomena., In: Magnetosphernye Issledovaniya, No. 8, pp. 126—129 (in Russian). Shpynev, G.B., Mishin, V.M. and Mishin, E.V., 1977. About some local peculiarities of substorms development and their nature. Issl. Geomagn. Aeron. Fiz. Solntza, 43: 3—13 (in Russian). Shpynev, G.B., Lunyushkin, S.B. and Mishin, V.M., 1987. Coast-effect in ionospheric currents. Issi. Geomagn. Aeron. Fiz. Solntza, 78: 106—116 (in Russian). Sobolev, G.A. and Husamiddmov, S.S., 1985. Pulsed electromagnetic Earth and ionosphere field disturbances accompanying strong earthquakes. Earthquake Pred. Res., 3: 33—45. Tribusch, H., 1978. Do aerosol anomalies precede earthquakes? Nature (London), 276(5688): 606—608. Tzerfas, K.E., 1971. The phenomena of atmospheric electricity preceding earthquakes. In: Tashkentskoje Zemletrjasenije, 1966. FAN, Tashkent (in Russian). Volkova, L.B. and Fedorenko, Yu.K., 1989. Regional dependence of electron concentration of the ionosphere. Kosm. Issl., in press. Vorobijev, A.A., 1970. About possibility of electric discharges in the Earth interior. Geol. Geofiz., No. 12: 3—13 (in Russian).
115
Terrogenic effects in the ionosphere: a review L.N. Popov
1,
Yu.K. Krakovetzkiy
1,
M.B. Gokhberg
2
and V.A. Pilipenko
2
‘Tomsk State University, Tomsk-10, 634010 (U.S.S.R.) of the Physics of the Earth, Moscow, 123810 (USSR.)
2 Institute
(Received December 8, 1987; revision accepted May 3, 1988)
Popov, L.N., Krakovetzkiy, Yu.K., Gokhberg, M.B. and Pilipenko, V.A., 1989. Terrogenic effects in the ionosphere: a review. Phys. Earth Planet. Inter., 57: 115—128. An attempt is made to collect and to analyze critically the existing experimental evidence of terrogenic effects in the ionosphere. By terrogenic effects we mean phenomena which indicate that the ionosphere ‘feels’ the structure and the properties of the underlying surface of the Earth. The review includes discussion of coast effect in aurora and in ionospheric currents, deep faults and aurora, equatorial electrojet and tectonic features, ionospheric concentration and Moho zone, magnetic anomalies and aurora, atmospheric electricity and orographic effect and seismogenic faults and ionospheric anomalies. Special attention is paid to the consideration of experimental data provided by photometric observations at high and middle latitudes.
1. Introduction The ionosphere is a complex dynamic system controlled by many parameters, primarily by solar—geophysical factors (solar activity, magnetic storms and substorms) and additionally by ‘internal’ factors, which have been intensively investigated: (1) acoustic motions of the atmosphere of different scales, the so-called ‘meteorological control of the ionosphere’ (for a comprehensive review see Danilov et al. (1987)); (2) technogenic activity, mainly electromagnetic emission of radio-transmitters, industrial power lines, and so on. Also, rather unusual experimental evidence began to appear recently, which indicates that the ionosphere can ‘feel’ the properties of the particular region of the Earth beneath it. All such effects will be called ‘terrogenic effects in the ionosphere’. Some of the terrogenic effects result from phenomena involving acoustic action on the ionosphere, i.e. a generation of acoustic waves, which
reach the ionosphere, by large-scale mountains (Gossard and Hooke, 1971); and sporadic heating and generation of wave disturbances in the E and F layers after earthquakes and volcanic eruptions (see review by Alperovitch et al., 1985). The physical mechanisms of these phenomena have been well studied in general, and they are fully described in many review papers. However, most of the terrogenic effects are related to electromagnetic interaction in the Earth—atmosphere—ionosphere system. Experimental data on electromagnetic terrogenic effects are very scanty and not firmly proved. All of them still need further thorough verification. The observed effects are small and cannot be extracted in all circumstances in such a complex many-parameter system as the ionosphere. Descriptions of particular terrogenic effects are dispersed over various papers, which are known only to a small group of geophysicists, so it seems useful to give a brief review of the experimental facts and discuss their possible physical implications.
