Showing papers on "Earth's magnetic field published in 1996"

Modeling the global magnetic field of the large‐scale Birkeland current systems

Abstract: Quantitative models are developed for representing the global distribution of the average magnetic field produced by the region 1 and 2 Birkeland current systems. The problem is solved in four following steps: (1) constructing a realistic tilt-dependent model of the Birkeland current sheets, based on the formalism of Euler potentials, (2) numerically computing their field at a large number of points within the modeling region, (3) finding a best-fit analytical approximation for that field, and (4) adding a current-free shielding field which confines the Birkeland field within the model magnetopause. At low altitudes, the model field-aligned currents reach the ionosphere along eccentric ovals, which fit the observed region 1 and 2 zones of Iijima and Potemra, and they continue there as horizontal currents. At larger distances, the nightside region 1 currents map to the plasma sheet boundary layer and are then diverted toward the tail flanks, while currents in the dawn-dusk and dayside sectors connect directly to the higher-latitude magnetopause. The region 2 current closes azimuthally near the equator, forming a spread-out partial ring current system. The described approach allows a great flexibility in the geometry of the Birkeland currents, making it feasible to infer their properties from spacecraft data.

Discovery of Ganymede's magnetic field by the Galileo spacecraft

Abstract: THE Galileo spacecraft has now passed close to Jupiter's largest moon—Ganymede—on two occasions, the first at an altitude of 838 km, and the second at an altitude of just 264 km. Here we report the discovery during these encounters of an internal magnetic field associated with Ganymede (the only other solid bodies in the Solar System known to have magnetic fields are Mercury, Earth and probably lo1). The data are consistent with a Ganymede-centred magnetic dipole tilted by ∼10° relative to the spin axis, and an equatorial surface-field strength of ∼750 nT. The magnetic field is strong enough to carve out a magnetosphere with clearly defined boundaries within Jupiter's magnetosphere. Although the observations require an internal field, they do not indicate its source. But the existence of an internal magnetic field should in itself help constrain models of Ganymede's interior.

A flexible, IMF dependent model of high-latitude electric potentials having “Space Weather” applications

Abstract: A method has been developed to derive the electric potentials in the high-latitude ionosphere resulting from any arbitrary combination of the interplanetary magnetic field (IMF) magnitude and orientation, solar wind velocity, and dipole tilt angle. This model is based on spherical harmonic coefficients that were derived by a least error fit of measurements from multiple satellite passes. These harmonic coefficients have been found to have systematic variations that can be reproduced by a combination of a Fourier series and a multiple linear regression formula. Examples of the model output are shown. In principal, this technique could be used as a fundamental building block to forecast geomagnetic disturbances, or “space weather”, from satellite measurements in the upstream solar wind.

Magnetic Orientation in Birds

Abstract: The magnetic field of the earth provides a very reliable, omnipresent source of spatial information for all living beings. For animals, perceiving the field lines of the geomagnetic field results in an anisotropy of space: the various directions do not appear equal but can be distinguished. The magnetic field structures space in the horizontal just like the gravity field structures space in the vertical by allowing the discrimination of “up” and “down.” At the same time, total intensity and inclination of the geomagnetic field vary in space in a way that is roughly correlated with geographic latitude, so that the ambient values contain information about geographic position on the earth.

Decay of the Dst field of geomagnetic disturbance after substorm onset and its implication to storm-substorm relation

Abstract: . In order to investigate the causal relationship between magnetic storms and substorms, variations of the mid-latitude geomagnetic indices, ASY (asymmetric part) and SYM (symmetric part), at substorm onsets are examined. Substorm onsets are defined by three different phenomena; (1) a rapid increase in the mid-latitude asymmetric-disturbance indices, ASY-D and ASY-H, with a shape of so-called `mid-latitude positive bay\'; (2) a sharp decrease in the AL index; (3) an onset of Pi2 geomagnetic pulsation. The positive bays are selected using eye inspection and a pattern-matching technique. The 1-min-resolution SYM-H index, which is essentially the same as the hourly Dst index except in terms of the time resolution, does not show any statistically significant development after the onset of substorms; it tends to decay after the onset rather than to develop. It is suggested by a simple model calculation that the decay of the magnetospheric tail current after substorm onset is responsible for the decay of the Dst field. The relation between the IMF southward turning and the development of the Dst field is re-examined. The results support the idea that the geomagnetic storms and substorms are independent processes; that is, the ring-current development is not the result of the frequent occurrence of substorms, but that of enhanced convection caused by the large southward IMF. A substorm is the process of energy dissipation in the magnetosphere, and its contribution to the storm-time ring-current formation seems to be negligible. The decay of the Dst field after a substorm onset is explained by a magnetospheric energy theorem.

