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NASA NTRS1984

Io's sodium directional features - Evidence for a magnetospheric-wind-driven gas escape mechanism

Elongated features in Io's sodium cloud, directed away from Jupiter and inclined both to the north and to the south of the satellite's orbital plane, have been observed. The north/south directions of the features are correlated with Io's magnetic longitude, suggesting a formation mechanism involving the oscillating plasma torus. It is shown by means of a model analysis that the features can result from a source of high-velocity (about 20 km/s) sodium combined with the oscillating neutral sodium sink provided by the plasma. The phase relationship between the features' directions and Io's magnetic longitude can be understood if escaping sodium is initially directed at near right angles to Io's orbital motion. The directionality of the features requires that the sodium flux from equatorial regions be higher than that from the poles. The initial directions and speeds of sodium atoms escaping Io to form the directional features can be understood in terms of a magnetospheric-wind-driven escape mechanism. The one sequence of directional feature observations that has been analyzed in detail implies a high-speed sodium source rate of about 10 to the 26th atoms/s.

Pilcher, C. B.↗
NASA NTRS1985

Coupled low-energy - ring current plasma diffusion in the Jovian magnetosphere

The outwardly diffusing Iogenic plasma and the simultaneously inwardly diffusing ring current plasma in the Jovian magnetosphere are described using a coupled diffusion model which incorporates the effects of the pressure gradient of the ring current into the cross-L diffusion coefficient. The coupled diffusion coefficient is derived by calculating the total energy available to drive the diffusion process. The condition is imposed that the diffusion coefficient takes on a local minimum value at some point in the region L = 7-8, at which point the gradient of the Io plasma density is specified as ramp value given by Siscoe et al. (1981). The hypothesis that the pressure gradient of the ring current causes the diminution of radial plasma transport is tested, and solution profiles for the Iogenic and ring current plasma densities are obtained which imply that the Io plasma ramp is caused by a high-density, low-energy component of the ring current hitherto unobserved directly.

Summers, D.↗
NASA NTRS1985

Thermalization of neutral-beam-injected ions by lower hybrid waves in Jupiter's magnetosphere

A theoretical analysis of lower-hybrid wave generation by a ring distribution of superthermal ions is presented. A kinetic instability of waves traveling across the magnetic field occurs when fast neutrals escaping from the Io plasma torus are reionized and accelerated to corotation speed in Jupiter's magnetosphere. Instability is possible for modest threshold values of V/a0 = 3 - 7 where V is the pickup gyrospeed and a0 is the background-ion thermal speed. Consequent effects include the relaxation of the pickup-ion distribution and the acceleration of superthermal electrons.

Barbosa, D. D.↗
NASA NTRS1985

On the configuration of the polar cusps in earth's magnetosphere

The interaction between the solar wind and the earth's vacuum dipole field leads to the formation of a discontinuity called the magnetopause. In the standard picture, the magnetopause confines the magnetic field in such a manner that the polar cusp field lines originate from high latitudes in the dayside ionosphere and end at the two magnetic neutral points. Wu (1983, 1984) has questioned this standard picture of the polar cusp. MHD simulations indicate the existence of a current sheet above the polar cusp region, called 'the cusp current sheet' by Wu. Wu (1983) concluded that the difference between his cusp configuration and the standard picture is due to the fact that his geometry results from a plasma model, whereas the standard picture is based on a vacuum concept. In the present investigation, Wu's conclusion is questioned, and it is demonstrated that the standard cusp configuration is not restricted to the vacuum magnetosphere.

Voigt, G.-H.↗
NASA NTRS1985

The substorm as an internal magnetospheric instability Substorms and their characteristic time scales during intervals of steady interplanetary magnetic field

In a study of the dynamics of dayside aurora, Horwitz and Akasofu (1977) adopted the basic methodology of examining substorms which occurred during intervals when the interplanetary magnetic field was steady. By using this approach, it was possible to remove the obvious ambiguity in the interpretation of dayside auroral dynamics which arises if interplanetary medium variations are not excluded. It is believed that for an understanding of the natural, internal instability behavior of the magnetosphere it will be necessary to employ the same methodology in many 'substorm' studies. The present investigation has the objective to present some examples of substorms occurring during intervals of steady interplanetary magnetic field. Subsequently, the approximate time scales of expansion and recovery for such substorms are determined. The obtained results are compared with a substorm model proposed by Hill and Reiff (1980).

Horwitz, J. L.↗
NASA NTRS1985

Magnetospheric plasma modeling (0-100 keV)

Spacecraft surface charging, which is primarily a current balance phenomenon, is in general a function of the dominant currents to and from the vehicle's surface. Within the near-earth magnetosphere the dominant currents to the surface are the ambient space plasma fluxes between approximately 0 and 100 keV. A major effort to understand the near-earth environment was initiated when spacecraft charging became a major issue. The present paper has the objective to summarize the basic features of the models which have resulted from this effort. A description is given of four categories of models, based primarily on the degree of empirical and theoretical input. Types of quantitative models are discussed, taking into account definitions, statistical models, analytic models, static models, and time-dependent models. Engineering models are also considered, giving attention to baseline models and 'worst-case' models.