116
2. Mutual induction in the ionosphere—Earth systern The current-carrying E layer of the ionosphere, separated from the highly conducting Earth by the atmospheric gap, which is small compared with typical horizontal dimensions of the ionospheric current system, represent a system of two inductively connected conductors. Naturally, the effects of mutual induction between ionosphere and Earth should manifest themselves in regions with intense ionospheric currents, and most of all at aurora! latitudes. 2.1 Coast effect in aurora A convenient object for investigation of electrodynamic interaction between Earth and ionosphere is an auroral arc. Its dynamics can be monitored by rather simple ground-based photometry. Real terrogenic effects in aurora are known to aurora specialists, and one of the best known is the coast effect, which was reported in 1961 by Soviet geophysicists D. Duma, and later by Nadubovitch, and Starkov (Nadubovich, 1967). A detailed description of earlier observations of coast effect and relevant references were given by Nadubovich (1967). The coast effect is the experimentally observed tendency for homogeneous auroral forms to align along the sea coastline, large rivers, bays, etc. Consequently, auroras are more frequently observed near a sea coast. This effect was statistically justified by independent groups of scientists from analysis of the ascafilms of an all-sky camera, which were obtained during several years of observations. Further research revealed other manifestations of the coast effect: fracture of an aurora! arc at a point of intersection with a coastline, and depression of auroras over islands, banks, etc. The most striking support for the coast effect was gained from the spacecraft Dynamics Explorer 1 (DE-1). Continuous global imaging of polar regions from DE-1 with auroral imaging instrumentation of the University of Iowa provided a large data set of unique information on aurora dynamics. In Fig. 1, we can see the visiblewavelength image at the 630-nm emission line of
intense auroral emissions over the western coast of Norway at 20.45 UT on 20 October 1981. An aurora! arc system is observed to transverse the Atlantic Ocean in the late evening hours of local time and to bifurcate at the Norwegian coastline. The northern branch follows the western coastline closely whereas the southern branch initially follows the southern coastline before proceeding eastward across southern Sweden. The existence of the coast effect in aurora led to the suggestion of the existence of a similar effect in the ionospheric current distribution. Indeed, this effect was found experimentally by Shpynev et al. (1977, 1987) from data of the magnetic station network in the Soviet Arctic. With the help of a specially designed algorithm for extraction of the part due to ionospheric currents from geomagnetic variations, charts of external and internal equivalent currents were constructed. The coast effect of these charts is seen usually as the increase of the current density over the coastline and as the elongation of the current lines alongside it. According to the current system cornputations (Shpynev et a!., 1987) the modulation rate of an original current as a result of the coast effect is > 50%. The coast effect has the highest probability of appearing near local midnight and at moderately disturbed auroral activity (AE 150 nT). The coincidence of morphological features shows that the coast effects in aurora and in ionospheric currents are just the two sides of the same phenomenon. It seems intuitively that the coast effect is the result of the pondermotive interaction between the west auroral electrojet and induced current in a near-coast zone of a sea. This point of view was assumed by early investigators of the coast effect and was then developed by Ponomarev (1964). In our view, the consideration of aurora! arc dynamics in the framework of a model with two currents in vacuum is an oversimplified idealization. An interesting idea was suggested by Shpynev et al. (1977): it may be that some substorms are related to the feedback instability of the magnetosphere—ionosphere current system. In local regions with small anomalies (— 1%) of ionospheric electric and magnetic fields caused by the Earth’s induction effect, the threshold for a substorm onset
117
Fig. 1. Example of geographic alignment of intense auroral emissions along the western coast of Norway (Mercator projection). The image was obtained with one of the two visible-wavelength imaging photometers carried on the NASA—GSFC spacecraft DE-1 as part of the University of Iowa’s auroral imaging instrumentation. The false-color coding presents the broad region of 630-nm emissions in red, with the more intense emissions in orange (reproduced by courtesy of Prof. L.A. Frank).
may be reduced. As a result, in these field tubes the explosive phase of a substorm begins earlier and may hamper the growth of a similar process in neighbouring tubes, so the substorm activity over terrogenic anomalies will be statistically enhanced. Until now, however, no theory has been developed for a physical mechanism of the coast effect. The essential point that this effect definitely demonstrates is that any theory of substorm and auroral arc generation should consider the ionosphere as an active factor in the ionosphere—magnetosphere interaction, 2.2. Deep faults and aurora Another impressive manifestation of an effect similar to the coast effect is the influence of crustal deep structures of the Earth on the formation of the spatial picture of aurora. The problem of relationship between aurora and geological structure was studied by Krakovetzky et al. (1984a) with the use of all-sky camera data. In Fig. 2, taken from that paper, the isochasms of aurora appearance frequency (isoaurora) over the Norilsk
