Showing papers on "Earth's magnetic field published in 1983"

A theory of long period magnetic pulsations, 3. Local field line oscillations

Abstract: The local magnetic field is shown to oscillate at its Alfven resonance frequency (ies) in response to a wide band source whose frequency range covers the resonance frequency (ies). The proposed mechanism explains certain observations of magnetic pulsations where the frequency is found to vary continuously as a function of latitude for a given event.

Geomagnetic field analysis ‐ I. Stochastic inversion

Abstract: Summary. Most of the Earth’s magnetic field and its secular change originate in the core. Provided the mantle can be treated as an electrical insulator, stochastic inversion enables surface observations to be analysed for the core field. A priori information about the variation of the field at the core boundary leads to very stringent conditions at the Earth’s surface. The field models are identical with those derived from the method of harmonic splines (Shure, Parker & Backus) provided the apriori information is specified appropriately. The method is applied to secular variation data from 106 magnetic observatories. Model predictions for fields at the Earth’s surface have error estimates associated with them that appear realistic. For plausible choices of a priori information the error of the field at the core is unbounded, but integrals over patches of the core surface can have finite errors. The hypothesis that magnetic fields are frozen to the core fluid implies that certain integrals of the secular variation vanish. This idea is tested by computing the integrals and their standard and maximum errors. Most of the integrals are within one standard deviation of zero, but those over the large patches to the north and south of the magnetic equator are many times their standard error, because of the dominating influence of the decaying dipole. All integrals are well within their maximum error, indicating that it will be possible to construct core fields, consistent with frozen flux, that satisfy the observations.

An ISEE 3 high time resolution study of interplanetary parameter correlations with magnetospheric activity

Abstract: The coupling between the solar wind and the geomagnetic disturbances was examined using data from the ISEE-3 spacecraft at an earth-sun libration point and ground-based data. One minute data were used to avoid aliasing in determining the internal magnetospheric response to solar wind conditions. Attention was given to the cross-correlations between the geomagnetic index (AE), the total energy dissipation rate (UT), and the solar wind parameters, as well as the spatial and temporal scales on which the magnetosphere reacts to the solar wind conditions. It was considered necessary to characterize the physics of the solar wind-magnetosphere coupling in order to define the requirements for a spacecraft like the ISEE-3 that could be used as a real time monitoring system for predicting storms and substorms. The correlations among all but one parameter were lower during disturbance intervals; UT was highly correlated with all parameters during the disturbed times. An intrinsic 25-40 min delay was detected between interplanetary activity and magnetospheric response in quite times, diminishing to no more than 15 min during disturbed times.

Plasma regimes in the deep geomagnetic tail: ISEE 3

Abstract: The spacecraft remained close to or within a previously unexplored part of the distant (60-220 earth radii) geomagnetic tail nearly continuously from January 1 to March 30, 1983. Analysis of the data reveals that all of the plasma regimes identified previously with near-earth measurements (plasma sheet, low-latitude boundary layer, plasma mantle, lobe, and magnetosheath) remain recognizable in the distant tail. These regimes, however, are found to be intermingled in a more chaotic fashion than near the earth. Within the plasma sheet at approximately 200 earth radii, typical flow velocities are about 500 km/s tailward, considerably higher than in the near-earth plasma sheet. Earthward flow within the plasma sheet is observed occasionally, indicating the temporary presence of a neutral line beyond 220 earth radii. Also found are strong bidirectional electron anisotropies throughout much of the distant plasma sheet, boundary layer, and magnetosheath.

Investigation of a Vine-Matthews Magnetic Lineation from a submersible: The source and character of marine magnetic anomalies

