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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Behavior of outer radiation zone and a new model of magnetospheric substorm.

This paper presents particle data obtained from synchronous altitudes and attempts to evaluate the origin and nature of particle flux variations observed during substorms. The correlated particle intensities, time variations, and energy spectrums are compared between the equatorial and auroral zones. The correlated particle and field observations during substorms are tied together and a model of magnetospheric substorms is derived. Among the features predicted by the model is the poleward expansion of visual auroras observed at the onset of magnetospheric substorms. The model also explains how substorms are triggered in a few minutes time scale during sudden commencements.

Parks, G. K.↗
NASA NTRS1972

Can the ionosphere regulate magnetospheric convection?

Following a southward shift of the interplanetary magnetic field, which implies enhanced reconnection at the nose of the magnetosphere, the magnetopause shrinks from its Chapman-Ferraro equilibrium position. If the convective return of magnetic flux to the magnetopause equalled the reconnection rate, the magnetopause would not shrink. Consequently, there is a delay in the development of magnetospheric convection following the onset of reconnection, which is ascribed to line tying by the polar cusp ionosphere. A simple model relates the dayside magnetopause displacement to the currents feeding the polar cap ionosphere, from which the ionospheric electric field, and consequently, the flux return rate, may be estimated as a function of magnetopause displacement. Flux conservation arguments then permit an estimate of the time scale on which convection increases, which is not inconsistent with that of the substorm growth phase.

Coroniti, F. V.↗
NASA NTRS1972

A study of the electric field in an open magnetospheric model

The qualitative properties of an open magnetosphere and its electric field are examined and compared to a simple model of a dipole in a constant field and to actual observations. Many of these properties are found to depend on the separatrix, a curve connecting neutral points and separating different field-line regimes. In the simple model, the electric field in the central polar cap tends to point from dawn to dusk for a wide choice of external fields. Near the boundary of the polar cap electric equipotentials curve and become crescent-shaped, which may explain the correlation of polar magnetic variations with the azimuthal component of the interplanetary magnetic field, reported by Svalgaard. Modifications expected to occur in the actual magnetosphere are also investigated: in particular, it appears that bending of equipotentials may be reduced by cross-field flow during the merging of field lines and that open field lines connected to the polar caps emerge from a long and narrow slot extending along the tail.

Stern, D. P.↗
NASA NTRS1973

A magnetospheric field model incorporating the OGO 3 and 5 magnetic field observations

A magnetospheric field model is presented in which the usually assumed toroidal ring current is replaced by a circular disk current of finite thickness that extends from the tail to geocentric distances less than 3 earth radii. The drastic departure of this model from the concept of the conventional ring current lies in the fact that the current is continuous from the tail to the inner magnetosphere. This conceptual change was required to account for the recent results of analysis of the OGO 3 and 5 magnetic field observations. In the present model the cross-tail current flows along circular arcs concentric with the earth and completes circuit via surface currents on the magnetopause. Apart from these return currents in the tail magnetopause, Mead's model is used for the field from the magnetopause current. The difference scalar field, delta B, defined as the difference between the scalar field calculated from the present model and the magnitude of the dipole field is found to be in gross agreement with the observed delta B. An updated version of the delta B contours from the OGO 3 and 5 observations, which is used for the comparison, is presented.

Sugiura, M.↗
NASA NTRS1972

Earth's magnetospheric processes; Proceedings of the Symposium, Cortina, Italy, August 30-September 10, 1971.

New data obtained by satellites, rockets, aircraft, and ground-based observations are interpreted in papers dealing with the magnetospheric structure and processes, particle distributions, magnetic and electric fields, plasma convection, particle acceleration and diffusion mechanisms, and substorm phenomena. Attention is given to particle populations in different magnetospheric regions, auroral particle precipitation patterns, effects of electric fields on plasma convection, VLF phenomena, a high-energy proton model for the inner radiation belt, substorm behavior of plasma sheet particles, and interpretations of magnetic field variations during substorms. Individual items are announced in this issue.

Mccormac, B. M.↗
NASA NTRS1972

Injun 5 observations of magnetospheric electric fields and plasma convection.

Recent measurements of magnetospheric electric fields with the satellite Injun 5 have provided a comprehensive global survey of plasma convection at low altitudes in the magnetosphere. A persistent feature of these electric-field observations is the occurrence of an abrupt reversal in the convection electric field at auroral-zone latitudes. The plasma convection velocities associated with these reversals are generally directed E-W, away from the sun on the poleward side of the reversal, and toward the sun on the equatorward side of the reversal. Convection velocities over the polar cap region are normally less than those observed near the reversal region. The electric-field reversal is observed to be coincident with the 'trapping boundary' for electrons with energies E greater than 45 keV. Near local noon the region of anti-sunward convection poleward of the electric-field reversal/trapping boundary corresponds to the low-altitude extension of the polar-cusp plasma.

Gurnett, D. A.↗
NASA NTRS1972

Thermal ions in the magnetosphere.

