The H and Z variations for the Five International Quiet Days, each in October 1978 and February 1979, were compared for Huancayo (12°S, 75°W, dip+1.9°) and Eusébio near Fortaleza (4°S, 39°W, dip -3.5°). The H ranges at both the locations fluctuated widely but not parallel to each other, indicating violent, rapid space-time variations of the equatorial electrojet characteristics. The Z patterns were generally dissimilar at the two locations and indicated large scale latitudinal meanderings of the electrojet center from day to day as also during the same day.

View full abstract

Satellites that observed magnetic field in the near-earth environment are mentioned, before the review of relevant studies of satellite data. The contributions, problems and prospects of satellite observations of equatorial electrojet fields are then critically discussed in relation to the major results from satellite data such as: the deviation of electrojet axis from the dip equator; the indications of return currents of the electrojet; the oscillation of electrojet field with altitutde; the indications of reversed signature of the electrojet; the proof of the existence of two components and separable parts of the Sq field (worldwide Sq and electrojet fields) observed at the dip equator; longitudinal and temporal variations of electrojet parameters; the constriction of electrojet width and its associated threshold intensity and total current; and the new evidence of meridional current from MAGSAT data. Finally, the main conclusions and recommendations for future satellite observations of equatorial electrojet fields are briefly listed.

View full abstract

The macroscopic MHD equation for the dynamic balance of the flowing plasma in the Jovian magnetodisc region has been solved for the aligned rotator model where the magnetic field rotates with the symmetrical configuration with respect to the magnetic equator that coincides with the perpendicular direction to the rotation axis. The equatorial region of the magnetodisc studied here is restricted in a limited azimuthal extent called “flow region”. The results indicate that the plasma is flowing out due to the centrifugal force forming the disc wind that blows outward with super magnetosonic velocity when the plasma approaches a critical region. The expansion of the disc current is also made as a result of the outflow of the plasma with slight differences of electron and proton velocity. In the region of the disc plasma, the magnetic field is frozen in the flowing plasma; there is a transient region, therefore, called here the internal magnetopause that separates the magnetic lobe of the Jovian magnetosphere from the flowing disc plasma. The plasma flow is interrupted at the inner boundary of the magnetopause forming the balance of the dynamic pressure between the solar wind and the disc wind. This inner boundary is sandwitching the intrinsic Jovian magnetic field with the outer boundary which is formed due to the solar wind interaction with the Jovian magnetic field.
When the solar wind pressure increases, the position of the magnetopause is compressed with heating up effects on the disc plasma. The plasma flow can not, then, exceed the magnetosonic velocity and no disc wind is formed. The balance feature of the disc wind and the solar wind, thus, controls the location of the Jovian magnetopause so called spongy nature after the observation by the field and particle instruments onboard Pioneer 10, 11, Voyager 1 and 2.

View full abstract

Polarization and wave form of geomagnetic pulsations observed at multiple stations on the ground below a streaming aurora, a propagating aurora and a large cluster of pulsating auroral patches were examined and compared with magnetic effects of current systems in and around model auroral patches. Conclusion is that both the polarization and the wave form of magnetic pulsations at various ground points are fully accounted for in terms of electric currents, a field-aligned pair current and a twin-vortex current, which are induced in an ionization tail in the ionosphere behind a moving auroral patch in conjunction with the convection electric field, and move following the patch migration. Since the irregular magnetic pulsation in the dawn auroral zone often tends to be continuously followed by auroral zone pc-3 in the daytime, the result strongly suggests that the auroral zone pc-3 may, some portion at least, also be caused by the same mechanism by the local fluctuations in conductivity in the ionosphere.

View full abstract

Magnetotelluric surveys by using natural noises in the Schumann frequency band (8-40Hz) were made to clarify the resistivity structure of two strike-slip type active faults, the Yamasaki and the Hanaori faults in southwest Japan. Micro-earthquakes are reported to occur frequently within narrow belts along them.
Broad low-resistivity zones were found along the Hanaori and the Yamasaki faults. Their widths are about 3km for the Hanaori fault and about 6km for the Yamasaki fault. Resistivity in the zone is smaller by 1 order of magnitude, relative to the surrounding earth. This fact suggests that the wide areas along the main axis of the two faults are weakly fractured and probably filled with water. The low-resistivity zone is a good indicator for a fault activity and its existence strongly suggests that the fault is active. The magnetotelluric method is one of the effective means to find such active faults.

