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The interaction between a positively biased body traveling through an ionospheric space plasma has direct application to electrodynamic tether (EDT) systems. A 2-D3v particle-in-cell model has been developed to study the plasma dynamics near a positively charged EDT system end-body and their impact on the current collected. The results show that the azimuthal current structures observed during the reflight of the tethered satellite system (TSS-1R) mission develop in the simulations and are found to enhance the current collected by the satellite by 67% when the magnetic field is ∼15° off of perpendicular to the orbital velocity. As the component of the magnetic field in the simulation's plane increases, the electrons are not able to easily cross the field lines causing plasma lobes to form in the +y and −y regions around the satellite. The lobes limit the current arriving at the satellite and also cause an enhanced wake to develop. A high satellite bias causes a stable bow shock structure to form in the ram region of the satellite, which limits the number of electrons entering the sheath region and thus limits the current collected. Electron-neutral collisions are found to destabilize the bow shock structure, and its current limiting effects were negated. Analytical curve fits based on the simulations are presented in order to characterize the dependence of the current collected on the magnetic field's orientation, space plasma magnetization, and satellite potential. The variations in the collected current induced by space plasma environmental changes may introduce new instabilities in an EDT system's dynamics.

Enhancements of the temperature of electrons in spacecraft plasma wakes have been reported for numerous cases [Samir and Wrenn, 1972; Troy et al., 1975; Oran et al., 1975) and this phenomenon has been discussed both empirically (Samir and Stone, 1986; Stone and Samir, 1986) and theoretically (Singh et al., 1987). However, very few measurements seem to have been made of the ion temperature within plasma wakes--possibly because the great majority of ion measurements were focussed on obtaining geophysical parameters and, hence, were confined to the region ahead of the spacecraft. Recently, however, an enhancement of the temperature of ions was discovered in data obtained in the wake of the Space Shuttle during the Spacelab-2 mission (Sorensen et al., 1997). At the time of that publication, this was the only known observation of this type. Herein, we report an additional case of ion temperature enhancement in a plasma wake. The data were taken during the Tethered Satellite System Reflight mission (TSS-IR) in the wake of the tethered satellite during passive (no current flow) operations. The measurements were obtained with the Differential Ion Flux Probe, or DIFP (Stone, 1977 and Stone et al., 1985).

The physical process of current collection by a “bare wire” electrodynamic tether in space is considered. The study uses an improved model that takes into account the resistance of the wire and the magnetic shielding induced by current flow in the tether. The plasma density, ne, electron temperature, Te, tether length, L, tether radius, rw and the angle of the geomagnetic field to the tether (90°-α) were all used as parameters. It is shown, for certain tether configurations and parameter values, that magnetic shielding reduces the collected current. In general, any parametric change that increases tether current, and hence, the strength of the current-induced magnetic field relative to the strength of the electric field between the tether and the ambient plasma, will increase the shielding effect. Tether current is increased directly with tether collection area (which depends on L and rw), plasma conductivity (which depends on ne and Te), and the motional emf along the tether (which increases with L and the angle 90°-α). It turns out that, as any of these parameters change so as to cause the overall tether current to increase, the overestimate of current that results from ignoring the magnetic shielding effect becomes correspondingly greater. Moreover, it is shown that a tether system in the thruster (or motor) mode suffers greater current reduction from magnetic shielding than does the same tether deployed in the generator mode. Finally, it is shown that, for certain tether system configurations combined with particular values of the governing plasma parameters, current-induced magnetic shielding can significantly reduce the collected current and, therefore, system efficiency. For example, in the case of an electrodynamic tether system in the thruster mode under conditions of ne=1.67×106 cm−3, α=60°, rw=2.5 mm, and Em=34 V/km, magnetic shielding will reduce the collected current 10% at a point L=0.65 km along the tether (4.3 A instead of 4.8 A) and this increases to more than 40% at L= 1.3km (9.6 A instead of 13.4A).

