Direction Of Earth's Magnetic Field Research Articles

Archaeomagnetic analyses on fumiers burned under controlled experimental conditions

The improvement of the archaeomagnetic dating method requires compiling new and older data of the Earth's magnetic field (EMF) variations for the last millennia. Combustion events from fumier sequences have been proposed as good directional EMF recorders. However, they are subjected to diverse taphonomical processes and how these affect the archaeomagnetic record has not yet been studied. In order to evaluate it, here we report the first archaeomagnetic and rock-magnetic results on samples from experimentally recreated fumiers since 2014 under controlled conditions. A facies description with unprecedent resolution was used to study the variation of magnetic properties in depth. Rock-magnetic analyses indicate a homogenous magnetic mineralogy dominated by pseudo-single domain magnetite as main carrier in all facies, with not very high and similar contribution of the finest (superparamagnetic) grains. The low values of anisotropy of magnetic susceptibility (AMS) indicate that the studied sample set is mainly isotropic. The directional behaviour in well-preserved burned facies (here described as G, LM and DGB), are jointly characterized by highly reversible thermomagnetic curves, high Koenigberger (Qn) ratio values and intense, univectorial and normal polarity orthogonal NRM demagnetization diagrams. On the contrary, specimens affected by mechanical alteration processes are less magnetic and show anomalous directional behaviours. The high thermomagnetic reversibility of ashes indicates that they reached ca. 600–700 °C, in line with the thermocouples’ data. Temperatures of 460 °C were obtained for the DGB facies (subyacent black carbonaceous facies). Sampling of ashes located on the top of these combustion events should be avoided for archaeomagnetism. Being just beneath the last stabling episode they are the most prone to undergo mechanical alterations and do not preserve well the Earth's magnetic field direction. Despite their unlithified nature and the multiple taphonomic processes that fumier sequences may undergo, under certain quality criteria, they are valid geomagnetic field recorders providing both information of archaeological and geophysical interest.

Is it possible to identify temporal differences among combustion features in Middle Palaeolithic palimpsests? The archaeomagnetic evidence: A case study from level O at the Abric Romaní rock-shelter (Capellades, Spain)

Archaeomagnetic dating is probably one of the most known applications of magnetic methods to archaeology but there are others still underutilized and of particular interest to Palaeolithic archaeology. Here, we report a novel application of archaeomagnetism as a technique to determine temporal diachronies among combustion features from the same surface within palaeolithic palimpsests. The approach is based on the subtle directional changes of the Earth's magnetic field through time (secular variation, SV) and on the ability of burned materials to record such variations under certain conditions. Three Middle Palaeolithic hearths from level O (ca. 55 ka BP) at the Abric Romaní rock-shelter (NE Spain), were archaeomagnetically investigated. The studied surface (black homogeneous carbonaceous facies), recorded the magnetic enhancement produced by fire with a tenfold increase in concentration-dependent magnetic parameters in the uppermost centimetre with respect to its unburned or deeper counterparts. Pseudo-single domain (PSD) Ti-low titanomagnetite was identified as the main remanence carrier. The irreversibility of thermomagnetic curves suggests that these samples did not undergo enough high temperatures as to record a full thermoremanence (TRM). Additionally, the occasional occurrence of maghaemitized magnetite is interpreted as an indication of a thermochemical remanent magnetization (TCRM), making these samples unsuitable for absolute palaeointensity determinations. Two well-defined (α95 < 5°) and statistically indistinguishable archaeomagnetic directions were obtained with their mean directions within their respective confidences circles at the 95% level. The lack of directional changes and the similarity in the magnetic properties suggest that these hearths recorded simultaneously or closely confined in time the Earth's magnetic field direction at the time of cooling. These results agree well with archaeological evidence which indicates a synchronic occupation of this activity area. The possibility of determining temporal differences among combustion features in prehistoric sites arises as a promising tool in palimpsest dissection studies and may help to reconstruct occupation patterns of prehistoric groups. The practical limits of the method are discussed as well as its potential to identify post-depositional mechanical alteration processes.

