Showing papers on "Earth's magnetic field published in 1989"

A new magnetic coordinate system for conjugate studies at high latitudes

Abstract: Studying magnetically conjugate phenomena at very high latitudes requires a magnetic coordinate system that is smooth and well defined at the geographic poles In addition, it should provide for accurate comparisons at different altitudes In this report the authors present a variation on the corrected geomagnetic coordinate system that is well defined and smooth over the entire globe It provides an analytic expression relating geographic coordinates, including altitude, to the magnetic coordinates The coordinate system is produced by tracing magnetic field lines using the IGRF85 reference magnetic field model with time derivatives updating the model to 1988 An expansion of the relationship in terms of spherical harmonics has been determined, which then provides the required well-defined and smooth relationship over the entire globe Independent expansions for different altitudes show a smooth functional relationship of the coefficients of the expansion with altitude, and therefore simple interpolation schemes can be used to provide an appropriate expansion at any altitude between 0 km and approximately 600 km By reversing the process, the inverse expansions relating the magnetic coordinates to geographic coordinates have also been determined The effects of the seasonal variation in the Sun's declination along with the variation in the Sun's declinationmore » along with the variation in the Sun's apparent position due to the eccentricity of the Earth's orbit result in a variation of nearly 1 hour of magnetic local time for a fixed UT over the course of a year In many applications this variation may be important and should be included when presenting data in terms of magnetic latitude and MLT« less

Spectral characteristics of plasma sheet ion and electron populations during undisturbed geomagnetic conditions

Abstract: The spectral characteristics of plasma-sheet ion and electron populations during periods of low geomagnetic activity were determined from the analysis of 127 one-hour average samples of central plasma sheet ions and electrons. Particle data from the ISEE-1 low-energy proton and electron differential energy analyzer and medium-energy particle instrument were combined to obtain differential energy spectra in the plasma sheet at geocentric radial distances above 12 earth radii. The relationships between the ion and electron spectral shapes and between the spectral shapes and the geomagnetic activity index were statistically investigated. It was found that the presence of interplanetary particle fluxes does not affect the plasma sheet particle spectral shape.

Solar wind-magnetosphere coupling during intense magnetic storms (1978-1979)

Abstract: The solar wind-magnetosphere coupling problem during intense magnetic storms was investigated for ten intense magnetic storm events occurring between August 16, 1978 to December 28, 1979. Particular attention was given to the dependence of the ring current energization on the ISEE-measured solar-wind parameters and the evolution of the ring current during the main phase of the intense storms. Several coupling functions were tested as energy input, and several sets of the ring current decay time-constant were searched for the best correlation with the Dst response. Results indicate that a large-scale magnetopause reconnection operates during an intense storm event and that the solar wind ram pressure plays an important role in the energization of the ring current.

A comparison of techniques for magnetotelluric response function estimation

Abstract: Spectral analysis of the time-varying horizontal magnetic and electric field components yields the magnetotelluric (MT) impedance tensor. This frequency dependent 2×2 complex tensor can be examined for details which are diagnostic of the electrical conductivity distribution in the Earth within the relevant (frequency dependent) inductive scale length of the surface observation point. As such, precise and accurate determination of this tensor from the electromagnetic time series is fundamental to successful interpretation of the derived responses. In this paper, several analysis techniques are applied to the same data set from one of the EMSLAB Lincoln Line sites. Two subsets of the complete data set were selected, on the basis of geomagnetic activity, to test the methods in the presence of differing signal-to-noise ratios for varying signals and noises. Illustrated by this comparison are the effects of both statistical and bias errors on the estimates from the diverse methods. It is concluded that robust processing methods should become adopted for the analysis of MT data, and that whenever possible remote reference fields should be used to avoid bias due to uncorrelated noise contributions.

