Natural Geomagnetic Field Research Articles

Influence of geomagnetism on the activity and orientation of the eel, Anguilla anguilla (L.), as evident from laboratory experiments

Abstract– Studies were performed with both a mechanical and a photosensory arrangement of the experimental tanks. Silver (migratory) eels were more active than yellow (stationary) eels. Comparison of pooled data on yellow eels tested under natural geomagnetic field conditions with those on eel tested under unnatural ones prcded no evidence of difference in activity. Silver and yellow eels showed stronger activity during the 6 days around new moon compared with the phase around full moon as well as a peak before new moon and a smaller peak before full moon. Under each kind of unnatural field condition, the activity relationships reversed or were cancelled. The directional choice of eels under natural conditions (controls) pointed to magnetic NW and mostly also in the opposite direction. When the magnetic north was changed by 90°, the tested yellow eels followed this change to about the same degree. The additional change of both the inclination (from 68° or 45° or 30°) and total intensity (by a reduction to 31% of the natural field) resulted in a change of the preferences of about 90° relative to magnetic north. Reversal of magnetic north by 180°, which is identical to a change of the inclination from 68° to 112°, also resulted in a change of the preferences by about 90°. Three experiments using compensation of both the horizontal and the vertical geomagnetic field led to circular directional preferences at random in two cases and in one case to about NW of the residual field magnetic north. The navigational abilities of the eel, on the basis of its magnetic sensitivity, are discussed.

Shifted magnetic fields lead to deflected and axial orientation of migrating robins, Erithacus rubecula, at sunset

The migratory orientation of the robin was tested in shifted magnetic fields during the twilight period after sunset, under clear skies and under simulated total overcast. The horizontal direction of the geomagnetic field was shifted 90° to the right or left in relation to the local magnetic field, without changing either the intensity of the field or its angle of inclination. Experiments were conducted during both spring and autumn, with robins captured as passage migrants at the Falsterbo and Ottenby bird observatories in southern Sweden as test subjects. Generally, the orientation of robins was affected by magnetic shifts compared to controls tested in the natural geomagnetic field. Autumn birds from the two capture sites differed in their responses, probably because of different migratory dispositions and body conditions. The robins most often changed their orientation to maintain their typical axis of migration relative to the shifted magnetic fields. However, preferred directions in relation to the shifted magnetic fields were frequently reverse from normal, or axial rather than unimodal. These results disagree with suggested mechanisms for orientation by visual sunset cues and with the proposed basis of magnetic orientation. They do, however, demonstrate that the geomagnetic field is involved in the sunset orientation of robins, probably in combination with additional visual or non-visual cues that contribute to establish magnetic polarity.

An automatic system for data logging and verification of multi-channel, multi-size geophysical data

William Gilbert announced in 1600 that the Earth is a great magnet. Since that time, numerous methods and techniques have been developed to utilise the many properties of the Earth’s natural magnetic field. One such geophysical technique utilises the time changes in the natural geomagnetic field, which occur with frequencies from kilohertz to periods of the order of one day, to investigate the electrical properties of the Earth. When time changes with periods of the order of several seconds to one day are used, the technique is referred to as the magnetotelluric (MT) method. The MT method is a way of determining the distribution of the Earth’s electrical conductivity structure from measurements of the naturally occurring time changes in the electric and magnetic fields on the ground surface. For a complete description of the geo-electromagnetic field, five orthogonal field components must be recorded. The five data components are referred to as X, Y, H, D and Z, all being functions of time. The first two data components are the potential differences between two points on the Earth’s surface. X is a measure of the induced electric field in the north-south direction, Y is a measure of the electric field in the east-west direction. H, D and Z represent the three orthogonal components of the magnetic field. H is a measure of the magnetic field in the north-south direction, D is a measure in the east-west direction and Z is the vertical component. In practice, a sixth data channel is also recorded. This channel is temperature which is used to correct the instruments response to thermal effects. The six-channel MT instrumentation produces analogue output of the field values. These voltages are sampled at a discrete rate that defines the sampling interval simultaneously on all six channels. A discrete six-channel time series, suitable for computer analysis, is thus formed. The goal of each field MT system is to obtain such data over a long continuous period of time (say six months) to act as a data base for subsequent analysis. The data base will consist of time variations of the Earth’s electric and magnetic fields recorded every 5 s. Five such field systems are arranged in a spatial array, giving 30 data channels of simultaneous magnetotelluric data. Such an MT-based field monitoring system is to be set up in western Turkey, a region which is seismically active and earthquake prone. The purpose of the experiment is to use the recorded data base to investigate stress-induced changes in the crust of the region which may be detected by the fields recorded.