Garrett, H. B.↗
NASA NTRS1985

Pulsar magnetospheres in binary systems

The criterion for stability of a tangential discontinuity interface in a magnetized, perfectly conducting inviscid plasma is investigated by deriving the dispersion equation including the effects of both gravitational and centrifugal acceleration. The results are applied to neutron star magnetospheres in X-ray binaries. The Kelvin-Helmholtz instability appears to be important in determining whether MHD waves of large amplitude generated by instability may intermix the plasma effectively, resulting in accretion onto the whole star as suggested by Arons and Lea and leading to no X-ray pulsar behavior.

Ershkovich, A. I.↗
NASA NTRS1985

ISEE-1 and 2 observations of field-aligned currents in the distant midnight magnetosphere

Magnetic field measurements obtained in the nightside magnetosphere by the co-orbiting ISEE-1 and 2 spacecraft have been examined for signatures of field-aligned currents (FAC). Such currents are found on the boundary of the plasma sheet both when the plasma sheet is expanding and when it is thinning. Evidence is often found for the existence of waves on the plasma sheet boundary, leading to multiple crossings of the FAC sheet. At times the boundary layer FAC sheet orientation is nearly parallel to the X-Z GSM plane, suggesting 'protrusions' of plasma sheet into the lobes. The boundary layer current polarity is, as expected, into the ionosphere in the midnight to dawn local time sector, and outward near dusk. Current sheet thicknesses and velocities are essentially independent of plasma sheet expansion or thinning, having typical values of 1500 km and 20-40 km/s respectively. Characteristic boundary layer current densities are about 10 nanoamps per square meter.

Elphic, R. C.↗
NASA NTRS1984

The electromagnetic field for an open magnetosphere

The boundary-layer-dominated models of the earth EM field developed by Heikkila (1975, 1978, 1982, and 1983) and Heikkila et al. (1979) to account for deficiencies in the electric-field descriptions of quasi-steady-state magnetic-field-reconnection models (such as that of Cowley, 1980) are characterized, reviewing the arguments and indicating the most important implications. The mechanisms of boundary-layer formation and field direction reversal are explained and illustrated with diagrams, and it is inferred that boundary-layer phenomena rather than magnetic reconnection may be the cause of large-scale magnetospheric circulation, convection, plasma-sheet formation and sunward convection, and auroras, the boundary layer acting basically as a viscous process mediating solar-wind/magnetosphere interactions.

Heikkila, W. J.↗
NASA NTRS1985

Particle and field stress balance within a planetary magnetosphere

A technique is developed for experimentally estimating the local tensor stresses within a planetary magnetic field configuration characterized by local spacecraft measurements. Key to the technique is the determination of the shapes of field lines using the symmetry properties of the system coupled with local and instantaneous measurements of the field line inclination angles. The technique is applied here to the inner and middle Saturnian magnetosphere using data returned by the Magnetic Field Experiment on the Voyager 1 spacecraft. It is concluded that the ring current has substantial radial structure, heretofore not shown. Outside about 13 R(s) the newly derived field stresses match remarkably well the funtional variation of the centrifugal corotation stresses of the cool particle population measured previously by the Plasma Science Experiment. Inside about 13 R(s) the key structure in the derived field stresses, a prominent local maximum, matches the approximate position of an apparent strong pressure gradient in the energetic particles characterized by the Low-Energy Charged Particle detectors.

Mauk, B. H.↗
NASA NTRS1985

Particle acceleration in Saturn's outer magnetosphere - In memoriam Alois Schardt

Fluctuations in field strength and particle flux observed during the outbound pass of Voyager 2 through the Saturnian magnetosphere are discussed. The observations of injections of energetic electrons and ions, associated plasma wave activity, and magnetic field perturbations are described. These data imply the existence of an acceleration or heating site some distance from the Voyager 2. A correlation between impulsive injections of 0.35-2 MeV electrons, increase in the hot ion flux of 28-215 KeV, a dip in magnetic field magnitude, and a signal from the plasma wave instrument in the 562 Hz channel was detected. An explanation of these observations is provided.