region are shown. In the same figure the system of deep faults of the transition zone from the Siberian platform to the East-Siberian plate is shown by a dashed line. We can see that, when isoauroras approach the fault, they change their normal direction relative to latitude, and after crossing the fault, resume their direction. It looks as if an aurora shows a tendency to align along a fault or even to be ‘expelled’ from a fault. The existence of the geological transition zone, revealed by auroral observations, was firmly proved by deep seismic sounding data. A detailed study by Krakovetzky et a!. (1984b) showed that these ionospheric anomalies are geographically stable and their location does not depend on season or geomagnetic activity. The stable and regular character of the inhomogeneous structure of the aurora’s statistical spatial distribution is an additional argument in favor of the reality of the terrogenic effect. 2.3. Equatorial electrojet and tectonic features The inductive coupling between Earth and the ionosphere should be effective also in another
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region of intense ionospheric currents—the equatonal electrojet. Indeed, Duhau and Osella (1985) succeeded in eliciting the mutual inductive effects at equatorial latitudes. They analyzed the data obtained at three magnetometer chains which crossed the dip equator (Fig. 3a). By separation of geomagnetic daily variation into an external and an internal part, Duhau and Osella (1985) obtamed the latitudinal profile of integrated E region current density. The mean Earth conductivity along geomagnetic chains in Nigeria and central Africa (Fig. 3a) is asymmetric relative to the dip equator—the conductivity of a continental shield is smaller than that of a continental platform. Also, latitudinal profiles of east—west ionospheric
current turned out to be asymmetric (Fig. 3b); the current intensity was anticorrelated with mean Earth conductivity. In the Peruvian chain (Fig. 3a), however, the conductivity inhomogeneity as a result of Cenozoic volcanic zone is almost centered at the dip equator. This is coincident with the fact that the profile of the ionospheric current is almost symmetrical there (Fig. 3b). These correlations between tectonic features and the ionospheric current were also confirmed by direct measurements with VHF radars (Crochet, 1977; Duhau and Osella, 1985). These effects were still not supported by theoretical estimates which should show that the variations of the upper boundary of the highly conducting region of the
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mantle is sufficient to induce the equatorial ionospheric current redistribution. 2.4. Ionospheric concentration and Moho zone Another manifestation of influence of crustal electric properties on the ionosphere was discovered by Volkova and Fedorenko (1989). They elaborated a method which enabled them to obtam a ‘pure’ N( h )-profile excluding the influence of the solar zenith angle. Processing by this method
a large number of ionograms from a middle-latitude network of stations, ranging from 40 to 700 N and 20° to 83°E, revealed that ‘pure’ electron concentration distributions showed clear and stable regional anomalies. In Fig. 4a, b, taken from Volkova and Fedorenko (1989), the meridional and latitudinal profiles of a normalized ‘pure’ electron density, averaged over a 1-month period, at the level h = 110 km are shown. Anomalies in geographical distribution of N, which can be seen in Fig. 4, diminish with height and do not reveal
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regularities of aurora distribution over the Yakutsk region. The reality of this effect was supported by investigations of Krakovetzky et al. (1984b). Figure 5, of Krakovetzky et al. (1984b), illustrates the relationship between the frequency of aurora appearance p over the Nonilsk area and anomalous magnetic field ~Ta. Comparison of two coincident
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themselves at the F layer. It is striking that these anomalies coincide with variations of Moho zone depth. As can be seen in Fig. 4 the N is larger where the Moho border is closer to the Earth’s surface. The discovered dependence is regular and was observed at various levels of solar activity and times of day. It may be that this effect has something in common with the effect, described above, of aurora depression over land. In both effects, possibly, additional raising of the ionosphere occurs over crustal regions with enhanced conductivity. .
3. Influence of regional magnetic anomalies on aurora The influence of the largest world magnetic anomalies (for example, the South-Atlantic anomaly) on the overlying ionosphere is a known and well-understood effect. However, explorations of the fine structure of the auroral oval demonstrated that regional magnetic anomalies also took part in the formation of an aurora’s visible forms. The relationship between spatial distnibution of aurora and local structure of the Earth’s magnetic field was first indicated by Samsonov and Zaretzky (1963), who studied the statistical
profiles along the latitude 69°N in Fig. 5 shows that maxima of isoaurora fell into the regions of reduced geomagnetic field. It therefore follows, from the above investigations, that an auroral arc is ‘expelled’ from a local positive magnetic anomaly. A simple explanation cannot be given for this effect. It is doubtful whether small alterations of the loss-cone over regional magnetic anomalies are sufficient to produce noticeable change of the particle precipitation into an auroral arc. It should be noted that the result obtained in the Nonilsk area was not confirmed in another geological region (Samsonov and Vasil’eva, 1986), possibly because of differences in the geological structure and physical properties of the underlying surface or in the data-processing approaches (Degtyarev et al., 1987). Once again, this situation indicates the complexity of terrogenic effects and of the inadequacy of our knowledge of them.
