UT variation of internal Sq currents and the oceanic effect during 1980 March 1–18

Summary. UT variation of the internal part of S _q currents is examined using the geomagnetic data during 1980 March 1–18, and the effect of the ocean is found. The ratio of the internal currents to the external increases when the external current vortex comes above the Atlantic and Pacific Oceans, and the internal current vortex shifts to the oceans when the external vortex approaches the edge of the oceans. The existence of the ocean increases the total induced current by about 30 per cent and this amount is consistent with model calculations for S _q by previous workers.     

The change in Sq(H) amplitude on Abnormal Quiet Days

Summary. Analysis of geomagnetic data has shown that the superposed northward magnetic field, which reduces the S _q( H ) amplitude at northern mid-latitude stations on Abnormal Quiet Days, and increases the amplitude at stations on the equatorward side of the S _q focus, builds up in amplitude over four to five days before the AQD occurs, and subsides over a similar period after the AQD. It is inferred indirectly that the azimuthal component B_y of the interplanetary magnetic field varies similarly. Data for the opposite meridian show that the imposed field reverses to a southward direction at lower latitudes. The inferred currents to account for these fields are believed to flow in the ionosphere, but to arise from magnetospheric electric fields induced by the solar wind-transported IMF.     

Palaeomagnetism of Dhar traps and drift of the subcontinent during the Deccan volcanism

Summary. Palaeomagnetic investigations were carried out on nine lava flows around the Dhar region, which constitute the northern part of the Deccan traps. The stability of remanent magnetism of these specimens was analysed by an alternating magnetic field, thermal demagnetization and memory tests. Six flows exhibited characteristic components of magnetization, with a mean direction of D =143°, I =+46° ( K = 107.1, α₉₅=5.5°). This gives a VGP located at 29° N, 67° W (δ_p=4.5°, δ_m= 7.0°). The lower site with normal magnetization and the upper five sites with reverse magnetization indicate a geomagnetic field reversal during the initial phases of Deccan volcanism in the Early Tertiary period. A rapid northward migration of about 18° in latitude and a simultaneous anticlockwise rotation of 37° is calculated for the subcontinent.     

The annual and daily variations of the Dst index

Summary. This investigation concerns both the annual variation and the daily variation of the Dst index for 18 years. The data display a 6-month wave, a 12-month wave and a 24-hr wave the phase of which apparently does not vary in the course of the year. This contrasts with the variations observed with planetary indices derived from subauroral K indices, which undergo a modulation in sin²Ψ_M (Ψ_M is the angle between the solar wind and the dipole axis) with the well-known change in phase of the daily variation from one solstice to another. The origin of the 6-month wave is attributed to the mechanism suggested by Malin & Isikara: the 6-month variation of the mean latitude of the ring current. The 12-month wave may be due to the northern latitude of most of the Dst observatories. The daily variation is probably due to the asymmetric part of the ring current which is not averaged out because of the non-uniform longitudinal distribution of the Dst observatories.     

Observation of electric currents in the Alaska oil pipeline resulting from auroral electrojet current sources

Summary. Currents in the 1.28 × 10³ km Alaska oil pipeline, induced from the ionospheric, auroral electrojet, were measured at three sites, near Fairbanks, Paxson and Valdez, Alaska, using a gradient configuration of two fluxgate magnetometers. The observed pipeline current magnitudes, which reached 50 A during times of mild geomagnetic activity, displayed a linear relationship with the electric earth potential. Using the induction relationship between the electric and magnetic fields and the typical spectral composition of the geomagnetic field at high latitudes, I obtained a spectral appearance of the current that shows a maximum in the range of 4.5- to 10-min period. Near Fairbanks the pipeline current amplitudes, I (Amperes), could be represented, approximately, by I = 0.65 B _x T ^−0.5, where B _x(nT) is the north—south geomagnetic field variation amplitude and T (min) is its apparent period. There is much less pipelines current at the sites south of Fairbanks. A previously established relationship between the local electric field and the planetary geomagnetic activity index, Ap , permitted a prediction of the pipeline current surge amplitudes in the Fairbanks region as approximately I = 5.0 Ap . Current surges larger than 500 A may be expected rather often in the Alaska pipeline during large geomagnetic storms.     