Abstract: We report here the first attempt to map directly the boundary of a Vine-Matthews magnetic stripe on the seafloor. Our objectives are to study the processes of oceanic crustal accretion as recorded in the reversal transition zone and to investigate the formation of the magnetic source of Vine-Matthews magnetic anomalies. Our deep-tow and ALVIN-based magnetic studies focus on the Matuyama/Brunhes reversal transition on the flanks of the East Pacific Rise near 21°N. While the sea level magnetic anomalies are less than average in clarity for the East Pacific Rise, a ‘three-dimensional’ inversion of deep-tow data reveals a sharp, strike-linear polarity transition less than 1.8 km wide (Macdonald et al., 1980a). These measurements have been augmented by mounting a vertical magnetic gradiometer on ALVIN and making 280 reliable polarity determinations along and across the polarity transition zone. Even on long traverses across both sides of the boundary, we find that nearly every magnetic target has the correct polarity, i.e., the same polarity as the regional magnetic lineation. This homogeneity in polarity of the magnetic lineations is surprising. The magnetic polarity transition in the outcropping volcanic section is sharp and linear along strike, delineated in some cases by a clear geologic contact of opposing flow fronts of different ages. Several weakly magnetized outcrops mapped within the transition zone may have erupted during the time in which the geomagnetic field was reversing. The reversal boundary mapped on the seafloor from ALVIN is displaced 250 m to 500 m NW away from the spreading axis relative to the position of the average boundary as derived from inversion of the deep-tow and sea level magnetic data. This offset provides a means for estimating the spillover of lava flows away from the spreading axis during the time the crust was formed. The combination of deep-tow and ALVIN measurements suggests that circa 0.7 m.y. ago the crustal accretion zone (magnetized volcanic, intrusive, and plutonic rocks) was 2000–2800 m wide, while the zone of recent volcanism alone was only 1000–2000 m wide. The determination for the most recent reversal agrees well with submersible observations at the present spreading center where the zone of recent volcanism (neovolcanic zone) varies between 600 and 2000 m in width. This very orderly picture for the formation of magnetic lineations and crustal accretion processes appears to conflict with complex Deep Sea Drilling Project (DSDP) magnetic results from the Atlantic. We suggest that the crustal generating processes and resulting magnetic structure vary significantly with spreading rate. On the slow-spreading Mid-Atlantic Ridge, major episodes of volcanism are likely to be infrequent (∼104 years), the magma chamber may be non-steady state, and the neovolcanic zone shifts or varies in width considerably. This sporadic, start and stop spreading process will contribute to a highly heterogeneous and complex crustal and magnetic structure as seen in DSDP holes. In addition, significant faulting and tilting may disrupt slow-spreading crust. For intermediate- to fast-spreading centers, more frequent volcanism (∼50–600 years), a nearly steady state magma chamber, and a narrow, stable neovolcanic zone will create a less complex magnetic and crustal structure both as seen from ALVIN and as inferred from clear sea level magnetic anomalies in the Pacific.

Rapid diffusion of the poloidal geomagnetic field through the weakly conducting mantle: a perturbation solution

Abstract: Summary. A systematic regular perturbation procedure is developed to account for weak mantle conduction as unsteady electromagnetic fields are extrapolated downward from the Earth’s surface to the core-mantle boundary. The mantle is treated as a radially symmetric conductor of highly variable conductivity. The unique poloidal-toroidal decomposition of a magnetic vector potential leads first to three-dimensional and then, after spherical harmonic analysis, to one-dimensional linear diffusion equations for the two defining scalar functions. Emphasis is placed on a regular perturbation solution to the inverse poloidal diffusion problem, for the case where diffusion through the mantle is rapid on the time-scale for changes in forcing at the core-mantle boundary. The perturbation theory is evaluated with reference to a range of proposed conductivity profiles and two geomagnetic field models. It is found that uncertainty in the conductivity is at present less important than errors in the field models, and that the first-order corrections to the main field and secular variation at the core surface are likely to be negligible and small, respectively.

Solar wind control of the low-latitude asymmetric magnetic disturbance field

Abstract: The extent to which the low latitude dawn-dusk magnetic asymmetry is controlled by the dawn-dusk solar wind motional electric field VBs and/or substorm processes measured by the westward auroral electrojet index AL is studied using the technique of empirical linear prediction filters. A new index, (ASYM), defined as the difference between dawn and dusk deviations in the X (geomagnetic Northward) magnetic field component, is used to measure the dawn-dusk asymmetry. Quantitative information which characterizes the coupling processes are provided by the empirically determined filters obtained from this analysis. Results indicate that some currents are directly driven by the solar wind-magnetosphere interaction and that their magnetic perturbations contribute to both the AL and ASYM indices. However, a portion of the AL index that is uncorrelated with VBs is correlated with ASYM which suggests that internal magnetospheric processes also contribute to AL and ASYM.

