Showing papers in "Journal of geomagnetism and geoelectricity in 1995"

Ionospheric Electrodynamics Using Magnetic Apex Coordinates.

Abstract: The equations of ionospheric electrodynamics are developed for a geomagnetic field of general configuration, with specific application to coordinate systems based on Magnetic Apex Coordinates. Two related coordinate systems are proposed: Modified Apex Coordinates, appropriate for calculations involving electric fields and magnetic-field-aligned currents; and Quasi-Dipole Coordinates, appropriate for calculations involving height-integrated ionospheric currents. Distortions of the geomagnetic field from a dipole cause modifications to the equations of electrodynamics, with distortion factors exceeding 50% at some geographical locations. Under the assumption of equipotential geomagnetic-field lines, it is shown how the field-line-integrated electrodynamic equations can be expressed in two dimensions in magnetic latitude and longitude, and how the height-integrated and field-aligned current densities can be calculated. Expressions are derived for the simplified calculation of magnetic perturbations above and below the ionosphere associated with the three-dimensional current system. It is shown how the base vectors for the Modified Apex coordinate system can be applied to map electric fields, plasma-drift velocities, magnetic perturbations, and Poynting fluxes along the geomagnetic field to other altitudes, automatically taking into account changes in magnitude and direction of these vector quantities along the field line. Similarly, it is shown how Quasi-Dipole coordinates are useful for expressing horizontal ionospheric currents, equivalent currents, and ground-level magnetic perturbations. A computer code is made available for efficient calculation of the various coordinates, base vectors, and related quantities described in this article.

Mapping the Gauss Coefficients to the Pole and the Models of Paleosecular Variation

Abstract: The present magnetic field is usually described by models such as the IGRF which are described by Gauss coefficients obtained by spherical harmonic analysis of the geomagnetic field. The use of Gauss coefficients in paleosecular variation (PSV) studies will therefore make it possible to compare field structures obtained through paleomagnetism with those observed directly. For making such comparisons, it is important to understand how the magnetic field maps into geomagnetic poles and what are their statistical properties, because geomagnetic poles are frequently used in PSV studies. We show how Gauss coefficients map into pole positions, and we develop statistical tools to treat the angular scatter of magnetic poles calculated from models based on Gauss coefficients. We use these methods to evaluate the models of Constable and Parker (1988) and McFadden et al. (1988) which are typical of PSV models using Gauss coefficients. Our results suggests the importance of the (l = 2, m = 1) harmonic in the dynamo process of the earth.

Archaeomagnetism: An Error Assessment of Fired Material Observations in the British Directional Database

Abstract: The direction of the geomagnetic field in Britain is now moderately well established for the last 2, 000 year based on analyses of the directions of magnetic remanence isolated at some 466 archaeological sites in Britain and parts of N.E. France. The vast majority of these observations are of fired, in situ archaeological materials, with only 21 site observations being based on sediments. Most of these findings are only available in virtually inaccessible field reports and theses, or similar such publications, so the summary mean site British values have been placed into a database; this has then been extended to include may directional observations on a global basis. These data are now available as Dbase and Access databases, as well as in ASCII form. It is hoped that such data will enable more detailed secular variation studies to be undertaken and encourage other workers to add their observations to this database, particularly those that may otherwise be observed scatter with that expected from archaeomagnetic, geomagnetic and archaeological site sources suggest that the precision of the mean direction is only a partial estimate of the total error, evaluated as being of the order of ±5° for any given site that may have taken place after the last heating. There is also clear evidence for invalid age assignments in some of the published data but increasing archaeomagnetic data are now enabling such errors to be re-evaluated and the technique is thus improving as more data accumulate.