Palaeomagnetism of the Proterozoic Zig-Zag Dal Basalt and the Midsommersø Dolerites, eastern North Greenland

Summary. Palaeomagnetic investigations are reported from 24 sites in the Proterozoic Zig-Zag Dal Basalt Formation and 12 sites in the Midsominersø Dolerites of eastern North Greenland. The Zig-Zag Dal Basalt is a typical tholeiitic flood basalt sequence, and dolerite intrusions in the underlying sandstones are thought to be genetically related to the basalts.
After a detailed AF demagnetization programme 19 sites in the basalts and 10 sites in the dolerites reveal one stable component of magnetization, probably of TRM and/or CRM origin residing in small single domain titano-magnetite grains. The degree of anisotropy has not affected the direction of the remanent magnetization. The maximum axis of the anisotropy ellipsoid is parallel to the flow direction of the magma, whereas the minimum axis is perpendicular to the flow plane.
Only one polarity of the geomagnetic field was found. The mean palaeomagnetic pole positions for the two rock types are not significantly different (basalt: 12.2°S, 62.8°E with A ₉₅= 3.8°; dolerites: 6.9°S, 62.0°E with A ₉₅ = 5.1°). After correction for Phanerozoic drift of Greenland the two mean poles compare closely to a relevant North American APW-curve for 1250–1350 Ma, in good agreement with Rb-Sr isochron ages of 1250 Ma obtained for the intrusives. The palaeogeographical position of Greenland was near equator with the major geographical axis orientated E-W.     

Modelling of solar quiet magnetic field variations near a conductivity anomaly

Summary. A simplified model of the solar quiet-time ionospheric current system is used to calculate the induced currents in a model earth. The conductivity is assumed to be constant below a depth of about 400 km and zero above that depth. The current induced in the north—south conductivity anomaly under the Rocky Mountains is then estimated from the time-varying potential difference between points at 30 and 45° latitude at the surface of the conducting sphere. The purpose of these calculations is to investigate whether variations in the latitude of the northern hemisphere current system vortex will substantially alter the relationship between the observed magnetic field components at the Earth's surface and the local magnetic field gradient caused by the conductivity anomaly. We find that a 10° shift in the latitude of the ionospheric current focus causes a change of 6 per cent or less in the transfer function from the field components to the gradient in the total field. Thus such latitude shifts cannot explain much of the magnetic field gradient variation at periods near 24 hr that has been observed near Boulder, Colorado.     

Palaeomagnetism of the Fongen-Hyllingen gabbro complex, southern Scandinavian Caledonides; plate rotation or polar shift?

Summary. Palaeomagnetic results are presented from the c . 160 km² Caledonian synorogenic layered Fongen-Hyllingen gabbro complex (of probable late Silurian age) located about 75 km SE of Trondheim, Norway, in the allochthonous Seve-Kdli Nappe Complex. A total of 80 oriented samples from eight sites in the northern part of the gabbro were investigated. After detailed af demagnetization two stable high coercivity components emerge: one with a well defined NW direction with D =325°, I =−21° (α₉₅=8°, N =8), and another, less well defined, probably younger, SW direction with D = 237°, I = 6° (α₉₅= 9°, N = 8). Correction for dip of these two directions gives D = 329°, I =−7° (α₉₅= 10°) and D = 238°, I =−11° (α₉₅= 12°), respectively. The corresponding pole positions are P ₁ : 19° N, 225° E and P ₂: 19° S, 308° E, respectively. The reversed pole -P ₂ of the SW direction lies close to other NW European palaeomagnetic poles of Caledonian, Upper Silurian-Lower Devonian age. However, the dominant pole PI is far away from these, and could be due to a late Caledonian geomagnetic excursion of considerable duration; or it could record a c . 90° rotation around a vertical axis of a crustal block within the Scandinavian Caledonides. Block rotation could have been related to nappe translation, although geological observations do not at present appear to support the occurrence of such an event.     