In this paper we present the measurements and interpretations of the Tethered Magnetometer (TEMAG) experiment flown on the Tethered Satellite Mission Reflight TSS-IR. Data were taken during the active operating phase when current steps of variable amplitude were injected along the electrically conducting tether. The magnetic field shows clear signatures in the components and amplitude, dependent upon the orientation of the magnetometer boom with respect to the spacecraft ram velocity vector. In the eight events studied, current effects could be observed in the magnetic field data only when the boom was pointing toward the ram side hemisphere of the satellite. The picture emerging from the observed results suggests the formation of a current sheet on the ram side, possibly associated with the aerodynamic shock and compression of the geomagnetic field ahead of the satellite. Although the limited data do not allow full description of the phenomenology, they place important observational constraints on any theoretical model.

Current collection by an infinitely long, conducting cylinder in a magnetized plasma, taking into account the magnetic field of the collected current, is discussed. A region of closed magnetic surfaces disconnects the cylinder from infinity. Owing to this, the collected current depends on the ratio between this region and the plasma sheath region and, under some conditions, current reduction arises. It is found that the upper bound limit of current collection is reduced due to this change of magnetic field topology. The effect can be substantial even if the orbit-limited model of current collection is valid. This model is used to find the reduction of the total current collected by a cylinder (e.g., a bare tether). It is shown that this effect strongly depends on plasma density. The results are applied to a tether system in the ionosphere, and it is found that current reduction can be significant for long tethers in typical dayside ionospheric conditions.

We present measurements of currents, orbiter potentials, and plasma spectra during high-voltage arcing coincident with the tether break event on the Tethered Satellite System reflight. In addition to the unexpectedly high tether currents observed, plasma spectra indicate the presence of ion and electron populations with broad energy ranges. These data were used in combination with satellite and orbiter current collection models to investigate the circuit behavior of the system's components. We find that arcing at the lower end of the tether supported the current flow in the tether during each phase of the break event, but with different mechanisms dominant depending on the location of the break point. With the break point inside deployer control structures, current arced to the orbiter ground, charging it to high negative potentials and allowing secondary ionization of neutral molecules near orbiter conducting surfaces. The most likely source of these neutrals is air trapped inside the tether at 1 atm of pressure that escaped through the hole in the tether insulation. When the break point was exposed to the exterior environment the tether current arced directly to the plasma. As long as the break point remained near the orbiter, the collection of electrons by conducting surfaces caused it to float at a low level of negative charging. The source of the energetic electrons detected in the payload bay remains uncertain. However, they can only have come from a region within the sheath that was more negatively charged than the orbiter's conducting surfaces.

An electrically floating metallic bare tether in a low Earth orbit would be highly negative with respect to the ambient plasma over most of its length, and would be bombarded by ambient ions. This would liberate secondary electrons, which, after acceleration through the same voltage, would form a magnetically guided two-sided planar e beam, and result in auroral effects (ionization and light emission) upon impact on the atmospheric E layer, at about 120–140 km altitude. This paper examines in a preliminary way the feasibility of using this effect as an upper atmospheric probe. Ionization rates can reach up to 105 cm−3 s−1 if a tape, instead of a wire, is used as tether. Contrary to standard e beams, the beam from the tether is free of spacecraft charging and plasma interaction problems, and its energy flux varies across the cross section, which is quite large; this would make possible continuous observation from the satellite, with high resolution, both spectral and vertical, of the induced optical emissions. Ground observation might be possible at latitudes around 40°, for night, magnetically quiet conditions.