Observations of Solar Hα Filament Disappearances with a New Solar Flare-Monitoring-Telescope at Hida Observatory

Abstract: The outine of a new solar Flare-Monitoring-Telescope is given, which was built at the Hida Observatory of Faculty of Science, Kyoto University, in the first financial year of the Japan's STEP project. The telescope is unique among currently-operating solar flare-patrol-telescopes, because it can simultaneously observe five solar images, which are four full disc images in Hα center, Hα + 0.8 A, Hα - 0.8 A and continuum, and one solar limb image in Hα center. After its start of operation it was confirmed that this telescope system is very useful to study the dynamical features of Hα filament disappearance, because it can, for the first time, continuously record the full solar disc simultaneously in three wavelengths of the Hα line. Two typical examples of Hα filament disappearance observed with the telescope are demonstrated and are comparatively studied. It is found that the more dynamical and more energetic Hα filament eruption causes the larger enhancement of soft X-ray bright arcade structure observed with the Soft X-ray Telscope of Yohkoh.

Magnetospheric Cavity Resonance Oscillations with Energy Flow across the Magnetopause.

Abstract: We discuss the condition for the presence of a magnetospheric global oscillation having the properties of a cavity resonance oscillation by using a numerical method and assuming that the magnetopause is not a perfect reflector. The cavity resonance oscillation occurs when there is a plasmaspheric trough of the Alfven speed. The plasmaspheric cavity resonance oscillation is evanescent in the outer magnetosphere, where the Alfven speed is larger than in the plasmasphere. The plasmaspheric mode is not completely confined within the plasmasphere, but can tunnel into the outer magnetosphere and have finite amplitude at the magnetopause. The nonzero electromagnetic field perturbation at the magnetopause leads to leakage of the Poynting flux across the magnetopause, and the plasmaspheric cavity resonance oscillation is damped by the escaping flux. The perturbations of the damped cavity resonance oscillation exhibit phase shift toward the magnetopause, which does not imply propagation of the wave.

Multi-Station System for Solar Wind Observations Using the Interplanetary Scintillation Method

Abstract: We describe a multi-station system dedicated to measurements of the solar wind plasma using the interplanetary scintillation technique. The observation system, which is operated at a frequency of 327 MHz, consists of four stations located at Toyokawa, Fuji, Sugadaira and Kiso. The newest station was constructed at Kiso in 1993 to make a four-station system. The present system, however, has insufficient sensitivity to measure enough IPS sources for observing the solar wind with good spatial and temporal resolution. As a result, for example, one synoptic map of solar wind speeds requires the superposition of data for three solar rotations. Therefore we have begun the project to upgrade the antenna system at each station. To start with, the Fuji antenna has successfully been improved with an optimum design of the primary feed system and the development of a low noise preamplifier. The Fuji antenna system after the improvement achieved the minimum detectable flux density of 0.3 Jy (bandwidth: 10 MHz, time constant: 100 ms) which is nearly five times better than the previous system. This upgrade project is expected to be fruitful during the coming decade for solar wind study.

Paleosecular Variation of Direction and Intensity from Two Pliocene-Pleistocene Lava Sections in Southwestern Iceland

Abstract: Two lava sections in southwestern Iceland were revisited ; sectoin IG of Kristjansson et al. (1988) of Brunhes chron age and section K of Wilson et al. (1972) of early Matuyama chron age. Paleodirections obtained from 22 lavas at the former section do not seem to have averaged out the secular variation completely because of their small angular standard deviation, and this suggests rapid accumulation of the lavas. The latter section consists of 29 lavas and contains the Gauss to Matuyama transition near its base. The time interval during which the lavas at this section were erupted also seems to be short, judging from the small directional dispersion. Thelliers' paleointensity method was also applied to the samples from this section. A characteristic low paleointensity was obtained from the transitional lava flow. However, low paleointensities were also obtained from the lavas which erupted some time after the transition, and which record directions close to those of an axial geomagnetic dipole.