A study of geomagnetic storms

Summary. An attempt is made to find interplanetary magnetic field and solar-wind parameters which control the development of geomagnetic storms. For this purpose, the interplanetary energy flux is estimated in terms of the Poynting flux ( E × B /4π), and its time variations are compared with the rate of energy dissipation in terms of the ring-current particle injection u _i( t ), Joule dissipation in the ionosphere u_j ( t ) and auroral particle injection u_p ( t ) for 15 major geomagnetic storms.
It is shown that the growth of geomagnetic storms, namely the time variations of the rate of the total energy dissipation, u ( t ) = u _i( t ) + u _j( t ) + u _p( t ), is closely related to the Poynting flux by the following relation:
where l ₀≅ 7 R _E and θ' is a measure of the angle between the interplanetary magnetic field vector and the magnetospheric field vector at the front of the magnetosphere in the equatorial plane. Further, it is shown that within a factor of 2 for each storm period.
A large increase of u ( t ) is associated with substorm activity. Thus, the energy flux ɛ( t ) entering the magnetosphere is dissipated through magneto-spheric substorm processes within the magnetosphere, and their accumulated effects can be understood as geomagnetic storm phenomena.     

Palaeomagnetism of a sequence of red beds of the Middle and Upper Sections of Pagnazo Group (Argentina) and the correlation of Upper Palaeozoic-Lower Mesozoic rocks

Palaeomagnetic data from 182 hand samples collected in a rock sequence of about 620-m of red beds of Late Palaeozoic to Early Triassic age exposed in north-western Argentina (30.3° S 67.7° W), are given.
After cleaning, the majority of the Upper Palaeozoic samples (Middle Section of Paganzo Group) show reversed polarity and yield a palaeomagnetic pole at 78° S 249° E (α₉₅= 3°). They also record a polarity transition which we have correlated with the Middle Permian Quebrada del Pimiento Normal Event. The position of the palaeomagnetic pole and the K-Ar age of a basalatic sill at the base of the sequence support this correlation.
Stable remanent magnetization has been isolated in the majority of samples from the Upper Section of the Paganzo Group; it is predominantly reversed and reveals three normal events and also three geomagnetic excursions suggesting an Illawarra Zone age (post Kiaman, Late Tatarian-Early Scythian). The palaeomagnetic pole of the reversely magnetized samples is located at 75° S 285° E(α₉₅= 13°).
The red beds involved in this study are correlated with red beds from the Corumbataí Formation (State of Paraná, Brazil) and with igneous rocks from the Quebrada del Pimiento Formation (Province of Mendoza, Argentina).
The South American Middle and Upper Permian, Upper Permian—Lower Triassic, Lower, Middle and Upper Triassic and Middle Jurassic palaeomagnetic poles reflect a quasistatic period with mean pole at 82° S 244° E, (α₉₅= 4°) which followed the South American Late Palaeozoic polar shift.     