Direct penetration of the polar electric field to the equator during a DP 2 event as detected by the auroral and equatorial magnetometer chains and the EISCAT radar

Abstract: The quasi-periodic DP 2 magnetic fluctuations (period of 30–40 min) appearing coherently at the auroral and equatorial latitudes during the day are analyzed based on the high time resolution magnetometer data recorded at the International Monitor for Auroral Geomagnetic Effects (IMAGE) stations in Scandinavia and at the Brazilian and African equatorial stations. It is shown that the correlation between the DP 2 magnetic fluctuations at both latitudes is excellent (correlation coefficient of 0.9). No discernible time shift has been found within the resolution of 25 s. The European incoherent scatter (EISCAT) radar observations in Scandinavia show that the DP 2 fluctuations at auroral latitudes are caused by an ionospheric Hall current which is controled by the convection electric field. The DP 2 fluctuations exhibit a strong decrease in magnitude with decreasing latitude, however, it is enhanced considerably at the dip equator with an amplitude comparable to that at the subauroral latitude. The considerable equatorial enhancement of the magnitude of the DP 2 fluctuations with an enhancement ratio of 4 is due to the concentration of the electric current along the highly conductive dayside equatorial ionosphere. These observational facts can be explained in terms of an ionospheric current which is generated by the magnetospheric electric field at the high latitude and extends to the equatorial ionosphere almost instantaneously. From the viewpoint of the electric field penetration, we conclude that the magnetospheric electric field penetrates to the equatorial ionosphere through the polar ionosphere almost instantaneously within the time resolution of 25 s. The nearly instantaneous propagation of the electric field to the equator can be explained primarily by a parallel plane transmission line model composed of the conductive Earth and ionosphere. In addition to our finding of the fast propagation of the DP 2 electric field, it is found that an impulsive magnetic change with a timescale of 100 s appears at the dayside dip equator with a time delay of about 10 s, which requires to include the effect of the high conductivity of the dayside equatorial ionosphere in future studies of the propagation model.

Rotation and magnetism of Earth's inner core

Abstract: Three-dimensional numerical simulations of the geodynamo suggest that a superrotation of Earth9s solid inner core relative to the mantle is maintained by magnetic coupling between the inner core and an eastward thermal wind in the fluid outer core. This mechanism, which is analogous to a synchronous motor, also plays a fundamental role in the generation of Earth9s magnetic field.

Magnetic storms and magnetotail currents

Abstract: The magnetospheric magnetic field is highly time-dependent and may have explosive changes (magnetospheric substorms and geomagnetic storms) accompanied by significant energy input into the magnetosphere. However, the existing stationary magnetospheric models can not simulate the magnetosphere for disturbed conditions associated with the most interesting magnetospheric physics events (intensive auroras, particle injection in the inner magnetosphere, and precipitations at the high latitudes, etc.). We propose a method for constructing a nonstationary model of the magnetospheric magnetic field, which enables us to describe the magnetosphere during the disturbances. The dynamic changes of the magnetosphere will be represented as a sequence of quasistationary states. The relative contributions to the Dst index by various sources of magnetospheric magnetic field are considered using a dynamic model of the Earth's magnetosphere. The calculated magnetic field is obtained by using the solar wind and geomagnetic activity empirical data of the magnetic storm of March 23–24, 1969 and the magnetic disturbance of July 24–26, 1986. The main emphasis is on the current system of the magnetospheric tail, the variations of which enable a description of the fast changes of Dst.