Secular Variation of the Earth's Magnetic Field

Abstract: There have been many models of the secular variation (SV) of the Earth's magnetic field proposed over the past few decades. Most if not all of them are models of Earth's magnetic field during so-called normal times, in other words, times during which the field is not reversing or undergoing an excursion. However, in studying data from Icelandic lava flows and from other areas, it has become obvious that there is no real distinction between so-called normal SV and the larger deviations of the Earth's field which are classified as excursions and/or reversals, except by the magnitude of the deviation, in which there is a continuous variation. Recently, reversal patterns have been studied in more detail, and it has been found that the exact start or end of a reversal is frequently only fuzzily defined, there being precursors to reversals, or events following reversals, which cannot be described either as belonging to the reversal per se or being a part of the normal SV pattern. In this paper, information from spherical harmonic models of the field during the past three centuries will be used to show that the field today has an abnormally large latitudinal variation of virtual geomagnetic pole (VGP) angular standard deviation, and should not be used to constrain SV models. Data for lava flows and other rock types from a number of different areas will be used to determine the scatter of the field at various observation latitudes, using VGPs from all latitudes, so that eventually a SV model may be produced which does not reject low latitude VGP information. Other factors will be described which any SV model should be able to produce.

The Mechanisms of Damping of Geomagnetic Pulsations

Abstract: The physical mechanisms that might determine the damping rates and resonance widths of Pc3-5 pulsations are considered. To estimate the role of ionospheric dissipation, a numerical model, based on a calculation of the complex eigenfrequencies of Alfven field line oscillations within the approximation of a “thin” ionosphere and for a realistic ionosphere has been used. At middle latitudes the Joule dissipation in the ionosphere may account for the experimentally determined width of the resonance peak of the fundamental Alfven mode. The meridional spatial structure of higher harmonics is determined by nondissipative mechanisms. The calculations predict a rapid decrease of the Q factor of the magnetospheric Alfven resonator at L < 1.5. The effect is related to the violation of the thin ionosphere approximation at low latitudes. Experimental support for the calculations was found while analyzing the meridional distribution of the damping rates of Psc3 pulsations observed along the 210° magnetic meridian network.

Minimum Norm Inversion of Magnetic Anomalies with Application to Aeromagnetic Data in the Tanna Area, Central Japan.

Abstract: An improved method of magnetization intensity mapping is proposed to interpret magnetic anomalies in terms of subsurface structure, where the side effect of source bodies surrounding the observational area is taken into account. The problem, which usually becomes underdetermined, can be solved iteratively by the conjugate gradient method. The asymptotic iteration gives the minimum Euclidean norm solution, to be suitable for subsurface modeling. This scheme was tested against synthetic data to evaluate its performance and applied to actual aeromagnetic data. From the analysis of aeromagnetic anomalies in the Tama area, a model of the distribution of magnetization intensity was derived, which revealed good correlation with active faults. Lower magnetization intensities appeared to the west of the southernpart and to the east of the northernpart of the Tanna fault, and in a narrow zone between the Ukihashi central and west faults. They were well correlated to highly fractured zones, and the quadrant pattern around the Tanna fault was interpreted as the effect of the compressional stress field brought about by strike-slip faulting.

Radar Observations of Magnetosphere-Ionosphere Coupling at Mid and High Latitudes

Abstract: Radar observations from Millstone Hill have identified many interesting phenomena associated with magnetosphere-ionosphere coupling processes at the equatorward edge of the auroral region. A number of important magnetospheric boundaries are found near the latitude of the radar facility (55° magnetic) and these result in the structure and dynamics which characterize the pre-midnight sub-auroral ionosphere. The equatorward extent of the plasma sheet particle population lies on field lines near the plasmapause and precipitation from the plasma sheet alters the ionospheric conductances, currents and fields. During disturbed conditions, an intense (>100 mV/m) polarization electric field can be setup when freshly-injected plasma sheet ions lie equatorward ofthe electrons and this drives the latitudinally-narrow polarization jet or Sub-Auroral Ion Drifts (SAID) which directly or indirectly result in a variety of ionospheric phenomena in the pre-midnight sector. Associated with the region of strongest convection, a deep, narrow F region trough forms while, equatorward of this, sunward advection of plasma from later local times and lower latitudes leads to the region of Storm-Enhanced Density (SED). The pre-midnight polarizationjet produces strong frictional heating near the Fpeak resulting in an expansion of the topside and heavy ion outflow which populates the magnetosphere with ionospheric 0'. Stable Auroral Red arcs (SAR arcs) occur within the narrow trough, coincident with the polarization jet electric fields and the elevated electron temperatures found there.