Self‐consistent theory of three‐dimensional convection in the geomagnetic tail

Abstract: The self-consistent theory of time-dependent convection in the earth's magnetotail of Schindler and Birn (1982) is extended to three dimensions to include more realistic tail geometry and three-dimensional flow. We confirm that a steady state solution implies unrealistic tail geometry or large particle or energy losses that are unrealistic during quiet times and conclude therefore that as in the 2-dimensional case the magnetotail becomes time-dependent for typical convection electric fields. Explicit solutions are derived, even analytically, for the three-dimensional flow and the electric and magnetic field in a realistic tail geometry, and quantitative examples are presented. Consequences of time-dependent convection are demonstrated considering two idealized cases of magnetosphere response to solar wind changes: (1) uniform compression as the likely consequence of increasing (static, dynamic or magnetic) solar wind pressure; and (2) compression only in the z direction perpendicular to the plasma sheet as the probable consequence of a dawn to dusk external electric field (E/sub y/>0), corresponding to a southward interplanetary magnetic field component (B/sub z/ 0 with geomagnetic activity. Several other features, already present in the 2-dimensional theory, are confirmed.« less

The Earth’s Ring Current: Causes, Generation, and Decay

Abstract: The development of currents due to arbitrary distributions of trapped particles in the geomagnetic field is described These currents form the Earth’s ring current and are responsible for world wide decreases of the surface magnetic field observed during magnetic storms It is shown that we do not yet know the relative abundances of the ions forming the ring current Because of this we do not understand how various sources mix to produce the ring current Several possible generation mechanisms are discussed Finally, the decay of the ring current is discussed and is shown to be due primarily to charge exchange with important secondary effects attributable to wave-particle interactions

The shift of the auroral electron precipitation boundaries in the dawn-dusk sector in association with geomagnetic activity and interplanetary magnetic field

Abstract: On the basis of the auroral precipitating electron data along the dawn-dusk meridian from a Defense Meteorological Satellite Program satellite (DMSP-F2), we show that the electron precipitation region extends poleward, often to the geomagnetic latitudes ≳85° from the average oval location, during quiet periods (namely, during periods of a large positive Bz component) This result may be interpreted as an indication that only a small amount of geomagnetic flux interconnects with the northward IMF, resulting in a contracted “open” region (the polar cap) The control of the location of the poleward boundary by the north–south component of the IMF is also statistically examined The rate of latitudinal movement is about 04° to 08° per 1 nT for positive IMF Bz values and about 07° to 11° per 1 nT for negative IMF Bz values The shift of the equatorward boundary is not obvious, however, for positive IMF Bz values As geomagnetic activity increases, both the poleward and equatorward boundaries shift toward lower latitudes The equatorward shift of the poleward boundary is greater than that of the equatorward boundary, resulting in a thinner electron precipitation region during disturbed periods than during quiet periods

Computer synthesis of geomagnetic palaeosecular variations

Abstract: Computer models, designed to synthesize palaeosecular variations of the geomagnetic field, cast doubt on some widely accepted palaeomagnetic dogmas. Westward (or eastward) drift cannot be uniquely deduced from the sense of rotation of the geomagnetic vector: the use of unit vectors in statistical analyses leads to systematic errors which render investigation of the global symmetry of the palaeofield more difficult than previously supposed. At least some geomagnetic ‘excursions’ may be regarded as large-amplitude secular variations.

Geomagnetic secular variations 0–14 000 yr bp as recorded by lake sediments from Argentina

Abstract: Summary. Palaeomagnetic and sedimentological studies carried out on the bottom sediments from three lakes in western Argentina are described. At the within-lake level, five lithological units could be broadly defined and were useful as a general guide in defining detailed correlations based on the magnetic susceptibility (k) and intensity of remanent magnetization (J). In one of the three lakes, el Trebol, it was possible to derive a closer, lithologically based, correlation and this was consistent with that based on the magnetic parameters. Inter-lake correlation was derived from the inclination and declination patterns considered as a pair, also taking account of a set of radiocarbon age determinations to remove any ambiguities. All the individual measurements were transformed to a time-scale prior to stacking to produce type-curves. The records for Laguna el Trebol and Brazo de Campanario extend back to about 6000 radiocarbon years before present while Lago Morenito provides a record extending back to about 14000 radiocarbon years before present. The records exhibit variations with similar overall characteristics to those previously obtained for Europe and North America, but there appears to be no correlation with the northern hemisphere patterns on a swing to swing basis. Both clockwise and anticlockwise precession of the geomagnetic vector is evident both from spectral analysis of declination/inclination pairs and VGP paths.