Planet Within a Planet: Rotation of the Inner Core of Earth

Abstract: The time dependence of the orientation of Earth's inner core relative to the mantle was determined using a recently discovered 10-degree tilt in the axis of symmetry of the inner core's seismic-velocity anisotropy. Two methods of analyzing travel-time variations for rays traversing the inner core, on the basis of 29 years of data from the International Seismological Centre (1964-1992), reveal that the inner core appears to rotate about 3 degrees per year faster than the mantle. An anomalous variation in inner-core orientation from 1969 to 1973 coincides in time with a sudden change ("jerk") in the geomagnetic field.

Ionospheric F 2 layer seasonal and semiannual variations

Abstract: An extensive series of computations, using the Coupled Thermosphere-Ionosphere-Plasmasphere model (CTIP), has been undertaken to investigate the semiannual variation in peak noontime electron density, a common feature of the Fa-layer, particularly at low latitudes and in the southern hemisphere at mid-latitudes. Results from the model reveal such a variation, most prominently, at mid-latitudes, in the South American sector. An analysis of this phenomenon shows that it is intimately related to the large offset of the geomagnetic axis from Earths spin axis in the southern hemisphere. Because of this offset, a given geographic latitude in the South American sector corresponds to a lower magnetic latitude than in other sectors and is thus farther from the energy inputs associated with the auroral regions. As a result, the composition changes are much smaller during the winter months than at other longitudes, the mean molecular mass being essentially constant for a 4-month period centered on the winter solstice. This result is understood in terms of the global thermospheric circulation. In the absence of any composition changes, noon ionospheric density is influenced primarily by the solar zenith angle. This angle reaches a maximum at the winter Solstice, leading to diminished ion production, a minimum in N(m)F2, and therefore a semiannual variation overall. On the basis of the model results, the semiannual variation is seen as a feature of the midlatitude ionosphere at geographic longitudes opposite to the location of the geomagnetic pole. This phenomenon is seen in both northern and southern hemispheres, though the effect is much larger in the southern hemisphere as a result of the greater magnetic offset.

