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At least 1,783 records · Page 99
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NASA NTRS1990

Solar wind dynamic pressure variations: Quantifying the statistical magnetospheric response

Solar wind dynamic pressure variations are common and have large amplitudes. Existing models for the transient magnetospheric and ionospheric response to the solar wind dynamic pressure variation are quantified. The variations drive large amplitude (approx 1 R sub E) magnetopause motion with velocities of approx. 60 km/s and transient dayside ionospheric flows of 2 km/s which are organized into double convection vortices. Ground magnetometer signatures are more pronounced under the auroral ionosphere, where they reach 60 to 300 nT, and under the equatorial electrojet. A statistical comparison of transient ground magnetometer events seen at a South Pole station and geosynchronous orbit indicates that all but the weakest ground events are associated with clear compressional signatures at the dayside geosynchronous orbit.

Sibeck, D. G.↗
NASA NTRS1990

A satellite-ground study of the dynamics of the bulge region of the Earth's magnetosphere

Data from multiple ground stations and satellites were used to see how the plasmasphere in the dusk sector is modified during magnetic storms. There is clearly some type of erosion process, during which the plasmasphere is diminished in size, and it is believed that the excess plasma is peeled off and carried (convected) away toward the outer boundary of the magnetosphere (the magnetopause). However, very little is know about the physics of the erosion process, and about how the plasmasphere recovers during the quiet periods that follow such disturbances. Case studies from three multiday periods in 1982 produced a number of new findings which are summarized in this two page document.

Carpenter, Donald L.↗
NASA NTRS1990

A mechanism for bursty radio emission in planetary magnetospheres

Bursty radio emissions are often observed from the polar magnetospheres of the earth, Jupiter, Saturn, and Uranus in addition to the smooth radio emissions commonly detected. It is shown that in plasma regimes in which the electron plasma frequency is less than the electron cyclotron frequency, anisotropic electron beams or gyrating electron beams can excite directly broadband electromagnetic radiation. The largest growth is for right-hand X-mode radiation with frequencies above the electron cyclotron frequency. This instability can produce bursty, broadband emission, consistent with some of the properties of the radiation observed from the magnetized planets.

Wong, H. K.↗
NASA NTRS1990

Response of energetic particles to magnetospheric ultra-low-frequency waves

ULF waves in the space physics context are the lowest-frequency plasma waves propagating in the earth's magnetosphere. Although the propagation of the waves is explained in MHD theory, excitation of the waves involves not only MHD but also kinetic processes. This article describes how in situ magnetic field and particle measurements can be used to distinguish the basic properties of ULF waves, including the azimuthal wave number of the standing wave structure and the propagation direction. Examples are taken from observations with the AMPTE/CCE spacecraft.

Takahashi, Kazue↗
NASA NTRS1990

Recent advances in magnetospheric substorm research

More than two decades of magnetospheric exploration have led to a reasonably clear morphological picture of geomagnetic substorms, which is often summarized in terms of the near-earth neutral line model of substorms. Although this qualitative theory is quite comprehensive and explains a great many observations, it is hard pressed to explain both recent observations of consistently earthward flow within 19 RE and also the prompt onset of magnetic turbulence at 8 RE at the time of substorm onset. Other theories have recently been proposed which tend to be more quantitative, but which explain a more limited number of substorm observations. The challenge seems to be to understand the esential physics of these various quantitative theories and integrate them into a larger structure such as provided by the near-earth neutral line model.

Fairfield, D. H.↗
NASA NTRS1991

Magnetospheric boundary dynamics - DE 1 and DE 2 observations near the magnetopause and cusp

Results are presented of a detailed analysis of plasma and field measurements taken close in time by the DE 1 and DE 2 satellites during the September 6, 1982 magnetic storm. High-altitude data show that, during the storm, the DE 1 satellite entered the magnetosheath by crossing a magnetopause with a large normal magnetic field component, and data from DE 2 show a well-defined cusp current system occurring on open magnetic field lines. The interactions of the solar wind/IMF with the magnetosphere low-altitude signatures of the magnetopause that most adequately explain the high-altitude observations of DE 1 and the near-earth cusp observations of DE 2 are considered, and the relationship between the measurements made by the two satellites are discussed.

Maynard, N. C.↗
NASA NTRS1990

Artificial electron beams in the magnetosphere and ionosphere

The Plasma Diagnostics Payload of the Echo 7 satellite carried TV cameras and photometers by means of which the luminosity around an electron beam in the polar ionosphere could be studied. It was found that, while the beam Larmor spiral could be clearly seen near 100 km, above this only a column due to suprathermal electrons was observable. At high altitudes, the emission of neutral gas both generated powerful luminosity and substantially reduced accelerator potentials. An analysis of conjugate echoes indicates that inferred magnetospheric electric fields do not map well into the ionosphere, as well as the presence of strong pitch-angle scattering.