Paleomagnetism of Young New Zealand Basalts and Longitudinal Distribution of Paleosecular Variation.

Abstract: A paleosecular variation (PSV) study was carried out on monogenic basalt volcanoes of the northern part of North Island, New Zealand. A new method of calculating angular standard deviation (ASD) by fitting a combination of two Fisher distributions to the cumulative histogram of VGP data is presented, which avoids the necessity of applying an arbitrary cut-off angle to exclude intermediate magnetic directions. The resultant ASD of 11.5° for northern New Zealand is smaller than that predicted by previous PSV models. Using this new method, recalculation of published data from elsewhere spanning the last 10Ma suggests that the ASD commonly has been over-estimated and that the global distribution of ASD exhibits a significant longitudinal variation. This relatively long-term non-axisymmetric distribution of ASD implies that the core dynamo process may be strongly influenced by the thermal and/or mechanical regime of the solid mantle.

Auroral Observations in Finland-Visual Sightings during the 18th and 19th Centuries

Abstract: Systematic naked eye sightings of northern lights started in Finland in 1748. Such observations were carried out for about 100 years at two places in Southern Finland. All observations were compiled and published by Hallstrom (1847). Auroral occurrence frequencies in this compilation obey well the semiannual variation and the 11-year sunspot cycle when compared with other coeval auroral and magnetic data available in Europe. In the middle of the 100-year auroral series there was an anomalous period of very low auroral occurrence, known as Dalton's solar activity minimum, lasting about three decades from 1795 to 1825. As a signature of Dalton's minimum, auroras were totally absent from the sky in Southern Finland for the 10 years 1806-1815.

Paleointensity Determinations with Measurements at High Temperature

Abstract: Paleointensity determinations following the principle of the Konigsberger Thellier Thellier (KTT) method, but with measurement at high temperatures have been carried out. The high temperature measurements were made with a continuous thermal demagnetization system, which operates as a modification of a 2G horizontal access cryogenic magnetometer. The high temperature measurements permit comparison of the NRM lost and TRM acquired within temperature stages incrementally, rather than in the cumulative manner of the KTT method. When this approach was applied to a Hawaii 1960 lava erupted in a known geomagnetic field intensity, the NRM/TRM ratio for one specific temperature range of 400-450° C gave anomalously high values, while the expected NRM/TRM ratios were obtained from all other temperature ranges. This anomalous behavior is related to the destruction of a phase carrying NRM in this temperature range. The advavantage of making measurements at high temperature is that it permits independent determinations of the incremental ratio of NRM lost/TRM gained. This ability to isolate anomalous temperature ranges may permit reliable intensity determinations in materials, such as lunar rocks and meteorites, whose complicated behaviour on heating has precluded intensity determinations in the past.

A Review of MU Radar Observations of the Thermosphere and Ionosphere

Abstract: The MU radar is situated in a longitude sector very different from those of the American and European incoherent scatter radars. Its location, together with its unique operating characteristics, gives it a particular importance to worldwide studies of the thermosphere and ionosphere. This paper summarizes the MU radar's capability for studying the ionosphere at heights above 100 km (Section 1), and broadly reviews the principal results obtained in the first ten years of operation on the large-scale structure of the ionospheric E- and F-layers (Section 2); thermospheric winds, electric fields and gravity waves (Section 3); and field-aligned irregularities (Section 4). Data from other instruments in Japan, such as ionosondes and magnetometers, have proved vital to the interpretation of MU radar data.