Magnetic Fields at Neptune

Abstract: The Voyager 2 magnetic field experiment discovered a complex and powerful magnetic field in Neptune, as well as an associated magnetosphere and magnetic tail. As the spacecraft exited the magnetosphere, the magnetic tail appeared to be monopolar. The auroral zones are probably located far from the rotation poles, and may possess complex geometry. The Neptune rings and all its known moons are imbedded deep within the magnetosphere (except for Nereid, which is outside when it lies sunward of the planet); the radiation belts have a complex structure due to the absorption of energetic particles by the moons and rings of Neptune, as well as losses associated with the significant changes in the diurnally varying magnetosphere configuration.

Electrical conductivity of the earth's lower mantle

Abstract: ELECTRICAL conductivity is an important physical property of the Earth's mantle because it controls the transmission of geomagnetic signals from the core to the surface. The lower mantle, from a depth of 670 km down to the core–mantle boundary (2,990 km), is probably composed of (Mg,Fe)SiO3 perovskite and magnesiowustite, (Mg,Fe)O. Analysis of the transient and secular variations of the geomagnetic field yields values of the lower-mantle conductivity of the order of 1 S m−1 at a depth of 1,000 km, increasing to ∼100 S m−1 at the core–mantle boundary1,2. Information about the physical mechanism of the conductivity and its dependence on temperature and pressure would help to constrain the temperature profile in the Earth. In the only study published so far, Li and Jeanloz3,4 reported values of the conductivity of the silicate perovskite and of a mixture of perovskite and magnesio-wiistite lower than 10−3 S m−1 and concluded that the lower mantle is an insulator, thus casting doubt on the geomagnetic results. We report here the results of measurements of the d.c. electrical conductivity of a mixture of perovskite and magnesiowiistite resulting from disproportionation of olivine, and of pure perovskite. The measurements were made in an externally heated diamond-anvil cell at pressures of ∼40 GPa and temperatures from 25 °C to ∼400 °C. Conductivity increases with increasing iron content, increasing temperature and increasing pressure. The activation energy (0.35 eV for 11% Fe) decreases with increasing iron content. The results are compatible with an electron-hopping conduction mechanism. Extrapolation to the temperature appropriate for a depth of 1,100 km, which corresponds to the pressure of our experiments, yields a conductivity of the lower mantle of the order of 1 S m−1; extrapolation to the temperature and pressure of the core-mantle boundary yields values between 50 and 100 S m−1, in agreement with geomagnetic determinations.

Ground-based and satellite observations of traveling magnetospheric convection twin vortices

Abstract: Close inspection of observations from the International Magnetospheric Study Scandinavian Magnetometer Array revealed the existence of a peculiar type of short-period magnetic variations. In typical cases it can be described as a positive deflection of the geomagnetic D component from the quiet time level with the disturbance of the H component approximately representing the negative time derivative of the D component. Thus these events are similar to magnetic variations recently reported by Lanzerotti et al. (1986) and discussed in terms of possible signatures of flux transfer events as well as to observations by Friis-Christensen et al. (1988) in connection with readjustments of the magnetopause to a sudden change in solar wind dynamic pressure. The typical duration of such a transient magnetic perturbation is about 10 min, with amplitudes of about 20 nT. Between 1975 and 1979 about 80 of these events have been detected, with a sharp occurrence maximum at magnetic forenoon. Most events were observed during moderately quiet times (KP≃2). A more detailed analysis of two such events exhibits a two-cell ground-equivalent current vortex structure. Simultaneous magnetic field observations from GEOS 2 for one of these events support our interpretation of these events as representing pairs of downward (in the west) and upward (in the east) field-aligned currents in the magnetosphere, moving over the observer from the dayside to the nightside. The physical mechanism generating these traveling magnetospheric convection twin vortices and the associated field-aligned currents is unclear. Flux transfer events, however, are not necessarily associated with the observed short-period transient magnetic field variations.

