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At least 1,477 records · Page 82
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NASA NTRS1973

Magnetopause structure and dynamics of the magnetosphere

It is shown how satellite magnetometer data at a magnetopause penetration can be used to determine the vector normal to the magnetopause current layer and the magnetic field component along this normal. Results from 22 Explorer 12 boundary penetrations indicate normal field components of less than 5 gamma in two-thirds of the cases. Measured field variations within the current layer demonstrate the existence of two fundamentally different types of boundary structure, the rotational and the tangential discontinuity. The rotational discontinuity seems to occur predominantly during magnetic storms. Finally, the calculated normal vector is compared with the normal to the surface of the Mead-Beard magnetosphere model.

Sonnerup, B. U. O.↗
NASA NTRS1973

Magnetospheric mapping with quantitative geomagnetic field models

The Mead-Fairfield geomagnetic field models were used to trace field lines between the outer magnetosphere and the earth's surface. The results are presented in terms of ground latitude and local time contours projected to the equatorial plane and into the geomagnetic tail. With these contours various observations can be mapped along field lines between high and low altitudes. Low altitudes observations of the polar cap boundary, the polar cusp, the energetic electron trapping boundary and the sunward convection region are projected to the equatorial plane and compared with the results of the model and with each other. The results provide quantitative support to the earlier suggestions that the trapping boundary is associated with the last closed field line in the sunward hemisphere, the polar cusp is associated with the region of the last closed field line, and the polar cap projects to the geomagnetic tail and has a low latitude boundary corresponding to the last closed field line.

Fairfield, D. H.↗
NASA NTRS1973

Thermal and suprathermal plasma densities in the outer magnetosphere

Using the low-frequency cutoff of electromagnetic noise trapped in the magnetosphere at frequencies above the local plasma frequency, it is now possible to make very accurate, + or - 1%, electron density measurements in the low density region between the magnetopause and plasmapause. This technique for measuring the total plasma density was used, together with measurements of the suprathermal proton intensities with the LEPEDEA instrumentation on the IMP-6 spacecraft, to determine the thermal proton densities in the region between the plasmapause and magnetopause. Although the thermal protons usually account for a significant fraction, approximately 50%, of the total proton density in this region, in some cases, particularly at the larger radial distances the density of the thermal protons sometimes drops to a very small fraction, 5%, of the total density and nearly all of the plasma consists of suprathermal particles.

Gurnett, D. A.↗
NASA NTRS1974

Explorer 45 (S 3-A) observations of the magnetosphere and magnetopause during the 4-5 August 1972, magnetic storm period

The Explorer 45 satellite performed extensive field and particle measurements in the heart of the magnetosphere during the double magnetic storm period of August 4-5, 1972. Both ground level magnetic records and the magnetic field deformations measured along the orbit by the satellite indicated the existence of only a moderate ring current. This was confirmed by the measurements of the total proton energy density less than those observed during the December 1971 and June 1972 magnetic storms. The plasmapause in the noon quadrant was eroded continuously from the onset of the first storm at the beginning of August 4 to an altitude below L = 2.07 at about 18 hours on August 5. During the orbit containing the second sudden commencement a large amount of low frequency electric and magnetic field noise was encountered throughout the entire orbit. A noteworthy observation during this orbit was the contraction of the magnetopause to distances inside the satellite at L = 5.2.

Hoffman, R. A.↗
NASA NTRS1974

Role of sudden commencements in triggering magnetospheric substorms

Sudden commencement events are examined in terms of available auroral-zone and low-latitude magnetic field, data, interplanetary plasma and magnetic field data, and magnetospheric electron flux and magnetic field data from the geostationary satellite ATS 1.

Newell, R. E.↗
NASA NTRS1974

Energetic particles in the Jovian magnetosphere

A detailed account of the Pioneer 10 encounter with Jupiter is presented. Flux time histories of electrons and protons are given over a wide energy band. Proton and electron energy spectra are given at various Jovicentric distances. Proton spectra are shown to transform from a power law with indices in the 3-4.2 range to more nearly exponential forms in the inner regions. Extensive data are presented on the angular distributions of protons and electrons at various locations in the Jovicentric magnetosphere. In addition, a harmonic analysis of 1-2 MeV proton angular distributions was performed. Alpha/proton ratios are given as a function of Jovian radius and are compared to the earth and solar wind values.