4. Local distortions of the global electricity circuit We have considered above the intensive current systems of the ionospheric auroral and equatorial electrojets, generated originally by the magnetospheric plasma convection. Now we consider
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cuit (Hays and Roble, 1979) predicted that over the largest mountain systems of the Earth areas iO~ km) with reduced ionospheric potential can emerge. The values of the horizontal electric field in the ionospheric anomalies can reach 1 mVm’. It would be difficult to extract any anomalies of the ionospheric parameters caused by such small electric field, because of the permanent meteorological factors over large mountain systems, such as rising mountain waves. The next problem of atmospheric electricity, which we now deal with, concerns the phenomena in which small change of the atmospheric electric field will result in a substantial deviation in the course of more energetic processes. As an example, we recollect the effect of the modulation of —
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S
~
Fig. 6. Schematic diagram of the global atmospheric electricity circuit. The following regions are marked: 5, storm current generator; R, enhanced output of radioactive emanations; 0, orographic features. Atmospheric current system is indicated by arrows.
another global current system—the atmospheric electricity circuit. In this circuit the near-equatorial centres of thunderstorm activity serve as a current generator. A generated current then spreads over the upper atmosphere and the ionosphere, as is shown schematically in Fig. 6, and closes at middle and high latitudes through the fair weather vertical currents. The thunderstorm generator supports the ionospheric potential at 2 x i05 V and this potential difference produces regular atmospheric electric field 2 x 102 Vm1 near the Earth’s surface. The question arises: can the regional peculiarities of the Earth’s crustal structure induce local distortions in the global electricity circuit? The answer to this question is found to be positive. The best-known effect of terrogemc distortions of the global electricity circuit is an orographic one. This effect is due to the fact that large mountains may substantially reduce a local columnar resistance between the Earth’s surface and the ionosphere. Thus the distribution of atmospheric electric fields and currents over a mountam system will be disturbed, as indicated in Fig. 6. Experiments on balloons in the stratosphere demonstrated the increase of current and electric fields over orographic anomalies (Ogawa and Tanaka, 1976). Large-scale orographic features can induce electric potential distortions, even at ionospheric heights. Detailed numerical calculations of the three-dimensional model of the electricity cir—
—
thunderstorm initiation by solar activity. Solar modulation of cosmic nay intensity results in a change of the ionization rate by 10% only. Nevertheless, even these small changes of the charge density and electric field may be a trigger for thunderstorm initiation when the meteorological conditions are favorable (Herman and Goldberg, 1978). Another terrogenic factor which modulates thunderstorm activity is the selective dependence of lightning probability on the local geological structure of the Earth’s surface (Saraev et al., 1974). Statistical analysis showed that the surface areas with more complex geological structure, with enhanced quartz in rocks, etc., were more frequently under lightning strikes than others. —
5. Influence of seismoactive faults on the ionosphere. Earlier we discussed the effects of the influence of deep geological faults and sharp inhomogeneities of the Earth’s crust on the dynamics of ionospheric processes. For seismoactive faults, however, the variety of potentially possible mechanisms to influence the ionosphere is considerably wider. As we confined ourselves in this review to consideration of only electromagnetic phenomena in the Earth—atmosphere—ionosphere system, we shall not discuss disturbances in the ionosphere as a result of acoustic impact of seismic shocks (a
122
detailed bibliography on this subject can be found in Alperovitch et al. (1985)). We shall concern ourselves with electromagnetic phenomena which arise at the final stage of earthquake preparation, several hours or days before a shock, and with their influence on the properties of the overlying ionosphere. In our view, seismo-ionospheric interactions as well as other terrogenic phenomena have a fairly weak impact on the ionosphere. On the other hand, a noticeable manifestation of such effects results from a terrogenic effect on the rate of energy dissipation and on the redistribution of energy in the course of considerably more powerful natural ionosphere—atmospheric processes.
5.1. Experimental observations Most studies devoted to a search for ionospheric effects produced by seismic activity before earthquakes were based on a hypothesis concerning the emergence of intense large-scale electrostatic fields in the preparation zone. Indeed, some fragmentary information on atmospheric electric field disturbances in seismoactive areas before weak shocks can be found in earlier papers (Kondo, 1968; Tzerfas, 1971). Moreover, the emergence of seismogenic intense electric fields were indirectly indicated by observation of phenomena such as near-surface glow of the air before an earthquake, the spontaneous switching on of luminescent lamps and breakdown of cables (Tzerfas, 1971; Derr, 1973). . Analyses of numerous reports of natural luminous events in various regions led Derr and Persinger (1986) to the conclusion that one class of luminous phenomena (mostly nocturnal lights) may result from small rock fractures as tectonic strain accumulates over a large region, whereas another class of earthquake lights may result from the release of that strain by fault breakage. It follows therefore that, in addition to hypothetical electric fields which appear several hours or days before an earthquake itself, an electric field impulse, with intensity sufficient to induce light effects, may be produced when the main rupture occurs (Lockner et al., 1983). It should be taken