Zeeman effect in atmospheric O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; measured by ground-based microwave radiometry

Abstract. In this work we study the Zeeman effect on stratospheric O2 using ground-based microwave radiometer measurements. The interaction of the Earth magnetic field with the oxygen dipole leads to a splitting of O2 energy states, which polarizes the emission spectra. A special campaign was carried out in order to measure this effect in the oxygen emission line centered at 53.07 GHz. Both a fixed and a rotating mirror were incorporated into the TEMPERA (TEMPERature RAdiometer) in order to be able to measure under different observational angles. This new configuration allowed us to change the angle between the observational path and the Earth magnetic field direction. Moreover, a high-resolution spectrometer (1 kHz) was used in order to measure for the first time the polarization state of the radiation due to the Zeeman effect in the main isotopologue of oxygen from ground-based microwave measurements. The measured spectra showed a clear polarized signature when the observational angles were changed, evidencing the Zeeman effect in the oxygen molecule. In addition, simulations carried out with the Atmospheric Radiative Transfer Simulator (ARTS) allowed us to verify the microwave measurements showing a very good agreement between model and measurements. The results suggest some interesting new aspects for research of the upper atmosphere.

Edward Irving FRSC CM. 27 May 1927 — 25 February 2014

Dr Edward (Ted) Irving, one of Canada's most respected geoscientists, died on 25 February 2014 in Saanichton, British Columbia, Canada, aged 86 years, leaving his wife, Sheila, children Katie, Susan, Martin and George, seven grandchildren and one great-grandchild. After his early work as a student at Cambridge, England, he moved first to Australia and then to Canada. Over more than 60 years his scientific career was devoted mainly to the use of magnetic remanence recorded in ancient rocks to address fundamental geological questions. This seemingly simple technology proved to have remarkably many applications. Through his measurements and analyses of rock samples that recorded the magnetic field at the time of their formation, Ted was in the forefront of demonstrating that continental drift was real, at a time when the theory was out of favour. His meticulous work on rocks from many areas of the world was instrumental in showing how continents have been constantly moving, breaking up and colliding to make new larger continents and then breaking up again. He published more than 200 articles in international scientific journals. His reference text Paleomagnetism and its applications to geological and geophysical problems is still widely used. Applying remanent magnetism to study the motion of continents, and to other important geological problems, required careful analyses and interpretations. These included showing that the secular change in the Earth's magnetic field direction averaged over time aligns with its rotation pole, that the Earth's magnetic field has reversed its polarity at irregular intervals of a few million years, and that overprinting by re-magnetizations of rocks at different geological times can be separated by special laboratory techniques. Other contributions included important research in ancient climates, continental glaciations, the origin of mountain systems, and the relative displacements of parts of continents (terranes), especially the inferred large northward movement of parts of western North America, a conclusion that remains controversial. His most important results depended critically on his developing and using the best field sampling methods, laboratory instrumentation and procedures, and methods of data analysis. During his career he established world-class palaeomagnetic laboratories in Cambridge and Canberra, and in Ottawa and Victoria in Canada. Ted Irving had broad interests and knowledge. He was a serious gardener and horticulturalist and wrote several scholarly articles on plants, especially on the biogeography of rhododendrons and magnolias. He received numerous awards and medals and wide recognition, including election as a Fellow of the Royal Society of London, a Fellow of the Royal Society of Canada, a Fellow of the American Geophysical Union, an Honorary Fellow of the Geological Society of London, and a Foreign Associate of the US National Academy of Sciences. He also received many awards and medals from professional geological societies. Ted Irving received honorary doctorates from three universities, and the Order of Canada, in recognition of his outstanding scientific contributions.

Geomagnetic excursions date early hominid migration to China

Global‐scale geomagnetic reversals, which are periods when the direction of Earth's magnetic field flips, leave imprints in magnetic minerals present in sediments. But so do smaller‐scale, even local, changes in Earth's magnetic field direction. Paleomagnetists believe that the smaller‐scale events represent “failed reversals” and refer to them as “geomagnetic excursions.” Scientists use geomagnetic excursions in sedimentary basins as markers to tie together events of Earth's history across the globe.