Trainor, J. H.↗
NASA NTRS1974

Barium cloud evolution and striation formation in the magnetospheric release on September 21, 1971

The joint NASA-Max Planck Institute Barium Ion Cloud (BIC) Experiment on September 21, 1971 involved the release of 1.7 kg of neutral barium at an altitude of 31,500 km at a latitude of 6.93 deg N. and a longitude of 74.40 deg W. A theoretical model describing the barium neutral cloud expansion and the ion cloud formation is developed. The mechanism of formation of the striational features observed in the release is also discussed. Two candidate instabilities, which may contribute to striation formation, are examined. The drift instability stemming from the outwardly directed drag force exerted on the ions by the outstreaming neutrals is rejected on the grounds that the ion density is too low during the collision-dominated phase of the cloud expansion to support this kind of instability. The joint action of Rayleigh-Taylor and flute instabilities plausibly accounts for the observed striational structure. This same mechanism may well be operative at times of sudden injection of plasma into the inner magnetosphere during geomagnetic storms and may thus contribute to the formation of field-alined inhomogeneities which serve as whistler ducts.

Adamson, D.↗
NASA NTRS1973

Equatorward shift of the cleft during magnetospheric substorms as observed by Isis 1

Isis 1 satellite observations of the cleft position during magnetospheric substorms show that the cleft shifts equatorward as the interplanetary B sub z component turns southward and substorm activity increases and that it shifts back toward higher latitudes as substorm activity subsides and B sub z returns northward. Also, unusually low latitudes for the cleft (less than 70 deg invariant latitude) were found during geomagnetic storms with significant Dst values and large negative B sub z values. Significant shifts occur in the cleft location with no accompanying effect seen in the AE index; however, B sub z is observed to be southward during these periods.

Yasuhara, F.↗
NASA NTRS1973

Electromagnetic radiation trapped in the magnetosphere above the plasma frequency

An electromagnetic noise band is frequently observed in the outer magnetosphere by the Imp 6 spacecraft at frequencies from about 5 to 20 kHz. This noise band generally extends throughout the region from near the plasmapause boundary to near the magnetopause boundary. The noise typically has a broadband field strength of about 5 microvolts/meter. The noise band often has a sharp lower cutoff frequency at about 5 to 10 kHz, and this cutoff has been identified as the local electron plasma frequency. Since the plasma frequency in the plasmasphere and solar wind is usually above 20 kHz, it is concluded that this noise must be trapped in the low-density region between the plasmapause and magnetopause boundaries. The noise bands often contain a harmonic frequency structure which suggests that the radiation is associated with harmonics of the electron cyclotron frequency.

Gurnett, D. A.↗
NASA NTRS1973

Cyclotron resonance wave amplification in the magnetosphere and energetic particle stability

Detailed electron distribution measurements in the outer magnetosphere now available indicate that the energetic electron anisotropy may be very energy dependent, there being positive anisotropies near the magnetic equator for low-energy particles and negative anisotropies in this region for high-energy particles. Previous wave growth calculations assuming the energetic particle anisotropy to be independent of energy are inaccurate at high energies. The net one-hop whistler mode wave amplification is calculated as a function of frequency for a measured electron distribution at L = 7 for a number of assumed equatorial plasma densities. It is found that an increase in cold plasma density results in a change from a net wave damping to significant wave amplification.

Lucas, C.↗
NASA NTRS1974

Energetic electrons in the magnetosphere of Jupiter

Observations of energetic electrons (above 7 MeV) show that the outer magnetosphere of Jupiter consists of a thin disklike, quasi-trapping region extending from about 20 to 100 planetary radii. This magnetodisk is confined to the vicinity of the magnetic equatorial plane and appears to be an approximate figure of revolution about the magnetic axis of the planet. Hard trapping is observed within a radial distance of about 20 Jovian radii. The omnidirectional intensity of electrons with energy above 21 MeV between 3 and 20 Jovian radii is described by a provisional expression in terms of radial distance and magnetic latitude. There is tentative evidence for mild effects of the Galilean satellite Europa and possibly Io and Ganymede but not Callisto.

Van Allen, J. A.↗
NASA NTRS1973

Magnetosphere structure and radiation mechanisms of pulsars

A presentation is made of the two main models of the radiation mechanism of pulsars: the light cylinder model and the polar cap model. They have to account for gamma radiation, radio and X-ray emission, and in the case of the Crab pulsar, optical radiation. It is hypothesized that pair creation is essential for radio emission. None of the models yields a theoretical pulse-width-period distribution or braking index that correlates well with observations. A revision of the polar cap model suggests that a glitch may be present in the magnetosphere, necessitating the presence of a large centrifugal force strong enough to tear open the magnetic field lines into a Y-type configuration. This model yields a braking index in line with observations.

