Showing papers on "Earth's magnetic field published in 1993"
TL;DR: In this paper, it was shown that sedimentary magnetizations varied in accordance with the magnetic field, albeit not always in a simple, linear fashion, and these results spurred the hope that variations in the Earth's magnetic field might indeed be recoverable from appropriate sedimentary sequences.
Abstract: Sediments have proved irresistible targets for attempts at determining the relative variations in the Earth's magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution. The assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited. Early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short, no dependence on magnetic field was expected. Nonetheless, a number of experiments showed that sedimentary magnetizations varied in accordance with the field, albeit not always in a simple, linear fashion. Experiments in which the sediments were stirred in the presence of a field (to simulate bioturbation) showed a reasonably linear relationship with the applied field, and these results spurred the hope that variations in the Earth's magnetic field might indeed be recoverable from appropriate sedimentary sequences. Examination of existing paleointensity data sets allows a few general conclusions to be drawn. It appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the Earth's magnetic field. The dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences. Moreover, when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences, there is considerable agreement among the various records. When viewed over timescales of hundreds to thousands of years, relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another, suggesting that they are dominated by nondipole field behavior. When viewed over timescales of a few tens of thousands to hundreds of thousands of years, however, the records show coherence over large distances (at least thousands of kilometers) and may reflect changes in the dipole field. On the basis of a sequence spanning the Brunhes and Matuyama chrons, the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years, but these oscillations are not clear in the record prior to about 300 ka; thus they are probably not an inherent feature in the geomagnetic field, and the correspondence of the period of oscillation to that of obliquity is probably coincidence.
565 citations
TL;DR: A long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes: major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals, whereas stable polarity states are characterized by a slow (∼0.5 Myr) relaxation process as mentioned in this paper.
Abstract: A long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes. Major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals, whereas stable polarity states are characterized by a slow (∼0.5 Myr) relaxation process. Reversals can be seen as the consequence of a progressive degradation of the dipole field.
372 citations
TL;DR: In this article, a snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge, producing a latitudinally narrow region of storm-enhanced plasma density (SED) and increased total electron content which is advected toward higher latitudes in the noon sector.
Abstract: Associated with the large-scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions, solar-produced F region plasma is transported to and through the noontime cleft from a source region at middle and low latitudes in the afternoon sector. As a result of the offset between the geomagnetic and geographic poles, the afternoon sector region of strong sunward convection is shifted to increasingly lower geographic latitude throughout the interval between 12 UT and 24 UT. A snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge, producing a latitudinally narrow region of storm-enhanced plasma density (SED) and increased total electron content which is advected toward higher latitudes in the noon sector. The Millstone Hill incoherent scatter radar regularly observes SED as a spatially continuous, large-scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid- or low-latitude source region. For local times away from noon, the latitude of most probable SED occurrence moves equatorward by 6° for an increase of 2 in the Kp index. During strong disturbances the topside SED is observed to be convecting sunward at ∼750 m s−1 with a flux of 1014 m−2 s−1. This feature accounts for the pronounced enhancement of ionospheric density near dusk at middle latitudes observed during the early stages of magnetic storms (called the dusk effect) and constitutes a source for the enhanced F region plasma observed in the polar cap during disturbed conditions.
364 citations
TL;DR: In this paper, high-resolution magnetic field and plasma measurements of an interplanetary magnetic cloud and its interaction with the earth's magnetosphere on January 14/15, 1988 are interpreted and discussed.
Abstract: High time resolution interplanetary magnetic field and plasma measurements of an interplanetary magnetic cloud and its interaction with the earth's magnetosphere on January 14/15, 1988 are interpreted and discussed. It is argued that the data are consistent with the theoretical model of magnetic clouds as flux ropes of local straight cylindrical geometry. The data also suggest that this cloud is aligned with its axis in the ecliptic plane and pointing in the east-west direction. Evidence consisting of the intensity and directional distribution of energetic particle in the magnetic cloud argues in favor of the connectedness of the magnetic field lines to the sun's surface. The intensities of about 0.5 MeV ions is rapidly enhanced and the particles stream in a collimated beam along the magnetic field preferentially from the west of the sun. The particles travel form a flare site along the cloud magnetic field lines, which are thus presumably still attached to the sun.