Asymmetric V-shaped streaks recorded on board DEMETER satellite above powerful thunderstorms

[1] We report here observations of both symmetric and asymmetric forms of V-shaped streaks on VLF spectrograms recorded on board the DEMETER satellite. Recent investigations have shown that V-shaped streaks are associated with intense and numerous 0+ whistlers generated in the VLF range by active thunderstorms. To understand the origin of the different spectral forms of the V-shaped events, a systematic survey of these phenomena is performed by means of a visual inspection of the VLF spectrograms covering more than 5 years of DEMETER observations. Asymmetric events are more frequently observed over high-latitude regions. The influence of the magnetic field inclination on the observed asymmetry is investigated. First, a full wave method is used to compute, at the lower ionospheric boundary, the electromagnetic pattern that is due to a streak of lightning. This allows us to show a small asymmetry especially for high frequencies (f > 10 kHz). Then, cold plasma dispersive properties are used to determine the characteristics of the waves propagating from the mesosphere to the satellite altitude. We find that, at the plasma cutoff altitude, the propagation is more efficient for waves having wave-normal directions parallel, or quasi parallel, to the Earth's magnetic field direction. Finally, a detailed analysis of an asymmetric V-shaped event is performed. The precise localization of the lightning associated with the V-shaped event is provided by the METEORAGE network. A relationship between the local inclination of the magnetic field above active thunderstorms and the asymmetry of the V-shaped event is pointed out for this particular event.

Passive magnetic attitude stabilization system of the EduSAT microsatellite

Passive magnetic attitude stabilization systems are simple, easy to realize, cheap, and do not require software development and on-board energy consumption. Owing to these features, passive magnetic attitude stabilization systems were selected for the EduSAT (Educational Satellite) microsatellite, a student-built satellite funded by the Italian Space Agency, scheduled to be launched in the last quarter year of 2010. The passive magnetic attitude stabilization system is based on a permanent magnet, which provides a restoring torque to align an oriented axis of the satellite with the Earth's magnetic field direction, and an energy dissipation system, which can consist of a set of permeable rods magnetized by the oscillation of the geomagnetic field along their axis. UNISAT-3 attitude determination results after 1 year from its launch demonstrated the necessity of an accurate design and manufacturing process of soft magnetic strips. Predicting system performance in orbit and evaluating the obtainable accuracy are not trivial: the main problem is knowing the effective magnetization of the permeable rods. The paper deals with sizing, choice of material, manufacturing process, and arrangement of a set of permeable rods on board the EduSAT microsatellite on the basis of previous flight experience.

A 2D micro-fluxgate earth magnetic field measurement systems with fully automated acquisition setup

In this paper we present an Earth magnetic field measurement system and an automated acquisition setup to characterize it. The measurement system consists of a fluxgate sensor and an integrated front-end circuit, both realized in CMOS technology. The couple of orthogonal axes of the sensor makes the system suitable for realizing an electronic compass device. Indeed, we can measure not only the amplitude of the Earth magnetic field (whose full-scale value is of the order of 60 μT), but also its direction. The complete measurement system achieves a maximum angular error of 1.5° in the measurement of the Earth magnetic field direction. Furthermore, an acquisition setup was developed to evaluate the measurement system performance. It consists of a precision mechanical plastic structure, in tower form, a microcontroller-based interface circuit, that provides a digital output through an RS232 serial interface, a PC software suitably developed to post-process the data from the acquisition system and a couple of Helmholtz coils to evaluate the linearity of the system. This setup allows us to perform a completely automated and numerically controlled characterization of the measurement system.

An “Inverse Conglomerate” Paleomagnetic Test and Timing of In Situ Terra Rossa Formation at Bloomington, Indiana

Directions of remanent magnetism in an ∼2-m interval of terra rossa claystone at Bloomington, Indiana, cluster around the current Earth magnetic field direction, a fact consistent with authigenic formation of the terra rossa over the last 0.78 m.yr., the date of the latest earth’s magnetic field reversal, and suggesting a minimum terra rossa formation rate of ∼2: 5m =m:yr: Paleomagnetic directions in limestone “floaters” within the terra rossa are indistinguishable from directions of the bedrock Mississippian Harrodsburg Limestone, indicating that the floaters are in situ and perfectly preserve their original stratigraphic position. Thus, the red claystone containing the floaters is interpreted as an authigenic, isovolumetric replacement of the Harrodsburg—the replacement of which the floaters escaped—and precludes the possibility that the floaters and the surrounding claystone are a mudflow that had carried limestone blocks in it. The implication that the claystone is authigenic independently confirms the petrographic observations reported previously that the red clay grew in situ, replacing the underlying limestone at a moving metasomatic front. Rock magnetic studies of the terra rossa clays show evidence of both goethite and maghemite as main magnetic carriers of this remanence. Both minerals are independently known to occur in pisolites of the terra rossa. The ferric-ironbearing red clays also are weakly magnetic.

