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At least 685 records · Page 38

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NASA NTRS1976

An evaluation of recent quantitative magnetospheric magnetic field models

Magnetospheric field models involving dipole tilt effects are discussed, with particular reference to defined magnetopause models and boundary surface models. The models are compared with observations and with each other whenever possible. It is shown that models containing only contributions from magnetopause and tail current systems are capable of reproducing the observed quiet time field just in a qualitative way. The best quantitative agreement between models and observations take place when currents distributed in the inner magnetosphere are added to the magnetopause and tail current systems. One region in which all the models fall short is the region around the polar cusp. Obtaining physically reasonable gradients should have high priority in the development of future models.

Walker, R. J.↗

NASA NTRS1976

Heavy ions from the Galilean satellites and the centrifugal distortion of the Jovian magnetosphere

The Galilean satellites constitute a potentially significant source of plasma to the Jovian atmosphere. The paper examines the distortion of the Jovian magnetosphere that would result from injection of ions at the Galilean satellites. The magnetic distortion produced is a localized perturbation near the equatorial plane, in contrast to the large-scale distortion produced by Jovian atmospheric ions. The study estimates the relative perturbation currents resulting from the two sources in terms of the unknown ion production rate at the satellites. It is found that the two perturbations would be of comparable magnitudes if the ion source flux at all of the Galilean satellites were comparable to the photo-ion escape flux from Jupiter's atmosphere. More specifically, ions produced by the Galilean satellites should be confined near the equatorial plane of Jupiter's magnetosphere in a sheet whose thickness is determined by a centrifugal scale height given essentially by the ion thermal speed divided by the corotation frequency.

Hill, T. W.↗

NASA NTRS1976

The magnetosphere

The terrestrial magnetic field shields the earth from the supersonic wind of the expanding solar atmosphere, forming a cavity called the magnetosphere. Since the velocity of the solar wind is supersonic, a detached shock wave stands in front of this cavity. The flow past the cavity is viscous, drawing the field lines back into a long tail. In this paper we review briefly the nature of the magnetosphere, the outstanding problem areas in this field, and what space missions are needed to attack these problems.

Russell, C. T.↗

NASA NTRS1976

Characteristics of magnetospheric radio noise spectra

Magnetospheric radio noise spectra (30 kHz to 10 MHz) taken by IMP-6 and RAE-2 exhibit time varying characteristics which are related to spacecraft position and magnetospheric processes. In the midfrequency range (100-1000 kHz) intense noise peaks rise a factor of 100 or more above background; 80% of the peak frequencies are within the band 125 kHz to 600 kHz, and the peak occurs most often (18% of the time) at 280 kHz. Bandwidths of the peaks range from about 100 kHz to more than 500 kHz; most often the lower cutoff is at about 100 kHz and the upper at 380 kHz for a total bandwidth of 280 kHz. This intense mid-frequency noise was detected at radial distances from 1.3 Re to 60 Re on all sides of the earth (i.e., all local times) during magnetically quiet as well as disturbed periods. Maximum occurrence of the mid-frequency noise is in the evening to midnight hours where splash-type energetic particle precipitation takes place.

Herman, J. R.↗

NASA NTRS1977

A versatile detector system to measure the change states, mass compositions and energy spectra of interplanetary and magnetosphere ions

An instrument is described for measuring the mass and charge state composition as well as the energy spectra and angular distributions of 0.5 to 350 kev/charge ions in interplanetary space and in magnetospheres of planets such as Jupiter and earth. Electrostatic deflection combined with a time-of-flight and energy measurement allows three-parameter analysis of output signals from which the mass, charge states, and energy are determined. Post-acceleration by 30 kV extends the energy range of the detector system into the solar wind and magnetosphere plasma regime. Isotopes of H and He are easily resolved as are individual elements up to Ne and the dominant elements up to and including Fe. This instrument has an extremely large dynamic range in intensity and is sensitive to rare elements even in the presence of high intensity radiation, and is adapted for interplanetary, deep-space, and out-of-the-ecliptic missions, as well as for flights on spacecraft orbiting Jupiter and earth.