217 citations
TL;DR: In this paper, UFL electromagnetic emissions from the Spitak (Armenia) earthquake site have been detected at the Dusheti and Vardzia (Georgia) observatories in a frequency range 0.005-1 Hz.
206 citations
TL;DR: In this article, the authors considered the effects of a finitely conducting inner core, in particular the resulting electromagnetic coupling between inner and outer core, and suggested that a geomagnetic reversal could only occur as a result of a particularly large fluctuation, large enough and lasting long enough to reverse the field throughout the inner core as well.
Abstract: IN view of its relatively small size (one-third the radius of the outer core), many geodynamo models neglect the inner core entirely1, or otherwise treat it as a non-conducting insulator2,3. In a previous steady-state model4, we considered some effects of a finitely conducting inner core, in particular the resulting electromagnetic coupling between inner and outer core. Here we include a prescribed buoyancy force, which is geophysically more realistic, and also yields time-dependent rather than time-independent solutions. The field in the finitely conducting inner core does not then adjust instantaneously to the field in the outer core, but has a diffusive timescale of its own of a few thousand years. Rather large, rapid fluctuations in the outer core are then effectively averaged out by the inner core, producing a relatively stable external dipole field. We speculate that a geomagnetic reversal could only occur as a result of a particularly large fluctuation, large enough and lasting long enough to reverse the field throughout the inner core as well.
203 citations
TL;DR: In this paper, the authors analyzed 22 AMPTE/IRM satellite passes through the low-latitude magnetopause region for which the magnetic shear, i.e., the field rotation angle on transit from the magnetosheath to the magnetosphere, was less than 30°.
Abstract: We have analyzed 22 AMPTE/IRM satellite passes through the low-latitude magnetopause region for which the magnetic shear, i.e., the field rotation angle on transit from the magnetosheath to the magnetosphere, was less than 30°. We found that on all passes a key time could be identified where the proton temperature, and usually also in the electron temperature and the temperature anisotropies, show net and often discontinuous changes. A change in plasma flow direction also occurs at this time. The small field rotations that occur have no fixed relationship to the key time. Earthward of the key time, the plasma has the characteristics of the magnetopause boundary layer, i.e., reduced density and bulk velocity, and in particular an electron temperature anisotropy with Te∥ > Te⊥, until further and usually more dramatic changes in plasma thermal properties mark the entry into the magnetosphere proper. Boundary layer durations varied widely, from 4s to 14 min. On the magnetosheath side we observed a layer of plasma density depletion (by a factor of two or more) and concurrent magnetic field pile-up in less than half the crossings. This depletion layer was 3 min wide on average and often characterized by a drop in Tp∥, as predicted. From the observational evidence we conclude that the changes in plasma thermal and flow properties at the key time mark the crossing of the magnetopause under conditions of low magnetic shear. It is tempting to attribute these changes to the crossing of a topological boundary, e.g., a transition from open interplanetary to closed geomagnetic field lines. There is evidence, however, that in some cases the field lines immediately earthward of the key time are not closed. The consistent presence of a boundary layer inside the low-shear magnetopause confirms earlier inferences that solar wind plasma can enter the magnetosphere regardless of field orientation. Inspection of the velocity distribution functions indicates that the plasma is heated upon entry. Taking measured plasma velocities along the magnetopause normal direction, we have inferred an average magnetopause speed of 11 km/s, an indication that the low-shear magnetopause moves more slowly. The duration of the temperature transition translates into a thickness of often less than 50 km, comparable with the typical proton gyroradius. For the plasma depletion layer and the boundary layer we obtained average thicknesses of 0.4 and 0.3 RE, respectively.