into account that under favorable meteorological conditions (e.g. overcooled water vapor, aerosols, mist, drizzle, etc., in the atmosphere), electric fields of considerably lower intensity than 3 x 106 Vm1 (critical breakdown voltage under normal conditions) may ignite the crown discharge (Tnibusch, 1978). The impulse of an electric field at the moment of a shock may produce a short synchronous disturbance of the ionosphere. However, direct experimental evidence which could convincingly prove the emergence at that moment of intense electric fields has not yet been obtained. As regular direct observations of electrostatic fields are extremely rare, efforts were made to collect information on the existence of such fields with the help of data obtained by vertical ionosphere sounding stations. Many publications have reported successful efforts to discover ionospheric anomalies preceding earthquakes. Some effects were identified by Sobolev and Husamiddinov (1985), and by Datchenko et al. (1972) in the course of a retrospective analysis of vertical ionospheric sounding station data in Central Asia. These authors discovered an anomalous increase (up to 20%) in the critical frequency f 0F2 of the night-time ionosphere at the periods of nearby seismicity. The plasma density in the F layer began to increase, on average, several days before a shock, and an earthquake itself occurred while the anomaly was waning. Unusual reflections and traces on ionograms which appeared before the earthquake were also seen (Sobolev and Husamiddinov, 1985). The results, presented in a number of articles in this issue, also confirm the existence of seismo-ionospheric precursors. Inhomogenities of E and D layers over the epicenters of future earthquakes were also recorded on the basis of disruptions in propagation conditions for MF and VLF radiowaves. Nestorov (1979) observed unusual fading of MF radiotraces 1 h before the Vrancha earthquake. De and Sarkar (1984) also recorded short-period fadings at long many-hop paths = 40 kHz), accompanying a large number of earthquakes. This fading often began many hours before an earthquake and lasted for hours after it. Gokhberg et al. (1987) reported the extensive statistics of clear phase anomalies at VLF radiotraces from the ‘Omega’ navigation sys(f
123
600
20-210182
20
22
0
2
21-22. 01.82
4
18
20
22
0
2
22-23.01.82
4
18
20
it
0
2
/
4r”S\ 23-24.01.82
4
6
22
0
2
4
24 -23 01.82r
6
18
20
22
0
2
4
Fig. 7. Variations of night airglow intensity (green oxygen line) over the Trans-Caucasus fault, recorded at Abastumam observatory. Arrow marks the moment of seismic shock.
tern, which lasted — 1 h and preceded near-path earthquakes at night. To investigate seismo-ionospheric effects, Fishkova et al. (1985) proposed that use should be made of information obtained from photometric observations of night airglow. The analysis of data accumulated over many years of observations at the Abastumani observatory showed a number of anomalous increases in emission intensity of the green oxygen line (557.7 nm) several hours before the nearby earthquakes of the Trans-Caucauses fault. An example of such events taken from Fishkova et al. (1985) is shown in Fig. 7. The intensity of green oxygen emission characterizes the dynamics of the ionized component of the ionosphere in the E layer. It is interesting to note that for the red oxygen line (630.0 nm), which has a photometric center at the altitudes of the F layer, increased emission intensity was not observed. 5.2 Possible physical mechanisms of ionospheric anomaly formation The above facts gave an impetus to studies of various aspects of quasistationary electric fields of seismic origin. Various hypotheses were developed on the physical mechanism of generation of these fields. It was assumed that disturbances of electric fields in the near-Earth atmosphere may result from charge separation at dielectric grains of the crust (Vorobijev, 1970), piezoelectrical phenomena (Finkelstein et al., 1973), triboelectrical phenomena (Parkhomenko and Balbachyan, 1981), the generation of large-scale current systems in the hypocenter as a result of coherent action of mech-
anoelectrical transformers (Gokhberg et al., 1985), or enhanced output of radioactive emanations (Pierce, 1976). In most experimental studies, the authors inexplicitly proceeded from the assumption that as typical dimensions of the earthquake preparatory zone are comparable with or larger to the height of the lower ionosphere, large-scale seismic electric fields can penetrate the ionosphere and bring about the emergence of the observable anomalies. That assumption can be substantiated by simple analytical estimates. Let us suppose that an anomalous electric field E~°(f)has emerged on the Earth’s surface in the region with finite radius i~. If the typical frequency f of a non-stationary field E0° is such that f 1 kHz, then the region between the Earth’s surface and the ionosphere proves to be a near-field zone of the source on the Earth’s surface. In this case, electric fields can be calculated by means of quasistatics equations. The solution to the similar problem of penetration into the ionosphere of the field of a point charge above the Earth (modelling a thunderstorm cloud) was obtained by Park and Dejnakarintra (1975). Their calculations need to be modified for application to the case of a source on the Earth’s surface of finite size. The following properties of electric field attenuation with height h can then be established (Gokhberg et al., 1985). At low altitude, where the atmospheric conductivity is low, the field decreases rather slowly by the law E(h) (h/r0)~ (n = 0—2). As height increases the conductivity is enhanced, by the exponential law ~ (h) a~exp(h/a). At altitudes where conductivity currents become larger than ‘~
=
124
acts as a physical agent of seismo-ionospheric relationships. Let us consider some mechanisms of possible influence on the ionospheric plasma of electric fields of seismic origin. If we suppose that an
6 5
10km 2
“
50 km
,
1
o 0
10_i
0
1
10
f (Hz)
Fig. 8. Plot of attenuation factor 8 = 1og(E~/E°) dependence on frequency f for two radii r 0 of a region with anomalous near-Earth electric field. Typical parameters of conductivity scale a and near-Earth conductivity a~were chosen.