The Earth's ring current: Causes, generation, and decay: Tutorial lecture

Abstract: The development of currents due to arbitrary distributions of trapped particles in the geomagnetic field is described. These currents form the Earth's ring current and are responsible for world wide decreases of the surface magnetic field observed during magnetic storms. It is shown that we do not yet know the relative abundances of the ions forming the ring current. Because of this we do not understand how various sources mix to produce the ring current. Several possible generation mechanisms are discussed. Finally, the decay of the ring current is discussed and is shown to be due primarily to charge exchange with important secondary effects attributable to wave-particle interactions.

A theoretical approach to the morphology and the dynamics of diffuse auroral zones

Abstract: Direct observations of the high-latitude ionosphere have established the continuous presence of large-scale emissions, referred to as diffuse auroras. Neglecting localized structures, such as discrete arcs, we focus upon a quantitative description of the effects of large-scale convection on diffuse precipitation. The transport of 100 eV-10 keV electrons from the geomagnetic tail earthward by the convection electric field, and their pitch angle diffusion into the loss cone by wave-particle interactions are believed to be the main cause of diffuse auroral electron precipitation. A self-consistent treatment balancing wave generation and particle diffusion appears to be still beyond the present state of the art. For the main purpose of diffuse precipitation modeling, we propose a simplified approach to this problem and test its validity against direct observations of the location and dynamics of diffuse auroral emissions. Using basically the strong pitch angle diffusion limit in the way proposed by Kennel [1969], we derive a set of fluid equations describing the three-dimensional transport of plasma-sheet electrons. Their integration provides the latitude and local time distribution of precipitation fluxes and characteristic energies at the top of the ionosphere as a function of the large-scale dawn to dusk electrostatic potential drop. The calculated expansion of the auroral oval with magnetic activity, deduced from our model, approaches the experimental results. So this simplified theoretical study permits us to reproduce and explain the main characteristics of the diffuse auroral zone. However, for high values of magnetic activity, the theoretical results are found to be shifted poleward relative to the observations. This suggests that the assumption of strong pitch angle diffusion overestimates the efficiency of wave-particle interactions.

Matching long term periodicities of geomagnetic reversals and galactic motions of the solar system

Abstract: To understand the phenomenon of frequent reversals of axial geocentric dipole fields it is essential to understand the spectral structure of geomagnetic reversal series and search for possible exogenetic (cosmic) factors associated with its dynamic behaviour. A scheme of Walsh spectrum analysis (which is more efficient and appropriate for binary processes as compared to Fourier Spectrum Analysis and Maximum Entropy Method), has been applied, for the first time, to the available world-wide paleomagnetic measurements during phanerozoic (last 570 million years). The results postulate long-term cyclicity in magnetic stratigraphy with reversal periods of 285, 114, 64, 47 and 34 million years with distinct resolution. The similar analysis was further repeated by dividing the total record in two sub-series. These results indicate mean periods of 71, 47 and 32- m.y. These peaks are statistically significant at 90% confidence level. These results, thus, question the widely accepted theory of randomness of geomagnetic reversal for long-period sequence. Surprisingly, the maximum spectral power is found for the Cosmic year (285 m.y.) Term (period of complete revolution of solar system around the Milky way galactic centre). The other reversal periods correspond nicely with the solar system's periods of galactocentric radial motion, interaction of spiral density wave with galactic orbit and solar oscillation in and outside of orbital plane. Such a remarkable correlation and harmony between observed gravitational phenomena and terrestrial records of electromagnetic processes on the cosmic scale appear to be of fundamental importance in macroscopic physics.

The dynamic response of the high-latitude thermosphere and geostrophic adjustment

Abstract: The theory of geostrophic adjustment in which the adjustment process is critically dependent on the ratio of the scale size of the forcing to the Rossby radius of deformation, is applied to the qualitative explanation of certain features of the response of the high latitude thermosphere to inputs of energy and momentum. When the ratio is small, large wind speeds are efficiently generated by a momentum source, and when small, changes in the mass field associated with a heat source are more effectively produced. The Rossby radius variation with height explains the qualitatively different responses of the high latitude E and F regions during disturbed geomagnetic conditions as shown by both observations and numerical models. The present analytic solutions are in good agreement with model results concerning energy apportionment between divergent and rotational components of the wind field as a function of wavenumber.