Palaeomagnetism of Lower Cambrian sediments from the Olenek River section (northern Siberia): palaeopoles and the problem of magnetic polarity in the Early Cambrian

New palaeomagnetic data from the Lower and Middle Cambrian sedimentary rocks of northern Siberia are presented. During stepwise thermal demagnetization the stable characteristic remanence (ChRM) directions have been isolated for three Cambrian formations. Both polarities have been observed, and mean ChRM directions (for normal polarity) are: Kessyusa Formation (Lower Cambrian) D = 145°, I = -40°, N = 12, α₉₅= 12.8°; pole position: φ= 38°S, A = 165°E; Erkeket Formation (Lower Cambrian, stratigraphically highly) D = 152°, I = - 47°, N = 23, α₉₅= 6.8°; pole position: φ= 45°S, A = 159°E; Yunkyulyabit-Yuryakh Formation (Middle Cambrian) D = 166°, I = - 33°, N = 38, α₉₅= 4.6°; pole position: φ= 36°S, L = 140°E. These poles are in good agreement with the apparent polar wander path based on the bulk of existing Cambrian palaeomagnetic data from the Siberian platform. In Cambrian times, the Siberian platform probably occupied southerly latitudes stretching from about 35° to 0°, and was oriented 'reversely' with respect to its present position. Siberia moved northwards during the Cambrian by about 10° of latitude. This movement was accompanied by anticlockwise rotation of about 30°. The magnetostratigraphic results show the predominance of reversed polarity in the Early Cambrian and an approximately equal occurrence of both polarities in the part of the Middle Cambrian studied. These results are in good agreement with the palaeomagnetic polarity timescale for the Cambrian of the Siberian platform constructed previously by Khramov et al. (1987).     

Approaches to Modelling the Surface Albedo of a High Arctic Glacier

Broadband surface albedo measurements, made during the summer melt season at three weather stations on John Evans Glacier (79°40 ' N, 74°00 ' W), varied strongly with the solar zenith angle ( θ _z ). Tests were carried out to assess the impact of diurnal variations in surface albedo on seasonal net shortwave radiation ( K ^* ) totals. Removing the diurnal signal from albedo measurements by daily averaging of hourly measurements, or by applying midday measurements to all hours of the day, changed K ^* by up to 16%. Ignoring measurements made at θ _z & 75°, to account for measurement (cosine) error at high θ _z , decreased K ^* by between 5 and 18%. Given the sensitivity of K ^* to diurnal patterns in surface albedo, experiments were carried out with two albedo models. One model accounted for albedo variations with θ _z and one did not. The model driven by θ _z , when implemented within a surface energy balance model for John Evans Glacier, produced better melt estimates. This suggests that diurnal variations in surface albedo should be accounted for in energy balance models of glacier melt.     

Palaeomagnetism of the Tudor gabbro, Ontario: evidence for divergence between Grenvillia and interior Laurentia

Summary. After thermal and alternating field (AF) cleaning, the characteristic high blocking temperature A component of natural remanent magnetization (NRM) of the Tudor gabbro of southern Ontario has a mean direction D = 326°, I =–46° ( k = 132, α₉₅= 4.8°, N = 8 sites). The corresponding palaeopole, 133°E, 12°N ( dp = 4°, dm = 6°), confirms the palaeopole 137°E, 17°N (α₉₅= 8.4°) reported earlier by Palmer & Carmichael, based on AF cleaning only. The A NRM has unblocking temperatures > 515–525°C which exceed the estimated 500°C peak temperature reached locally during ∼ 1050 Ma Grenvillian regional metamorphism. The A NRM therefore predates metamorphism and is probably a primary thermoremanence (TRM). The age of the Tudor NRM has previously been taken to be about 675 Ma, but recent ⁴⁰Ar/³⁹Ar dating by Baksi has shown that this is the time of post-metamorphic cooling to 200–250°C. Hornblendes record initial cooling of the intrusion to 590±20°C at 1110 Ma and this is the best estimate of the age of the A remanence. Successful Thellier-type palaeointensity determinations on 11 Tudor samples confirm that the A NRM is a TRM and indicate a palaeofield at this time of 18–27 μT, about 50–70 per cent of the present field intensity at 27° magnetic latitude. The anomalous Tudor A palaeopole, which lies well to the west of both 1000–800 Ma Grenvillian palaeopoles and 1100–1050 Ma poles from Interior Laurentia, is interpreted as recording divergence between Grenvillia and Interior Laurentia just before the Grenvillian orogeny, rather than a post-metamorphic extension of the apparent polar wander path as previously assumed.     

