A description of the external and internal quiet daily variation currents at North American locations for a quiet-Sun year

Summary. An order 4, degree 12 spherical harmonic analysis of the smoothed quiet geomagnetic daily variations was used to separate the external and internal geomagnetic S _q field at North American locations for the quiet-Sun year, 1965. These fields were represented by a month-by-month display of equivalent current vortex systems with dominant, pre-noon foci. The focus reached 40° latitude near the June solstice and about 30° latitude near the December solstice. The daily range of S _q current amplitudes was largest in late July to early August and smallest in mid-December. Semi-annual variations of S _q currents dominated only the equatorial region. Daily maxima in mid-latitudes, occurred mostly near local noon in December to February and about 1 hr before noon in June to mid-October. The ratio of external to internal current amplitude vxied from about 1.5 to 1.9 in the middle latitudes with both annual and semiannual changes. An error treatment indicated that the analysis reproduced the form of the surface field with a correlation coefficient of about 0.9 and the amplitudes of the surface field to about 10 per cent of the S _q daily range.     

On the nature of abnormal quiet days in Sq(H)

Summary. It is well known that the time of occurrence of the minimum in the horizontal component of the Earth's magnetic field ( H _min) on quiet days at a mid-latitude station on the poleward side of the Sq focus shows considerable variability from day to day. This variability has previously been discussed in terms of the incidence of so-called 'abnormal quiet days'(AQD), arbitrarily defined for the station Hartland as quiet days when H _min occurred outside an interval of ± 2½ hr centred on the most common time of 1130 LT. AQDs have some interesting properties, which have been documented, but their precise nature and cause have not been elucidated. In this paper we report the results of a study of AQDs as identified at Hartland using a chain of Northern hemisphere stations situated approximately along the 0° longitude meridian and extending on both sides of the Sq focus. It is found that there are two effects on AQDs: (i) a northward component field varying in LT is superposed at all latitudes throughout the day, so reducing the amplitude of the normal Sq(H) variation at stations poleward of the focus and increasing it on the equatorward side, (ii) a southward perturbation field, of most probable magnitude 6.0 nT for the period studied, is imposed for about 4 hr at a fixed UT at all latitudes, so constituting an 'AQD event' which can lead to the occurrence of H _min at an anomalous local time for a station poleward of the focus. It is shown that the AQD event may be of large geographical extent and that it is related to the interplanetary magnetic field. All the main properties of AQD occurrences are explained, and it is suggested that much of the day-to-day variability in the amplitude and phase of the normal Sq(H) variation probably also arises from the occurrence of AQD events at times close to the diurnal turning points.     

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.     

Crust and upper mantle shear velocities from controlled sources

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A two ship refraction profile was undertaken on the Australian continental shelf during the Banda Sea geophysical program, carried out by the Woods Hole Oceanographic Institution, the Scripps Institution of Oceanography and the Geological Survey of Indonesia. S waves originating close to the sea bottom were observed to distances of up to 1150 km at an array of stations in northern Australia.
These observations are interpreted as implying S mantle velocities of 4.60 km s^-1 from a depth of 45 km to a depth of 76 km and 4.72 km s^-1 below a depth of 76 km.
Ratios of the P and S travel times (V_p/V_s) have been determined to be 1.74 in the crust rising to a value of greater than 1.79 below a velocity discontinuity at a depth of 200 km. It is inferred that this high value arises because the effect of temperature is greater for S than for P .
Using the data from this and other studies in the shield region of Northern Australia it has been found that the _S travel times are significantly less than predicted by the Jeffreys—Bullen tables.     

Transient and impulse responses of a one-dimensional linearly attenuating medium—II. A parametric study

Summary. We investigate one-dimensional waves in a standard linear solid for geophysically relevant ranges of the parameters. The critical parameters are shown to be T*= t_u/Q_m where t _u is the travel time and Q_m the quality factor in the absorption band, and τ⁻¹ _m , the high-frequency cut-off of the relaxation spectrum. The visual onset time, rise time, peak time, and peak amplitude are studied as functions of T* and τ _m. For very small τ _m , this model is shown to be very similar to previously proposed attenuation models. As τ _m grows past a critical value which depends on T* , the character of the attenuated pulse changes. Seismological implications of this model may be inferred by comparing body wave travel times with a'one second'earth model derived from long-period observations and corrected for attenuation effects assuming a frequency independent Q over the seismic band. From such a comparison we speculate that there may be a gap in the relaxation spectrum of the Earth's mantle for relaxation times shorter than about one second. However, observational constraints from the attenuation of body waves suggest that such a gap might in fact occur at higher frequencies. Such a hypothesis would imply a frequency dependence of Q in the Earth's mantle for short-period body waves.     