View full abstract

Remanent magnetization directions were obtained from 28 red sandstone samples of the Lower Cretaceous Atajaña Formation. Paleomagnetic studies were also performed on 103 igneous rock samples taken from 19 dikes in the Arica dike swarm, which intrudes into the Atajaña Formation sandstone. They showed 10°-15° counterclockwise rotation of the Arica region, northernmost Chile, with respect to the stable South American platform. These data suggest that the northernmost Chilean region underwent a tectonic rotation at least after the Cretaceous period, as the southern end of the Peruvian block.

View full abstract

This review of archaeomagnetism covers advances made during the last three years in (i) instrumentation for determining the direction and magnitude of the archaeomagnetic field, (ii) analytical and sampling techniques and (iii) the production of large data sets of both secular variation and field magnitude.

View full abstract

During the last four years the directly measured geomagnetic polarity time scale covering the last 5Ma of geological time has not been changed significantly, but several short intervals of inverse polarity appear to have been authenicated from measurements on volcanic rocks. The polarity time scale extending to about the Middle Jurassic (-165Ma) is based upon interpretation of marine magnetic anomaly data. For the interval extending to -80Ma ago several new time scales have been published in recent years, with calibration by means of the directly determined scale for the last 3.5Ma, and then by use of a number of calibration points relating individual magnetic anomalies to the numerical time scale, usually through biostratigraphy and less commonly by direct dating of volcanic sequences. Considerable improvements have been effected, but further changes will occur in the future. From about 80Ma ago to the limit of the marine magnetic anomaly data, calibration is far less well controlled. Only the broad pattern of geomagnetic field reversals is known from the Middle Jurassic to the Middle Paleozoic, with little information available for earlier times.

View full abstract

Paleomagnetic research efforts concerning field behavior during polarity transitions have significantly increased in number and scope over the past few years. This report attempts to summarize the state of our present knowledge regarding geomagnetic reversal by highlighting recent advances. In addition, possible directions for future work will be explored.

View full abstract

The ongoing decrease of the earth's dipole moment has repeatedly been interpreted as a decay of the whole geomagnetic field. However, the dipole part of the secular variation loses its predominant role, if the field is reduced to the depth of the source layer, some 100km below the core/mantle boundary, as inferred from the spatial energy density spectrum of the field. It is shown that the mean magnetic energy density at that depth is already balanced if only two more spectrum terms, i.e., the quadrupole and the octupole constituents, are included. This result strongly supports the concept of the presently observed secular variation to be caused primarily by structural changes of the current system in the source layer, associated with an exchange of energy between spherical harmonics of different degrees, without involving a growth or decay of the field as a whole.

View full abstract

Magnetic change associated with various mechanical models can be generated from four kinds of sources: i.e. ‘free air’ magnetic change (M1), magnetic terrain effect (M2), effect of source volume change (M3) and piezomagnetic field (M4). For the Mogi model as an example, relative magnitude is compared among these contributions. It is shown that |M4|>>|M2|-|M3|>>|M1|.

View full abstract

In view of a possible seismic gap in the western part of the North Anatolian Fault Zone, we carried out electric and magnetic observations in the Iznik-Mekece area, aiming primarily at searching for location of an active fault and disclosing an anomalous structure associated with it. For reference, we also made similar observations at the Ismetpasa area where an earthquake of magnitude 7.6 took place in 1944 and fault lines are well known. The preliminary results of observations are summarized as follows: in most cases, magnetic anomalies are found along fault lines, and the ground resistivity tends to be lower near fault lines. Comparison of these anomalous features in both the observation areas indicates that the active fault found in the Iznik-Mekece area is similar to the fault in the Ismetpasa area. It is suggested, therefore, that the fault in the Iznik-Mekece area may be one of the candidates for future earthquake generation in the seismic gap region.

View full abstract