Gloeckler, G.↗

NASA NTRS1977

Observations of large transient magnetospheric electric fields

Transient electric field events were observed with the long, double probe instrumentation carried by the IMP-6 satellite. Nine, clearly defined, exceptionally large amplitude events are presented here. The events are observed in the midnight sector at geocentric distances 3.5 to .5.5 R sub e at middle latitudes within a magnetic L-shell range of 4.8 to 7.5. They usually have a total duration of one to several minutes, with peak power spectra amplitudes occurring at a frequency of about 0.3 Hz. The events occur under magnetically disturbed conditions, and in most cases they can be associated with negative dH/dt excursions at magnetic observatories located near the foot of the magnetic field line intersecting IMP-6. The magnetospheric motions calculated for these electric fields indicated a quasi-stochastical diffusive process rather than the general inward magnetospheric collapsing motion expected during the expansive phases of auroral substorm activity.

Aggson, T. L.↗

NASA NTRS1977

Charge exchange lifetimes for ions in the magnetosphere

Latest and best measurements of physical quantities involved in complete calculation of the charge exchange lifetime of mirroring magnetospheric ions are coalesced and summarized. It is critical that the charge exchange lifetimes for ions be known as accurately as possible in order to apply the charge exchange mechanism to ion phenomena within the earth's magnetosphere.

Smith, P. H.↗

NASA NTRS1976

Field-aligned currents in the Jovian magnetosphere - Pioneer 10 and 11

Field-aligned currents, which are known to play an important role in the dynamics of the earth's magnetosphere, would also be expected to flow in the Jovian magnetosphere during magnetically active periods. Such currents are difficult to observe largely because of their limited spatial extent and intermittent nature, but on the Pioneer 11 inbound pass to Jupiter on day 336, 1800 GRT, near the L shell of Ganymede, the helium vector magnetometer recorded a structured perturbation with the characteristic signature of a field-aligned current. No analogous structure was found on a similar Pioneer 10 inbound pass (day 337, 1200 GRT). The perturbation could have been caused by turbulence with the wake of Ganymede through which the spacecraft was moving or the perturbation could represent the first observation of a Jovian substorm. The observations are discussed in terms of both models, and difficulties are examined.

Kivelson, M. G.↗

NASA NTRS1976

Interchange stability of a rapidly rotating magnetosphere

A rotation-dominated magnetosphere is unstable to magnetic flux-tube interchange motions if and only if the plasma content of a unit magnetic flux tube is a decreasing function of distance from the spin axis. For a spin-aligned dipole field, an approximate expression is obtained for the marginally stable distribution. Plasma filling the Jovian magnetosphere from internal sources would initially violate this stability criterion so that interchange motions would act to establish the marginally stable distribution.

Hill, T. W.↗

NASA NTRS1976

Examples of plasma flows within the earth's magnetosphere

Examples of observed plasma flows in the dayside magnetosphere near the magnetopause, within the ring current in the local evening sector, and at two positions simultaneously in the plasma sheet are presented. These measurements were gained with plasma instruments on the IMP 6 and 7 satellites. Flow velocities inside the magnetopause in the dayside magnetosphere are typically 25 to 75 km/s and are directed generally parallel to the tangent to the nearby magnetopause with a small component directed into this boundary. Bulk flow speeds within the ring current ranged from the instrument threshold of about 20 km/s to speeds of 50 km/s. Strong tailward 'jetting' of plasma, in the range of 200 to 300 km/s, at geocentric radial distances of about 35 earth radii in the plasma sheet is found to be often associated with the occurrence of magnetic substorms.

Frank, L. A.↗

NASA NTRS1976

Magnetospheric whistler ducts observed by ISIS satellites

The latitudinal width of the magnetospheric whistler duct has been estimated by the first and final invariant latitudes of whistler echoes and the conservation of the magnetic flux for the centered dipole field, using 105 whistler echoes in ISIS VLF data received at Kashima, Japan for 1972-1973. The latitudinal distribution of whistler duct occurrence shows a maximum at invariant latitudes of 40-45 degrees near the maximum occurrence latitude of ground whistlers. The radial width of magnetospheric whistler duct in the geomagnetically equatorial plane increases with invariant latitude of the geomagnetic flux tube in which whistlers propagate.

Ondoh, T.↗

NASA NTRS1976

Natural precedents to active magnetospheric experiments

Some of the natural analogs of proposed seeding experiments in the magnetosphere are: electric charged particle injections during substorms; lightning-induced whistlers providing wave injections; 'detachment' of plasma from the plasmapause, which is equivalent to light-ion seeding in the outer magnetosphere. In the present paper, the characteristics of some natural precedents to active experiments are reviewed, and their relative advantages and drawbacks are examined. One of these precedents - wave generation in detached plasma regions - is then discussed in some detail; the discussion reveals the generation of ELF electromagnetic noise when the energetic electrons exceed the stably trapped limit, and also indicates that a critical dimension for the seeded volume exists transverse to the geomagnetic field.