Magnetospheric Plasma Pressures in the Midnight Meridian: Observations From 2.5 to 35 RE

Abstract: Plasma pressure data from the ISEE 2 fast plasma experiment (FPE) are statistically analyzed to determine the plasma sheet pressure versus distance in the midnight local time sector of the near-earth (12-35 earth radii) magnetotail plasma sheet In regions where the bulk of the plasma pressure is contributed by particles in the energy range of the FPE (70 eV to 40 keV for ions), the statistically determined peak plasma pressures vary with distance similarly to previously determined lobe magnetic pressures Estimates of plasma pressures in the 'transition' region (7-12 earth radii), where the magnetic field topology changes rapidly from a dipolar to a taillike configuration, are compared with the observed pressure profiles Quiet time observations and estimates are combined to provide profiles of the equatorial plasma pressure along the midnight meridian between 25 and 35 earth radii

Pressure-driven magnetopause motions and attendant response on the ground

Abstract: The terrestrial magnetopause suffered considerable sudden changes in its location on 9–10 September 1978. These magnetopause motions were accompanied by disturbances of the geomagnetic field on the ground. We present a study of the magnetopause motions and the ground magnetic signatures using, for the latter, 10 s averaged data from 14 high latitude ground magnetometer stations. Observations in the solar wind (from IMP 8) are employed and the motions of the magnetopause are monitored directly by the spacecraft ISEE 1 and 2. With these coordinated observations we are able to show that it is the sudden changes in the solar wind dynamic pressure that are responsible for the disturbances seen on the ground. At some ground stations we see evidence of a “ringing” of the magnetospheric cavity, while at others only the initial impulse is evident. We note that at some stations field perturbations closely match the hypothesized ground signatures of flux transfer events. In accordance with more recent work in the area (e.g. Potemra et al., 1989, J. geophys. Res., in press), we argue that causes other than impulsive reeonnection may produce the twin ionospheric flow vortex originally proposed as a flux transfer even signature.

Multisatellite and ground-based observations of transient ULF waves

Abstract: Transient ULF pulsations associated with variations in solar wind plasma density observed by the IMP 8 satellite are presently studied in light of observations obtained during a fortuitous alignment of the AMPTE and Viking satellites with respect to the EISCAT Magnetometer Cross. It is found that the isolated 10-min oscillation in solar wind plasma density produced magnetic field compression oscillations within the magnetosphere at the same frequency, thereby enhancing resonant oscillations at approximately twice the frequency which were already present. Support is seen for the periodic solar wind density variation's exciting of a tailward-traveling large-scale magnetosphere wave train which excites local field line resonant oscillations.

A three‐dimensional numerical model of ionospheric plasma in the magnetosphere

Abstract: A three-dimensional particle trajectory tracing in empirical models of the geoelectric and geomagnetic fields is used to study the ionospheric contribution to magnetospheric plasma. Various ionospheric outflows are examined and results on ion transport are presented in terms of density, composition, and energy. Results are presented for two opposite magnetospheric configurations, ground state and storm phases. An estimate of the contribution of ionospheric O(+) to the hot plasma sheet is given. The simulation results are compared with observational data.

The Equatorial Electrojet

Abstract: The typical quiet day variations of the equatorial electrojet (EEJ) current intensity with time of the day, season, sunspot number, and geomagnetic latitude are presented in terms of the corresponding variations of ΔH which is the deviation of the horizontal component (H) of the geomagnetic field from its steady nighttime level. The observed height structure of the current density in the EEJ as measured in rocket flights is presented, along with the theoretically computed structure. Theoretical model results on the polarization electric fields and east-west currents as generated by the local interactions of height-varying winds in the EEJ show large height gradients and reversals for both currents and electric fields; experimental evidence for the reality of such height structures is also shown. The characteristics of the counter-electrojet events are presented and the possible causative mechanisms are discussed critically.

Simple models of fluid flow at the core surface derived from geomagnetic field models

Abstract: The fluid flow at the core surface is determined for a given model of temporal magnetic field variation, assuming steady flow and adopting the frozen-flux approximation. Evidence is presented for ageostrophic flow in the form of flow across the geographical equator. A simple pattern of the large-scale toroidal part of the flow is found. The pattern is nearly symmetric about the equator. Changes in the flow pattern are obtained from flow maps covering 5-year intervals beginning in 1915.