Physics of the upper polar atmosphere

Abstract: 1. The Sun as a radiation source.- 1.1 General about the Sun .- 1.2 The solar atmosphere.- 1.3 The electromagnetic radiation from the Sun .- 1.4 Planck's radiation law.- 1.5 The greenhouse effect.- 1.6 Radiowave emissions from the Sun.-1.7 The sunspots - Solar cycle.-1.8 The electromagnetic radiation from the disturbed Sun.- 1.9 Particle emissions from the quiet Sun.-1.10 Fluid flow in a nozzle.- 1.11 The solar wind equation.- 1.12 The frozen-in field concept 1.13 The garden hose effect.- 1.14 Exercises.- 2. The atmosphere of the Earth 2.1 Nomenclature.- 2.2 The temperature structure of the atmosphere.- 2.3 Atmospheric drag on satellites.- 2.4 The atmosphere as an ideal gas.- 2.5 The exosphere.- 2.6 Height-dependent temperature.- 2.7 The adiabatic lapse rate.- 2.8 Diffusion.- 2.9 The equation of motion of the neutral gas.- 2.10 The geostrophic and thermal winds.- 2.11 The wind systems of the upper atmosphere.- 2.12 Observations of the neutral wind.- 2.13 Collisions between particles.- 2.14 Collisions in gases with different temperatures.- 2.15 Drag effects.- 2.16 Thermospheric neutral winds.- 2.17 The E-region winds.- 2.18 Observations of E-region neutral winds.- 2.19 The vertical motion.- 2.20 Exercises.- 3. The Earth's magnetic field and magnetosphere.- 3.1 An historical introduction.- 3.2 Description of the Earth's magnetic field.- 3.3 Mathematical representation of the Earth's magnetic system.- 3.4 Secular variations in the Earth's magnetic field.- 3.5 Tracing the magnetic field lines.- 3.6 E-field mapping along conducting magnetic field lines.- 3.7 The source of the magnetic field of the Earth.- 3.8 The unipolar inductor.- 3.9 The magnetic field away from the Earth.- 3.10 The magnetic tail.- 3.11 Magnetic field merging.- 3.12 Effects of the magnetic force.- 3.13 The energy flux into the magnetosphere.-3.14 Some aspects of the energy balance.- 3.15 Magnetic field convection.- 3.16 High-latitude convection patterns and field-aligned currents 3.17 Exercises .- 4. The ionosphere.- 4.1 The production of ionization by solar radiation.- 4.2 The ionization profile of the upper atmosphere.- 4.3 The Chapman ionization profile.- 4.4 The recombination process.- 4.5 The O+ dominant ionosphere.- 4.6 Ambipolar diffusion.- 4.7 Multicomponent topside ionosphere.- 4.8 Diffusion in the presence of a magnetic field.- 4.9 The E-layer ionization and recombination.- 4.10 The time constant of the recombination process.-4.11 The D-region ionization and recombination.- 4.12 Equatorial fountain effect.- 4.13 Ferraro's theorem.- 4.14 The magnetospheric convection close to the Earth 4.15 Exercises .-5 Currents in the ionosphere.- 5.1 The steady-state approach.- 5.2 Rotation of the ion velocity by height in the ionosphere.- 5.3 The current density in the ionosphere.- 5.4 Height-dependent currents and heating rates.- 5.5 Heating due to collisions.- 5.6 Heating of an oscillating electric field.- 5.7 Currents due to gravity and diffusion.- 5.8 Exercises.- 6. Magnetic fluctuations in response to height-integrated currents.- 6.1 Height-integrated currents and conductance.- 6.2 Magnetic field fluctuations from auroral currents.- 6.3 Equivalent current systems.- 6.4 Equivalent currents at different latitudes.- 6.5 The Sq current system.- 6.6 Mapping of E-fields in the ionosphere.- 6.7 Polarization fields around an auroral arc.- 6.8 Currents related to an auroral arc.- 6.9 Exercises.- 7 The aurora.- 7.1 An historical introduction.- 7.2 The height of the aurora.- 7.3 The occurrence frequency of the aurora.- 7.4 The global distribution of the aurora.- 7.5 The auroral appearance.- 7.6 Auroral particles.- 7.7 Precipitation patterns of auroral particles.- 7.8 The energy deposition profiles of auroral particles.- 7.9 Deriving energy spectra from electron density profiles.- 7.10 Excitation processes in the aurora.-7.11 The quenching process.- 7.12 The proton aurora.- 7.13 Exercises References Symbols Index.

The 13.5‐day periodicity in the Sun, solar wind, and geomagnetic activity: The last three solar cycles

Abstract: We make a detailed analysis of the 13.5-day periodicity of the solar chromosphere, the near-Earth solar wind, interplanetary magnetic field and geomagnetic activity during the last three solar cycles. The 13.5-day periodicity is a real quasi-periodicity whose amplitude varies sizably with time, attaining occasionally values larger than, for example, the amplitude of the 27-day periodicity. In case of heliospheric and geomagnetic variables, intervals of large 13.5-day periodicity are due to the occurrence at 1 AU of two high-speed streams per solar rotation. According to the tilted solar dipole model, such two-stream structures appear if the heliospheric current sheet is sufficiently narrow and tilted. We show that, during the main two-stream structures, the interplanetary magnetic field (IMF) indeed had a persistent two-sector structure, and the heliosheet was sizably tilted. Multiple IMF sector structure is thus excluded as the main cause for 13.5-day periodicity in solar wind and geomagnetic activity. We determine the exact time and phase (solar longitude) of all intervals of significant 13.5-day periodicity during the last three solar cycles. We find that even the longest intervals of two-stream structure (up to 2 years) consist of separate activations. Each of the main activations of the 13.5-day (as well as 27-day) periodicity has a nearly equal length of a few (about 4) solar rotations only. This gives new, interesting information about the solar dynamics related to the development of the dipole tilt. Using the phase of the main 13.5-day activations, we could determine the longitudinal position of the solar dipole tilt for all major activations. We note that this position can abruptly change by even 90 deg between two successive 13.5-day activations. For each of the three solar cycles studied, the largest two-stream structures were found in the late declining phase of the cycle. On the other hand, the main activations of the 13.5-day periodicity of solar variables, which are due to two active solar longitudes approximately 180° apart, tend to occur around solar maxima.