On the connection between the critical value of the "vertical" electrical anisotropy and earthquake prediction

Summary. It is shown that at any orientation of a dipole of direct current at the surface of a uniform anisotropic halfspace there exists a unique critical value η_cr of the "vertical" electric anisotropy η. For η > η_cr not all field lines are closed, and three straight field lines pass through the centre of the dipole. Also there exists a unique value η_eq > η_cr, where equatorial η is equal to zero. If η > η_eq then equatorial ρ_app is negative since the electric vector is turned more than through 90° at the equator of dipole, and it is shown that η_cr= 1.79, η_eq= 1.93 if the axis of dipole is directed symmetrically with respect to the axes of anisotropy. Routine observations of variations of ρ_app with the aim of earthquake prediction should be carried out taking into account the possibility of great fluctuations on η far from the epicentre of an earthquake.     

Experimental study of the anisotropy of longitudinal and transverse waves from local earthquake records

Summary. The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δ t_s1-_s2 ) between the arrival times of shear waves with different polarizations.
The velocities of P -waves vary with azimuth from 5.3 to 6.27 km s^-1 and the velocities of S -waves vary from 3.15 to 3.5 km s^-1.
The delay times Δ t_S1 - _S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S 1 and S 2 propagating at different velocities, and the character of the distribution of Δ t_S1 - _S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.     

Palaeomagnetic results from two basic volcanic formations in the western Himalayas and a Phanerozoic polar wandering curve for India

Summary. We present palaeomagnetic results from the Durgapipal and Rudraprayag formations, which are basic volcanic formations in the Lesser Himalayas of Uttar Pradesh State. NRM measurements and AF demagnetization stability tests were made on specimens cored from oriented block samples collected at representative sites. Mean stable remanent magnetic directions were used for calculating the Virtual Geomagnetic Pole (VGP) positions; where necessary tectonic corrections were applied.
The virtual geomagnetic north poles were found to be located at:

• (a). 

  Durgapipal (Permian): λ _p = 10° S, L_p = 42° W;
• (b). 

  Rudraprayag (Silurian-Devonian): λ _p = 30° S, L_p = 12° W.

A new, continuous Phanerozoic apparent polar wandering curve for the Indian subcontinent has been plotted from the available palaeomagnetic data and the VGP positions reported in this paper. As a result, the gap in the Indian palaeomagnetic data from the Lower Carboniferous to the Cambrian has been partially filled. The locations of the pole positions for the two formations on the Phanerozoic polar wandering curve for the Indian subcontinent, have been found to coincide with the stratigraphic ages assigned to them on the basis of rather limited geological and palaeontological evidence.
The Cambrian and Permian poles for the Salt Range in the NW Himalayas and the Permian pole for the Kumaon Himalayas are grouped along with the pole positions of contemporaneous formations of the Peninsular Shield. The palaeomagnetic data thus suggests that the two formations are autochthonous in nature.     

Linearity spectrum analysis of multi-component magnetizations and its application to some igneous rocks from south-eastern Australia

Summary. A method that enables the objective resolution of almost parallel multi-component magnetizations is described and demonstrated. A feature distinguishing this method from others is its simultaneous analysis of demagnetization data from a group of specimens, rather than the analysis of data from one specimen at a time. The only prerequisite is that the specimens are derived from a homogeneous source. Thus for a formation carrying a simple single component magnetization, all specimens from the formation may be simultaneously reduced. For a more complicated two component magnetization it is shown that only specimens from a particular site can be considered homogeneous, and for a complex three component system each sample often requires undivided attention. Thus the workload is proportionally increased to achieve analyses of comparable reliability from data of variable quality.
New pole positions from Mesozoic intrusions of the Sydney Basin, NSW are: from the Marsden Park Breccia pipe 48°S, 127°E ( A ₉₅= 6°); the St Marys Breccia pipe 46°S, 150°E ( A ₉₅= 8°); the Prospect Dolerite 60°S, 142°E ( A ₉₅= 13°) and 53°S, 180°E ( A ₉₅= 6°); and from the Dundas Breccia pipe 58°S, 162°E ( A ₉₅= 36°) and 31°S, 195°E ( A ₉₅= 16°). The last two formations possess multi-component magnetizations. These pole positions are consistent with previous results from south-eastern Australia.     

