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Fluxgate magnetometer data obtained by OGO-5 near perigee were used to establish the existence of large field depressions in the magnetosphere under conditions of varying degree of disturbance at distances ranging from 2.3 to 3.6 R sub E at all local times. The results also provide the average delta B at these distances when Dst, as being derived at present, is zero.
Whistler propagation in the magnetosphere was studied in detail to find accurate and economical means of determining the path latitude and the electron concentration along the path from whistler parameters of nose frequency and travel time at the nose. Longitudinal propagation in field aligned whistler ducts of cold plasma was assumed, and the earth's magnetic field was approximated by a centered dipole. The effects of whistler propagation in the earth-ionosphere waveguide and through the conjugate ionospheres were treated as small perturbations. Several alternative methods are described so that the most economical method may be chosen depending on the desired accuracy and the availability of a computer or a calculator.
A band of natural radio noise between the local electron plasma frequency and the upper hybrid resonance frequency is observed by the IMP-6 satellite. The band exists over a large range of geocentric radial distances extending from inside the plasmapause boundary to greater than 10 earth radii in the outer magnetosphere. The center frequency of the noise band decreases with increasing radial distance, and changes abruptly at the plasmapause boundary. The broadband electric field strength of this noise is very small, seldom exceeding 10 microvolts/meter, and probably could not be detected without using long electric antennas of IMP-6. It is believed that this noise is produced by incoherent Cerenkov emission from super-thermal electrons. In some cases a second very narrow noise band was observed at a frequency slightly above the second harmonic of the electron gyrofrequency.
A comprehensive study of the temporal behavior of trapped protons, alpha particles and ions (Z 2) in outer zone of the earth's magnetosphere has been made. These observations were made by the Injun V satellite during the first 21 months of operation, August 1968 to May 1970. Rapid increases in the observed number of particles followed by slower exponential decay characterize the data. Comparisons are made with the temporal behavior of interplanetary particles of the same energy observed by Explorer 35. Increases in the trapped fluxes generally correspond to enhanced interplanetary activity. The energy spectra of protons and alpha particles at L = 3 have similar shapes when compared on an energy per charge basis while the respective polar cap spectra have similar shape on an energy per nucleon basis. Apparent inward trans-L motion of energetic protons is observed. These particles are diffused inward by a process involving fluctuating electric fields. The loss of trapped low altitude protons, alpha particles and ions (Z 2) is controlled by coulombic energy loss in the atmosphere.
It is suggested that the definition of the growth and expansion phases of substorms should rest on magnetospheric convection theories and data because of the ambiguities in interpretation of ground magnetometer records from which these concepts arose originally. Data such as that discussed in the report offers strong evidence for the validity and usefulness of the concepts of the growth and expansion phases.
A model of pulsar magnetospheres is described which has evolved inductively from the work of Sturrock, where the radiation is produced near the surface of a neutron star. Some of the theoretical ideas of others, particularly those of Sturrock, are discussed. The braking index n and period-pulse-width distribution of pulsars are first reinvestigated by relaxing the conventional assumption that R sub Y = R sub L, where R sub Y is the radius of the neutral points marking the transition from closed to open magnetic field lines, and R sub L is the radius of the light cylinder. This is replaced by the parameterization R sub Y = R sub * (1- eta )power R sub L (eta), where R sub * is the neutron star radius. If the ratio frequency radiation is created near the surface and beamed along open field lines, it is found that a good fit to the period-pulse-width distribution can be obtained for eta in the range 0.5 = or eta = or 0.7. The relation n = 1 + 2 eta then gives n = 2.2 + or - 0.2, which is in good agreement with the values measured for the Crab pulsar.
The delta B distribution deduced from the Ogo 3 and 5 satellites shows large field depressions in the equatorial region inside the plasmasphere. On the basis of the delta B distribution, it is shown that what is usually considered the quiet-time ring current is an equatorial sheet current that is an extension of the neutral sheet current in the magnetospheric tail. The primary source of the large field reductions inside the plasmasphere is a population of protons with energies of 0.1-1 Mev initially detected by Davis and Williamson (1963) and Davis (1965) on Explorer 12, 14, and 15. Low-energy protons extensively measured on Ogo 3 by Frank (1967, 1971) are primarily responsible for the current near and outside the plasmapause.
Magnetospheric research over the period from 1969 to 1971 is surveyed by describing significant results obtained from studies of the bow shock, magnetosheath, magnetopause, polar cusp, convection-driving electric fields, and the composition of trapped particles with charge greater than one. Particle and field observations are related to general questions of particle origin and destination as well as the mechanism by which the entire system remains in equilibrium.
Significant results of magnetospheric charged particle measurements conducted within the past two years are reviewed in an attempt to provide a general description of relationships among particle populations in the magnetosheath, plasma sheet, extraterrestrial ring current region, electron trough, pseudotrapping region, and stable-trapping region. Special attention is given to the characteristics of protons, electrons, alpha particles, and particles with charge greater than three in the stable trapping region.