Winckler, John R.↗
NASA NTRS1991

ULF waves in the magnetosphere

Research efforts in the area of magnetospheric ULF waves in the 1987-1990 period are reviewed. Attention is given to externally excited hydromagnetic waves including field line resonance, the global cavity mode, bow-shock-associated upstream waves, and Kelvin-Helmholtz waves. Consideration is given to internally excited Pc 4-5 pulsations and the role of these pulsations in the diffusion of ring-current ions based on the observed properties of the pulsations.

Takahashi, Kazue↗
NASA NTRS1991

Direct VLBI detection of the magnetosphere surrounding the young star S1 in Rho Ophiuchi

VLBI 6-mm data are presently used to investigate the circularly polarized radio core previously identified around the young B3 star S1 in Rho Ophiuchi. The measured angular diameter and brightness temperature are found to be consistent with gyrosynchrotrom radiation emission from mildly relativistic electrons. A simple model based on a pole-on dipolar magnetic field of about 2 kG at the stellar surface suggests itself as consistent with the main observed features of the S1 magnetosphere; an important feature of the model is its taking the influence of the X-ray-emitting plasma into account. S1 may represent a new type of young stellar object, characterized by very extended magnetic fields.

Andre, Philippe↗
NASA NTRS1991

PC 3,4 magnetic pulsations observed simultaneously in the magnetosphere and at multiple ground stations

Periods of magnetic conjugacy between ISEE and the magnetometer array of the Institute for Geological Sciences have been examined to search for the simultaneous occurrence of Pc 3,4 magnetic pulsations. When compressional waves are seen in space, waves are also observed on the ground at the same frequency and with similar waveforms. The wave amplitude on the ground at midlatitudes is similar to that in space at ISEE but at high latitudes the amplitudes are larger than in space. The one occurrence of a purely transverse signal at ISEE was not observed on the ground. These results confirm that Pc 3,4 wave energy is most readily transported through the magnetosphere by compressional fluctuations.

Odera, T. J.↗
NASA NTRS1991

Particle simulations of collisionless magnetic reconnection in magnetospheric current sheets

The driven collisionless magnetic reconnection processes at the dayside magnetopause and in the magnetotail are investigated through 2.5 D particle simulations. During reconnection, magnetic field configurations with both a single X line and multiple X lines are obtained. Magnetic reconnection enhancements, which are indicated by the peaks in the inductive electric field, are observed in the simulations. Coincident with the magnetic reconnection enhancements, bursts of energetic particles, electrons with an energy spectrum of about E exp -3.8 and ions with an energy spectrum of about E exp -4.4, are generated during the reconnection processes. Field-aligned particle heat fluxes are also generated. Intense plasma waves associated with the reconnection processes are observed. It is found that the fluctuating magnetic field has a power spectrum of about f exp -3.8 and the fluctuating electric field has a power spectrum of about f exp -1.8, respectively. A review of the simulation studies of driven collisionless magnetic reconnection and slow shocks in the magnetospheric current sheets is presented.

Ding, D. Q.↗
NASA NTRS1990

Low frequency propagation in the earth's magnetosphere

Using a model to simulate wave propagation, estimates were obtained on the effect of the earth's magnetosphere on the imaging potential of the Low-Frequency Space Array mission for observations above the ionosphere at frequencies below about 10 MHz. Results of this simulation show that, for imaging at 1.5 MHz, large orbital radii will be required. It is concluded that successful imaging from within the plasmasphere may depend upon the feasibility of correction schemes.

Dennison, Brian↗
NASA NTRS1990

Investigation of nonlinear E.M. phenomena in the tethered magnetospheric cloud

Nonlinear effects of parametric and of heating type, produced in a plasma under the action of an electric field E(sub 0)(e exp i(omega)t), are considered in this work in connection with the Tethered Magnetospheric Cloud (TMC) accompanying the Tether Satellite System (TSS). The theoretical results show that these phenomena should appear in the ionosphere at high altitudes Z greater than or = (150-200) km, particularly, at Z approximately = 300 km of the TSS system orbit. Therefore, it is of a special interest to search these phenomena by such a unique experiment as the forthcoming first TSS-I and by the future, perhaps modified TSS missions. Because of the parametric decay instability, new branches of wave may be excited both around the electron and ion Lengmuir frequencies Omega (sub 0) = 2(pi)f(sub 0) and Omega (sub 0) = 2(pi)f(sub 0) under the influence of high frequency (HF), (f less than or = 10(exp 6) to few 10(exp 6) Hz), strong electric waves. The heating of all the kinds of particles is growing up very quickly in the ionosphere with altitude in the extra low and very low frequency ranges, F approximately = (1 to 10(exp 4) Hz, discussed below. The temperatures (energies), for example, of the electrons accelerated by the electric field become larger than the ionization potential in this frequency range already at altitudes Z greater than or = (150 - 200) km when the amplitude of the electric field is the absolute value of E(sub 0) approximately = (1-2) mV/m. The sources of these electric fields may be in the TSS-I mission, that so called Phantom Loop (PL) - the Tethered Electrodynamic Tail (TET), and the different kind of electromagnetic oscillations produced by different kinds of instabilities in the TMC plasma. The growth rates of these instabilities will become very high in the TSS surrounding magnetoplasma. However, in the future TSS missions, special artificial sources (generators) of electric fields should be used for these investigations.