The step 210° Magnetic meridian network Project

Abstract: Imaging the Earth's magnetosphere by using ground-based magnetometer arrays is still one of the major techniques for investigating the dynamical features of solar wind-magnetosphere interactions The organized ground network data of magnetic fields make it possible (1) to study the magnetospheric processes by distinguishing between temporal changes and spatial variations in the phenomena, (2) to clarify the global latitudinal structures and propagation characteristics of magnetic variations from high to equatorial latitudes along the magnetic meridian (MM), and (3) to understand the global generation mechanism of magnetospheric phenomena During the international Solar Terrestrial Energy Program (STEP) period of 1990-1997, multinationally coordinated magnetic observations are being conducted along the 190°, 210°, and 250° MMs from high latitudes through middle and low latitudes to the equatorial region, spanning L = 850-100, in cooperation with 29 organizations in Australia, Indonesia, Japan, Papua New Guinea, Philippines, Russia, Taiwan, and the United States In this paper, we review the 210° MM Magnetic Observation Project and its initial results

The polar wind

Abstract: The polar wind is an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes to the magnetosphere along geomagnetic field lines. The polar wind plasma consists mainly of H+, He+, and O+ ions and electrons. Although it was initially believed that O+ ions play a major role only at low altitudes, it is now clear from observations that relatively large amounts of suprathermal and energetic O+ ions are present in the polar magnetosphere. Recently, thermal O+ outflow has been observed at altitudes of 5000–10,000 km together with H+ and He+ ions. The polar wind undergoes four major transitions as it flows from the ionosphere to the magnetosphere: (1) from chemical to diffusion dominance, (2) from subsonic to supersonic flow, (3) from collision-dominated to collisionless regimes, and (4) from heavy to light ion composition. The collisions are important up to about 2500 km, after which the ions and electrons exhibit temperature anisotropies. The direction of the anisotropy varies with geophysical conditions. The polar wind outflow varies with season, solar cycle, and geomagnetic activity. The O+ flux exhibits a summer maximum, while the H+ flux reaches a maximum in the spring. The He+ flux increases by a factor of 10 from summer to winter. At both magnetically quiet and active times the integrated H+ ion flux is largest in the noon sector and smallest in the midnight sector. The integrated upward H+ ion flux exhibits a positive correlation with the interplanetary magnetic field. In the sunlit polar cap the photoelectrons can increase the ambipolar electric field, which in turn increases the polar wind ion outflow velocities. The outflowing polar wind plasma flux tubes also convect across the polar cap. When the flux tubes cross the cusp and nocturnal auroral regions, the plasma can be heated and become unstable. Similar mixing of hot magnetospheric plasma with cold polar wind may result in instabilities. A number of free energy sources in the polar wind, including temperature anisotropy, relative drift between species, and spatial inhomogeneities, feed various fluid and kinetic instabilites. The instabilities can produce plasma energization and cross-field transport, which modify the large-scale polar wind outflow.

Geomagnetic intensity and 14C abundance in the atmosphere and ocean during the past 50 kyr

Abstract: We have used a new sedimentary record of geomagnetic field intensity, combined with published volcanic data, to examine the extent to which geomagnetic changes have affected 14C production in the past. The production record was then converted into an atmospheric Δ14C signal using a 4-box model of past oceanic circulation. The results indicate that changes in geomagnetic field intensity account for at least 80% of the Δ14C shift documented by published U-Th data, which are of the order of 500 per mil with respect to a constant production hypothesis. Model simulations show that changes in past oceanic circulation account for only ±100 per mil of Δ14C variation, which is within the uncertainty of the geomagnetic correction. In terms of dating, these results show that the radiocarbon ages have to be shifted by 2 to 3.5 kyr towards older ages during the 20–40 ka interval.

