Showing papers on "Earth's magnetic field published in 1992"

A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic

Abstract: We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the world's ocean basins. This is the first time, since Heirtzler et al. (1968) published their time scale, that the relative widths of the magnetic polarity intervals for the entire Late Cretaceous and Cenozoic have been systematically determined from magnetic profiles. A composite geomagnetic polarity sequence was derived based primarily on data from the South Atlantic. Anomaly spacings in the South Atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles. Fine-scale information was derived from magnetic profiles on faster spreading ridges in the Pacific and Indian Oceans and inserted into the South Ariantic sequence. Based on the assumption that spreading rates in the South Atlantic were smoothly varying but not necessarily constant, a time scale was generated by using a spline function to fit a set of nine age calibration points

A thermosphere/ionosphere general circulation model with coupled electrodynamics

Abstract: A new simulation model of upper atmospheric dynamics is presented that includes self-consistent electrodynamic interactions between the thermosphere and ionosphere. This model calculates the dynamo effects of thermospheric winds, and uses the resultant electric fields and currents in calculating the neutral and plasma dynamics. A realistic geomagnetic field geometry is used. Sample simulations for solar maximum equinox conditions illustrate two previously predicted effects of the feedback. Near the magnetic equator, the afternoon uplift of the ionosphere by an eastward electric field reduces ion drag on the neutral wind, so that relatively strong eastward winds can occur in the evening. In addition, a vertical electric field is generated by the low-latitude wind, which produces east-west plasma drifts in the same direction as the wind, further reducing the ion drag and resulting in stronger zonal winds.

Model of the formation of the low-latitude boundary layer for strongly northward interplanetary magnetic field

Abstract: A model for the formation of the low-boundary layer in which a magnetosheath flux tube reconnects in the north and south beyond the cusp when the IMF is strongly northward is presented. For northward IMF the geomagnetic field captures solar wind flux tubes through intermittent reconnection at the cusp region. These newly captured flux tubes shorten and sink into the magnetosphere while the flux tubes reorient themselves as they become assimilated into the magnetosphere. There is no significant acceleration for the particles within the flux tubes. In the magnetosphere the interchange instability disperses the flux tube azimuthally along the magnetopause to form a boundary layer. Subsequent reconnection forms sublayers of the boundary layer and different sublayers represent different ages after reconnection. The interchange instability is stable radially, which keeps sharp boundaries between sublayers and between the boundary layer and magnetosphere.

Observations of weak double layers on auroral field lines

Abstract: The wave instrument on the Swedish Viking satellite performed measurements of plasma density and potential fluctuations with high temporal and spatial resolution. In the auroral regions, double-layer structures were observed which have scale lengths of about 100 m and where the plasma density is reduced by up to 50%. The structures are negatively charged and have net potential drops of up to a few volts directed upwards in the same direction as the electric currents along the geomagnetic field lines; i.e., the structures are dissipative elements of the magnetospheric electric circuit. In some regions double-layer structures are so ubiquitous that it is possible that series of such double layers may form a large-scale equipotential pattern, with a substantial potential drop along the field lines, thus contributing to the energization of the electrons that produce the aurora and upward flowing ions. >

Paleointensity of the geomagnetic field during the last 80,000 years

Abstract: High-resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores. Three duplicate records were used to estimate the regional coherency of the data within a single area (Tyrrhrenian Sea) while the two others document the field variations in the eastern Mediterranean and the southern Indian Ocean. Careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes. The time-depth control was provided by oxygen isotopes, and small-scale variations in the deposition rates were constrained by means of tephrachronology. The synthetic curve calculated from the Mediterranean records provides a continuous record of the intensity variations during the last 80,000 years (80 kyr), which correlates well with the sparse volcanic data available for the period 0–40 kyr. The fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the Pacific Ocean establishes the truly dipolar character of these variations. The dipole field moment is characterized by large-scale changes as shown by the existence of pronounced drops (at 39 and 60 kyr) alternating with periods of higher intensity. The record suggests a periodic nature for these intensity variations; however, the period studied is not sufficiently long to state this conclusively. These results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time.

