Orientation Of Geomagnetic Field Research Articles

Theoretical Analysis on Affecting Factors of Power Line Harmonic Radiation

Power line harmonic radiation (PLHR) is an artificial electromagnetic pollution source in the near-earth space environment. Up until now, there has been little quantitative research on its formation mechanism. This paper details a model and solver for electromagnetic waves propagation in a stratified anisotropic ionosphere. In our model, these electromagnetic waves are generated by an arbitrary electric dipole source above a nonideal conductive ground. The matrix-exponent method and a full-wave method are combined to analyze the model, avoiding the problem of numerical overflow and swamping in numerical calculations. The computing results obtained from the proposed method and the observed electric field data by using DEMETER satellite are compared and found to be in good agreement. Furthermore, this paper considers the effects on PLHR propagation in the ionosphere due to dipole source frequency, earth’s conductivity, electron density, geographic latitude, and geomagnetic field orientation. Results show that when the frequency increases, the strength of PLHR penetrating into the ionosphere has an increasing tendency. If the harmonic current remains constant, lower ground conductivity and electron density of the ionosphere result in higher radiation power. PLHR propagates along the direction of the geomagnetic field in the ionosphere, and its strength decreases with the increasing geomagnetic inclination at the same height. In addition, the radiation power of less than 1 W may trigger PLHR.

Micromagnetic analysis of the maghemite platelet chains in the iron-mineral-based magnetoreceptor of birds

Experiments demonstrate that large varieties of animals have the ability to sense the magnetic field. Especially, some birds can use the geomagnetic field for navigation. According to the avian magnetoreception mechanisms, birds can build magnetic map for orientation and navigation by sensing the intensity and incline of geomagnetic field. However, researches on the magnetoreceptor of birds are still at the stage of discovery and identification, and it is necessary to study the iron-mineral-based structures to know whether they have the physical and magnetic properties to act as magnetoreceptor in living system. In this paper, the maghemite platelet chains in the iron-mineral-based magnetoreceptor have been studied with three-dimensional simulation based on the theories of micromagnetics. The reaction of the maghemite platelet chains with geomagnetic field has been investigated. Result indicates that the maghemite platelet chains with vortex domain can response to the change of the orientation of geomagnetic field.

Modeling the effects of secular variation of geomagnetic field orientation on the ionospheric long term trend over the past century

A middle‐ and low‐latitude ionospheric theoretical model is used for the first time to assess the effects of the secular variations of geomagnetic field orientation on ionospheric long‐term trends over the past century. It is found that the varied geomagnetic field can produce ionospheric long‐term trends in both foF2 and hmF2. Since the amplitudes of geomagnetic field change depend on location, the modeled trends by secular variations of geomagnetic field orientation differ from place to place. The modeled ionospheric trends have obvious seasonal and diurnal variations. These variations show typical regional features. By comparison with existing results, we suggest that the changes of the global geomagnetic field may partly contribute to the inconsistent seasonal and local time variation patterns in ionospheric trends from different observations. The average trends of foF2 and hmF2 are −0.0047 MHz/a and −0.0107 km/a during 1900–2005, which is near or even comparable with those of other trend origins. In comparison with the trend results from worldwide observations, we conclude that the effects of geomagnetic field orientation on the ionospheric long‐term trend cannot be ruled out, especially in areas with large geomagnetic field variations.