Archaeomagnetic Intensity in Finland during the Last 6400 Years : Evidence for a Latitude-Dependent Nondipole Field at -AD 500

Abstract: Archaeomagnetic field intensity in Finland (Lat. 61°N, Long. 22° E) has been determined for the past 6400 years with the Thellier technique of bricks, potsherds and baked clays. The normalized archaeointensity curve for Finland, based on 23 mean values, shows an increase from ∼4360 BC to the maximum at AD 500, after which it decreases to the present value. The high intensity at AD 500 as found in Finland, but not in Bulgaria and Japan, is not a consequence of the applied Thellier technique since we are able to reproduce the known field intensities in the laboratory, and some of the dated bricks yield values which are in broad agreement with Nurmijarvi Magnetic Observatory data. We also show that variations in grain size of the magnetic carriers, cooling rate or magnetic refraction are unlikely to cause systematic errors in intensity larger than a few percent. Based on results of self-made bricks we show that the archaeointensity, however, depends on the angle between the laboratory and ancient field directions. If these are not aligned errors up to ±10% in archaeointensity can occur and are probably caused by anisotropy of remanence or by other factors not yet understood. The high intensity at AD 500 in Finland can be modelled by a radial nondipole field source at the core-mantle boundary producing enhanced field at higher latitudes (Finland) and relatively weaker fields at lower latitudes (e.g. Bulgaria and Japan). Extrapolation of the present field (IGRF 1990) back in time shows similar field behaviour.

Short-Period (5-33 min) Variations in Vertical Drift of F-Region Plasma near the Magnetic Equator

Abstract: Measurements of F region vertical drift, Vz made at Kodaikanal (dip 4°N) on 133 days during March 1991-February 1992 under non-equatorial spread-F(ESF) conditions, are analysed for the characteristics of short-period (5-33 min) variations in Vz in the evening hours (1630-0130 IST). The spectral content of the fluctuations in Vz is found to depend on local time. The amplitude of the fluctuations, quantified in terms of the variance (σ2), increases during the period of postsunset enhancement of upward vertical drift and decreases thereafter reaching a minimum during 2200-0000 IST. In consonance with this, while functuations in Vz in the entire band 5-33 min manifest in the postsunset hours, the longer-period (13-33 min) components dominate thereafter (2200-0000 IST). The amplitude of the fluctuations and its day-to-day variability are higher in local summer than in local winter. On a day-to-day basis, the amplitude of Vz functuations increases with peak height in the interval 1756-2004 IST corresponding to the epoch of the evening enhancement in Vz. The results support the view that the short-period variations in F region vertical drift near the dip equator during the evening hours could be due to electric fields associated with the commonly occurring atmospheric gravity waves.

Holocene Geomagnetic Excursion at Mylodon Cave, Ultima Esperanza, Chile

Abstract: Paleomagnetic data from a section of a sedimentary sequence at Mylodon Cave (Seno de Ultima Esperanza, Chile) are reported. According to previous 14C data, the sediments have dates between 13, 500 and 5, 360 y.b.p. The section that is analyzed here is younger than 12, 000 y.b.p. Natural Remanent Magnetization (NRM) was measured from 39 successive cores, and stability of the NRM was analyzed by thermal demagnetization. Only five samples were rejected because they did not carry a characteristic remanent magnetization. Analysis of the rest of samples shows that the upper part of the section (younger than 10, 000 y.b.p.) recorded directions far removed from the present magnetic field, probably corresponding to a field excursion. Two areas where transitional VGP are concentrated were noticed: one in Africa and the other near Australia.

Paleointensity of the Geomagnetic Field in Egypt from 4000 BC to 150 AD Using the Thellier Method.