Evidence for 18.6-yr MN term in Japanese air pressure and geophysical implications

Summary Evidence is found in Japanese station pressure records for the 11-yr solar cycle term and a long-period signal near 20 yr which we identify as the 18.6-yr M _N lunar nodal term in atmospheric tides. Amplitudes of the identified M _N term are highly variable with a nominal amplitude of 0.1 mb and, for the seven available stations, the term is in phase (0.0±2.3yr) with the M _N term in 30 temperature records in north-eastern North America. These results are important because of my proposal that enhanced drought conditions in the western United States on a time-scale of ˜ 20 yr are neither 'recurrent' nor 'rhythmic', but rather periodic with a period 18.6yr. And the influence of the Rocky Mountains (as well as the Tibetan Plateau) on standing atmospheric waves vis-à-vis precipitation is the common forcing function for the signal in temperature and drought conditions.     

A comparative study of the ARM and Isr of some natural magnetites of MD and PSD grain size

Summary. ARM and I _sr stability with respect to alternating field and low-temperature treatment were measured for a set of artificial rock specimens. Each of the specimens contained a known amount of a natural magnetic mineral of a known grain size. The following minerals were used: a homogeneous natural magnetite, a natural titanomagnetite with submicroscopic Fe-rich inclusions, and a natural titanomagnetite and a natural titano-maghemite that both show exsolution phenomena. The grain size of the fractions varied from < 5 to 250 μm. It is found that the stability of ARM and I _rs of multi-domain grains with respect to alternating fields and low-temperature treatment is similar, whereas for the smallest grains studied (< 5 μm) ARM is more stable than I _sr. Multi-domain ARM data from this study are in agreement with current multi-domain ARM theory. Low-temperature behaviour of ARM and I _sr can be explained either in terms of a model in which stable moments in the bulk of the grains are screened by the surrounding matrix or in terms of a model in which regeneration of the bulk remanence plays a role and in which stable moments are situated on the surfaces of the grains.     

Three-dimensional VP and VP /VS models of the upper crust in the Friuli area (northeastern Italy)

3-D images of P velocity and P - to S -velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting V _P and V _P / V _S 1-D models were computed before the 3-D inversion. V _P was measured on two rock samples representative of the investigated upper layers of the Friuli crust. The tomographic V _P model was used for modelling the gravity anomalies, by converting the velocity values into densities along three vertical cross-sections. The computed gravity anomalies were optimized with respect to the observed gravity anomalies. The crust investigated is characterized by sharp lateral and deep V _P and V _P / V _S anomalies that are associated with the complex geological structure. High V _P / V _S values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associated with sharp variations in the V _P / V _S anomalies. The V _P images show a high-velocity body below 6 km depth in the central part of the Friuli area, marked also by strong V _P / V _S heterogeneities, and this is interpreted as a tectonic wedge. Comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the Friuli area.     

The tidal dynamics of the Irish and Celtic Seas

Summary. Current meter data collected over periods of more than 14 day from the Irish and Celtic Seas are harmonically analysed and presented in maps of tidal stream information. Making use of the analysed current data, and by constructing time series of frictional and inertial stresses which are also harmonically analysed, harmonic constituents of the surface tidal slopes at current meter stations are obtained. Using these with data collected from offshore tide gauges, and in conjunction with coastal tide data, cotidal maps are drawn with some confidence for M ₂, S ₂, O ₁ and K ₁, the M ₂ chart resolving the discrepancy which exists between the different charts of the Celtic Sea already produced. Cotidal maps for M ₃ and M ₄ are also presented.
The mean over a tidal cycle of the energy flux for M ₂, S ₂ and O ₁ is also presented in the form of the total energy flux in these constituents which crosses different sectional lines. A flux of 44 × 10⁶ kW is observed to enter the Celtic Sea from the Atlantic and this is compared with previous estimates. An energy budget is also performed for M ₂, including all the effects of astronomical forcing and Earth tides to enable comparison to be made between the true energy inflow and the estimated frictional dissipation. Finally, comparison is made between the mean of the instantaneous energy flux and the sum of the energy fluxes associated with the major harmonics.     