Installation age of limestone masonry determined from its viscous remagnetization

AbstractMany rocks passively acquire some time‐dependent or “viscous” remanent magnetism (VRM) at ambient temperatures, without any extraordinary energetic intervention. This magnetization overprints existing remanent magnetization so that it is effectively a remagnetization subparallel to the contemporary geomagnetic field, averaging the geomagnetic field orientation. Certain limestone masonry remagnetizes viscously over an archaeologically useful interval (100 to 8000 Ka) so that the degree of remagnetization is monotonically (but not linearly) related to the construction age. The laboratory unblocking temperature (TUB) that removes the viscous magnetization is a simple monotonic measure of relative age. The longer a piece of masonry remained stabilized in a certain orientation, the greater is its viscous remagnetization and the higher is its TUB. Monuments of known age with a similar limestone source permit us to establish a calibration curve of T UB against historical ages. The resulting calibration curve may then be used to predict the ages of otherwise‐undated masonry. Viscous remanent magnetism dating provides precision of <50a in medieval monuments in England and <150a precision for classical to Neolithic monuments in Cyprus; precision depends on the remagnetization rate of the limestone in question. Our calibration curves, for the Jurassic Oolitic Limestone of England and for the Lefkara‐Pakhna Chalks of Cyprus, allowed us to investigate the authenticity of a medieval English synagogue in Lincoln, England, and of a medieval house in Cyprus. Multiple archaeologic VRMs show that masonry was recycled in historical times. © 2006 Wiley Periodicals, Inc.

Variations of quasi-electrostatic fields and ionosphere potential above lightning discharge at equatorial latitudes

Lightning discharges by thunderstorms cause generation of electromagnetic pulses and of quasi-electrostatic fields (QESF) in the atmosphere above, which occur in different time-scales. QESF penetrate into the mesosphere and the lower ionosphere where they are big enough to generate considerable electric charge transfer there and, in some cases, to cause red sprites. These processes may have an important contribution to the global atmospheric electric circuit. Significant transient variations of the ionospheric potential above the thunderstorm take place as well. QESF depend on the atmospheric conductivity and in the ionosphere they are affected also by its anisotropy determined by geomagnetic field orientation. QESF after a lightning discharge are investigated theoretically in this work in the case of equatorial latitudes (by horizontal geomagnetic field), where thunderstorms are important contributors to the global circuit. Results for DC electric fields in the lower equatorial ionosphere above a thundercloud obtained by earlier models demonstrate some specific features of the spatial distribution of these fields, which appear due to geomagnetic field orientation. Thus, the electric fields can be shifted by tens or more kilometers to east of the cloud charge region; also their horizontal scale is much bigger than in the case of middle latitudes. Here, a presence of similar specific features of quasi-electrostatic field distributions and ionospheric potential variations caused by a lightning stroke is studied. A situation when no secondary ionization is generated is considered. A model based on the Maxwell equations for potential electric fields is proposed. Computations of QESF in the middle atmosphere and of the ionospheric potential variations are provided as dependent on conductivity and its anisotropy in D-region. The obtained results for the ionosphere show that the electric fields in the equatorial lower ionosphere are comparable to these formed in the case of middle latitudes. However, their horizontal scales are much bigger and depend on conductivity profiles. Similar features are valid also for the ionospheric potential variations and for their horizontal scales.

Effect of horizontal gradients on ionospherically reflected or transionospheric paths using a precise homing-in method

A homing-in method is presented for determining ionospheric reflected or transionospheric paths between fixed transmitter and receiver locations in the presence of ionospheric gradients or ripples. Both initial elevation and azimuth are automatically adjusted to find the path that arrives exactly at the receiver. The method can be used for any 3D ionospheric model to find precise ray paths and phase and group delays for both magneto-ionic modes. The method takes full account of path location, geomagnetic field orientation and the bending of the ray path resulting from horizontal as well as vertical gradients of electron density. It can also find multiple paths e.g. low and high angle, 1- and 2-hops for both ordinary and extraordinary modes. Examples of its use are given for both terrestrial HF links and Earth to Satellite paths. For paths reflected from the ionosphere, the effect of gradients of both critical frequency and height of maximum electron density are determined and the comparative effect of gradients on high and low angle and 1- and 2-hops paths for both magneto-ionic modes investigated. Path variation with frequency for a fixed link is also studied and the bandwidth of the ionospheric background channel (dispersive bandwidth) and its reciprocal (the pulse rise time), important for wideband digital HF broadcasting or spread spectrum HF communications, is estimated for a range of frequencies, for high- and low-angle rays and 1- and 2-hop paths. For Earth–satellite paths, the effect of the ionosphere and horizontal ionospheric gradients is determined for a range of frequencies and elevation angles. It is shown that the method can also enable the determination of second-order errors in satellite navigation methods, such as GPS, due to ionospheric gradients and the effect of the geomagnetic field.