Biogenic Ferrimagnetism: A New Biomagnetism

Abstract: In its simplest form, biomagnetism is usually defined as the study of the magnetic fields originating in living systems, while the term magnetobiology refers to the effects of magnetic fields on organisms Most biomagnetic research currently in progress today and discussed in other chapters deals with magnetic fields generated either by ionic currents flowing within an organism, those induced para- or diamagnetically by the application of external fields, or those produced by the transient high-field alignment of the magnetic moments of magnetic contaminants However, there is another proper branch of biomagnetism that warrants discussion here: magnetic moments and fields produced by biochemically precipitated ferrimagnetic minerals Unlike paramagnetic or diamagnetic substances, a ferrimagnetic particle of the proper size and shape will spontaneously produce a permanent magnetic moment which under normal biological conditions cannot be demagnetized On a microscopic scale, the magnitude of the local magnetic field adjacent to these particles may be quite large, and the particles will interact strongly with the weak geomagnetic field

On reconnection and plasmoids in the geomagnetic tail

Abstract: The nonlinear evolution of the collisional tearing mode is numerically determined for a two-dimensional current sheet configuration whose magnetic field lines are tied at one end to a stationary surface. The configuration is analogous to that occurring in the geomagnetic tail at the start of a substorm. The numerical results suggest that the formation of a near-earth neutral line at substorm onset is due to the asymmetric tearing that occurs because the field lines in the geomagnetic tail are partly line-tied, or anchored, by the earth's ionosphere. The results also suggest that during substorm recovery the neutral line in the tail moves away from the earth at a speed on the order of the speed of the plasma flowing into the neutral line region. Overall, the solution is consistent with proposed reconnection models of the substorm but suggests that the recovery phase may be more complex than previously expected, due to the growth and coalescence of multiple magnetic islands (i.e., plasmoids).

Flux of Atmospheric Neutrinos

Abstract: New calculations of the flux of neutrinos produced by cosmic rays in the atmosphere are reported The authors have taken account of effects of the geomagnetic cutoff and of solar modulation separately for upward- and downward-going neutrinos of both electron and muon flavor with energies from 200 MeV to 10 GeV The geomagnetic cutoff in particular must be handled carefully because it induces behavior very similar to a neutrino oscillation signal

On the theory of electromagnetic induction in the Earth by ocean currents

Abstract: of the electrical conductivity structure of the earth. TM modes are closely associated with the Coriolis force deflection of water currents and with coastline effects that limit the source currents to the ocean basin. PM modes are induced by nondivergent electric currents that are fully contained within the ocean. Surface gravity waves and a Kelvin wave are examined in detail. The Kelvin wave result of Larsen (1968) is reevaluated, and because the upper lithosphere was modeled as an insulator, significant errors in his magnetic induction values, caused by neglect of the TM mode, are revealed. The sensitivity of the TM mode magnetic field to lithospheric electrical conductivity suggests the use of tidal induced electromagnetic fields to probe the earth's conductivity structure. Electromagnetic fields are induced in the earth by exter- nal, ionospheric and magnetospheric, current systems and have long been used to investigate the electrical conduc- tivity structure of the earth by the geomagnetic depth sounding or magnetotelluric methods. An additional natural source, the dynamo interaction of ocean currents with the ambient geomagnetic field, is important in the world oceans. Since the crust and mantle of the earth are electrical conductors that couple to the ocean both conduc- tively and inductively, observations of low-frequency elec- tromagnetic fields in the ocean contain information about both the electrical conductivity of the earth and the circu- lation of the oceans. Electromagnetic fields produced by ocean flows are dis- cussed by Cox et al. (1971), Sanford (1971), and Larsen (1973). Solutions of the Maxwell equations for surface gravity waves were obtained by Weaver (1965) for an infinitely deep ocean and were extended to long waves in a finite depth ocean by Larsen (1971). The surface and internal wave problem was also investigated by Podney (1975), who presented a general method for solving fluid induction problems using a magnetic vector potential when the flow is incompressible. All of these studies indicate that the induced electromagnetic fields are small, amount- ing to fractions of a/xV/m or a few nanoteslas (nT) near the sea surface. At lower frequencies, tidal signals have been detected in both seafloor- and island-based elec- tromagnetic data (Larsen, 1968). Low-frequency, meso- scale and large-scale, ocean-induced electromagnetic fields are discussed by Cox (1980, 1981), who emphasized the influence of shallow electrical conductivity structure on the observed fields.