Late Paleozoic remagnetization of the Trenton Formation in Ordovician petroleum reservoirs of southwestern Ontario

A paleomagnetic study of subsurface core samples from dolomitized carbonates of two producing reservoirs in the Upper Ordovician Trenton Formation, collected from four wells in southwestern Ontario yielded a paleomagnetic direction of D = 152.3°, I = − 12.3° ( N = 49, α 95 = 8.7). This characteristic remanent magnetization (ChRM) direction was azimuth-corrected by aligning the viscous remanence magnetization (VRM) with the present Earth's magnetic field direction. A drilling-induced magnetization (VRMdi) was present in less than half the specimens sampled in this study. In addition, where the VRM correction could not be made, a paleolatitudinal arc calculated from the inclination-only mean of I = − 9.0° ( N = 34, α 95 = 3.0°) intersected the apparent polar wander path in the Late Permian–Early Triassic. These paleodirections are similar to the paleomagnetic directions observed in Ordovician Trenton carbonates from the Michigan Basin and New York State, U.S.A., suggesting a related regional late Paleozoic remagnetization.

Anisotropic Structure of Small-Scale Ionospheric Turbulence

We consider various theoretical models for the spectrum of small-scale ionospheric turbulence. The particular role of the generalized model of the ionospheric-turbulence spectrum, which takes into account that the anisotropy (extension) of small-scale irregularities of the upper ionosphere along the Earth's magnetic field direction depends on the transverse scale of those irregularities, is emphasized. The results of the. rst target experiments on radio sensing of the midlatitude ionosphere by signals from on-orbit satellites at frequencies 150 and 400 MHz under conditions of increased solar activity are presented. The experiments were performed at the radiophysical facility in the Nizhny Novgorod region in 2003. We studied statistical characteristics of the amplitude fluctuations of the received signals for different angles ϑ between the line of sight from a satellite to a ground-based reception point and the Earth's magnetic field direction. It was found in the course of the experiments that the spectrum slope of amplitude fluctuations of the received radiation is a function of the angle ϑ. The obtained result agrees with the generalized model of the ionospheric-turbulence spectrum and can be an argument in favor of the pronounced anisotropic structure of small-scale electron-density irregularities of the midlatitude ionosphere under disturbed geophysical conditions.

Ｃｏ系アモルファス磁性薄帯／プレーナコイル積層型フラックスゲート二軸センサの試作

A two-axis flux gate magnetic field sensor, with a multilayer structure composed of a Co-based amorphous magnetic ribbon and planar coils, was developed. Since the two planar spiral coils for ac excitation and flux detection were located on the same plane, it was easy to fabricate by the photo-lithography technique. The fabricated two-axis flux gate functioned as an earth magnetic field direction sensor.

Polarization measurement of Auroral Kilometric Radiation by INTERBALL-2 satellite

Preliminary analyses of low and high resolution data are presented from the polar plasma waves measured on board INTERBALL-2 satellite in three frequency bands: ELF (5–1000 Hz), VLF(1–20 kHz) and LF (20–260 kHz). The propagation and polarization characteristics of the Auroral Kilometric Radiation (AKR) emission are estimated and studied. The obtained result shows that R-X and L-O modes of AKR emission just above source regions are found. The propagation characteristics are also determined: wave normal directions weakly oblique or quasi-parallel to the earth magnetic field direction.

Preliminary results from the MEMO multicomponent measurements of waves on-board INTERBALL 2