Roberts, D. H.↗
NASA NTRS1974

The origin and propagation of chorus in the outer magnetosphere

Wave normals of chorus in the outer magnetosphere have been determined for the first time from data obtained with the Ogo 5 search coil magnetometer. These measurements combined with simultaneous information concerning geomagnetic field, plasma density, and the electron energy and pitch angle distributions provide a consistent picture of the generation, propagation, and subsequent damping of chorus in agreement with theory. Specifically, the data are consistent with chorus generation within 25 deg of the equatorial plane on the dayside and within 2 deg on the nightside. Chorus is generated by a Doppler-shifted cyclotron resonance with electrons between 5 and 150 keV but only when the pitch angle distribution is peaked at 90 deg to the local magnetic field and the anisotropy exceeds a critical value.

Burton, R. K.↗
NASA NTRS1974

Motion of magnetospheric particle clouds in a time-dependent electric field model

A computer code has been developed to study quantitatively the drift motion of magnetospheric particles in a time-dependent electric field. These calculations were applied to the case of proton and electron injections from the plasma sheet during substorms; the model predictions were checked against observations on board the geosynchronous satellite ATS 5 by DeForest and McIlwain (1971). It was found that it is possible to simulate the observed proton spectrograms with an adequate choice of a time-dependent electric field model. The resulting kinematics is physically quite simple and in its gross features does not depend too strongly on the particular fine structure of the model.

Roederer, J. G.↗
NASA NTRS1974

Stably trapped proton fluxes in the Jovian magnetosphere

A model of the energetic proton fluxes in the Jovian magnetosphere is constructed based on the inward radial diffusion of protons from the solar wind and the plasma turbulent precipitation loss of protons from the radiation belts. Outside 12 Jovian radii the proton fluxes follow a loss-free radial diffusion profile. Inside 12 Jovian radii the proton fluxes should be near the stably trapped limit flux set by convective marginal stability to the electromagnetic ion cyclotron wave and the quasi-electrostatic ion loss-cone wave.

Coroniti, F. V.↗
NASA NTRS1974

Generation of Alfven waves in the magnetosphere by parametric interaction between whistlers

A theory is developed for the generation of Alfven waves by the mixing of whistler waves. Calculations are carried out for interaction in the magnetosphere where the whistlers fall in the VLF frequency range and the Alfven wave falls in the ULF(1-5 Hz) range. Typical amplitudes of 2-5 milligammas for the Alfven wave are calculated, and it is shown that these values might be increased by one order of magnitude through suitable variation of the experimental parameters. The study thereby develops a possible explanation for naturally occurring Pc 1 micropulsations and demonstrates the feasibility of artificial generation of such micropulsations by ground-launched whistlers.

Harker, K. J.↗
NASA NTRS1974

Aligned magnetohydrodynamic solution for solar wind flow past the earth's magnetosphere

Exact numerical solutions of the MHD equations for a perfect dissipationless gas with aligned magnetic field are given for conditions representative of steady supersonic solar wind flow past an axisymmetric model of the earth's magnetosphere. The solution is based on use of a transformation that relates without approximation the equations of magnetohydrodynamics to those of gasdynamics of a pseudogas that has an unusual equation of state. The results confirm the applicability of the previously existing gasdynamic solutions for the typically modest intensities of the interplanetary magnetic field that lead to Alfven Mach numbers of about 10 or greater. For smaller values, however, significant effects are indicated with the flanks of the bow wave moving away from the earth and the nose moving toward the earth. The results are consistent with direct observations in space.

Spreiter, J. R.↗
NASA NTRS1974

Thermal and suprathermal plasma densities in the outer magnetosphere

By using the low-frequency cutoff of electromagnetic noise trapped in the magnetosphere at frequencies above the local plasma frequency, it is now possible to make very accurate electron density measurements in the low density region between the magnetopause and plasmapause. This technique for measuring the total plasma density has been used, together with measurements of the suprathermal proton intensities with the Lepedea instrumentation on the Imp 6 spacecraft, to determine the thermal proton densities in the region between the plasmapause and magnetopause. Usually, the high-energy tail of the thermal proton distribution is sufficient intense to be detected at low energies within the Lepedea energy range. When the high-energy tail of the thermal distribution can be detected, it is possible to determine the temperature of the thermal protons. The temperature in a typical case is found to be 80,000 K. This high temperature for the thermal protons, which are presumably of ionospheric origin, agrees with earlier Ogo 5 measurements reported by Serbu and Maier (1970).

Gurnett, D. A.↗