Geomagnetic anomaly detected at hydromagnetic wave frequencies

We report the discovery, in northwestern Illinois, of a geomagnetic anomaly, using hydromagnetic wave frequencies as the source spectrum. Three portable magnetometer stations with computer‐compatible digital data acquisition systems were operated in a longitude array at Piano and Ashton, Illinois, and Cascade, Iowa (total separation ∼200 km), in 1981–1982. Analysis of the natural geomagnetic field fluctuations in the hydromagnetic wave regime reveals that the vertical components of the detected fluctuations are essentially 180° out of phase between Plano/Ashton and Cascade for variations with periods ∼30–120 s. The observations can be modeled in terms of a shallow (∼10–20 km) north‐south oriented geomagnetic anomaly of enhanced conductivity located between Ashton and Cascade, approximately parallel to the Mississippi River valley.

Conductivity anomalies: lower crust or asthenosphere?

The indication from surface measurements of a zone of relatively high conductivity (resistivity<200 ohm-m) at depths between 20 and 50 km has become so general over the Earth that regions without this zone can be considered anomalous. However, the depths indicated often span the lower crust and the uppermost mantle, so that before any effect can be definitely attributed to one or the other, the depth resolution in the electromagnetic measurements must be carefully considered. This paper applies the eigenvector decomposition of generalized linear inverse theory to soundings by Schlumberger resistivity, by magnetotellurics, by man-made electromagnetic fields formed by controlled current flow in grounded electric transmission lines, and by natural magnetic field variation studies to improve the bounds on depth, thickness and conductivity of a conductive layer. It is shown that many of the methods are capable of giving the depth to the top of a conductor with remarkable accuracy. Joint inversion of two or more of them offers an advantage in the separation of thickness and conductivity of both conductive and resistive layers. Natural geomagnetic field transfer functions, while accurately mapping the position of the edge of a conductor, do not provide the resolution of the other techniques, largely because the frequencies that can be practically measured at present are much too low.

Relative Importance of Stars and the Magnetic Field for the Accuracy of Orientation in Night-Migrating Birds

In the natural geomagnetic field or a field of comparable intensity (0.46 Gauss), the vector length rm of the test series, calculated from the headings of the individual test nights, was similar in the presence and absence of stars, a slight increase under stars being observed only for the robin. The concentration ai around the selected direction was, however, strongly increased by the stars; artificial and natural ones having the same effect. In a reduced magnetic field (0.34 Gauss), no preferred heading direction was found, but again the concentration ai was greatly increased by the presence of artificial stars.

ANALOGUE MODEL MEASUREMENTS FOR ELECTROMAGNETIC VARIATIONS NEAR A COASTLINE

An analogue model for studying the behavior of the natural geomagnetic and telluric field variations for the coastline problem is discussed. Measurements of amplitudes and phases for the horizontal electric, the horizontal magnetic, and the vertical magnetic field components for sloping earth–sea interfaces and for a sea over an upwelling conducting zone within the mantle in the form of a step are discussed for both the E and the H polarizations.

ANALOGUE MODEL MEASUREMENTS FOR ELECTROMAGNETIC VARIATIONS NEAR VERTICAL FAULTS AND DYKES

An analogue model for studying the behavior of the natural geomagnetic and telluric field variations for various geological structures is described. Measurements of amplitudes and phases for the horizontal electric, the horizontal magnetic, and the vertical magnetic field components are obtained and discussed for various conducting vertical faults and dykes for both the H and the E polarizations.

The biological effects of magnetic fields—A survey

The literature relevant to the biological effects of applied magnetic fields has been reviewed. Reported effects vary with the type of field applied and the duration of application. The production of magneto phosphenes by the application of alternating fields (of certain frequencies) is a well accepted phenomenon. The major question, at this time, seems to be the mode of action responsible. Medium to high strength, non-uniform, steady-state fields have well documented effects upon rapidly growing tissues. Steady-state fields with strengths approximating the natural geomagnetic field are reported to produce alterations in the pattern of biological cycles.