Spatial and temporal behavior of ULF pulsations observed by the Goose Bay HF radar

Abstract: Techniques which allow the instantaneous amplitude and phase to be determined as functions of geomagnetic lattitude, longitude, and time are employed to carry out a detailed analysis of HF radar data of a ULF pulsation event in the postmidnight sector on January 11, 1989. Field line resonances with several different frequencies occur simultaneously at different latitudes. These can be associated with cavity mode frequencies of 1.3 mHz, 1.9 mHz, 2.7 mGz, and 3.3 mHz. These frequencies are constant to better than 10 percent over a local time period of nearly 4 hr. The field-aligned currents driven by the resonances can be as large as 5 micro-A/sq m at ionospheric heights. The data support a picture of modes driven by solar wind impulses.

Radial profiles of quiet time magnetospheric parameters

Abstract: The quiet time magnetosphere is studied using ion measurements covering energies from about 1 keV to about 4 MeV from selected passes of the Charge Composition Explorer during low geomagnetic activity. The quiet time plasma pressure in the midnight sector was found to be comparable to that deduced by Spence et al. (1989) and Kan et al. (1992) from inverting the magnetic field models of Tsyganenko and Usmanov (1982) and Tsyganenko (1987). The radial profile of the total plasma pressure exhibited a peak at L = 3-4 and decreased monotonically from L = 4 and L = 9. Contrary to theoretical expectations for the magnetospheric closure of the region 2 field-aligned current system, no large-scale earthward decrease in plasma pressure was found to occur in these outer L shells.

40Ar/39Ar Dating of the Brunhes-Matuyama Geomagnetic Field Reversal.

Abstract: Magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale (GPTS) to date events in the range 0 to 5 million years ago. A critical tie point on the GPTS is the potassium-argon age of the most recent (Brunhes-Matuyama) geomagnetic field reversal. Astronomical values for the forcing frequencies observed in the oxygen isotope record in Ocean Drilling Project site 677 suggest that the age of this last reversal is 780 ka (thousand years ago), whereas the potassium-argon-based estimate is 730 ka. Results from 4039; Ar incremental heating studies on a series of lavas from Maui that straddle the Brunhes-Matuyama reversal give an age of 783 + 11 ka, in agreement with the astronomically derived value. The astronomically based technique appears to be a viable tool for dating young sedimentary sequences.

Transverse ion acceleration by localized lower hybrid waves in the topside auroral ionosphere

Abstract: Up to now, observations had been unable to show conclusively a one-to-one correspondence between perpendicular ion acceleration and a particular type of plasma wave within the O(+) source region below 2000 km. In this paper we demonstrate that intense (100-300 mV/m) lower hybrid waves are responsible for transversely accelerating H(+) and O(+) ions to characteristic energies of up to 6 eV. This wave-particle interaction takes place in thin filamentary density cavities oriented along geomagnetic field lines. The measurements we discuss were conducted in the nightside auroral zone at latitudes between 500 km and 1100 km. Our results are consistent with theories of lower hybrid wave condensation and collapse.

On the formation and evolution of plasmoids: A survey of ISEE 3 geotail data

Abstract: ISEE 3 magnetometer and electron plasma measurements from the 1983 Geotail Mission were surveyed to determine the magnetic and plasma properties of plasmoids and their evolution with distance downtail. Events were selected on the basis of a bipolar magnetic signature in either the geocentric solar magnetospheric Bz and/or By component; most had Bz bipolar signatures. We found 366 events consistent with this signature while ISEE 3 was in the plasma sheet. ISEE 3 observed plasmoids all along its trajectory whenever it was in the plasma sheet. Plasmoids are characterized by high-speed plasma flow. Plasmoid length was determined using both the magnetometer and the electron plasma velocity data. We found the average length of plasmoids is 16.7 ± 13.0 RE, significantly smaller than previous estimates. Many plasmoids have a well-defined magnetic core field, characterized by a field strength maximum at the center of the pass through the structure. Plasmoids appear to be relatively stable structures once their formation process is complete. The size, velocity, magnetic core strength, and Bz field amplitude of plasmoids do not depend on distance beyond 100 RE downtail. The average electron temperature inside plasmoids drops by a factor of 2 and the electron density increases by a factor of 2 as plasmoids propagate from near Earth distances (within 100 RE of the Earth) to the deep tail. We conclude that the stable size of the plasmoids, the density increase and the temperature decrease are consistent with a flux of cold electrons into the plasmoid. The strong correlation of interplanetary magnetic field By an hour before the event with the strength and direction of By observed inside plasmoids, the existence of events with the bipolar signature in both the By and Bz components, and the possible mass flux all are consistent with plasmoids being “open” magnetic structures.