187 citations
TL;DR: In this article, a two-dimensional array of 42 temporary magnetometer stations was run in Scandinavia, supplementary to the permanent observatories and concentrated in the northern part of the region.
Abstract: As a contribution to the International Magnetospheric Study (IMS, 1976–1979) a two-dimensional array of 42 temporary magnetometer stations was run in Scandinavia, supplementary to the permanent observatories and concentrated in the northern part of the region. This effort aimed at the time-dependent (periods above about 100 s) determination of the two-dimensional structure of substorm-related magnetic fields at the Earth's surface with highest reasonable spatial resolution (about 100 km, corresponding to the height of the ionosphere) near the footpoints of field-aligned electric currents that couple the disturbed magnetosphere to the ionosphere at auroral latitudes. It has been of particular advantage for cooperative studies that not only simultaneous data were available from all-sky cameras, riometers, balloons, rockets, and satellites, but also from the STARE radar facility yielding colocated two-dimensional ionospheric electric field distributions. In many cases it therefore was possible to infer the three-dimensional regional structure of substorm-related ionospheric current systems. The first part of this review outlines the basic relationships and methods that have been used or have been developed for such studies. The second short part presents typical equivalent current patterns observed by the magnetometer array in the course of substorms. Finally we review main results of studies that have been based on the magnetometer array observations and on additional data, omitting studies on geomagnetic pulsations. These studies contributed to a clarification of the nature of auroral electrojets including the Harang discontinuity and of ionospheric current systems related to auroral features such as the break-up at midnight, the westward traveling surge, eastward drifting omega bands, and spirals.
166 citations
TL;DR: In this paper, the amplitude of the last three solar cycles has been predicted using geomagnetic disturbance indices, assuming that these indices are indicative of solar phenomena such as the occurrence of coronal holes.
Abstract: Predictions of the amplitude of the last three solar cycles have demonstrated the value and accuracy of the group of prediction methods known as the ‘precursor’ techniques. These are based on a correlation between cycle amplitude and phenomena observed on the Sun, or originating from the Sun, during the declining phase of the cycle or at solar minimum. In many cases, precursor predictions make use of the long record of geomagnetic disturbance indices, assuming that these indices are indicative of solar phenomena such as the occurrence of coronal holes. This paper describes a precursor technique for predicting the amplitude of the solar cycle using geomagnetic indices. The technique is accurate — it would have predicted each of the last 11 cycles with a typical error of less than 20 in sunspot number. It has also advantage that a prediction of the lower limit of the amplitude can be made throughout the declining phase, this limit building to a final value at the onset of the new cycle.
143 citations
TL;DR: In this article, the monthly and daily samples of the Ap index for the interval from 1932 through 1982 were studied using the power spectrum technique, and results obtained for Bartel's period (about 27 days), the semiannual period, the dual peak solar cycle distribution of geomagnetic storms, and certain other medium-scale periodicities were examined in detail.
Abstract: The monthly and daily samples of the Ap index for the interval from 1932 through 1982 were studied using the power spectrum technique Results obtained for Bartel's period (about 27 days), the semiannual period, the dual-peak solar cycle distribution of geomagnetic storms, and certain other medium-scale periodicities are examined in detail In addition, results on the cumulative occurrence number of storms per decade as a function of the Ap and Dst indices for the storm are presented
141 citations
TL;DR: In this paper, the authors investigated the idea that both phenomena are caused by traveling atmospheric disturbances (TADs) and concluded that TADs are responsible for both positive ionospheric storms at middle latitudes and the geomagnetic activity effect at low latitudes.