Russell, C. T.↗

NASA NTRS1977

Heavy ion circulation in the earth's magnetosphere

A mechanism for heavy ion circulation in the magnetosphere is proposed. Singly charged ions heavy ions from the plasmasphere are convected intermittently to the dayside magnetopause, accelerated there, swept into the distant tail lobes and boundary layer, and convected earthward in the plasma sheet to reenter the magnetosphere.

Freeman, J. W.↗

NASA NTRS1976

Electrostatic turbulence in the magnetosphere

Plasma wave measurements from the IMP-6, IMP-8 and Hawkeye-1 satellites show that a broad region of intense low-frequency electric field turbulence occurs on the high latitude auroral field lines at altitudes ranging from a few thousand kilometers in the ionosphere to many earth radii in the distant magnetosphere. A qualitatively similar, but less intense, type of electric field turbulence is also observed at the plasmapause during magnetic storms. In the auroral regions the turbulence occurs in an essentially continuous band on the auroral L-shells at all local times around the earth and is most intense during periods of auroral activity. In this paper we summarize the basic characteristics of this electric field turbulence and consider the possible role this turbulence may play in the heating and acceleration of plasma in the magnetosphere.

Gurnett, D. A.↗

NASA NTRS1976

The magnetosphere of Mercury

Data on Mercury's magnetosphere and on the plasma, planetomagnetic, and energetic particle environment of the planet obtained in three encounters (Mariner 10 flybys) are compared, and tasks for future research are outlined. The Mercury bow shock and magnetopause are much closer to the planet than the earth counterparts are to the earth. The magnetotail with embedded plasma sheet-field reversal region, global deflection of the solar wind by an intrinsic dipolar magnetic field, variations in solar wind momentum flux, and absence of such features as ionosphere, plasmasphere, and radiation belts, are described. Energetic electrons are accelerated in the magnetotail, however, and the interplanetary magnetic field variations distort Mercury's magnetosphere to produce a southward field associated with substorm-like disturbances.

Ness, N. F.↗

NASA NTRS1977

Electric and magnetic fields in the high-latitude magnetosphere

The configuration of high-latitude electric and magnetic fields is reviewed. Various results suggest that high-latitude magnetic field lines from the outermost regions of the dayside magnetosphere converge toward a point near the noon meridian. Plasma flows, the midday cusp, and a dawn-dusk electric field across the polar cap are characterized. The electric fields associated with plasma flows produce Hall currents on the polar cap which vary with sector structure. Some evidence indicates that polar cap convection may reverse during intervals of strong northward interplanetary field. It is concluded that most observations are consistent with an open field magnetosphere model.

Fairfield, D. H.↗

NASA NTRS1977

Impulsive penetration of filamentary plasma elements into the magnetospheres of the earth and Jupiter

Assuming that the solar wind plasma is usually nonuniform over distances of 10,000 km or less, it is shown that filamentary plasma elements stretched out from the sun can penetrate impulsively and become engulfed into the magnetosphere. The diamagnetic effects associated with these plasma inhomogeneities are observed in outer magnetospheres and magnetosheaths as dips or directional discontinuities in the magnetic field measurements. From the mean penetration distances of these diamagnetic plasma elements one can deduce a mean deceleration time, as well as an approximate value of the integrated Pedersen conductivity in the polar cusp of the earth and Jupiter.

Lemaire, J.↗

NASA NTRS1977

The magnetospheric boundary layers - A geometrically explicit model

A geometrically explicit three-dimensional model of the magnetosphere is proposed and compared with existing models. The proposed model is a bimodal combination of the classical open and closed magnetosphere. The plasma sheet is topologically contained in a closed field line volume which is exposed to the magnetosheath along the flanks. Boundary layer plasma at the flanks of the closed field line volume freely flows along field lines to the ionosphere and forms a complete border to the plasma sheet. The plasma sheet derives from the closed boundary layer plasma, which is in the same topological volume. The closed boundary layer between the clefts on the dayside is covered by plasma hood (the open field line equatorward extension of the mantle) when dayside merging occurs.

Crooker, N. U.↗