Localized lower hybrid acceleration of ionospheric plasma.

Abstract: Observations of the transverse acceleration of ions in localized regions of intense lower hybrid waves at altitudes near 1000 km in the auroral ionosphere are reported. The acceleration regions are thin filaments with dimensions across geomagnetic field lines of about 50-100 m corresponding to 5-10 thermal ion gyroradii or one hot ion gyroradius. Within the acceleration region lower hybrid waves reach peak-to-peak amplitudes of 100-300 mV/m and ions are accelerated transversely with characteristic energies of the order of 10 eV. These observations are consistent with theories of lower hybrid wave collapse.

Secular variation of the aurora for the past 500 years

Abstract: Knowledge of solar variability and its effects on the Earth is essential since the Sun affects almost every aspect of our lives. Direct observations of the Sun, usually of sun- spots, with some continuity, exist only since about 1700. Understanding of long-term solar variability must then de- pend on proxy data, such as visual auroral observations, measurements of magnetic activity, and the radiocarbon re- cord. These also give us information on the interaction be- tween the solar wind, the interplanetary field, and the terrestrial magnetosphere, as well as, for the radiocarbon record, hellospheric conditions. This paper uses a data base of visual auroral observations for a period of about 500 years, from 1450 to 1948, comprising about 45,000 observations, in addition to the well-known sunspot series and the magnetic activity index aa, from 1868 to 1990. The secular variation of the aurora is examined and compared, where possible, to sunspot data and magnetic activity data. Blackman-Tukey power spectra are used to determine periodicities. The study confirms the variability of the periodicities in both frequency and amplitude. In particular, the well-known 11.l-year cycle disappears during the Maunder minimum and at the end of the eighteenth and beginning of the nineteenth century. While the 11.l-year period is normally strongly dominant for sun- spots, other shorter periods become important, and even dom- inant, for auroras and magnetic activity. Consequently, the temporal behavior of these three variables differs. Prolonged solar activity minima are clearly evident. In addition to the known Sptrer, Maunder, Dalton, and 1901-1913 minima, a previously unrecognized minimum about 1765 is clearly ev- ident in the data. Comparison of the depth of these minima shows that the Dalton minimum may be the deepest, or at least rivals the Maunder minimum in importance. This min- imum clearly deserves further study. Combining the polar data base with that of mid-latitudes provides for the first time a globally comprehensive historical record of auroral occur- rence. The data provide confirmation of the anticorrelation of auroral occurrence in the polar regions with sunspot ac- tivity, as a result of displacement of the auroral oval with changes in solar and magnetic activity. Assuming the validity of some current models of the solar origin of geomagnetic activity, the data provide a basis for understanding the var- iation over time of the general magnetic field of the Sun, in particular the polar field.

Dipolar reversal states of the geomagnetic field and core–mantle dynamics

Abstract: Palaeomagnetic records from lavas suggest that at least two specific inclined dipolar field configurations have dominated the reversal process for the past ten million years. These long-lived states provide a way to explain directional rebounds, aborted reversals and the recording in sediments of what appear to be preferred longitudinal paths of the virtual geomagnetic pole. The polar orientations correlate with near-radial flux concentrations recognizable when today's field is stripped of its axial dipole, and with lower-mantle seismic anomalies, suggesting a tie to deep-Earth dynamics.