the displacement currents, the electric field becomes severely (exponentially) attenuated. And finally, beginning from altitudes of — 70 km, where the anisotropy of ionospheric conductivity becomes important, the transverse component of the electric field E1 is shifted with no attenuation along geomagnetic field lines, and the longitudinal component (alongside the geomagnetic field) decreases rapidly. of This E reflects the equipotentiality the behavior field linesof in the ionosphere for large-scale disturbances. Estimates of the attenuation of E between the Earth’s surface and the lower ionosphere, based on the above considerations, are summarized in Fig. 8. The value of the resultant attenuation 8 = log(Eff/E~?) for various frequencies f was estimated for optimal conditions—night-time ionosphere at a point directly above the extended anomaly, i.e. r 0 >> (r, a). In the day-time ionosphere the field’s attenuation is greater by 1—1.5 orders of magnitude. The rough estimates given in Fig. 8 show that to produce a disturbed field, say E1 10 mVm in the ionosphere it is necessary to have near the Earth’s surface a large-scale stationary anomalous electric field E0° 3 x l0~Vm which is larger by only an order of magnitude than the normal value of the atmospheric potential gradient. For non-stationary disturbances the attenuation factor in accordance with Fig. 8 is even smaller. The above simple estimates thus prove the validity of the hypothesis that the quasistationary electric field ~,
~,
anomalous electric field has a constant component, then the ionospheric layers will undergo, under the influence of such a field, a regular vertical drift with velocity W. We shall confine ourselves to the solution of a one-dimensional equation of the ionization balance in a night midlatitude F layer, in which the vertical electric drift is taken into account as a small parameter (although that equation has been examined elsewhere .
.
in detail by numerical methods). In this case, simple estimates can be obtained for the distortion of general parameters of the F layer under the influence of an applied electric field: the height hm of maximum F2 layer T —~
—~
(la)
a density A~of the ionized component ~N, T/3/TW 1~2T /T h >
—
tion in the ionosphere is as follows. Diffusional flow of plasma from the plasmasphere into the night-time ionosphere becomes slower when moving downward. As a result, the density N~increases. At low altitudes, however, the plasma density rapidly diminishes because of recombination. As a consequence, the concentration maximum2, ofwhere the F alayer is formed at a certainbalance height diffusion—recombination h mF T~) takes place. The influence of electric (TD fields on the plasma density is typical of the physics of terrogenic effects. The applied electric field does not produce additional ionization of the ionosphere but merely shifts the ionospheric layers
125
slightly and redistributes concentration with altitude. As is evident from eqns. (1), hm increases under the influence of the upward drift. At the same time N1 increases at the maximum of the
However, the typical size of microcracks is many orders of magnitude less than the length of VLF radiowaves, so it is hardly to be expected that
layer and above and decreases below it. To produce anomalies that can be reliably registered (z~ N1/N1 10%; Sobolev and Husamiddinov, 1985; Gokhberg et a!., 1983) it is sufficient, in accordance with eqn. (1), to have an electric field — 1 mVm’. Thus, the parameters of the night F layer may serve as sensitive indicators of rather weak electric fields. In view of the fact that a permanent component of a near-Earth electric field penetrates to ionospheric altitudes with great attenuation, as it is evident from Fig. 8, it is worthwhile to consider the possibility of non-stationary disturbances influencing the ionosphere. The impact of a low-frequency electric field on the ionosphere plasma will result in an ion Joule heating. The rate of increase in ion temperature T1 can be estimated on the basis of the equations of dynamics and heat balance for ions
microcracks may be an effective source of EME and generate considerable signals at great distances in a far-zone. Nevertheless, there are events where seismogenic EME in the VLF range was recorded at distances up to l0~ km from the epicentre (Gokhberg et a!., 1979; Sobolev and Husamiddinov, 1985), so it is reasonable to assume that the emission from a seismic source will be noticeable at ionospheric altitudes as well. The power of a seismic emission is clearly insufficient to serve as a direct source of any substantial disturbance in the ionosphere. None the less, there exists a possibility in principle which makes it feasible for a low-power ground-based radioemitter (0.1—1 kW) to produce a noticeable disturbance in the ionosphere. This possibility is implemented through the resonant interaction between EME and energetic trapped electrons of the inner magnetosphere and is revealed as a stimu-
L~T
lated precipitation (Helliwell et al., 1973). of particles, An additional the ‘trimpi flux of effect’ en-
—
f ~Ej/Epj~p \2
The value of E~1,the ‘ion plasma field’, is determined by 3T M = .L (~o~ + e where w~is an ion gyrofrequency, p1 is collision frequency, 1~is temperature of neutrals, M1 and are ion and neutral masses. According to estimates, the value of ~ is minimal for E layer of the night-time ionosphere 50 mVm ‘. However, for a noticeable contribution from ion heating to the heat balance of the night-time ionosphere (say aT~/at 10 deg h l) it is necessary to have electric fields with intensity no less than several hundred mVm ~. __!~
.