The distribution and dynamics of thermal (approximately 1 eV) plasma are of fundamental importance for understanding many magnetospheric processes. Above the ionosphere the bulk of the thermal plasma is found in the plasmasphere, which displays varying characteristics in the different LT regions. These different characteristics are reviewed with specific interest placed on the H(+) ion density profiles, since the H(+) ions are the main component of the plasmasphere. Plasmasphere dynamics and morphology can be explained in terms of a time-varying convection model of the magnetosphere which incorporates the bulge region as part of the main flow pattern of the plasmasphere.

Chappell, C. R.↗
NASA NTRS1972

Electric fields in the magnetosphere.

Two techniques, tracking the motions of Ba(+) clouds and measuring the differences in floating potential between symmetric double probes, have been highly successful in: (1) demonstrating the basic convective nature of magnetospheric electric fields, (2) mapping the global patterns of convection at upper ionosphere levels, and (3) revealing the physics of electric currents in the ionosphere and the importance of magnetosphere-ionosphere feedback in altering the imposed convection. The basic pattern of anti-solar convection across the polar cap and night toward day convection in both the evening and morning sectors at auroral belt latitudes persists at all levels of activity. The dawn-dusk potential drop across the polar cap (anti-solar convection) ranges from 20 to 100 kilovolts with the most typical values in the center of this range. The sum of morning and evening (night toward day convection) potential drops in the adjacent auroral belts roughly equals the polar cap drop in the opposite sense as expected.

Heppner, J. P.↗
NASA NTRS1973

A theoretical and experimental study of non-ducted VLF waves after propagation through the magnetosphere.

The nonducted propagation of artificial VLF waves through the magnetosphere has been studied by means of data obtained by the FR-1 satellite experiment, in which fixed frequency (16.8 kHz) waves radiated by a transmitter at the ground (at L equals 2.1) are received by the satellite, at 750 km altitude, both in the zone close to the transmitter and in the conjugate zone. From the wave-normal directions and Doppler shifts measured in the conjugate zone, it has been possible to identify waves that have been propagated from the Northern to the Southern Hemisphere in both ducted and nonducted modes. In this paper, the main features of the nonducted wave have been interpreted by ray tracing in models of the magnetosphere. The results improve our understanding as to why very few whistlers are observed on the ground at low latitudes.

Cerisier, J. C.↗
NASA NTRS1973

High-frequency electrostatic waves in the magnetosphere.

High-frequency electrostatic microinstabilities in magnetospheric plasmas are considered in detail. Rather special plasma parameters are found to be required to match the theoretical wave spectrum with satellite observations in the magnetosphere. In particular, it is necessary to have a cold and a warm species of electrons such that (1) the warm component has an anomalous velocity distribution function that is nonmonotonic in the perpendicular component of velocity and is the source of free energy driving the instabilities, (2) the density ratio of the cold component to the hot component is greater than about 0.01, and (3) the temperature ratio of the two components for cases of high particle density is no less than 0.1. These requirements and the corresponding instability criteria are satisfied only in the trapping region; this is also the region in which the waves are most frequently observed. The range of unstable wavelengths and an estimate of the diffusion coefficient are also obtained. The wave are found to induce strong diffusion in velocity space for low-energy electrons during periods of moderate wave amplitude.

Young, T. S. T.↗
NASA NTRS1973

On the relationship between the growth and expansion phases of substorms and magnetospheric convection.

The definition of the growth and expansion phases of substorms is considered in terms of temporal sequences of magnetospheric convection. It is suggested that the definition of these phases should rest on magnetospheric convection theories and data because of the ambiguities in the interpretation of ground magnetometer records from which these concepts arose originally. Reviewed data are shown to offer strong evidence for the validity and usefulness of the concepts of substorm growth and expansion phases.

Mozer, F. S.↗
NASA NTRS1973

A magnetospheric field model incorporating the OGO 3 and 5 magnetic field observations.

A magnetospheric field model is presented in which the usually assumed toroidal ring current is replaced by a circular disk current of finite thickness that extends from the tail to geocentric distances less than 3 earth radii. The drastic departure of this model from the concept of the conventional ring current lies in that the current is continuous from the tail to the inner magnetosphere. This conceptual change was required to account for the recent results of analysis of the OGO 3 and 5 magnetic field observations. In the present model the cross-tail current flows along circular arcs concentric with the earth and completes circuit via surface currents on the magnetopause. Apart from these return currents in the tail magnetopause, Mead's (1964) model is used for the field from the magnetopause current. The difference scalar field, Delta B, defined as the difference between the scalar field calculated from the present model and the magnitude of the dipole field, is found to be in gross agreement with the observed Delta B.