Abstract: Anomalous increases of the ionization density at middle latitudes (positive ionospheric storms) and anomalous increases of the neutral gas density at low latitudes (the geomagnetic activity effect) are prominent features of upper atmospheric storms. The present study investigates the idea that both phenomena are caused by traveling atmospheric disturbances (TADs). According to theory, such TADs are generated during magnetic substorm activity and propagate with high velocity from polar to equatorial latitudes. To examine the above hypothesis, magnetic, ionospheric, and neutral atmospheric data are compared for five different disturbance events. These case studies demonstrate that (1) there is a good temporal correlation between magnetic substorm activity at high latitudes, daytime positive ionospheric storms at middle latitudes, and the geomagnetic activity effect at low latitudes; (2) the initial phase of positive ionospheric storms propagates with high velocity toward lower latitudes; (3) this velocity is roughly consistent with the time lag of the geomagnetic activity effect at low latitudes; (4) the ionospheric disturbance is a conjugate phenomenon of global extent; and (5) it cannot be explained as an electric field effect. In summary, our data are fully consistent with the idea that TADs are responsible for both positive ionospheric storms at middle latitudes and the geomagnetic activity effect at low latitudes.
TL;DR: In this paper, the authors show that the present geomagnetic field morphology and pattern of secular variation have persisted for several million years, as would occur if the solid mantle controls flow at the top of the core.
Abstract: HISTORICAL geomagnetic measurements covering the past 400 years reveal a symmetrical pattern of four relatively stationary flux concentrations ('lobes') at the surface of the liquid core and regions of rapid change extending from the Atlantic to the Indian Ocean1. Palaeomagnetic data define a time-average over several thousand years2 which might reflect the stationary parts of the present field, but unfortunately the historical record is too short to provide a satisfactory average3. Here we model palaeomagnetic directions from the past 2.5 Myr using the same methods as for modern data4, and find the two northern lobes in the same position as today, over Arctic Canada and Siberia. In southern regions, by contrast, the field appears to have been smoothed out, as might be expected from the current rapid secular variation1. We propose that the present geomagnetic field morphology and pattern of secular variation have persisted for several million years, as would occur if the solid mantle controls flow at the top of the core.
04 Feb 1993
TL;DR: In this article, the pulsating cusp model was used to detect a series of discrete, poleward-moving plasma structures that are consistent with the pulsing-cusp model.
Abstract: THE plasma precipitating into the Earth's dayside auroral atmosphere has characteristics which show that it originates from the shocked solar-wind plasma of the magnetosheath1'2. The particles of the magnetosheath plasma precipitate down a funnel-shaped region (cusp) of open field lines resulting from reconnection of the geomagnetic field with the interplanetary magnetic field3. Although the cusp has long been considered a well defined spatial structure maintained by continuous reconnection, it has recently been suggested4–6 that reconnection instead may take place in a series of discontinuous events; this is the ‘pulsating cusp model’. Here we present coordinated radar and satellite observations of a series of discrete, poleward-moving plasma structures that are consistent with the pulsating-cusp model.
TL;DR: In this paper, the authors present a compilation of ∼400 intermediate virtual geomagnetic poles from 121 volcanic records of excursions and reversals less than 16 Myr old, sampled at various longitudes and latitudes.
Abstract: ACCORDING to a recent compilation of sedimentary records from the past 12 Myr, the reversing geomagnetic field displays a marked long-term longitudinal organization which seems to be correlated with the thermal structure of the lower mantle1. However, the crucial palaeomagnetic observation that intermediate virtual geomagnetic poles are preferentially confined to longitudes over the Americas or antipodal to this sector2,3 might be an artefact caused by distortion and smoothing of the geomagnetic signal by sediments4–6. Here we present a compilation of ∼400 intermediate poles from 121 volcanic records of excursions and reversals less than 16 Myr old, sampled at various longitudes and latitudes, which does not confirm previous inferences1 and reinforces doubts4–6 about the ability of sediments to properly record a rapidly varying field. In contrast to virtual poles from excursions, those from reversals are uniformly distributed in longitude, which indicates that the reversing field is statistically axisymmetrical. Thus, we conclude that there is no evidence for control of transitional fields by the temperature distribution in the lowermost mantle.
