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All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

The origin of polarity asymmetries in the history of the geomagnetic field

The behavior of magnetohydrodynamic stationary modes in the presence of an imposed weak magnetic field originating separately from the dynamo is studied. A rare class of stationary states is found that exhibit high sensitivity to the presence of weak imposed fields. The amplitude of the difference between the total fields of opposite polarity is much larger than the amplitude of the imposed nondynamo fields. It is proposed that Earth's magnetic field operates in such a mode, highly sensitive to the presence of an ambient field. An argument is given to explain why the terrestrial dynamo should choose to operate in one of these rare states. Implications are discussed for the general mechanism of dynamo magnetic field equilibrium in planets.

Levy, E. H.↗
NASA NTRS1987

Enhancements of energetic ions associated with travelling compression regions in the deep geomagnetic tail

This paper presents a representative example of an enhancement in energetic ion flux associated with the International Sun-Earth Explorer 3 (ISEE 3) spacecraft's encounter with a traveling compression region (TCR). Data from the energetic particle anisotropy spectrometer (EPAS) instrument on ISEE 3 are studied, along with magnetic field data from the vector helium magnetometer. It is concluded that the ion enhancements seen are spatial in nature, thus supporting the idea that TCRs are the lobe signatures of plasmoids moving along the magnetotail, away from earth.

Murphy, N.↗
NASA NTRS1986

Geomagnetic temporal change: 1903-1982 - A spline representation

The secular variation of the earth's magnetic field is itself subject to temporal variations. These are investigated with the aid of the coefficients of a series of spherical harmonic models of secular variation deduced from data for the interval 1903-1982 from the worldwide network of magnetic observatories. For some studies it is convenient to approximate the time variation of the spherical harmonic coefficients with a smooth, continuous, function; for this a spline fitting is used. The phenomena that are investigated include periodicities, discontinuities, and correlation with the length of day. The numerical data presented will be of use for further investigations and for the synthesis of secular variation at any place and at any time within the interval of the data - they are not appropriate for temporal extrapolations.

Langel, R. A.↗
NASA NTRS1986

Twisting of the geomagnetic tail

Theory and models indicate that the IMF causes the magnetotail to produce an observable twist, predicting that the torque should bring the north lobe south of the GSM xy plane on one flank of the magnetotail, and the south lobe north of the plane on the other flank. In this paper, ISEE-3 geotail observations are compared with simultaneously made IMP 8 IMF observations supplemented by inferred IMF B(y) polarities to demonstrate the IMF-induced magnetotail twist. The results indicate that twisting is the single most important factor accounting for observations of the north lobe south of the GSM xy plane and the south lobe north of the plane. The value of the observed magnetotail twist about its aberrated axis is about 18 deg.

Sibeck, D. G.↗
NASA NTRS1987

The effects of magnetic B(y) component on geomagnetic tail equilibria

A two-dimensional linear magnetohydrostatic model of the magnetotail is developed here in order to investigate the effects of a significant B(y) component on the configuration of magnetotail equilibria. It is concluded that the enhanced B(y) values must be an essential part of the quiet magnetotail and do not result from a simple intrusion of the IMF. The B(y) field consists of a constant background component plus a nonuniform field existing only in the plasma sheet, where it is dependent on the plasma paramater beta and the strength of the magnetic B(z) component. B(y) is strongest at the neutral sheet and decreases monotonically in the + or - z direction, reaching a constant tail lobe value at the plasma sheet boundaries. The presence of a significant positive B(y) component produces currents, including field-aligned currents, that flow through the equatorial plane and toward and away from earth in the northern and southern halves of the plasma sheet, respectively.

Hilmer, Robert V.↗
NASA NTRS1985

The solar/interplanetary/magnetosphere/ionosphere connection - A strategy for prediction of geomagnetic storms

A physically-based strategy for the prediction of geomagnetic/ionospheric disturbances, the Solar-Terrestrial/Environmental Model (STEM 2000), is proposed, with application to the prediction of periods of spacecraft charging. Synoptic solar observations provide input for MHD models for flare occurrence, propagation of coronal disturbances, and high speed solar wind streams. A three-dimensional interplanetary MHD model determines solar wind parameters including the energy flux and the cross-magnetosphere tail electric field. Observational earth data are used to predict local time, high latitude ionospheric disturbances which have an impact on the ionospheric structure.