Alpert, Yakov L.↗
NASA NTRS1991

A mysterious plasma wave emission and the determination of plasma densities in Neptune's inner magnetosphere

One of the strongest plasma wave signals observed during the Voyager 2 encounter with Neptune is a narrowband emission between 3.0 and 4.3 kHz that was detected over a period of roughly 2 hours around closest approach. The emission occurs below the electron cyclotron frequency and the low-frequency cutoff of the radio continuum radiation. Of the naturally occurring signals in the earth's auroral zone and in Jupiter's magnetosphere this emission most resembles trapped Z mode waves found near the left-hand cutoff frequency. Using this identification, a plasma density profile is obtained that is independent of the plasma temperature. These densities greatly exceed those measured by the plasma science instrument on Voyager but are lower than estimates based on other models of Neptunian plasma wave phenomenology. If this wave mode is not a natural emission, it might arise from an unusual interaction of the spacecraft with the cold, dense ambient plasma.

Moses, S. L.↗
NASA NTRS1991

Voyager 2 electron observations in the magnetosphere of Neptune

This paper describes electron data obtained during the Neptune encounter by the Voyager 2 plasma science experiment. The densities and temperatures of low-energy (10-5950 eV) electrons and the electrostatic potential of the spacecraft near Neptune are derived. The data indicate that the escape of charged particles from Triton and the local ionization of atoms in the neutral torus originating from Triton are the major plasma sources. It is inferred that this neutral torus of hydrogen atoms has a density of about 300/cu cm and an inner boundary at 8 R(N). The data near Neptune exhibit signatures suggesting that both precipitation into Neptune's atmosphere and ring absorption are important plasma loss mechanisms. Plasma transport in the magnetosphere appears to be very fast; the diffusion coefficient is 1 x 10 exp -7 L exp 3 R(N) exp 2/sec.

Zhang, Ming↗
NASA NTRS1991

On the lack of a magnetic signature of Triton's magnetospheric interaction on the Voyager 2 flyby trajectory

There is strong, albeit indirect, evidence for a sub-Alfvenic and transonic interaction between Triton and the Neptunian magnetosphere. A new inspection of magnetic field data around Triton encounter shows no evidence for the interaction. It is shown that this is due to the geometry of Triton's Alfvenic wings, which were probably too far from the spacecraft during the encounter. The situation is complicated by finite ion gyroradius effects, which tend to smear out the simple Alfven wings such as encountered at Io.

Neubauer, F. M.↗
NASA NTRS1991

On the limit of field-aligned current intensity in the polar magnetosphere

Field-aligned current (FAC) is here defined by 4 pi j = alpha B, where alpha is constant along a magnetic field line. The upper limit value of alpha in the polar magnetosphere, possible source regions of the strongest FAC and the relationship of them to some auroral and ionospheric irregularity cross-field scale sizes are discussed. Cross-field dimensions of the strongest FAC are related to the gyroradii of source particles (O(+), He(2+), He(+), H(+), e) in the current-generating region. It is suggested that experimental determination, and mapping of the values of alpha, may assist with the search for the generators of such currents in near-earth space including in the nearby solar wind. The upper limit of alpha is associated with the breakup of FAC systems.

Cole, Keith D.↗
NASA NTRS1992

Preliminary study of the CRRES magnetospheric barium releases

Preliminary theoretical and computational analyses of the Combined Release and Radiation Effects Satellite (CRRES) magnetospheric barium releases are presented. The focus of the studies is on the evolution of the diamagnetic cavity which is formed by the barium ions as they expand outward, and on the structuring of the density and magnetic field during the expansion phase of the releases. Two sets of simulation studies are discussed. The first set is based upon a 2D ideal MHD code and provides estimates of the time and length scales associated with the formation and collapse of the diamagnetic cavity. The second set uses a nonideal MHD code; specifically, the Hall term is included. This additional term is critical to the dynamics of sub-Alfvenic plasma expansions, such as the CRRES barium releases, because it leads to instability of the expanding plasma. Detailed simulations of the G4 and G10 releases were performed. In both cases the expanding plasma rapidly structured: the G4 release structured at time t less than about 3 s and developed scale sizes of about 1-2 km, while the G10 release structured at time t less than about 22 s and developed scale sizes of about 10-15 km. It is also found that the diamagnetic cavity size is reduced from those obtained from the ideal MHD results because of the structure. On the other hand, the structuring allows the formation of plasma blobs which appear to free stream across the magnetic field; thus, the barium plasma can propagate to larger distances traverse to the magnetic field than the case where no structuring occurs. Finally, a new normal mode of the system was discovered which may be excited at the leading edge of the expanding barium plasma.

Huba, J. D.↗