TL;DR: In this article, an extension of Mayaud's aa-index series (from 1868 to the present) for two solar cycles 1844-1868 was presented, where the magnetic data sources are hourly declination readings at the Helsinki magnetic-meteorological observatory carried out in 1844−1880.
Abstract: This paper deals with an extension of Mayaud's aa-index series (from 1868 to the present) for two solar cycles 1844–1868. The magnetic data sources are hourly declination readings at the Helsinki magnetic-meteorological observatory carried out in 1844–1880. The new data set provides a reliable and homogeneous series of declination for computations of equivalent aa–indices. For the time period 1868–1880 the linear correlation coefficient between the two indices was 0.96 on the monthly basis. Behaviour of the geomagnetic activity during the course of the sunspot cycles 9–12 revealed similar periodicities and linear dependencies (Ohl's and Feynman's relationships) as later activity data analysis has shown.
01 Jul 1993
TL;DR: In the northern hemisphere, the aurora borealis is visual evidence of simultaneous fluctuations in the earth's magnetic field (geomagnetic field) as mentioned in this paper, which can affect a number of man-made systems, including electric power systems.
Abstract: In the northern hemisphere, the aurora borealis is visual evidence of simultaneous fluctuations in the earth's magnetic field (geomagnetic field) These geomagnetic disturbances (GMD's), or geomagnetic storms, can affect a number of man-made systems, including electric power systems The GMD's are caused by the electromagnetic interaction of the solar wind plasma of protons and electrons with the geomagnetic field These dynamic impulses in the solar wind are due to solar flares, coronal holes, and disappearing filaments, and reach the earth from one to six days after being emitted by a solar event Instances of geomagnetic storms affecting telegraph systems were noted in England in 1846, and power system disturbances linked to GMD's were first reported in the United States in 1940 This Working Group report is a summary of the state of knowledge and research activity to the present time, and covers the GMD/Geomagnetically-induced currents (GIC) phenomena, transformer effects, the impact on generators, protective relay effects, and communication system effects It also summarizes modeling and predicting GIC, measuring and monitoring GIC, mitigation methods, system operating guidelines during GMD's, and alerting and forecasting procedures and needs for the power industry
TL;DR: In this article, the hidden ocean of the core (HOC) is considered and global axisymmetric eigenoscillations of the H-layer are possible that are similar to MAC-waves.
Abstract: The dynamics of the stably stratified layer at the top of the core, which we call the H-layer or the hidden ocean of the core (HOC), is considered. It is shown that global axisymmetric eigenoscillations of the H-layer are possible that are similar to MAC-waves. These oscillations have periods of the order of a few decades (∼65 yr) if N ∼ 2Ω where N is the Brunt-Vaisala frequency of the H-layer and Ω is the frequency of the Earth's rotation. H-layer oscillations can be excited by an instability mechanism that resembles baroclinic (sloping) instability, and they in turn can excite torsional oscillations (TO) in the bulk of the core. The joint action of these two oscillations provide a mechanism for the generation of the decade geomagnetic secular variations, and the associated variations in the length of day
TL;DR: In this article, the authors tested the orientation of hatchlings tethered inside a circular arena surrounded by a coil system that could be used to reverse the vertical and horizontal components of the ambient field.
Abstract: Recent experiments have demonstrated that hatchling loggerhead sea turtles can orient using the earth's magnetic field. To investigate the functional characteristics of the loggerhead magnetic compass, we tested the orientation of hatchlings tethered inside a circular arena surrounded by a coil system that could be used to reverse the vertical and horizontal components of the ambient field. Hatchlings tested in darkness in the earth's magnetic field were significantly oriented in an eastward direction. Inverting the vertical magnetic field component resulted in an approximate reversal of orientation direction, whereas reversing both the vertical and horizontal components together did not. The hatchlings failed to orient in a horizontal field of earth-strength intensity. These results provide evidence that the magnetic compass of loggerheads is an inclination (axial) compass, functionally similar to that of birds.