Sugiura, M.↗
NASA NTRS1973

Quantitative magnetospheric models derived from spacecraft magnetometer data

Quantitative models of the external magnetospheric field were derived by making least-squares fits to magnetic field measurements from four IMP satellites. The data were fit to a power series expansion in the solar magnetic coordinates and the solar wind-dipole tilt angle, and thus the models contain the effects of seasonal north-south asymmetries. The expansions are divergence-free, but unlike the usual scalar potential expansions, the models contain a nonzero curl representing currents distributed within the magnetosphere. Characteristics of four models are presented, representing different degrees of magnetic disturbance as determined by the range of Kp values. The latitude at the earth separating open polar cap field lines from field lines closing on the dayside is about 5 deg lower than that determined by previous theoretically-derived models. At times of high Kp, additional high latitude field lines are drawn back into the tail.

Mead, G. D.↗
NASA NTRS1973

Magnetospheric electric field measurements during sudden commencements

Direction measurements of electric fields were made in the outer magnetosphere during two sudden commencements in 1972. These measurements were observed with the double floating probe experiment carried aboard the IMP 6 satellite. The initial variations of the measured electric field consisted of an increase from a background of about 1 mv/meter to some 10 mv/meter at about 7 rE (earth radi) and to some 4 mv/meter at 3 rE. These initial electric field disturbances were longitudinal, oriented counter clockwise about an axis pointed north. A solution of Maxwell's third equation is derived for these measurements using a quasi-static version of Mead's model of the magnetosphere B (t). This solution seems to describe well the magnitude and direction of the initial perturbation of the electric field vectors observed during these two sudden commencements. After the initial increase, the measured electric field rings several times with periods of the order of minutes. This observed oscillatory behavior correlates with magnetic observatory records taken near the foot of the magetic field line passing through the satellite.

Aggson, T. L.↗
NASA NTRS1974

Identifications of the polar cap boundary and the auroral belt in the high altitude magnetosphere: A model for field aligned currents

Using the OGO-5 fluxgate magnetometer data, the polar cap boundary is identified in the high altitude magnetosphere by a sudden transition from a dipolar field to a more tail like configuration. The basic pattern of the magnetic field variations observed during the satellite's traversal of the auroral belt is presented. This pattern shows the existence of a field aligned current layer on the equator side of the polar cap boundary. Currents flow in the opposite directions in the two field aligned current layers. The current directions in these layers as observed by OGO-5 in the high-altitude magnetosphere are the same as those observed at low altitudes by the polar orbiting TRIAD satellite. The magnetic field in the region where the lower latitude field aligned current layer is situated is essentially meridional. Thus the equatorial current closure of this current system must be via the equatorial current sheet. The two field aligned current systems, one at the polar cap boundary and the other on the low latitude side of the auroral belt, are coupled through the Pedersen current in the ionosphere.

Sugiura, M.↗
NASA NTRS1973

A study of the electric field in an open magnetospheric model

Recently, Svalgaard and Heppner reported two separate features of the polar electromagnetic field that correlate with the dawn-dusk component of the interplanetary magnetic field. This work attempts to explain these findings in terms of properties of the open magnetosphere. The topology and qualitative properties of the open magnetosphere are first studied by means of a simple model, consisting of a dipole in a constant field. Many such properties are found to depend on the separation line, a curve connecting neutral points and separating different field line regimes. In the simple model it turns out that the electric field in the central polar cap tends to point from dawn to dusk for a wide variety of external fields, but, near the boundary of the polar cap, electric equipotentials are deformed into crescents.

Stern, D. P.↗
NASA NTRS1973

Magnetospheric current sheets

A review is presented of theories and observations of current sheets in the magnetosphere, emphasizing the magnetopause and tail current sheets. Theoretical models of the magnetopause current sheet can be divided into two types: specular reflection and fluid models containing no external magnetic field, and models with an interplanetary/magnetosheath magnetic field. Our understanding of the first type is much better than of the second, although magnetospheric observations indicate that the external magnetic field plays an important role. Most of the theoretical models of the tail current sheet attempt to arrive at an understanding of the reconnection process or start with the assumption that reconnection is important. Observations are reviewed and implications for the models are discussed.

Speiser, T. W.↗
NASA NTRS1973

LF radio noise from the earth's magnetosphere

Gyro-synchrotron radio noise emitted by electrons trapped in the earth's magnetosphere has been a subject of extensive research. Previous efforts, which considered frequencies greater than 1 MHz, have shown that this noise should not be detectable in the MF to HF range because its intensity is below the cosmic background noise level. The author has investigated the LF range and has found that appreciable noise is generated at these frequencies. In fact, the theoretical results for this LF noise agree very well with experimental data obtained by a radio astronomy experiment aboard the IMP 6 spacecraft. A comparison showed that the model predicted both variation in the observed noise intensity with Kp and the noise spectral characteristics. Consequently, it is concluded that detectable LF radio noise is emitted, by means of the cyclotron-synchrotron mechanism, by electrons trapped in the earth's magnetosphere, and that this noise is observable only for frequencies below about 300 kHz. For higher frequencies, the theoretical model and the experimental data reconfirm that this noise is below that of cosmic origin.

Frankel, M. S.↗