A nonlinear dynamical analogue model of geomagnetic activity

Abstract: Consideration is given to the solar wind-magnetosphere interaction within the framework of deterministic nonlinear dynamics. An earlier dripping faucet analog model of the low-dimensional solar wind-magnetosphere system is reviewed, and a plasma physical counterpart to that model is constructed. A Faraday loop in the magnetotail is considered, and the relationship of electric potentials on the loop to changes in the magnetic flux threading the loop is developed. This approach leads to a model of geomagnetic activity which is similar to the earlier mechanical model but described in terms of the geometry and plasma contents of the magnetotail. The model is characterized as an elementary time-dependent global convection model. The convection evolves within a magnetotail shape that varies in a prescribed manner in response to the dynamical evolution of the convection. The result is a nonlinear model capable of exhibiting a transition from regular to chaotic loading and unloading. The model's behavior under steady loading and also some elementary forms of time-dependent loading is discussed.

Particle scattering and current sheet stability in the geomagnetic tail during the substorm growth phase

Abstract: The particle scattering and current sheet stability features in the geomagnetic tail during the phase of substorm growth were investigated using Tsyganenko's (1989) magnetic field model. In a study of four substorm events which were observed both in the high-altitude nightside tail and in the auroral ionosphere, the model magnetic field was adjusted to each case so as to represent the global field development during the growth phase of the substorms. The model results suggest that the auroral brightenings are connected with processes taking place in the near-earth region inside about 15 earth radii. The results also suggest that there is a connection between the chaotization of the electrons and the auroral brightenings at substorm onset.

Traveling magnetospheric convection twin vortices: Another case study, global characteristics, and a model

Abstract: A case study of a transient geomagnetic field variation event associated with a traveling magnetospheric convection twin vortex is presented. The characteristics of this event are different from those of other presented cases, as the sense of rotation of the associated ionospheric current system is reversed, that is, it exhibits upward (downward) field-aligned currents in the tailward (dayside) part of the vortex structure. Other features are as usually observed of such transients. In particular, a clear tailward motion at high latitudes can be deduced from local as well as global magnetic field observations. At low dayside latitudes, magnetic field variations similar to sudden impulse (SI) variations are recorded. Almost simultaneous onset and no westward propagation are observed, too. As possible source mechanisms of such transient events, localized magnetic field reconnection as well as pressure pulses at the dayside magnetopause are discussed. If the switch-on of magnetic reconnection can be regarded as equivalent to an equivalent current flowing against the existing magnetopause current and if this countercurrent is spatially localized, an Alfven wave is generated with downward (upward) field-aligned current flow in the tailward (dayside) part of the wave, as observed for some of the reported transients. However, the SI-like behavior of the event studied in this paper, its reversed sense of rotation, and the vortex associated field-aligned current density excludes such localized magnetic reconnection as a possible source mechanism and argues in favour of the existence of pressure pulses at the magnetopause. It is shown that a spatially localized compression or dilatation of the dayside magnetopause is also associated with the onset of a current flowing parallel or antiparallel to the magnetopause current, respectively. The additional current associated with compression gives rise to a system of upward and downward field-aligned currents in the tailward and dayside part of the system, much as observed for the event analysed. As the pressure pulse is moving along the magnetopause, the generated Alfven wave pattern is that of an Alfven wing structure, with the ground-magnetic transient representing the passing by of the first ionospheric reflection site. Such an Alfven wing structure may also explain recent observations of multiple twin vortex systems.

Palaeomagnetic constraints on the geometry of the geomagnetic field during reversals.

Abstract: Palaeomagnetic records of the path of the pole during reversals of the Earth's magnetic field provide a test of the hypothesis that dipolar or low-order axisymmetric components of the field dominate during reversals. Multiple records of reversals during the past 12 Myr show no simple or consistent geographical pattern. Although a more robust analysis of the transitional field awaits a greater number of well-distributed sampling sites, the present data are not inconsistent with the simplest models, in which a field reminiscent of the non-dipole component of the present-day field becomes dominant.