Abstract. The MEMO (MEsure Multicomposante des Ondes) experiment is a part of the INTERBALL 2 wave consortium. It is connected to a total of six electric and nine magnetic independent sensors. It provides waveforms associated with the measurement of two to five components in three frequency bands: ELF (5–1000 Hz), VLF (1–20 kHz), LF (20–250 kHz). Preliminary analyses of low and high resolution data are presented. The emphasis is put on the estimation of the propagation characteristics of the observed waves.VLF hiss emissions are shown to be mainly whistler mode emissions, but other modes are present. An accurate estimation of the local plasma frequency is proposed when the low L = 0 cutoff frequency is identified. AKR emissions observed just above source regions are studied. R-X and L-O modes are found: the first at the lowest frequencies and the second at the highest. Both propagate with wave normal directions weakly oblique or quasi-parallel to the Earth's magnetic field direction. Propagation characteristics are also determined for a (non-drifting) fine structure of AKR. There is no fundamental difference with structurless events. Nightside and dayside bursts of ELF electromagnetic emissions are presented. It is not clear whether the two emissions belong to the "lion roar" emissions or not.Key words. Magnetospheric physics (auroral phenomena; plasma waves and instabilities; instruments and techniques)

Broadband ELF plasma emission during auroral energization: 1. Slow ion acoustic waves

High‐resolution measurements by the Freja spacecraft of broadband extremely low frequency (BB‐ELF) emission from dc up to the lower hybrid frequency (a few kHz) are reported from regions of transverse ion acceleration (TAI) and broad‐energy suprathermal electron bursts (STEB) occuring in the topside ionospheric auroral regions. A gradual transition of the broadband emission occurs near the local O+ cyclotron frequency (ƒO+ ≈ 25 Hz) from predominantly electromagnetic below this frequency to mostly electrostatic above this frequency. The emission below 200 Hz often reach amplitudes up to several hundred mV/m and density perturbations (δn/n) of tens of %. An improved analysis technique is presented, based on the quantity |δE/(δn/n)| versus frequency and applied to the Freja plasma wave measurements. The method can be used to infer the dispersion relation for the measured emission as well as give estimates of the thermal plasma temperatures. The BB‐ELF emission is found to consist partly of plasma waves with an ion Boltzmann response, which is interpreted as originating from the so‐called slow ion acoustic wave mode (SIA). This emission is associated with large bulk ion (O+) temperatures of up to 30 eV and low electron temperatures (1–2 eV) and therefore occurs during conditions when Te/Ti ≪ 1. The BB‐ELF emissions also contain other wave mode components, which are not equally easy to identify, even though it is reasonably certain that ion acoustic/cyclotron waves are measured. The ion Boltzmann component is characterized by a dominantly perpendicular polarization with respect to the Earth's magnetic field direction and a small magnetic component with amplitudes around 0.1–1 nT. The ion Boltzmann component dominates the lower‐frequency part (30–400 Hz) of the BB‐ELF emissions. The BB‐ELF emission have often an enhanced spectral power when certain waveform signatures, interpreted as solitary kinetic Alfvén waves (SKAW), or when large‐amplitude electric fields, possibly related to black aurora, are encountered in regions often associated with large‐scale auroral density depletions. A scenario where the SKAW provides the original free energy and via the BB‐ELF emission causes intense transverse ion heating (TAI) is suggested.

Turbulence spectrum of the upper ionosphere

The problem of defining the spectral form of ionospheric irregularities with dimensions from hundreds to thousands of meters is considered. A generalized model is proposed for the ionospheric turbulence spectrum, taking into account both the anisotropic properties of the large-scale fraction of irregularities and the dependence of the anisotropy (elongation) of small-scale irregularities of the upper ionosphere along the Earth magnetic field direction on the transverse scale of those irregularities. Relations have been derived to determine the basic parameters of the irregularity spectrum of the uppers ionosphere (anisotropy indices for large-scale and small-scale fractions) and the depth of a thin ionospheric layer through measurement of the spectral characteristics of amplitude and phase fluctuations of orbital satellite signals. Using this model of the plasma irregularity spectrum, we can explain consistently many well-known experimental data on spectral characteristics of the phase and amplitude fluctuations of orbital satellite signals both in the high-latitude and midlatitude ionosphere.

Severe remagnetization revealed from Triassic platform carbonates near Guiyang, Southwest China

An intensive paleomagnetic investigation has been conducted on the Middle and Lower Triassic platform carbonates near Guiyang, Southwest China in the hope of refining a Middle Triassic paleomagnetic pole previously reported from the region. Instead, severe late Cretaceous to Recent remagnetizations have been revealed from all the three rock formations studied. In all cases, the in situ characteristic remanent magnetization (ChRM) directions resemble or fall close to the local present-day Earth's magnetic field direction and fail the fold test, despite the presence of dual polarity in two formations. The major magnetic carriers of the remagnetizations are hematite and magnetite. The cause(s) for the remagnetization events cannot be ascertained at present, but weathering in recent time and migrating fluid in the past might be responsible. It is suggested that caution should be exercised when investigating platform carbonate rocks which are prone to late remagnetization.