A large polar crown coronal mass ejection and a “problem” goemagnetic storm: April 14–23, 1994

Abstract: In the 1960s and 1970s, systematic programs to associate geomagnetic storms with solar flares gave rise to a category of “problem storms” for which there were no clear associations or for which the associations were with solar activity that seemed too insignificant to properly account for the magnitude of the geomagnetic events. In the last 2 decades the role of filament eruptions and coronal mass ejections (CMEs) in driving geomagnetic activity has received increasing attention, and an answer to the problem storm question has been pieced together. Until recently the inability to view CMEs on the solar disk has impeded a precise demonstration of the inferred relationships. The data from the Yohkoh Soft X ray Telescope (SXT), however, allow the association of soft X ray coronal structures with the aftermath of CMEs. Since CMEs are now believed to be the main cause of aperiodic severe disturbances of Earth's magnetosphere, the ability to distinguish the aftermath of a CME on the disk improves our ability to predict geomagnetic storms. On April 14, 1994, a large scale coronal arcade formation was observed by SXT after the eruption of a long section of the southern polar crown neutral line. Data from the Mauna Loa coronagraph are consistent with this event being the result of a CME that formed in the helmet streamer underlying the heliospheric current sheet. The CME was followed by a severe geomagnetic storm, starting 68 hr later, and was also observed at 3.2 AU and high heliographic latitude by the Ulysses satellite. Since there was no significant flare associated with the eruption, and no significant filament disappearance, the subsequent geomagnetic events are an extreme example of a problem storm and of the great utility of soft X ray imagers for the prediction of space weather. In this paper we have assembled a wide range of data relating to the solar event, the evolution of the region that generated it, and the various observed consequences at Earth and in the interplanetary medium. The global magnetic field changes leading to the CME are outlined, a brief description of the coronal arcade formation is given, and the arcade's association with subsequent events at Earth and the interplanetary disturbance observed at Ulysses is described. We present this case as an example of the advantages to be derived from a comprehensive multidisciplinary approach to studying these potentially disruptive phenomena.

The 22‐year cycle of geomagnetic and solar wind activity

Abstract: The 22-year cycle in geomagnetic activity is characterized by high activity during the second half of even-numbered solar cycles and the first half of odd-numbered cycles. We present new evidence for this 22-year cycle using the aa magnetic index for the years 1844–1994. Over this 150-year interval, the 22-year cycle can be observed through differences between the decay phases of even- and odd-numbered cycles in (1) average values of a 27-day recurrence index; (2) the results of a χ2 “event” analysis of 27-day recurrences of both disturbed and quiet days; and (3) an apparent annual modulation of the 27-day peak in the power spectrum of the aa index. Currently, the 22-year variation is attributed to the Russell-McPherron solar wind - magnetosphere coupling mechanism working in conjunction with the Rosenberg-Coleman polarity effect. Contrary to this viewpoint, we argue that an intrinsic 22-year solar variation (other than polarity reversal), revealed in the systematic low-high alternation of even-odd sunspot maxima within the last six complete Hale cycles, is the dominant cause of the 22-year cycle in geomagnetic activity. This sunspot and related coronal mass ejection variation should lead directly to higher geomagnetic activity during the first-half of odd-numbered solar cycles. Various lines of evidence (including 1–3 above) indicate that 27-day recurrent wind streams are more prominent during the decline of even-numbered solar cycles, contributing to the higher geomagnetic activity observed at those times. These stronger recurrence patterns may be related to the more rapid expansion of polar coronal holes (faster movement of the coronal streamer belt to low latitudes) observed following the maxima of recent even-numbered cycles. The amplitudes of the 22-year sunspot and geomagnetic activity cycles over the last 150 years are shown to be highly correlated. The 22-year pattern of geomagnetic activity appears to be a reflection of the solar dynamo coupling of poloidal magnetic fields on the decline of one solar cycle to the toroidal fields at the maximum of the following cycle. It seems likely that the 22-year variation in sunspot/solar wind activity plays a role in the observed 22-year modulation of galactic cosmic ray intensity.