Modelling of the magnetic field of magnetospheric ring current as a function of interplanetary medium parameters

Abstract: The models are examined which are proposed elsewhere for describing the magnetic field dynamics in ring-currentDR during magnetic storms on the basis of the magnetospheric energy balance equation. The equation parameters, the functions of injectionF and decay τ, are assumed to depend on interplanetary medium parameters (F and τ during the storm main phase) and on ring-current intensity (τ during the recovery phase). The present-day models are shown to be able of describing theDR variations to within a good accuracy (the r.m.s. deviation 5 <δ < 15 nT, the correlation coefficient 0.85

Ultrahigh Resolution Marine Magnetic Anomaly Profiles: A Record of Continuous Paleointensity Variations?

Abstract: A distinctive pattern of small-scale marine magnetic anomalies (25-100 nT amplitude, 8-25 km wavelength: tiny wiggles) is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the Indian Ocean. By normalizing and stacking multiple profiles, it is demonstrated that this pattern of tiny wiggles is a high-resolution recording of paleodipole field behavior between chrons C24 and C27. The pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading (82 mm/yr half rate) profile from the southeast Pacific where a similar signal is observed, confirming the paleodipole field origin of the anomalies. Two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations. We conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles. The implications of this study are that (1) tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo; (2) the small-scale anomalies observed in the Jurassic quiet zones may be due to paleointensity fluctuations; (3) tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the Cretaceous quiet zones; and (4) much of the variance in anomaly profiles normally attributed to crustal eraplacement processes, particularly at fast and ultrafast spreading rates, is actually due to intensity variations in the paleomagnetic field.

Fluxgate magnetometer instrument on the CRRES

Abstract: Introduction and Scientific Objectives A PRIMARY objective of the CRRES mission is to examine the effects of the radiation belt environment on a variety of microelectronic components by making comprehensive and simultaneous environmental measurements. Other objectives include new static and dynamic models and descriptions of processes in the radiation belts. These objectives require knowledge of the Earth's magnetic field, and its variations, which are important for the study of all space plasma processes, including control of the source, loss, and energization of radiation belt particles. The fluxgate magnetometer makes the magnetic field measurements that are required to satisfy these major mission objectives. CRRES was launched on July 25, 1990, at 1921 UT. In the initial CRRES orbit (period = 9 h 52 min, inclination = 18.2 deg, perigee = 350 km, apogee = 6.3Re, and initial magnetic local time of apogee = 8 MLT), the magnetometer must have a large dynamic range to measure magnetic fields from fractions of a nanotesla to about 45,000 nT (1 nT = 10 ~ 9 T). These measurements will be used 1) together with the look angles of the particle experiments to obtain the pitch angle of the measured particles; 2) as a diagnostic of global and local geomagnetic disturbances and current systems; 3) as a diagnostic of low-frequency waves in the ambient environment and to study wave-particle interactions; 4) to provide plasma gyrofrequencies; 5) to measure v x B electric fields; 6) to support chemical release activities; and 7) to provide a secondary source of spacecraft attitude information.

Standing ULF modes of the magnetosphere: A theory

Abstract: Field line resonances with frequencies in the range 1 to 4 mHz have recently been observed by the JHU/APL HF Doppler radar during quiescent geomagnetic conditions. These structures are observed to have stable frequencies for durations of several hours, leading us to the conclusion that they may be standing waves (in the radial direction, as opposed to standing waves along a field line) of the magnetosphere driven by the solar wind. Using this premise, the propagation of fast mode ULF waves in the magnetosheath and the near earth magnetosphere are examined in an ideal, linearized MHD context. A model is presented in which fast waves propagate in the equatorial plane between the flanks of the bow shock and a turning point deep within the magnetosphere. Due to the magnetic field gradient near the earth, a field line resonance develops between the turning point and the plasmapause. Using a realistic set of magnetospheric parameters, it is possible to reproduce the set of observed frequencies and the respective positions of their field line resonances within the ionosphere (assuming a dipole mapping). However, because the model cavity frequencies are sensitive to magnetosheath parameters, this model does not explain the extreme stability with respect to geomagnetic conditions of the observed frequencies.