Derivation of a geomagnetic model to n=63

Abstract: SUMMARY A high degree model of the geomagnetic fielc is derived using an integral technique to extend coefficients beyond the limits allowable with least squares approaches. A previously derived model to n = 29 was first updated with new secular change data for the interval 1979 September-1980 June combined with the previously obtained analysis set of Magsat vector data. Residuals of a new selection of observed vector Magsat data were then analysed by solving for ionospheric-magnetospheric variations and removing their effect. The reduced B, components were then averaged over approximately 3” X 3” blocks of latitude and longitude, and coefficients derived using the Neumann method. These coefficients, when combined with those from the least squares solution, were seen to show significantly greater detail in the structure of the geomagnetic field which appeared to be realistic to n = 50.

Consequences of using simple analytical functions for the high-latitude convection electric field

Abstract: Analytical functions are developed to represent the convection electric field of Heppner and Maynard (1987), which are realistic representations of the field. The functions include an explicit variability with geomagnetic activity for southward interplanetary magnetic field (IMF). The authors have used a simple computer algorithm to model the ionosphere and investigate the consequences of using the Heppner-Maynard analytical functions for the high-latitude convection electric field. The Heelis et al. (1982) model, which has been widely used by investigators with computer simulations of the magnetosphere, ionosphere and/or thermosphere, is used here to provide a comparison. The Heppner-Maynard functions improve on the simple Heelis function for southward IMF. The Heppner-Maynard functions yield resonable configurations and magnitude for Joule heating and field-aligned currents for a southward IMF. The Heelis model may not yield reasonable results depending on the input parameters. The major differences between the Heppner-Maynard functions and the Heelis function for southward IMF are for regions near (1) the dayside cusp, (2) the Harang discontinuity and (32) the subauroral latitudes. In general, the Heppner-Maynard functions yield results that more closely compare with other investigators. For northward IMF, the functional representations of the Heppner-Maynard do not reproduce the field-aligned currents in the polarmore » cap which have been referred to as the NBZ current system.« less

Fine and Hyperfine Structure in the Spectrum of Secular Variations of Atmospheric 14C

Abstract: The coarse structure of the 14C spectrum consists of a secular trend curve that may be closely fit by a sinusoidal curve with period ca 11,000 yr and half amplitude ±51. This long-term trend is the result of changes in the earth's geomagnetic dipole moment. Consequently, it modulates solar components of the 14C spectrum but does not appear to modulate a component of the spectrum of ca 2300-yr period. The ca 2300-yr period is of uncertain origin but may be due to changes in climate because it also appears in the δ18O spectrum of ice cores. This component strongly modulates the well-known ca 200-yr period of the spectrum's fine structure. The hyperfine structure consists of two components that fluctuate with the 11-yr solar cycle. One component results from solar-wind modulation of the galactic cosmic rays and has a half-amplitude of ca ±1.5. The other component is the result of 14C production by solar cosmic rays that arrive more randomly but rise and fall with the 11-yr cycle and appear to dominate the fluctuation of the galactic cosmic-ray-produced component by a factor of two.

The daytime F layer trough and its relation to ionospheric-magnetospheric convection