Abstract: A total of 170 ceramic specimens from 16 archaeological sites representing 16 well determined ages from 4000 BC to 150 AD were investigated using the Thellier method. These paleointensities were compared with data from countries near Egypt after reduction to a common latitude to determine a secular variation curve of the geomagnetic field for the time period from 4000 BC to 150 AD. The field intensity increases from about 30 μT around 3500 BC to a maximum of 70 μT at about 400 BC when it starts to decrease until 150 AD. The Thellier double heating method was used together with the following additional test measurements : a) during the Thellier experiments the laboratory field F Lab . pointed in the direction of stable NRM, b) magnetic susceptibility was measured after each double heating step to check for chemical alteration, c) a TRM check was made, by repeating the pTRM induction at a lower temperature, d) recent samples which were fired in a known field were used to test the reliability of our experimental setup and method. Based on rock magnetic measurements magnetite was identified as the predominant carrier of magnetization in the ceramics and bricks.

Upper Mantle Electrical Conductivity in the Himalayan Region

Abstract: The electrical conductivity profile of the Earth at depths of about 50 to 500 km was determined using the quiet ionospheric current variations observed at a line of stations near 75° East longitude. We found conductivity values of about 0.06 S/m from 50 to approximately 350 km depth with slight relative maxima near 125 and 275 kin, interpersed by relative minima near 210 and 330 km. Thereafter, the conductivity increased sharply toward a value of about 0.18 S/m at 500 km with no indication of leveling off. A comparison with regional seismic wave-velocity models shows good correspondence between high conductivity and low-velocity zones. The conduction by hydrogen-saturated pyroxene is envisaged as a possible mechanism for the high conductivity and its variation in the upper mantle.

Propagation Characteristics of Auroral Hiss Observed by Akebono Satellite.

Abstract: V-shaped hiss and saucer emission observed by the Akebono satellite in the auroral region in the altitude range between 3, 000 km and 5, 000 km were studied on their wave normal directions using 5 components of electric and magnetic field wave data by Phillips-Tikhonov regularization method. It was found that the V-shaped hiss come down to the spacecraft and their azimuthal direction of wave normal is anti-symmetric with respect to the location of V-shaped frequency minimum. Ray tracing study is made to see how the frequency versus time characteristics and wave normal direction of the emissions should be at the satellite level. The results by ray tracing shows good agreement with the observation, under the assumption that the initial wave normal direction is circularly symmetric around the magnetic field, with the resonance cone angle.

Volcanomagnetic Effects on the Unzen Volcano (1990-1992).

Abstract: The summit eruption of Unzen volcano started at Jigokuato (J) and Kujukushima (K) craters on November 17, 1990, after 198 years quiescence. The volcanic activity intensified at Byobuiwa (B) crater, about 150 m west of J, on February 12, 1991, and a new dome was extruded at Jon May 20, 1991. Seven domes were extruded until early summer 1992. Preceding the major volcanic activity, a geomagnetic change of about 20 nT was detected by repeated surveys at the summit during the period of 1986 to the end of 1990. To elucidate the cause of the geomagnetic change, closely placed and continuous geomagnetic observations with proton magnetometers have been conducted near the summit of the volcano since January 1991. In association with the volcanic activity at B, geomagnetic changes amounting to about +18 and -5 nT were observed at close points in the northern and southern directions of B, respectively, from February to March 1991. During the early stages of the new dome extrusion, large geomagnetic changes of more than 80 nT were simultaneously observed at several points near J from May 12 to June 3, 1991. The changing rate of about 5 nT/day at mid-May 1991 decreased exponentially to about 0.1 nT/day by the end of 1991. The total change reached to more than 160 nT at three points closer than 400 in from J from early March 1991 to the end of June 1992, although it was only 10 nT at a point of about 500 m south of J. Efforts to clarify the causes of each geomagnetic change resulted in no systematic simple model through this eruption sequence. These magnetic changes were probably the result of many processes such as thermal demagnetization, removal effects of magma intrusion, strain/stress, rotation and displacement of magnetic materials and perhaps also electrokinetic effects.