Travel times of Pn -waves in the Aegean and surrounding area

Summary. Based on accurately located 23 very shallow earthquakes ( h = 1–14 km) in northern and central Greece by portable networks of seismic stations and by the joint epicentre method, the travel times of the P_n -waves from the foci of these earthquakes to the sites of 54 permanent stations in the Balkan region have been determined. The travel times of P_n -waves in the central and eastern part of the area (eastern Greece, south-eastern Yugoslavia, the Aegean Sea, Bulgaria, southern Romania, western Turkey) fit a straight line very well with the P_n velocity equal to 7.9 ± 0.1 km s^-1. On the contrary, the travel times of P_n -waves to stations in the western part of the area (Albania, western Greece) do not fit this curve because the P_n -waves travelling to these stations are delayed by more than 1 s due to the thicker crust under the Dinarides–Hellenides mountain range. Time delays for P_n -waves have been calculated for each permanent station in the Balkan area with respect to the mean travel-time curve of these waves in the central and eastern part of the area. Corrections of the travel times for these delays contribute very much to the improvement of the accuracy in the location of the shallow earthquakes in the Aegean and surrounding area.     

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.     

Inefficient propagation of Sn under south-eastem China

Summary. Records of 21 earthquakes with ray paths crossing south-eastern China have been examined to establish the nature of propagation of the seismic wave S_n . S_n is seen at stations within 1500km of earthquakes, but not beyond 1500km. No single localized region of attenuation can account for all the missing or weak S_n waves. The inefficient propagation of S_n through south-eastern China is surprising in light of efficient S_n propagation in other continental platforms which, like south-eastern China, appear to have been stable throughout the Cenozoic.     

An investigation of the causes of abnormal quiet days in Sq (H)

Summary. From a study of 'abnormal quiet days' (AQDs) along the 0° meridian between 14 and 60° N, it was found (Butcher & Brown) that the minimum in H at stations on the poleward side of the Sq ( H ) focus was formed by a small negative substorm event when the normal Sq ( H ) amplitude was reduced by a superposed northward field.
In this paper we consider both the AQD event and the superposed northward field as a function of longitude and also consider in more detail the latitude variation of the superposed northward field. From such a study it is concluded that: (1) the AQD event is definitely due to a small magnetospheric substorm event; (2) the superposed northward field varies smoothly with longitude falling to zero some 110° from the longitude of its maximum amplitude; (3) the superposed northward field has a variation with geomagnetic latitude tending to zero near 20° and 70°N with a maximum near 55° N in summer and 35° N in winter; (4) there is some evidence that the effect of the IMF penetrates into the mid-latitude E-region and its effect is latitude-dependent. Although the evidence supports the suggestion that the currents responsible for the superposed northward field flow in the E-region no suggestion as to the origin of the driving force of the currents is forthcoming.     

Stacking for the frequencies and Qs of 0S0 and 1S0

Summary. Using nine IDA records for the Indonesian earthquake of 1977 August 19, we have formed an optimal linear combination of the records and have measured the frequency and Q of ₀ S ₀ and ₁ S ₀. The frequency was measured using the moment ratio method. The attenuation was measured by the minimum width method and by the time-lapse method. The frequency and attenuation were measured simultaneously by varying them to obtain a best fit to the data. A 2000-hr stack, the sum of nine individual records, for ₀ S ₀ gave a frequency of 0.814664 mHz±4 ppm. The values for the Q of ₀ S ₀ for the three different methods of measurement were 5600,5833 and 5700, respectively. The error in the estimates of Q ^-1 is about 5 per cent for the minimum power method. For ₁ S ₀ a 300-hr stack yielded a frequency of 1.63151 mHz±30 ppm. The values of Q for this mode were 1960, 1800 and 1850, respectively, with an error in Q ^-1 of about 12 per cent for the minimum power method.     

The detection of single-domain greigite (Fe3S4) using rotational remanent magnetization (RRM) and the effective gyro field (Bg): mineral magnetic and palaeomagnetic applications

The intensity of rotational remanent magnetization (RRM) acquired by single-domain greigite at a rotation frequency of 5 rps was combined with measurements of anhysteretic remanent magnetization (ARM) to calculate the effective biasing field ( Bg ) that produced the RRM. Samples of single-domain greigite had Bg values between -137 and -84 μT, and a MDF_RRM of c. 80 mT. By contrast, a suite of natural and synthetic ferrimagnetic iron oxide samples, including single-domain magnetite and y Fe₃O₄ tape particles, acquired Bg values between -3 and -14 μT, and MDF_RRM ranged between 43 and 68 mT (when RRM was acquired). Multidomain magnetite did not acquire a RRM. Bg values at 5 rps were calculated from previously published data for magnetite particles of different grain sizes, which revealed a minimum Bg value of -24 μT and a MDF_RRM of 57 mT for the finest fraction (0.2-0.8 μm in diameter). In a geological example, measurements of Bg and MDF_RRM were used to detect the presence of greigite in a 4 m long Late Weichselian sediment core. Variations in inclination, declination and the intensity of the natural remanent magnetization (NRM) correlate with changes in magnetic mineralogy.     