Abstract: The daytime F layer trough is studied by means of an extensive network of ground-based ionospheric sounders in the northern hemisphere under conditions of solar maximum near winter solstice. The trough is observed to be a continuous band having an instantaneous extent of thousands of kilometers consisting of depletions in the daytime electron density, often by an order of magnitude. It lies in regions of sunward ionospheric-magnetospheric convection, an afternoon sector corresponding to the dusk cell, a morning sector corresponding to the dawn cell, and morphology and activity dependence consistent with convection. As detected in the diurnal distributions of f0F2, the trough is a persistent feature at high latitudes, appearing on each day of a 31-day period of continuous observation, and, although highly variable from day to day, is apparent in the monthly medians. The afternoon trough, which is detected independently by at least five and as many as 17 stations on each day, is generally continuous and stationary for a duration of many hours in magnetic latitude/magnetic local time coordinates. The trough contracts during quiet conditions so as to lie above 70° magnetic latitude but expands during disturbed conditions so as to extend from 75° to 52° magnetic latitude. The trough has a pronounced dependence on longitude, appearing principally in the afternoon in eastern magnetic longitudes but in the morning in western magnetic longitudes, an effect so prevalent that it produces large east-west local time asymmetries in the diurnal distributions of median daytime F layer electron densities throughout a wide range of latitudes. The longitudinal dependence is found to result from the relation between the two principal coordinate systems of the ionosphere-magnetosphere interaction: solar geomagnetic coordinates in which the convection pattern and the resultant daytime trough reside, and solar terrestrial coordinates in which solar ion production and the undisturbed daytime F layer in general reside; as a consequence of the fact that these coordinate systems vary with respect to one another with longitude, the trough varies within the daytime F layer with longitude.

A test of magnetic field draping induced Bz perturbations ahead of fast coronal mass ejecta

Abstract: ICE plasma and magnetic field data are examined to look for observational evidence of IMF draping ahead of fast coronal mass ejections (CMEs). The utility of the draping model for predicting the Bz perturbations and hence geomagnetic activity associated with the sheath regions ahead of such CMEs is also examined. A simple prediction scheme based on the upstream radial field component is developed and a set of interplanetary shock events previously associated with interplanetary type II bursts, and hence solar source locations, is used. Of 17 events the radial component predictor developed here correctly predicts the direction considered of the Bz perturbations for 13 events (76 percent). While this result is certainly not conclusive, it is considered to be supportive of the draping scenario.

Chronology and correlation of Quaternary magnetostratigraphy and nannofossil biostratigraphy in Norwegian-Greenland Sea sediments

Abstract: Paleomagnetic analyses of a series of sediment cores from the Norwegian—Greenland Sea document the existence of seven short-lived geomagnetic events during the last about 500.000 years. Calcareous nannofossil biostratigraphy, correlated to oxygen isotope chronostratigraphy, provides a time framework to infer age and duration of these periods of extreme divergence of the earth's magnetic paleofield from the regular normal polarity configuration of the Brunhes Chron. The geomagnetic events typically extend over 5 to 10 kyr. Two events are observed within the oxygen isotope stage 2–4 interval. A short event is recorded at the stage 4/5 boundary and another just below the stage 5/6 boundary. Additional three geomagnetic events are identified in oxygen isotope stage 6, at the stage 9/10 boundary and near the top of oxygen isotope stage 13, respectively.

Particle distributions in a two‐dimensional reconnection field geometry

Abstract: Particle trajectories in a two-dimensional steady state reconnection field geometry are studied in order to understand phase space density variations near the magnetopause and in the geomagnetic tail. Charged particles are traced back in time from a spacecraft position until they are in the source or inflow region for an entire gyroorbit. The inflow regions for the dayside magnetopause are the magnetosheath and the closed field lines inside the magnetosphere. The inflow regions for the magnetotail are the lobes north and south of the central plasma sheet. Using Liouville's theorem and known distribution functions in the source regions, the distribution function at the spacecraft can be calculated. Unlike previous studies of “remote sensing,” this one is in two dimensions and includes a perpendicular electric field. The question of whether “trapped” distribution functions can exist on “open” field lines on the dayside magnetopause is examined as well as the signatures of reconnection in the geomagnetic tail. We find that “trapped” distributions which have a peak at or near 90° pitch angle can exist on topologically “open” field lines. Thus the observations of such distributions in the low-latitude boundary layer do not establish that this layer is on closed field lines. In the tail the omnidirectional flux of keV ions should show a minimum in the center of the plasma sheet at distances up to about 25 RE earthward of the neutral line. Since no such minimum is apparent in the ISEE data, we conclude that the neutral line in the tail is generally tailward of 50 RE. We also find that adiabatic theory cannot be used as a means for visualizing or predicting the orbits of charged particles in these complex magnetic field topologies. In fact, we find that some orbits are chaotic in the sense that particles which start at nearly the same place in phase space follow entirely different and exponentially (in time) diverging orbits. These rather unintuitive results suggest that there is no simple or general method for interpreting pitch angle distributions observed near magnetospheric boundary layers which are magnetically connected to a neutral line.