Interferometric determination of broadband ELF wave phase velocity within a region of transverse auroral ion acceleration

Abstract: Broadband electric field fluctuations with typical amplitudes of 10–20 mV/m peak-to-peak and frequencies from 0 Hz to 3 kHz (BB-ELF) were observed coincident with a region of ≤200 eV transverse H+ acceleration (TAI) near the poleward edge of the pre-midnight aurora. The coherence and phase velocity of the electric fields were measured using a interferometric antenna array over the frequency range of ≈ 100 Hz to 3 kHz. These electric field fluctuations were found to have the following characteristics: 1) incoherence perpendicular to the geomagnetic field, 2) coherence parallel to the the geomagnetic field, 3) parallel phase velocity (ω/k∥) of 30–35 km/s upwards, 4) 0 < |k∥/k⟂| < 0.22. We show that these properties are compatible with the emission being electrostatic H+ cyclotron (EHC) waves. We also discuss possible generation mechanisms for the waves, and their relationship to the TAI.

The heliospheric magnetic field at solar minimum: ULYSSES observations from pole to pole.

Abstract: The fast transit of the Ulysses spacecraft from the south polar regions of the heliosphere to the north polar regions has allowed us to obtain a unique perspective on the configuration and properties of the heliospheric magnetic field at a time of solar minimum activity. We compare the magnetic field in the northern and southern hemispheres and find no evidence of asymmetry in the radial component of the field or in the variances of the field. We find that the magnetic field polarity corresponds to that observed at the Sun. The only difference between the hemispheres is that, while magnetic field lines in the southern hemisphere have a most probable azimuth angle approximately 24◦ more tightly wound than predicted by the Parker model, those in the northern hemisphere are in good agreement with the model.

An 84-kyr paleomagnetic record from the sediments of Lake Baikal, Siberia

Abstract: We have conducted a paleomagnetic study of sediment cores obtained from the Selenga prodelta region of Lake Baikal, Russia. This record, which spans approximately the last 84 kyr, contributes to a better understanding of the nature of geomagnetic field behavior in Siberia and is a useful correlation and dating tool. We demonstrate that the Lake Baikal sediments are recording variations in the geomagnetic field. The directional record displays secular variation behavior with a geomagnetic excursion at 20 ka and additional excursions appearing as large-amplitude secular variation at 41, 61, and 67 ka. Smoothing of the geomagnetic excursion behavior occurs in Lake Baikal sediments owing to the intermediate sedimentation rate (13 cm kyr−1). The Lake Baikal relative paleointensity record correlates to absolute paleointensity data for the last 10 kyr and to relative paleointensity records from the Mediterranean Sea and Indian Ocean for the last 84 kyr. This correlation suggests a strong global (i.e., dipole) component to these records and further supports the reliability of sediments as recorders of relative geomagnetic paleointensity. We show that a relative geomagnetic intensity stratigraphy has a potential resolution of 7 kyr by correlating continental and marine records. The geomagnetic intensity stratigraphy helps constrain the age of the difficult to date Lake Baikal sediments.

Nonlinear mean field electrodynamics of turbulent dynamos

Abstract: Mean field electrodynamics of turbulent dynamos taking into account Lorentz forces of the generated magnetic fields is derived and studied. Small‐scale magnetic fields that are much stronger than the mean field B0 modify the fluid turbulence in such a way as to suppress the α effect: α=α0(1+R)−1, where α0 is the kinematic value, and the reduction factor is proportional to the magnetic Reynolds number Rm, R=RmB20/(4πρv2), and v is the characteristic turbulent velocity. In two dimensions the analog is turbulent magnetic diffusivity suppression. Suppression becomes noticeable at very low values of the mean magnetic field, B20∼ρv2/Rm. The modification of turbulence by the small‐scale magnetic fields is discussed.

Variability of the tail lobe field strength

Abstract: A large magnetosphere database has been used to study spatial and temporal variations of the geomagnetic tail lobe field strength. The study uses 12,388 average tail lobe measurements from the region -15 > X > -70 R E obtained from 11 different spacecraft over a 20-year period. The average radial gradient is found to be represented by B=1659.2/R 1.46 +7.47 which can be used to eliminate the dependence of individual measurements on distance down the tail. A set of 5562 of the resulting normalized data points is associated with simultaneously measured solar wind dynamic pressure and interplanetary magnetic field (IMF) values which permit the study of relationships of B lobe with geomagnetic activity and with solar wind parameters on various timescales. Variations of annual averages of B lobe and AE exhibit a solar cycle dependence and are very similar to each other and to solar wind dynamic pressure variations. On an hourly time scale, tail lobe field strength normalized to a distance of 30 R E is found to depend on both solar wind dynamic pressure, P sw , and IMF B z . This dependence can be represented as B n 2 =-96.5+294.5√P sw +2.36B 2 IMF sin 2 (θ/2) where θ varies from 0° for northward to 180° for southward. The ability to quantitatively predict average B lobe given solar wind pressure and IMF B provides a baseline which should be useful in the study of more rapid substorm variations which are due to internal magnetospheric dynamics.