TL;DR: In this article, a simple analytic model for the directly driven, adiabatic solar wind-magnetosphere-ionosphere coupling is derived on the basis of frontside reconnection and force balance in the tail.
Abstract: A simple analytic model for the directly driven, adiabatic solar wind-magnetosphere-ionosphere coupling is derived on the basis of frontside reconnection and force balance in the tail. Magnetosphere-ionosphere coupling occurs through Alfven waves which are driven by changes of electric fields in the magnetosphere. The model provides an algorithm which calculates the directly driven part of the auroral electrojets from an observed solar wind variation. For two days in May 1979 the cross correlation between the prediction and observation of AE is greater than 0.9. This suggests that most of the geomagnetic activity during these two days was directly driven by the solar wind.
TL;DR: It is reported that leatherback sea turtle hatchlings orient to the geomagnetic field in complete darkness, a finding consistent with theoretical models proposing that magnetoreception involves a modulation of the response of retinal photoreceptors to light.
Abstract: Diverse animals can orient to the earth's magnetic field (1-6), but the mechanism or mechanisms undrlying magnetic field detection have not been determined. Behavioral (7-9) amd neurophysiological (10-12) results suggest that the transduction process underlying magnetic compass orientation in vertebrates is light-dependent, a finding consistent with theoretical models proposing that magnetoreception involves a modulation of the response of retinal photoreceptors to light (13, 14). We report, however, that leatherback sea turtle (Dermochelys coriacea) hatchlings orient to the geomagnetic field in complete darkness. Thus, light-dependence is not a universal feature of vertebrate magnetic compasses.
TL;DR: The problems facing forecasters of solar-terrestrial activity were discussed at the Solar-Terrestrial Predictions Workshop held in Ottawa, Canada in May 1992 as mentioned in this paper, and the results and recommendations of the scientists participating in this Workshop are presented.
TL;DR: In this paper, evidence is presented for the existence within the Earth's core of substantial quantities of uranium and thorium, and the concept of an accumulation of uranium in the core of the Earth functioning as a nuclear fission breeder reactor is presented.
Abstract: Ideas have previously been advanced suggesting the possibility that uranium exists within the Earth's core. In. the present paper, evidence is presented for the existence within the Earth's core of substantial quantities of uranium and thorium. The concept of an accumulation of uranium in the core of the Earth functioning as a nuclear fission breeder reactor is presented. Means for concentrating actinide elements within the Earth's core and for separating actinide elements from reactor poisons are disclosed. Nuclear reactor feasibility is demonstrated by Fermi's k∞ in excess of unity for times in the geological past. The concept that the Earth's geomagnetic dynamo is driven by nuclear fission energy is discussed as is the concept that the frequent, but irregular, polarity reversals of the geomagnetic field have their origins in intermittent nuclear reactor output. Although great uncertainty exists in estimates of the abundances of the actinide elements in the core of the Earth and in details of the chemistry of the core, the results of the present paper indicate if uranium and thorium exist in the core of the Earth as elements or compounds, as evidence indicates, the actinides: (1) would be the most dense matter in the Earth; (2) would tend to concentrate at the center of the Earth; (3) would tend to be separated on the basis of density from less dense reactor poisons; and (4) if accumulated 3000 million years ago, would be able to initiate self-sustaining nuclear fission chain reactions which may continue to the present through fuel breeding reactions. The magnitude of available nuclear fission energy release throughout geological time is of major geophysical importance and is more than sufficient to power the geomagnetic field.
01 Jan 1993
TL;DR: In this article, the effect of the ionosphere on the Global Positioning Systems (GPS) signal for a ground user is estimated based on relaistic modeling of the electron density of the Ionosphere and using a dipole moment approximation for the earth magnetic field.