Absolute S-velocity estimation from receiver functions

We present a novel method to recover absolute S velocities from receiver functions.
For a homogeneous half-space the S velocity can be calculated from the horizontal slowness and the angle of surface particle motion for an incident P wave. Generally, the calculated S velocity is an apparent half-space value which depends on model inhomogeneity and P -waveform. We therefore, suggest to calculate such apparent half-space S velocities from low-pass filtered (smoothed) receiver functions using a suite of filter-parameters, T . The use of receiver functions neutralize the influence of the P -waveform, and the successive low-pass filterings emphasize the variation of S velocity with depth.
We apply this   V _{S ,app.}( T )  technique to teleseismic data from three stations: FUR, BFO and SUM, situated on thick sediments, bedrock and the Greenland ice cap, respectively. The observed   V _{S ,app.}( T )  curves indicate the absolute S velocities from the near surface to the uppermost mantle beneath each station, clearly revealing the different geological environments. Application of linearized, iterative inversion quantify these observations into   V _S ( z )  models, practically independent of the S -velocity starting model. The obtained models show high consistency with independent geoscientific results. These cases provide also a general validation of the   V _{S ,app.}( T )  method.
We propose the computation of   V _{S ,app.}( T )  curves for individual three-component broad-band stations, both for direct indication of the S velocities and for inverse modelling.     

Rapid computation of magnetic anomalies with demagnetization included, for arbitrarily shaped magnetic bodies

Summary. The potential function ø for a magnetic body of susceptibility μ in a medium of susceptibility μ* satisfies the integral equation
Here Φ* is the potential function for the region without the heterogeneity and R is the distance from the point of observation to the point on the surface, s , of the body. δΦ /δn is the normal derivative, in the direction of the outward normal. The equation allows for the effects of demagnetization. For numerical purposes the surfaces can be divided into N facets over which δΦ/δ n is a constant. The unknown quantities δΦ/δn_j can be found from the system of equations defined by:
The prime on the summation sign denotes that the summation does not include the i th element. The magnetic field in the direction of the unit vector P( P ₁, P ₂, P₃ ) is given by:      

An experimental study of the intensity and stability of thermoremanent magnetization acquired by synthetic monodomain titanomagnetite substituted by aluminium

Summary. Titanomagnetites of composition Fe_2.4-δAl_δTi_0.6O₄ and Fe_2.6-δAl_δTi_0–4O₄(δ=0, 0.1 and 0.2 in both cases) were prepared in the monodomain state by pulverization of sintered synthetic material. In low fields, the thermoremanence (TRM) was found to be linear with inducing field and of high enough intensity to account for typical natural remanent magnetizations of fresh submarine basalts. The higher field TRM acquisition curves follow the Néel model curve for an assemblage of non-interacting identical particles in a general way only, the differences being due to interactions, or the range of particle blocking temperatures and volumes or other features of the samples not included in the model. The unblocking temperatures of low field TRM lie in a narrow range below the Curie point. The low field TRM is very resistant to alternating field demagnetization and provides a very striking illustration of the strength of the TRM mechanism in preserving a stable record of a weak magnetic field. The result of a Lowrie-Fuller test on the material is consistent with the monodomain state.     

The Influence of Surface Inhomogeneities On Deep Electromagnetic Soundings of the Earth

Summary. Asymptotic expressions for components of the electromagnetic field of a grounded electric dipole are considered for the model consisting of a thin surface-layer overlapping a stratified medium with a highly resistive screen on the roof. It is shown that the method of spatial derivatives makes it possible to obtain proper estimates of the impedance at distances of r ≥|λ₀| from the nearest edge of the surface anomaly (|λ₀| being the effective depth of the field penetration in the underlying section). the magnetotelluric methods allow one to obtain the true values of impedance, provided r ≥ max {|λ₀|, |/( S ⁻¹+ Z ⁻₀|^1/2} where S is the integrated conductivity of the surface layer, is the transverse resistance of the screen, and Z ⁻₀ is the Tikhonov—Cagniard impedance for the medium underlying the surface layer.