A scenario for solar wind penetration of earth's magnetic tail based on ion composition data from the ISEE 1 spacecraft

Abstract: Based on He(2+) and H(-) ion composition data from the Plasma Composition Experiment on ISEE 1, a scenario is proposed for the solar wind penetration of the earth's magnetic tail, which does not require that the solar wind plasma be magnetized. While this study does not take issue with the notion that earth's magnetic field merges with the solar wind magnetic field on a regular basis, it focuses on certain aspects of interaction between the solar wind particles and the earth's field, e.g, the fact that the geomagnetic tail always has a plasma sheet, even during times when the physical signs of magnetic merging are weak or absent. It is argued that the solar plasma enters along slots between the tail lobes and the plasma sheet, even quite close to earth, convected inward along the plasma sheet boundary layer or adjacent to it, by the electric fringe field of the ever present low-latitude magnetopause boundary layer (LLBL). The required E x B drifts are produced by closing LLBL equipotential surfaces through the plasma sheet.

Traveling magnetospheric convection twin-vortices : observations and theory

Abstract: Ground-based magnetometer arrays are an important tool to image dynamical features of the Earth's magnetosphere, and have more recently been used to study a peculiar type of geomagnetic field variation, impulsive magnetic events, occurring predominantly in the dayside magnetosphere. Magnetic field imaging has been able to show that such magnetic impulsive events are due to tailward-propagating magnetospheric convection twin-vortices with the vortex centers separated by 1000-2000 km and being located at high geomagnetic latitudes, on field lines probably mapping back to the dayside magnetopause. Several mechanisms have been suggested to generate such twin-vortex systems and are discussed in detail, among them magnetic reconnection and changes of the magnetopause pressure balance. Detailed analysis of a twin-vortex structure exhibits that they are associated with a pair of upward and downward flowing field-aligned current with current densities up to 10 −9 A/m 2 . That such large currents may eventually become unstable, and potentially lead to particle acceleration is supported by observations of dayside auroral activity associated with such twin-vortex systems. They may be looked at as a dayside equivalent of the nightside, substorm current wedge

Evidence that fin whales respond to the geomagnetic field during migration

Abstract: We challenge the hypothesis that fin whales use a magnetic sense to guide migration by testing for associations between geophysical parameters and the positions where fin whales were observed over the continental shelf off the northeastern United States. Monte Carlo simulations estimated the probability that the distribution of fin whale sighting was random with respect to bottom depth, bottom slope and the intensity and gradient of the geomagnetic field. The simulations demonstrated no overall association of sighting positions with any of these four geophysical parameters. Analysis of the data by season, however, demonstrated statistically reliable associations of sighting positions with areas of low geomagnetic intensity and gradient in winter and fall, respectively, but no association of sighting positions with bathymetric parameters in any season. An attempt to focus on migrating animals by excluding those observed feeding confirmed the associations of sighting positions with low geomagnetic intensity and gradient in winter and fall, respectively, and revealed additional associations with low geomagnetic gradients in winter and spring. These results are consistent with the hypothesis that fin whales, and perhaps other mysticete species, possess a magnetic sense that they use to guide migration.

Possible connection between surface winds, solar activity and the Earth's magnetic field

Abstract: RECORDS of 14C in tree rings provide a proxy for changes in solar activity on timescales of decades to centuries1,2. Here I report an association between Maunder-type solar cycles recorded in tree rings and fluctuations in surface wind intensity on a roughly 200-year timescale over a period of about 2,000 years in the mid-Holocene. The wind record is preserved as changes in the thickness of varved sediments in Elk Lake, Minnesota, which contain a large aeolian component. Changes in cyclonic activity and tropospheric winds have been reported1–5 to occur in this region a few days after strong coronal mass ejections (CMEs); increases of up to 7% in zonal flow have been reported5 at an altitude of ~ 300 m after strong CMEs in winter. The implication of this association is that century-scale changes in surface winds over Minnesota were the long-term (Maunder-scale) counterpart of short-term (daily) changes in winds after CMEs. The weak magnetic field strength of the Earth at this time6,7 might be relevant to a possible mechanism for the association.