Imaging upper-mantle discontinuity topography usingunderside-reflection data

This paper presents a method to invert underside-reflection ( P _d P or S _d S arrivals) data for lateral depth variations of upper-mantle discontinuities, combining traveltime and amplitude data. The method greatly improves the resolution of small-scale undulations obtained by existing imaging methods and does not suffer from the long-wavelength biases that are likely to be present in currently available models. Existing inversion methods account for the large size of the Fresnel zone of underside reflections, but not for its complexity, arising from the mini-max traveltime nature of PP- and SS -related waves. This neglect results in long-wavelength artefacts from small-scale undulations of the discontinuities, obscuring true long-wavelength depth variations. The inversion method presented in this paper uses a complex-valued sensitivity kernel, derived from the representation of underside reflections through a Kirchhoff integral formulation. The sensitivity kernel accounts for the varying sensitivity of the waveforms to discontinuity structure over the Fresnel zone. The method is applied to a large, synthetic data set. The data set consists of P _d P amplitudes and traveltimes. The results show that the new inversion method resolves depth variations on a lateral scale that is smaller than the size of the Fresnel zone of individual underside reflections (but larger than the dominant wavelength), retaining the resolution of large-scale variations. The results presented here suggest that the discontinuity depth variations induced by slab penetration of the 670 discontinuity could be resolved by current broad-band P ₆₇₀ P data sets.     

Annual variation of the geomagnetic field

Summary. Horizontal and vertical intensity data, obtained between 1957.0 and 1961.0 at 69 observatories, are analysed to determine the worldwide distribution of the annual variation of the geomagnetic field. Only data observed near local midnight are used, to avoid the small, but significant contamination from Sq. Over most of the world the variation is found to be small, with a clear dependence on latitude, but near the poles it is larger and more erratic. The non-polar variation is subjected to spherical harmonic analysis and separated into parts of internal and external origin. The polar variations are shown to be consistent with a north—south oscillation of the mean position of the auroral electrojets during the year. It is suggested that, with the exception of the polar effect, the annual variation is not due to ionospheric currents (as was hitherto believed), but results from an annual variation in the latitude of the ring current.     

Palaeomagnetism of alkaline lava flows from El Salto-Almafuerte, Córdoba Province, Argentina

Summary. Stable components of magnetization have been isolated in 15 lava flows (mean K-Ar age 123 ± 4 Myr) from the alkaline sequence outcropping at El Salto-Almafuerte, Province of Cordoba, Argentina. Magnetic and geologic stratigraphy, as well as K-Ar ages indicate that this sequence was probably extruded in the Lower Cretaceous during the first volcanic cycle of the Sierra de los Cóndores Group (Vulcanitas Cerro Colorado Formation).
The palaeomagnetic pole-position for El Salto-Almafuerte lava flows, computed from the mean of 15 virtual geomagnetic poles and denoted SAK₇, is: 25° E, 72° S ( k = 35, α₉₅= 6.5°); it is fairly close to other Lower Cretaceous palaeomagnetic poles for South America. The elongated distribution of Cretaceous palaeomagnetic poles suggest recurrent drift for South America in early Cretaceous time.
The palaeomagnetic and radiometric data for the igneous rocks from El Salto-Almafuerte support the magnetic reversal time-scale for the early Cretaceous suggested by oceanic magnetic lineations.