Dryer, M.↗
NASA NTRS1987

Simultaneous stochastic inversion for geomagnetic main field and secular variation. I - A large-scale inverse problem

The method of stochastic inversion is extended to the simultaneous inversion of both main field and secular variation. In the present method, the time dependency is represented by an expansion in Legendre polynomials, resulting in a simple diagonal form for the a priori covariance matrix. The efficient preconditioned Broyden-Fletcher-Goldfarb-Shanno algorithm is used to solve the large system of equations resulting from expansion of the field spatially to spherical harmonic degree 14 and temporally to degree 8. Application of the method to observatory data spanning the 1900-1980 period results in a data fit of better than 30 nT, while providing temporally and spatially smoothly varying models of the magnetic field at the core-mantle boundary.

Bloxham, Jeremy↗
NASA NTRS1988

Geomagnetic activity during the passage of the earth through Halley's tail in 1910

The magnetic disturbance expected when the earth passed through the center of Comet Halley's tail in 1910 apparently occurred 12 hr too early. A detailed study of the records reveals that the discrepancy is due to a change in the convention for determining the start of the day. The magnetic disturbance did in fact arrive at the expected time and no unusual aberration of the solar wind need be invoked to explain the timing. The disturbance consisted of two troughs in the horizontal component of the earth's magnetic field, separated by about 14 hr, presumably associated with wakes in the solar wind momentum flux on either side of the ion tail. The disturbance was independent of latitude, indicating that the responsible current system flowed far above the earth's surface. After the comet's passage the magnetosphere was left in a mildly disturbed condition, with a weak ring current present.

Russell, C. T.↗
NASA NTRS1987

Structure of the geomagnetic tail

Observational data on the structure of the earth magnetotail are summarized and discussed, with reference to theoretical models. Particular attention is given to the interplanetary control of the tail, the magnetic-field structure, the plasma-sheet boundary layer, the magnetotail plasma parameters, the chemical composition of the tail, and the evidence indicating that a steady-state magnetotail cannot exist. Extensive diagrams and graphs are provided.

Fairfield, D. H.↗
NASA NTRS1987

Model for the geomagnetic ion spectrometer in the magnetotail lobes

A two-dimensional kinetic tail-lobe ion spectrometer model for the transport of ionospheric ions from the polar-cleft ionosphere into the tail lobes is developed to simulate semiquantitatively the behavior of the observed O(+) ion streams in the magnetotail lobes. The consequences of the present model include: (1) the increase in velocity of the O(+) streams away from the tail midplane and (2) the existence of tongues of O(+) density from the cleft ionosphere into the tail lobes whose distribution configuration depends on the convection electric field and the source thermal energies. These consequences are reasonably consistent with the trends in lobe streams observed by recent spacecraft.

Horwitz, J. L.↗
NASA NTRS1988

Observations of a gradual transition between Ps 6 activity with auroral torches and surgelike pulsations during strong geomagnetic disturbances

This paper describes a long-lasting large-amplitude pulsation event, which occurred on January 10, 1983 in the ionosphere and magnetosphere and was characterized by Steen and Rees (1983). Over the 4-h period (0200-0600 UT), the characteristics of the pulsations in the ionosphere changed from being Ps 6 auroral torches toward substorms and back to Ps 6. At GEO, the corresponding characteristics were a modulation of the high-energy particle intensity and plasma dropouts. Based on the ideas presented by Rostoker and Samson (1984), an interpretation of the event is offered, according to which the pulsations are caused by the Kelvin-Helmholtz instability during an interval of strong magnetospheric convection. On the basis of this explanation, a new interpretation of the substorm time sequence is proposed.