Earth magnetic field fluctuations produced by filamentation instabilities of electromagnetic heater waves

Abstract: A mechanism associated with the filamentation instabilities of circularly polarized waves is investigated, showing the capability of producing significantly large earth magnetic field fluctuations in addition to ionospheric density fluctuations. This mechanism is suggested as the cause of earth magnetic field fluctuations reported by Stubbe and Kopka (1981) in the ionospheric heating experiments performed at the EISCAT facilities in Tromso, Norway.

Improving ionospheric maps using theoretically derived values of f0F2

Abstract: The results of a study to improve the global maps of monthly median values of the F2 region critical frequency, f0 F2, using values determined from a theoretical model are presented. Values of the midlatitude F2 region critical frequency that could be used to improve the prediction of f0 F2 in regions of the earth inaccessible to ground-based measurements were obtained. This was accomplished by including in the theoretical calculations realistic physical processes along with a realistic geomagnetic field model. Parameters were adjusted so that agreement was achieved between calculated and observed f0 F2 values as a function of local time at different stations, and then these same input parameters (i.e., neutral wind field, and neutral atmospheric model) were assumed valid at all regions of interest where the major difference is the geomagnetic field line configuration. Coefficients that yield global representations of f0 F2 were then determined using the theoretically derived f0 F2 values, and the predicted critical frequencies were compared with observed values to estimate the degree of improvement.

Recent developments in the dynamo theory of planetary magnetism

Abstract: of the geomagnetic field in terms of fluid motions in the outermost cor e has been revived and new methods of data analysis have been applied to this problem. Second, physically more realistic models of fluid motions in the liquid core have been introduced into the dynamo problem and attempts are being made to model particular episodes, such as reversals in the evolution in time of the geodynamo. Another fac tor that has stimulated interest in dynamo theory is the discovery and measurement of the magnetic fields of other planets. Jupiter, Saturn, and Mercury exhibit magnetic fields that appear to be caused by a dynamo process in the electrically conducting liquid cores of those planets. Because of the close similarity of the relevant physical

Late Precambrian Keweenawan Asymmetric Polarities as analyzed by axial offset dipole geomagnetic models

Abstract: The Great Logan Paleomagnetic Loop has been interpreted to be a paleomagnetic signature of the North American plate motion during the Keweenawan igneous activity (1200–1000 Ma). However, the presence of three successive asymmetric polarities, along with new geochemical data from the Mamainse Point area, makes this interpretation untenable. A new interpretation is presented according to which the asymmetric polarities represent long-term zonal nondipole geomagnetic field components. Wilson's single-dipole offset model can in general explain the inclinational asymmetries, but it yields intensity ratios (reversed/normal) significantly lower than those observed. A coaxial two-dipole model is proposed that better takes into account the observed intensity data. This model consists of a geocentric dipole and an offset dipole located at the core-mantle boundary. The offset dipole simulates the long-term zonal average of the spherical harmonic nondipole field. If the geocentric dipole reverses while the offset dipole retains its constant polarity, nonantiparallel reversal results. A method is presented by which the depth of the offset dipole and the ratio of the dipole strengths can be determined using only the observed inclinations. The two-dipole model suggests that inclinational asymmetries are due to a different nondipole/dipole ratio during the normal and the reversed polarity. The most probable variant of the two-dipole models predicts that this ratio is 35% and 49% for the normal and the reversed polarity, respectively. A new interpretation for the evolution of the Logan Loop is described according to which at least 50% of the hairpin-shaped loop is produced by oscillation of the dipole strength ratio. Apparent polar wander (APW) is still evident, but the shape of the loop and the APW speed along the loop is reduced. The two-dipole model is tested by worldwide paleomagnetic data of Keweenawan age.

IMF Control of the Earth’s Magnetosphere

Abstract: We review recent progress in the understanding of the IMF control on the Earth’s magnetosphere through the reconnection process. Major points include, (1) the identification of the magnetopause structure under the southward IMF polarity to be the rotational discontinuity and the resulting inference that the reconnection line is formed in the equatorial region, and (2) the confirmation from several observational aspects that under the northward IMF the reconnection takes place in the polar cusp. The point (1) is consistent with the observed correlations of geomagnetic indices with IMF but raises an important theoretical issue, and the point (2) is accompanied by an interesting issue of explaining why the polar cap electron precipitation is more energetic under such IMF conditions. Critical studies have reaffirmed the view that the energy supplied by reconnection is partly transported directly to the ionosphere to drive the DP-2 type current system but at the same time it is partly stored in the magnetic field of the tail to be unloaded 0.5 ~ 1 hr later to produce the expansion phase of substorm.