Abstract: Based on relaistic modeling of the electron density of the ionosphere and using a dipole moment approximation for the earth magnetic field, we are able to estimate the effect of the ionosphere on the Global Positioning Systems (GPS) signal for a ground user. The lowest-order (1/f(exp 2)) effect, which is of the order of .1 - 30 meters of zenith group delay, is subtracted out by forming a linear combination of the dual frequencies of the GPS signal. One is left with second- (1/f(exp 3)) and third-order (1/f(exp 4)) effects which are estimated typically to be approximately 0 - 2 cm, and approximately 0 - 2 mm at zenith respectively, depending on the time of day, time of year, the solar cycle and the relative geometry of the magnetic field and the line of sight. Given the total electron content along a line of sight, we derive an approximation to the second-order term which is accurate to approximately 90% within the magnetic dipole moment model; this approximation can be used to reduce the second-order term to the millimeter level, thus potentially improving precise positioning in space and on the ground. The induced group delay, or phase advance, due to second- and third-order effects are examined for two ground receivers located at equatorial and mid-latitude regions tracking several GPS satellites.
TL;DR: In this article, it is proposed that the mean magnetic field of the Earth was dipolar, but that its axis has swung from one stable position to another during the middle Tertiary and was therefore sometimes different from the rotation axis.
TL;DR: In this paper, the authors detected magnetic changes associated with volcanic activity of Aso volcano with a dense network of continuously recording proton-precession magnetometers during the period from June 1989 to June 1990 Magnetic date clearly indicate that changes in the magnetization within the volcano are most probably caused by temperature changes.
TL;DR: In this article, a survey of ground magnetometer observations at the times of 70 transient (2-10 min) events recorded at South Pole Station, Antarctica has been conducted, showing that most South Pole events correspond to sudden sharp variations in the equatorial magnetospheric and low-latitude ground magnetic field.
Abstract: We survey GOES 2/5/6 geosynchronous and Huancayo, Peru, ground magnetometer observations at the times of 70 transient (2-10 min) events recorded at South Pole Station, Antarctica. The simultaneous observations indicate that most South Pole events correspond to sudden sharp variations in the equatorial magnetospheric and low-latitude ground magnetic field. The exceptions occur when the South Pole events have weak amplitudes and/or Huancayo and GOES 2/5/6 are far from local noon. The corresponding features observed at GOES 5 and GOES 6 are generally similar, with a lag indicating antisunward motion. A similar antisunward motion may be inferred from the ground observations themselves. On a case-by-case and statistical basis, the characteristics of the events observed in South Pole ground magnetograms resemble those previously interpreted as sudden impulse and sudden storm commencement signatures at other high-latitude stations. These observations suggest that the transient events at South Pole form part of the magnetospheric and ionospheric response to a sudden change in the fraction of the solar wind dynamic pressure applied to the magnetosphere.
TL;DR: In this paper, the authors compute the field line integrals of the Hall and Pedersen conductivities and wind-driven currents from a sophisticated F region model and find that the F region may provide a significant contribution to the dynamo electric field at all local times in a limited altitude and latitude region.
Abstract: Previous studies of the low-latitude atmospheric dynamo have concluded that the E region dynamo is the principal source of electric fields during the day, with the F region dynamo being of significant importance in the postsunset period. By computing field line integrals of the Hall and Pedersen conductivities and wind-driven currents from a sophisticated F region model, we find that the F region may provide a significant contribution to the dynamo electric field at all local times in a limited altitude and latitude region. Though the peak value of the Pedersen conductivity in the E region is larger than the peak value of the F region Pedersen conductivity, the field line integrated Pedersen conductivity of the F region can be significantly larger than the integrated Pedersen conductivity of the E region. This effect is most pronounced for low-latitude flux tubes with apex altitudes slightly above the F peak. This is due to the unique geometry of the equatorial geomagnetic field in which the field line path length through the F region is much greater than the field line path length through the E region. This effect decreases with increasing latitude as the field lines become more vertical and the path lengths through the appropriate parts of E and F regions become more comparable. We examine the consequences of this effect upon the low-latitude drifts derived from three dynamo circuit assumptions and compare the results to average drifts observed at Jicamarca, Peru.