Steen, A.↗
NASA NTRS1988

A predicted energetic ion signature of a neutral line in the geomagnetic tail

Under certain assumptions one can predict the formation of a magnetic neutral line on theoretical grounds, generally as a result of an unstable current sheet. Neutral points also seen to form spontaneously in a wide variety of plasma simulatioons, from MHD to fully kinetic. Hence, such magnetic null regions are central to some models of magnetotail energization and magnetic substorms. Yet the observational evidence for such regions remains controversial. This paper presents a new signature of a neutral line: the field-aligned ion beam characteristic of current sheet acceleration becomes dispersed into a 'ridge' in the velocity space distribution function as a neutral line is approached. The results were obtained using single-particle simulations in a fixed magnetic field model. The origin of the ridge is shown to be due to initially earthward going ions which pass through the neutral line region. These ions come from a smaller part of the initially antiearthward flowing distribution, generating the depletion of the distribution function below the ridge.

Martin, R. F., Jr.↗
NASA NTRS1988

Ring current development during the great geomagnetic storm of February 1986

The variations of the ring current energy density and composition during the great magnetic storm of February 1986 were investigated using particle measurements obtained by the charge-energy-mass instrument on the AMPTE Charge Composition Explorer spacecraft. The ring current composition of this storm, which had a complicated main phase and a minimum Dst of -312 nT on February 9, was followed for five days from the prestorm quiet time to the early recovery phase. Results suggested that the very rapid initial Dst recovery (tau of about 9.3 hrs) in this storm resulted largely from the rapid loss of 75- to 100-keV O(+) via charge exchange in the inner portion of the ring current. It is proposed that a major O(+) + N(+) ring current component generally exists near the maximum phase of great storms.

Hamilton, D. C.↗
NASA NTRS1988

Modelling of time-dependent ion outflows at high geomagnetic latitudes

The output from the NCAR Thermospheric General Circulation Model was used to simulate the time and altitude-dependent F-region ion frictional heating rate. These ion-heating profiles, computed as a functionm of UT along the locus of specific flux tubes, were used as boundary conditions in the Gombosi et al. (1985) polar wind model to simulate the plasma heating history of a flux tube traversing the cusp region. Using derived time-varying frictional heating rates such as those experienced by these flux tubes, it is shown that transverse ion heating below 500 km can provide sufficient energy to perturb the velocity distribution of the major ion species.

Cannata, R. W.↗
NASA NTRS1989

Intensive MHD-structures penetration in the middle atmosphere initiated in the ionospheric cusp under quiet geomagnetic conditions

In connection with the recently detected quasiperiodical magnetic disturbances in the ionospheric cusp, the penetration of compressional surface magnetohydrodynamic (MHD) waves through the middle atmosphere is modelled numerically. For the COSPAR International Reference Atmosphere (CIRA) 72 model the respective energy density flux of the disturbances in the middle atmosphere is determined. On the basis of the developed model certain conclusions are reached about the height distribution of the structures (energy losses, currents, etc.) initiated by intensive magnetic cusp disturbances.

Mateev, L. N.↗
NASA NTRS1989

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

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.

Bloxham, Jeremy↗
NASA NTRS1989

CDAW 8 observations of plasmoid signatures in the geomagnetic tail - An assessment

Magnetotail observations from the ISEE 3 distant (1983) tail mission taken during the Coordinated Data Analysis Workshop 8 (CDAW 8) A and G events are investigated. The ISEE 3 magnetic field, plasma, and energetic particle measurements taken in these two plasmoids have been analyzed and compared with various equilibrium structures and propagating waves/tail oscillation modes. Results indicate general agreement with either the closed-loop (Hones, 1977) or very small pitch angle flux rope (Hughes and Sibeck, 1987; Birn et al., 1989) models of plasmoid structure and poorer agreement with other hypotheses. Calculations based upon typical plasmoid and tail parameters are presented, indicating that the J and B force associated with the disconnected lobe field lines may be sufficient to accelerate plasmoids up to the speeds observed by ISEE 3. Overall, the energy expended in accelerating the plasmoids down the tail appears comparable to that dissipated in the inner magnetosphere and ionosphere. The study produces strong evidence in favor of the plasmoid model of substorm tail dynamics.

Slavin, J. A.↗