Occurrence patterns for transient magnetic field signatures at high latitudes

Abstract: Impulsive events marked by durations ranging from 5 to 15 min and peak-to-peak variations in the H component exceeding 40 nT are common in observations by high-latitude day side ground magnetometers. We survey observations by a global network of high-latitude ground magnetometers to distinguish between their proposed causes. Peak event amplitudes and occurrence rates occur prior to local noon in the range of geomagnetic latitudes from 70° to 76°. Although most events can be associated with abrupt changes in the interplanetary magnetic field (IMF) orientation, few can be associated with abrupt changes in the solar wind dynamic pressure. Many events can be observed during intervals of steadily northward IMF orientation, but only a few can be observed during intervals of steadily southward IMF orientation. We suggest an interpretation of the events in terms of a model in which changes in the IMF orientation modulate the fraction of the solar wind pressure applied to the magnetosphere.

Geomagnetic changes across the last reversal recorded in lava flows from La Palma, Canary Islands

Abstract: Two adjacent sequences of 41 and 21 individual lava flows have been sampled in the island of La Palma. Although no detailed correlation can be established between the two sections, they both encompassed the Matuyama-Brunhes (M-B) reversal. Stepwise alternating field (AF) and thermal demagnetization in air and in vacuum accompanied by investigations of the rock magnetic properties allowed us to identify several lava flows with the same signature and thus most likely associated with very short episodes of intense volcanic activity. The intermediate virtual geomagnetic pole (VGP) positions do not cluster within any preferred geographic areas. The polarity change is followed by a rebound (or an excursion) with a cluster of VGPs over northeastern India. Whether or not this feature is truly associated with the transition or correlated with the Delta event (0.69 Ma) will hopefully be resolved by further dating. Paleointensity experiments were performed on 60 samples using the modified Thellier method [Coe, 1967]. Corrections for magnetomineralogical changes induced by heating were applied using the technique of partial thermoremanent magnetization (pTRM) checks [Valet et al., 1994b] with a significant increase in the success rate (from 38% to 58%). Twenty-four independent estimates of the paleofield range between 3.9 and 46.0 μT. The field intensity was reduced to at least one tenth of the present-day value during the transition. Low (transitional) field intensities are also observed during the rebound (or excursion). The lava flows preceding the polarity change show evidence for a significant drop in the dipole field intensity, while the posttransitional flows exhibit high field intensities. These features are compatible with the asymmetrical saw-tooth pattern recently observed in sedimentary records [Meynadier et al., 1994]. The characteristics of the M-B transition emerging from a compilation of the four available volcanic records are not inconsistent with the hypothesis of rapidly time-varying nondipole transitional components favored by the decrease of the dipole and its subsequent recovery in the opposite polarity state.

An improved technique for reduction to the pole at low latitudes

Abstract: Reduction to the pole at low latitudes based on a Wiener filtering approach has been improved by introducing a deterministic noise model. It is assumed that the noise power is a fixed fraction of the signal power. This allows the method to be fully automated. Further improvement is obtained by requiring the reduced-to-the-pole field to map into the observed field when projected to the geomagnetic latitude of the observed field. This is done by iteratively minimizing the difference between the measured field and the reduced-to-the-pole field projected to the geomagnetic latitude of the measured data. This results in a reduced-to-the-pole magnetic map that, when projected to the geomagnetic latitude of the given data, closely matches the measured data. The final reduced-to-the-pole field does not show any of the artifacts typical of reduction-to-the-pole at low geomagnetic latitudes. The method is demonstrated on a data set from an aeromagnetic survey flown over north-central Burkina Faso, West Africa.