TL;DR: In this paper, the first full cycle with measurements of solar wind parameters, geomagnetic activity measured by Ap was found to correlate with the square of the solar wind speed V, and Dst was found correlated with the product of V and the southward component of the interplanetary magnetic field, B[sub s].
Abstract: During solar cycle 20, the first full cycle with measurements of solar wind parameters, geomagnetic activity measured by Ap was found to correlate with the square of solar wind speed V, and activity measured by Dst was found to correlate with the product of V and the southward component of the interplanetary magnetic field, B[sub s]. Both of these correlations break down during cycle 21. In the case of Ap, the much stronger variation of B[sub s] in cycle 21 compared to cycle 20 makes clear that the B[sub s] contribution to activity is important on yearly as well as shorter time scales. The product B[sub s]V[sup 2] gives an excellent correlation with Ap over both cycles. In the case of Dst, the stronger variation of B[sub s] in cycle 21 causes a stronger variation in B[sub s]V, which is not reflected in Dst, perhaps because Dst also depends upon solar wind dynamic pressure in a nonlinear way. 12 refs., 4 figs.
TL;DR: In this article, the authors show that the velocities associated with the resonances in the E and F regions are often substantially greater than 1 km/s, and that the frequencies are in the interval from 1 to 4 mHz.
Abstract: Magnetohydrodynamic, field line resonances in the Earth's magnetosphere can have very large velocity shears and field-aligned currents. Auroral radar measurements of high-latitude resonances indicate that the velocities associated with the resonances in the E and F regions are often substantially greater than 1 km/s, and that the frequencies are in the interval from 1 to 4 mHz. Assuming that these resonances are oscillating at the fundamental mode frequency, and mapping these velocity fields along magnetic field lines to the equatorial plane shows that the velocity shears in the equatorial plane are of the order of 200 km/s over a radial distance of less than 2000 km (the amplitude of the velocity fluctuations is 100 km/s). Using a three-dimensional magnetohydrodynamic computer simulation code, we show that the resonances evolve through the development of Kelvin-Helmholtz instabilities near the equatorial plane. Within this framework, the instability is taking place on dipole magnetic field lines, and the resonances form a standing shear Alfven wave field due to the boundary conditions which must be satisfied at the polar ionospheres. We find that the nonlinear evolution of the Kelvin-Helmholtz instability leads to the propagation of vorticity from the equatorial plane to the polar ionosphere and that the vorticity leads ultimately to the dissipation of the resonance. This occurs within a quarter wave period of the shear Alfven field associated with the resonances.
TL;DR: In this article, the location of the Martian magnetopause and that of the bow shock were studied on the basis of three-dimensional solar wind proton spectra measured by the TAUS spectrometer on board Phobos 2 in its 56 circular orbits.
Abstract: The location of the Martian magnetopause and that of the bow shock are studied on the basis of three-dimensional solar wind proton spectra measured by the TAUS spectrometer on board Phobos 2 in its 56 circular orbits. The clear and strong dependence of the areomagnetopause position on solar wind ram pressure was revealed, while the position of the bow shock was practically independent of this parameter. In the power law expression telling the dependence of the Martian magnetotail thickness D on the solar wind ram pressure: D∼(ϱυ²)−1/k, the power index turned out to be k∼5.9±0.5. The close coincidence of this index with k = 6 for a dipole geomagnetic field, and the large areomagnetotail thickness compared with the planetary diameter, suggest that an intrinsic dipole magnetic field is likely to be an important factor in the solar wind interaction with Mars. On the other hand, the relatively stable position of the subsolar point of the Martian magnetopause and unambiguous induction effects observed by the Phobos 2 MAGMA magnetic experiment in the magnetotail indicate the essential role of an induced magnetic field, too. The weak dependence of the terminator bow shock position on the solar wind ram pressure may be related to the relatively stable position of the subsolar magnetopause.