Drifting F-Layer Patches over the Magnetic Pole.

Abstract: Establishment of a polar observatory at Eureka, Canada in 1991 has permitted studies of F-layer ionization patches drifting over the north magnetic pole, using a multi-channel photometer and a digital ionosonde. Optical characteristics of two typical patches are shown. A case study of a sequence of several patches observed on February 17, 1993 is reported. Their optical intensities as determined from 630 and 558 nm meridian photometer scans averaged 200 R and 50 R respectively above airglow background. Their drift velocities through zenith, as measured by a digital ionosonde (CADI), averaged about 0.3 km/s while their bulk motion as seen optically was somewhat greater. The first 3 patches of the sequence, from 0945 to 1010 UT, moved generally in an antisunward direction while three immediately following, from 1015 to 1100 UT, had a velocity component in the dawn to dusk direction with a velocity vector rotated a full 90° from the earlier motions.

Toward an Improved Distribution of Magnetic Observatories for Modeling of the Main Geomagnetic Field and Its Temporal Change.

Abstract: The magnetic field from Earth's core (the main field) is a global phenomena with measurable temporal variations with periods ranging from one year to millennia. Geomagnetic studies are thus heavily dependent on the availability of data well distributed over the globe and acquired over long periods of time. Satellite data provide the best geographic coverage, but are unlikely to be available except possibly at intervals of 10 to 30 years. Accurate mapping of the main field over long periods of time is mostly dependent upon a network of geomagnetic observatories, each of which contributes continuous, three-component, data of high accuracy. The overall accuracy of knowledge of the main field depends both upon the adequacy of the geographic distribution of those observatories and on the existence of periodic surveys by satellite. Analysis of models based on the existing observatory distribution reveals large geographic regions in which their accuracy is degraded such that studies of the field, its source dynamo, etc. are seriously limited. Model accuracy is studied for three distributions of 92, 162, and 252 equally spaced observatory sites and for degradation of those distributions by a large area with no data. The 92-site distribution is the most economically realistic. Expansion of the existing network so that a subset of observatories approximates this 92-site distribution can be accomplished by a phased program of collocating magnetometers at 20 sites already established, or now planned, for other geophysical networks such as FLINN, GEOSCOPE, IDA, and IRIS, at 10 additional land or island sites, and at 8 sea bottom sites. Specific locations for these sites are proposed. While not meeting all of the needs for study of current problems in geomagnetism, if implemented, this extension of the current observatory network would form a firm foundation for most such studies. Such implementation will only be accomplished if the burden for doing so is partially shouldered by most or all of the national agencies and organizations representing data users and if such users unit in expressing their own need.

Upper Crustal Structure in the Torni-Purnad Region, Central India Using Magnetotelluric Studies.

Abstract: Magnetotelluric studies were conducted over a 100 km long NS profile in the Satpura range and Tapti basin. Data were collected at 18 stations in this region with an interstation spacing of 3-10 km. The geoelectric substructure was obtained using a two dimensional modelling technique and has indicated the presence of Deccan traps and Gondwana sediments overlying a rather complex granitic basement. Two distinct layers of Deccan basalts were delineated with a total thickness varying from 2000 m in the central part to about 300-1000 m on either sides of this NS trending profile. The thickness of the Gondwana sediments below the traps was observed to vary rather strongly in the range, 300-2000 m. Two vertical conductive zones were delineated corresponding to the Khandwa lineament and the Burhanpur tear but the sensitivity studies indicated a large degree of nonuniquness associated regarding the shape and vertical extent of these conductors. An interesting finding of these studies is the presence of a prism shaped body in the central part of the survey profile in the Satpura horst region at depths between 3.5 and 14 km with a resistivity of 20 ohm-m. The body had a lateral extent of 8 km at the top and 40 km at the bottom in the NS direction. This observation is discussed along with the other geophysical results in the Satpura and Tapti regions.