Observations from Millstone Hill during the geomagnetic disturbances of March and April 1990

Abstract: The incoherent scatter radars at Millstone Hill operated continuously during the periods March 16–23 and April 6–12, 1990, providing observations of large-scale ionospheric structure and dynamics over a large portion of eastern North America. Major geomagnetic storms occurred during each of these periods, with deep nighttime ionospheric troughs and large magnetospheric convection electric fields observed equatorward of Millstone. The Millstone observations provide a comprehensive data set detailing storm-induced ionospheric effects over a 35° span of latitude during both of these intervals. At the latitude of Millstone the ionospheric peak height hmF2 rose above 600 km in the trough on March 22 and 23 and reached ≈500 km at night on April 11 and 12. Increased recombination, apparently due to the strong electric fields, the temperature dependent recombination rate coefficient, and neutral composition changes, greatly depleted the F2 region over a wide latitude range during the day on April 10, 1990. This resulted in an ionosphere dominated by molecular ions, with ionospheric peak heights below 200 km on this day. A number of frictional heating events during the disturbed periods are seen from comparison of ion temperature and velocity measurements. The most intense event took place near 1200 UT (≈0715 LMT) on April 10, 1990, when Kp reached 8. At 0110 UT on March 21, line of sight ion velocities in excess of 500 m s−1 were observed at the extreme southern limit of the Millstone steerable radar's field of view (40° apex magnetic latitude at an altitude of 700 km). These could be due to penetration of magnetospheric electric fields or electric fields associated with ring current shielding in the storm-time outer plasmasphere. About an hour later, ion outflow was observed just equatorward of Millstone. This is most likely due to heating from a latitudinally confined region of intense westward convection. Neutral meridional winds above Millstone were obtained by three different techniques employing radar and Fabry-Perot measurements. The latitude variation of the winds was also estimated from radar measurements of hmF2 and electric fields using the servo model method. Strong equatorward nighttime neutral wind surges were found during both the March and April disturbances, which reached the equatorward limit of the observations at F peak heights.

Paleomagnetism of 122 Ma plutons in New England and the mid-Cretaceous paleomagnetic field in North America: true polar wander or large-scale differential mantle motion?

Abstract: A paleomagnetic study of Cretaceous White Mountains plutonic complexes in New Hampshire and Vermont yields high unblocking temperature, dual polarity magnetizations in different types of igneous rocks. The resulting pole position for three plutons (71.9°N, 187.4°E, A95 = 6.9°, age = 122.5 Ma) agrees with previously published mid-Cretaceous poles for North America, which together give a mid-Cretaceous standstill reference pole slightly revised from Globerman and Irving [1988] at 71.2°N, 194.1°E (A95 = 3.7°, N = 5 studies). We argue on the basis of the wide geographic distribution of these studies, the variety in tectonic settings and rock types, positive reversal tests, and an overall reversal pattern consistent with geomagnetic polarity time scales, that this mean pole represents the North American mid-Cretaceous reference field for nominally 36 m.y. (124 to 88 Ma). The standstill pole limits to within ±4°, the motion of the North American plate relative to the Earth's spin axis. During the same mid-Cretaceous interval, the New England hotspot track (124 Ma Monteregian Hills, 122.5-Ma Cretaceous White Mountains, and 103- to 84-Ma New England seamounts) requires 11°±4° of north-poleward motion of North America, in direct conflict with the paleomagnetic standstill. A similar (∼13°) discrepancy is independently demonstrated between the spin axis and the Tristan da Cunha hotspot track on the African plate during the mid-Cretaceous interval. The hotspot/spin axis discrepancies ended by ∼90 Ma when it is shown that both Atlantic hotspots agree with North American and African dipole paleolatitudes and present-day locations. Nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis, leaving as possible interpretations true polar wander and large-scale (but differential) mantle motion. The southerly motion of the mid-Cretaceous Louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics. The motions of the three widely separated mid-Cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid-Cretaceous “superplume.”