Electric dipole antennas on magnetospheric spacecraft measure E field components of many kinds of electromagnetic waves. In addition, lower hybrid resonance emissions are frequently observed well above the ionosphere. The Ogo 5 plasma wave experiment has also detected new forms of electrostatic emissions that appear to interact very strongly with the local plasma particles. Greatly enhanced wave amplitudes have been found during the expansion phases of substorms, and analysis indicates that these emissions produce strong pitch angle diffusion. Intense broadband electrostatic turbulence is also detected at current layers containing steep magnetic field gradients. This current-driven instability is operative at the bow shock and also at field null regions just within the magnetosheath, and at the magnetopause near the dayside polar cusp. The plasma turbulence appears to involve ion acoustic waves, and the wave particle scattering provides an important collisionless dissipation mechanism for field merging.
Satellite-borne electric field measurements using the double-probe technique have now provided a comprehensive survey of convection electric fields at low altitudes in the magnetosphere. The most prominent features of the convection electric fields are reversals located at high magnetic latitudes, with generally anti-sunward convection poleward and sunward convection equatorward of the electric field reversal location. The electric field reversal is usually coincident with the trapping boundary for electrons with energies above 45 keV and is interpreted as the boundary between open and closed magnetic field lines.
Some properties of a model magnetosphere for Saturn are studied in order to determine the bounds that can be set on the surface field strength and the trapped particle population. The primary observational constraint is that nonthermal radiation similar to the Jovian radio emissions must be undetectable from earth. It is argued that for a Saturn surface field of approximately one gauss, those particles that are energized as they diffuse in from the magnetopause with conservation of magnetic moment will produce synchrotron radiation levels that are undetectable at a range of 9.5 AU. The plasma instabilities that heat the oncoming wind particles at the bow shock and others that can limit the stably-trapped flux levels are also discussed briefly.
The shape of the magnetosphere has been calculated self-consistently for inclinations of the earth's magnetic dipole from perpendicular to the solar wind. Inclination angles of 0-35 deg have been chosen in steps of 5 deg and various smooth trends in the surface characteristics with increasing inclination angle noted. The surface points and the complete field at the surface points have been calculated for the entire surfaces. The neutral point region has been given precise study in one degree steps and is found to be tangent to the solar wind velocity and to have a smooth continuous curvature.
Two types of magnetic field changes are repeatedly observed in the high-latitude nightside magnetosphere in association with magnetic substorms. One type of field change occurs on field lines associated with the high-latitude part of the auroral oval and is characterized by a sudden decrease in the field strength accompanied by an abrupt perturbation in the field declination angle. These changes are attributed to field-aligned sheet currents flowing on the high-latitude boundary of an expanding plasma sheet following substorms. A second type of field change observed on polar cap field lines is a decrease in field inclination during substorms. This type of change is regarded as a further manifestation of the changing field configuration during substorms and can be described in terms of azimuthal currents.
Scaling laws for possible outer planet magnetospheres are derived by extrapolation from terrestrial physics. These laws suggest that convection and its associated auroral effects are likely to be relatively less important than in the earth's case, and that there is a possibility that the outer planets have significant radiation belts of energetic trapped particles.
The initial expansion of a barium ion cloud release in the distant earth's magnetosphere is numerically investigated by means of a computer model with self-consistent electric and magnetic interactions. Initial conditions for the calculations are an expanding neutral cloud that is photoionized with a time constant of 20 sec. The results show that after the first 30 sec, the large fluctuations in the magnetic field and in other parameters are damped out and the magnetic field in the central region of the cloud is reduced to near zero. Also, after only 10 sec, a relatively high central ion density has developed and remains for the duration of the calculations.
The barium cloud injection deep into the magnetosphere of Sept. 21, 1971, has revealed that the quiet-time near-equatorial nightside magnetic field is significantly different from that predicted by existing quite-time models. The influence of currents flowing on or near the geomagnetic equator gives the field a more tail-like structure at a distance as close as 5 earth radii. Several models have been tried out to obtain a best fit to the experimental observations of the shape of the field-aligned ion cloud. Although this method does not lead to a unique solution, the most acceptable option emerging from this analysis is a quiet-time tilted image-dipole model with a Williams-Mead tail field and a disclike ring current on the geomagnetic equator.
A sudden commencement occurred at 2348 UT on Feb. 15, 1967, when the ATS-1 satellite was about 2 hr past local noon at a geocentric distance of 6.6 earth radii. Plasma was observed by the Suprathermal Ion Detector first to flow in the antisolar direction, as expected, but then to flow westward, for about 2 min, at about 50 km/sec. Analysis of ground magnetograms suggests that the surprising westward flow, which must have involved an electric field of about 10 mV/m at 6.6 earth radii, resulted from the ionosphere's reaction to the sudden commencement. Beginning about 2 min before the start of the westward flow at ATS-1, ground magnetometers near the foot of the ATS-1 field line typically recorded magnetic-field deflections of about 70 gamma, to the northeast. Taking the ground observations, assuming a height-integrated Hall conductivity of 1 mho, and a standoff distance of 7.2 earth radii inferred from Explorer 33 solar-wind data, a magnetospheric electric field is derived which agrees in magnitude and direction with that required to produce the observed flow at ATS-1.