Schardt, A. W.↗
NASA NTRS1985

Electrons and Cosmic Ray Produced Protons in Saturn's Inner Magnetosphere

The Cerenkov detector on Pioneer 11 previously observed Crand protons above 600 MeV in Saturn's inner magnetosphere, mixed with a poorly understood background of energetic electrons. The electron count is separated from the proton counts and the first-order angular distributions are established for each species. To do this the theoretical relationships among the harmonic coefficients of the count rate is used as a function of spacecraft roll angle. The majority of the counts were electrons with energy above several MeV; i.e., with drift periods shorter than the satellite orbital resonance. The electrons have isotropic pitch angle distributions, and the protons pancake over most of the region between Nimas and the rings, although there is a small region of dumbbell proton distributions in the vicinity of Janus and Epimetheus.

Northrop, T. G.↗
NASA NTRS1986

Magnetospheric multiprobes (MMP/CHEMSAT)

The magnetospheric multiprobes (MMP) are a set of ejectable, self contained, limited lifetime free flyers which are designed to make plasma diagnostic measurements at multiple locations within telemetry range of the space station's coorbiting platform and polar platform. When configured as CHEMSATs, one or more MMP's will conduct chemical releases as tracers or modifiers of the local plasma and field environment, while diagnostic measurements are made from other MMP's and from the nearby platform. The probes will be battery powered and will have lifetimes of a few days to several weeks. Up to 12 probes would be placed on the coorbiting platform and the polar platform every six months and two years respectively for use in the campaign mode of operation.

Burch, J.↗
NASA NTRS1985

Three-dimensional plasma observations near the outer magnetospheric boundary

The present study is concerned with an evaluation of some of the plasma and field processes by which the magnetospheric boundary layer acts as a magnetohydrodynamic (HD) dynamo. An analysis is conducted of 18 crossings of the dayside magnetopause and boundary layer, taking into account data provided by the quadrispherical electrostatic analyzers (Lepedea) of the International Sun-Earth Explorer (ISEE) spacecraft. These instruments sample ion and electron velocity distributions in three dimensions. Attenion is given to aspects of data presentation, magnetopause identification, details regarding the magnetopause crossings, cross-field plasma flow, comparisons of magnetosheath and boundary layer plasma flow, and six conclusions.

Eastman, T. E.↗
NASA NTRS1985

Jovian ionospheric conductivity and magnetospheric plasma outflow - Voyager 1

Using magnetic field and plasma data from the Voyager 1 inbound encounter, local values are derived for the mass outflow rate and the radial bulk velocity in the dayside Jovian magnetosphere, and for the height-integrated Pedersen conductivity of the Jovian ionosphere. These values, accurate within a factor of 2, are compatible with previous order-of-magnitude estimates of these quantities. The results suggest that the shape of the plasma sheet is more complicated than previously suspected. A search for evidence of enhanced plasma outflow in the active sector was inconclusive.

Hairston, M. R.↗
NASA NTRS1985

Magnetospheric interchange instability

It is shown that the conventional derivation of the MHD interchange instability criterion using an energy approach yields incorrect results. A special case involving straight, parallel flux tubes is considered in which the conventional energy argument should apply but disagrees with the result of an elementary dynamical argument. It is shown that the disagreement arises because the conventional derivation neglects the self-consistent changes in the magnetic field that result from the flux-tube interchange. When these self-consistent changes are included in the energy approach, a new interchange instability criterion is derived that agrees with the dynamical criterion. The new instability criterion is qualitatively different from the conventional one and suggests that the pressure gradient of energetic ions at Jupiter may not stabilize the Io torus against interchanges. The inner magnetospheres of earth, Jupiter, and Saturn may all be interchange unstable.

Cheng, A. F.↗
NASA NTRS1985

Equatorial magnetospheric particles and auroral precipitations

The shape of the equatorial magnetosphere and its effect on auroral precipitation is considered. The concept of an 'auroral shell' representing the inner limit of active auroral processes, is introduced. It is shown that the shell marks the equatorward edge of the auroral oval at low altitudes, and the edge of the injection boundary at high altitudes. The auroral 'ring' is defined as the intersection of the magnetic equator with the auroral shell. A simple equation is derived for computing the expected location of the auroral ring, given the local time and the level of magnetic disturbance.

Mcilwain, C. E.↗
NASA NTRS1985

The effects of density gradients on the convective amplification of upper hybrid waves in the magnetosphere

Intense (at least 10 mV/m) electrostatic plasma waves have been detected near the upper hybrid frequency between + or -50 deg MLAT during recent GEOS-1 crossings. Wave growth rate and convective amplification calculations were carried out in order to explain the occurrence of intense upper hybrid (IUH) events over such a wide range of latitudes. The effects of wave refractions were taken into account in the convective amplification calculations. Specific results are presented for the upper hybrid wave growth of an IUH event occurring at 10 deg MLAT. It is shown that a density gradient may be necessary to explain the observed amplification at 10 deg MLAT. At the equator, however, the long scale length of the magnetic field gradient enables large amplitudes to be attained without a density gradient. The results of a UH ray tracing analysis are discussed within the framework of current theories concerning magnetospheric continuum radiation.

Engel, J.↗

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