Palaeomagnetic Dating of Mineral Deposits: A Brief Review and a Western Australian Perspective

Published examples show that the palaeomagnetic method may furnish constraints upon the timing of a mineralisation for a variety of ore-deposits. Success of the palaeomagnetic method relies upon: 1. Knowledge of the apparent polar wander path (APWP) for the continent or terrane in which the deposit occurs. 2. Crystallisation of new magnetic minerals in a deposit or its wall-rocks during a principal phase of mineralisation, constituting a magnetisation event. 3. Duration of the mineralisation event having been sufficiently long to average palaeosecular variation of the Earth's magnetic field. 4. The ore deposit having a sufficiently isotropic magnetic fabric to record accurately the Earth's magnetic field direction. 5. Knowledge of the history of tectonic disturbance of the mineral deposit. 6. A lack of complete alteration of the magnetic phases, leading to total palaeomagnetic overprinting in a later, and different, magnetic field-direction. Even when a number of these requirements for dating are not met, the palaeomagnetic method still has the potential to discriminate syngenetic from epigenetic mineralisation or provide a minimum estimate of mineralisation-age. By sampling both the ore deposit and the country rocks surrounding the zone of mineral alteration, it can be established whether the two have comparable or different magnetisation histories. The former relationship characterises a syngenetic deposit; the latter an epigenetic deposit. In the case of total remagnetisation of an ore, only a minimum mineralisation age can be estimated from the remagnetisation direction. The usefulness and precision of the palaeomagnetic method varies from continent to continent and with mineralisation age. In favourable circumstances, a precision of within half a geological period is estimated during Phanerozoic times. Precision generally decreases with increasing mineralisation age. Potential applications of the method to Western Australian base-metal and iron-ore deposits serve to illustrate the dependence of the method on (i) the density of existing palaeomagnetic data and (ii) the rate and course of APW at the approximate time of mineralisation.

Separation of Paleomagnetic Secular Variation Components: A Method and Its Application to a Sedimentary Record from Japan.

A new method is introduced to analyze paleosecular variations of the earth's magnetic field direction. The method consists of the following four steps. First, a time series of a pair of declination and inclination is transformed into two components defined in ADF reference frame. Second, characteristic periods are estimated with a spectral analysis. Third, the secular variation components of the periods are separated through band-pass filters. Finally, the motion of field vectors is investigated with respect to its curvature on a polar projection for each secular variation component. The present method has been applied to the sedimentary secular variation record from central and southwest Japan for the last 12000 years. The results indicate two main periodic motions of field vectors with the periods of about 700 and 2350 years. The 2350 years component is dominated by smooth clockwise motions. This suggests that the westward drifting field has persisted almost steadily for the last 12000 years. However, the breakup of the clockwise motion took place between 4500 and 8100 years B. P., which may have been generated by an accidental temporary standing field. The 700 years component is characterized by the thin elliptic motion whose major axes remained almost constant during more than 1750 years long and rotational sense changed frequently. This elliptic motion may have been caused by intensity changes in standing fields added with week drifting fields.

Spin modulation of spectrally broadened VLF signals

The spectra of broadened VLF signals from ground transmitters received on the spinning ISIS spacecraft have a characteristic temporal dependence when the long receiving dipoles pass close to parallel to the earth's magnetic field direction. As the angle between the dipole and the field direction passes its minimum, the spectra evolve from having enhanced negative Doppler components to having enhanced positive Doppler components or vice versa. The relative orientation of the spin, spacecraft velocity, and magnetic field vectors determines whether the enhancement evolves from positive to negative Doppler or from negative to positive. Theoretical calculations of the Doppler spectra agree with observations from the ISIS spacecraft. These analyses show that wave vectors corresponding to broadened spectra are confined to a narrow range of azimuths about the magnetic field direction. The wave vectors are distributed through a range of polar angles with respect to the magnetic field direction, possibly as the result of scattering of the upcoming waves in the ionosphere below the spacecraft. The technique has been used to deduce the preferred azimuth in the case of one spectrum.