Paleointensity of the Earth's magnetic field and K‐Ar dating of the Louchadiere volcanic flow (central France): New evidence for the Laschamp excursion

Abstract: We report paleointensity results of the Earth's magnetic field from a Late Pleistocene lava flow (Louchadiere, Central France), which recorded an intermediate geomagnetic field direction (5 sites, mean declination=114.1°, inclination=58.2°, k=130, α95=6.7°). New K-Ar age determinations confirm that this flow is contemporaneous with the Laschamp and Olby flows, and that this excursion occurred around 45ka ago. Using the Thellier double heating method, reliable paleointensities have been obtained for ten samples from three different sites, providing an average field strength of 12.9 µT (±3.3). This low value and previous results of the Laschamp excursion from France and Iceland confirm that the Earth's magnetic field was in an intermediate state during the Laschamp excursion.

Some aspects of the relation between Pi 1-2 magnetic pulsations observed at L = 1.3-2.1 on the ground and substorm-associated magnetic field variations in the near-Earth magnetotail observed by AMPTE CCE

Abstract: The relation between Pi 1-2 pulsations on the ground and substorm-associated magnetic field variations in space has been studied using data obtained on the ground at low-latitude conjugate stations (L = 1.3-2.1) and in the near-Earth magnetotail by the AMPTE CCE spacecraft. The ground-based data were acquired in a campaign period from July 20 to September 16, of 1986, during which the apogee of CCE (8.8 R{sub E}) was located between 2330 and 0230 hours magnetic local time. Of 16 clear magnetic field dipolarizations observed at CCE, all had a corresponding Pi 2 pulsation on the ground, with a time lag of +1 to {minus}7 min. For most (13) of these cases, the time lag was equal to or shorter than 2 min. However, the authors also found Pi 2 pulsations that do not accompany a dipolarization at CCE. These results are consistent with previous observations, which showed that Pi 2 pulsations are a global indicator of the expansion phase onset of a substorm, whereas dipolarizations occur in a limited region in the near-Earth magnetotail. One of the 16 events, which occurred on August 28, 1986, is studied in detail because a 13-s Pi 1 pulsation was observed on themore » ground in addition to an ordinary Pi 2 pulsation. For this event, CCE also observed a {approximately} 13-s oscillation at {approximately} 8.1 R{sub E} in the midplane of the magnetorail near midnight (Takahashi et a., 1987). They suggest that field line resonance driven by a quasi-monochromatic oscillation in the near-Earth tail is the cause of the Pi 1 pulsation observed on the ground. The commonly observed Pi 2 pulsations could be attributed to other wave excitation mechanisms including transient response of the magnetospheric cavity to a substorm-associated impulse.« less

The Geomagnetic Spectrum For 1980 and Core‐Crustal Separation

Abstract: SUMMARY The spectrum of a high degree spherical harmonic model of the geomagnetic field is analysed to compute the constants for the core and crustal field contributions. Using a noise estimate of 0.091 nT2 at the mean Magsat radius of 6791 km, the power reduced to the Earth's surface is found to be 9.66 X 10' (0.286)" nT2 for the core, and 19.1 (0.996)" nT2 for the crust. These values show half the crustal power extrapolated to n = 0 compared with a previously published n = 23 model, and a white noise depth of only 14 km below the mean surface. The core spectrum power is 30 per cent less than previously estimated and becomes flat 80 km below the core-mantle boundary. The crustal power level is an eighth of that of an estimate based on one-dimensional analyses of Project MAGNET survey lines. The point where the energy density of the core and crustal components become equal at the Earth's surface is n = 14.2.