Dispersion equations of low-frequency geomagnetic disturbances in the E-layer

The effect of the altitude distributions of the Hall and Pedersen conductivities on the dispersion equations of lowfrequency geomagnetic disturbances (GMD) is studied. Single-layer and two-layer models of the E-region are examined with allowance for the earth's conductivity and wave radiation into the magnetosphere. The form of the dispersion equations and the physical mechanisms of GMD propagation along the E-layer are analyzed as functions of the geomagnetic-field orientation.

Propagation of low-frequency electromagnetic waves in the earth-ionosphere waveguide

Excitation of the earth-ionosphere waveguide by low-frequency electric and magnetic sources is considered. A planar model of the waveguide, bounded by an ideally conductive earth and a homogeneous ionosphere, is used for calculations. The direction of the external magnetic field is arbitrary. Expressions are obtained for fields in the ionosphere for the case of a terrestrial source localized in the waveguide, with neglect of beam dispersion. Wave-packet-cent er displacement is dependent on geomagnetic field orientation. Expressions are presented for fields created near the earth by an arbitrary system of ionospheric currents. In the case of waveguide excitation by localized ionospheric sources a pattern of ,effective sources n located at the upper boundary of the waveguide is obtained. Relative effectiveness of dipole sources of various types and orientations is considered. The results obtained may be used for studies of tow-frequency signals of artificial and natural origin. It is well known that propagation of low-frequency electromagnetic waves under terrestrial conditions is of a waveguide nature. This effect is created by reflection of the waves from the ionosphere and the earth, where indices of refraction are high. The question of wave propagation in the earth-ionosphere waveguide has been examined in numerous studies, of which we will note [1-5]. In the general case the problem is complex, since Maxwellis equations must be applied to a spherical earth surrounded by an inhomogeneous anisotropic ionosphere. At the present time this problem has not been solved. The present study will consider the model of a plane waveguide located between an ideally conductive earth and a homogeneous anisotropic ionosphere. Such an idealization is quite coarse, since ionospheric plasma parameters vary with altitude ia a complex fashion (see, for example, [3]). Moreover, neglect of the earthrs curvature allows consideration of fields only at distances small compared to the earth,s radius. Consideration of the attitude dependence of indices of refraction for normal waves makes it impossible to obtain analytical results. Numerical computation methods are required. Inasmuch as numerical methods do not always present the complete picture of wave propagation, it becomes important to consider relatively simple model problems, which allow production of analytical results and clarification of certain propagation peculiarities. This is the goal of the present study.

Further studies of “Spread- F” at Brisbane-I experimental

Rotating-spaced loops at Brisbane recorded the azimuths-of-arrival of pulsed 3.84 Mc/s signals from Armidale (202°, 355 km from Brisbane); also pulsed signals at the same frequency, from a transmitter on the site of the loops. These and other normal-incidence records indicate that the irregularities of the F2-layer responsible for “Spread- F” records at Brisbane, are ripples of considerable lateral extent with a “wavelength” varying from 20 km to over 100 km. The position of the ripple in each ionization contour changes with height in such a manner as to suggest a simple relationship with the geomagnetic field orientation. Seasonal variations of the phenomenon show a winter maximum and a summer sub-maximum, and there is a distinct inverse sunspot-cycle relationship. The type of spreading defined as range-spreading generally has a peak of occurrence around midnight, and that defined as frequency-spreading peaks during the sunset and sunrise periods, after an allowance is made for a diurnal variation associated with the changing f 0> F2 value.