—
5.3. Seismic trimpi effect Now we shall discuss impulsive electromagnetic emission (EME) as another possible agent of seismo-ionospheric relationships. The sources of this emission are the processes of shifting and cracking which take place alongside active faults and which are intensified before earthquakes.
ergetic precipitated electrons may produce an additiona! ionization of D and E layers, stimulate increase of airglow intensity in the lower ionosphere, etc. Numerical estimates (Inan et al., 1978) show that the energy density of precipitated partides may exceed by several orders the energy density in a wave. It is essential that the effectiveness of pitch-angle scattering by both coherent and non-coherent emission is practically the same (man, 1987). Thus, in this mechanism, in the spirit of the physics of terrogenic effects, the energy of fast particles accumulated in the radiation belt can be partly released under the influence of seismic radio emission. It is possible that a ‘seismic Trimpi effect’ is partly responsible for enhancement of airgiow intensity over a seismogenic fault (Fishkova et al., 1985) and for phase disturbances at VLF radiopaths passing over seismoactive regions (Gokhberg et al., 1987). 5.4. Quasi-orographic seismic effects The process, which is similar to the orographic effect in atmospheric electricity, may be an ad-
126
ditional mechanism of seismo-atmospheric relationships. Enhanced emanations of radioactive elements along faults, which become more intense before earthquakes, will lead to the decrease of an atmospheric near-surface potential gradient (Kondo, 1969; Pierce, 1976). Moreover, the reduced columnar resistance between the ionosphere and the Earth’s surface results, in a similar way to the orographic effect, in the increase of the fair weather vertical current and electric field at higher atmospheric layers. In a qualitative way,. this phenomenon is shown in Fig. 6. In accordance with the experimental facts on solar-thunderstorm relations mentioned in section 4, we may expect that even a small change of the atmospheric electric field may influence meteorological processes (creation of cloud, thunderstorm generation, etc.) in favorable conditions. These processes may then be responsible for the more substantial distortion of atmospheric electric field before an earthquake, and even for its reversal, as was observed by Tzerfas (1971). Moreover, it has been suggested that seismic processes may have an effect even on weather. Milkis (1986) analysed a long-term set of data from a wide network of meteorological stations in Middle Asia. Before strong earthquakes a gradual trend to atmospheric warming, and decrease of humidity and precipitation in comparison with normal values was observed. A typical feature of the periods preceding earthquakes was also the lower number of overcast days and reduced cloudness. Meteorological anomalies gave local spatial zones, which extended along deep faults. Besides a hypothetical ‘quasi-orographic effect’ caused by enhanced radioactive emanations, the additional heating along seismogenic faults as a result of crustal block movement may influence meteorological processes (Milkis, 1986).
6. Geographically stable anomalies in lower atmosphere and ionosphere Physical interpretation of the above-described terrogenic effects in the ionosphere is based on guesswork, rather than on a real theory. However, there are some indications of terrogenic anomalies
in the ionosphere and lower atmosphere for which there is no physical explanation. Other interesting facts are therefore also awaiting interpretation; for instance, it was noticed by Shaftan et al. (1986) that radioaurora at middle latitudes are spatially correlated with projections on the ionosphere of the borders of tectonic features. We should like to mention that there are mdications of geographically stable ionospheric anomalies, which may result from terrogenic effects but for which we cannot find unambiguous relationships with any peculiarities of properties of the Earth’s crust. Balev et al. (1976) discovered during ‘Cosmos-469’ observations at 260 km altitude local zones (102_103 km) of ion concentration diminished by one or two orders in the night ionosphere at low latitudes. These quasi-stationary holes of the night F layer were found to coincide with local regions of enhanced ionization in the D layer (Givishvily, 1986). The anomalies in the night ionosphere were revealed for the same geographical zones at different heights even over long timeintervals (up to 9 years) (Balev et al., 1976; Givishvily, 1986; Givishvily et al., 1987). These facts are awaiting not only interpretation, but also experimental verification.
7. Conclusion The main purpose of this report is to draw attention to a new class of geophysical phenomena, in many aspects still scarcely studied, namely terrogenic effects in the ionosphere. None of these effects yet have reliable physical interpretation, and their investigation may be of interest from various points of view. First, these effects represent a striking reflection of the fact that different geophysical environments—the Earth’s crust, atmosphere, ionosphere and magnetosphere—are in fact parts of a single system interacting with one another. Therefore an isolated examination of processes in one of the environments alone proves to be insufficient. Second, as we have shown, terrogenic phenomena also include seismo-ionospheric interactions, so their study by modern geophysical and radiophysical techniques, including satellite observa-
127
lions, will enable an understanding of the nature of seismo-electromagnetic phenomena at the final stage of earthquake preparation. The prospects for obtaining, by ionospheric methods, information on the existence of active faults, deep geological mnhomogeneities, increasing seismic activity, etc., are very attractive, and at the same time very realistic, Finally, it should be noted that, according to estimates, the direct influence on the ionosphere of the factors which bring about the manifestation of terrogenic effects is not strong. Nevertheless, the existence of the effects observed is indicative of the fact that even a slight outside influence can affect, under certain conditions, the dynamics of ionospheric processes. A profound understanding of the physical mechanisms of terrogenic influence on the ionosphere may help in the development of methods of active modification of ionospheric processes. Although, in this review, apparently quite different geophysical phenomena are collected, we think that for their investigation the development of the new common approach is needed. We imagine that terrogenic effects are mainly the manifestation of various aspects of the general physical situation—a small external influence on a system which is near the threshold of its internal instability. It seems to us, that this is a way to understand the physical mechanism of the appearance of terrogenic effects in the ionosphere. Because particular conditions, which are often beyond the control of an observer, are required for the manifestation of terrogenic effects, these effects cannot be monitored in every case, everywhere or in identical form. Standard statistical analysis therefore cannot be applied to study the reality of these effects.
Acknowledgements Helpful discussions with V. Vasil’eva, K. Davies, N. Gershenzon are gratefully acknowledged. We also thank L. Frank for kind permission to publish the DE-1 picture, and the referee for useful comments.
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Park, C.G. and Dejnakarintra, M., 1975. Penetration of the thundercloud electric fields into the ionosphere and magnetosphere. J. Geophys. Res., 78: 6623—6650. Parkhomenko, El. and Balbatchan, M.Ya., 1981. Triboelectric effects of rocks. Dokl. Akad. Nauk SSSR, 261(2): 325—328 (in Russian). Pierce, E.T., 1976. Atmospheric electricity and earthquake prediction. Geophys. Res. Lett., 3: 185—188. Ponomarev, E.A., 1964. Influence of sea currents on the dynamics of auroras. In: Geo- i Geliofizicheskije Effekty v Kosmitcheskikh Luchakh i Polarnikh Sijanijakh. Nauka, Moscow, pp. 107—112 (in Russian). Samsonov, V.G. and Vasil’eva, V.G., 1986. Local inhomogeneity of distribution of aurora. Geomagn. Aeron., 26(4): 691—694. Samsonov, V.P. and Zaretzky, N.S., 1963. The azimuthal and geographic distribution of aurora revealed by the data for Yakutsk network stations. Geomagn. Aeron., 2: 198—202. Saraev, V.A., Alekhina, N.M., Valaeva, E.A., Duizin, A.L. and Domashenko, V.G., 1974. About relationship of thunderstorm phenomena with geological elements. In: Geologija i Polezniye Iskopaemyje Sibiri. Vol. 1, 143—144, Tomsk (in Russian). Shaftan, B.A., Zlotnikov, M.F., Vinogradov, U.I. and Pigin, E.G., 1986. Discovery of spatial zones of generation of atmospheric inhomogeneities by seismic phenomena., In: Magnetosphernye Issledovaniya, No. 8, pp. 126—129 (in Russian). Shpynev, G.B., Mishin, V.M. and Mishin, E.V., 1977. About some local peculiarities of substorms development and their nature. Issl. Geomagn. Aeron. Fiz. Solntza, 43: 3—13 (in Russian). Shpynev, G.B., Lunyushkin, S.B. and Mishin, V.M., 1987. Coast-effect in ionospheric currents. Issi. Geomagn. Aeron. Fiz. Solntza, 78: 106—116 (in Russian). Sobolev, G.A. and Husamiddmov, S.S., 1985. Pulsed electromagnetic Earth and ionosphere field disturbances accompanying strong earthquakes. Earthquake Pred. Res., 3: 33—45. Tribusch, H., 1978. Do aerosol anomalies precede earthquakes? Nature (London), 276(5688): 606—608. Tzerfas, K.E., 1971. The phenomena of atmospheric electricity preceding earthquakes. In: Tashkentskoje Zemletrjasenije, 1966. FAN, Tashkent (in Russian). Volkova, L.B. and Fedorenko, Yu.K., 1989. Regional dependence of electron concentration of the ionosphere. Kosm. Issl., in press. Vorobijev, A.A., 1970. About possibility of electric discharges in the Earth interior. Geol. Geofiz., No. 12: 3—13 (in Russian).