Ground-based radar observations during NASA's Plasma Motor Generator (PMG) electrodynamic tether mission are reported, which indicate the existence of ion-acoustic or soliton wave packets produced as ionospheric excitations by the orbiting PMG tether system. These appear to be the first such radar measurements of traveling ionospheric disturbances propagating along geomagnetic field lines during an active orbiting ionospheric tether-plasma experiment. The presence of such excitations gives added importance to earlier studies of the plasma radiation modes involved in tether electrodynamics and the part they play in the ionospheric energy balance mechanisms. In addition, tether-current measurements studied during postflight data analysis show a pronounced current decrease when the spacecraft passed from daylight to darkness, which may be related to current limiting. The significance of this observed day/night correlation in conjunction with the soliton-like wave packet radiation modes calls for a detailed investigation of the PMG tether-current measurements in terms of local and global interactions of the PMG with Earth's ionosphere, the initial work for which we show here. This includes an analysis of the radiation impedance due to Alfven and lower hybrid bands, ion-acoustic (soliton) modes, and whistler waves. A decomposition of the measured tether currents into Hall, Pedersen, Cowling, and Birkeland conductivities is presented.

We have surveyed measurements taken by the Shuttle Potential and Return Electron Experiment (SPREE) in the shuttle payload bay during the satellite deployment phase of the Tethered Satellite System (TSS 1) mission. The shuttle was flying in an inverse airplane mode with its engine bells facing the ram direction to facilitate ion current collection. SPREE ion spectral measurements indicate that with no thrusters firing, the shuttle charged to, at most, a few tens of volts negative. We identified 13 intervals during which the tether was electrically connected to the shuttle by a low-impedance shunt and thruster firings occurred. In previously reported examples of neutral gas releases from charged spacecraft the magnitude of the vehicle potential decreased. Thruster firings during our events either left the TSS circuit unchanged or caused the current measured in the tether to diminish and the shuttle to become more negatively charged. In the latter case it appears that thruster gas impeded access of ambient ions to the current collecting surfaces of the shuttle. The changes in the distribution of the motionally induced potential between the tether and sheath calculated using Ohm's law are in agreement with SPREE measurements.

We report on the analysis of megahertz modulation of electrons as measured by the Shuttle Potential and Return Electron Experiment (SPREE) during dc firing of the shuttle electrodynamic tether system (SETS) fast pulsed electron generator (FPEG). The SPREE and FPEG were flown aboard the space shuttle Atlantis flight STS 46 as part of the Tethered Satellite System (TSS 1) mission. The principal data reported here are from the SPREE multiangular electrostatic analyzers (ESAs) and Space Particle Correlator Experiment (SPACE). The ESAs, mounted on rotary tables, measured electrons and ions in the energy range from 10 eV to 10 keV over a solid angle of 2π sr. The SPACE is a signal processing system that analyzes the pulse stream from the SPREE ESAs to identify bunching of the electrons and ions produced by coherent wave-particle interactions (WPIs). The SPACE detects modulations in the electron fluxes in frequency range 0- to 10-MHz. This paper concerns 2- to 4-MHz modulations of the electron flux detected by the SPACE when the FPEG was firing in a dc mode at pitch angles close to 90°. During such operations, FPEG emitted a current of 100 mA at an energy of 1 keV. For these times, electrons with energies from 10 to 1850 eV were measured by the SPREE. For energies between ∼10 and 100 eV the electron flux is basically isotropic. At higher energies the flux increases for pitch angles near 90°. The electron distribution functions generally decrease monotonically with increasing energy up to 100 eV. At energies >100 eV the distributions either monotonically decrease or exhibit a peak or plateau at energies near the beam emission energy. Megahertz modulations were observed for electrons with energies from 10 to 1180 eV, on both positive and negative slopes in the distribution function and throughout the 2π sr sampled by the ESAs. The occurrence and strength of the modulations exhibit no clear dependence on the pitch angle at which the electrons are measured. However, they appear to be limited to low parallel velocities (

The tethered sounding rocket payload OEDIPUS A was launched over aurora in the evening of January 30, 1989. A conducting tether between the instrumented subpayloads was extended to 958 m during the flight which attained an apogee altitude of 512 km. Electron fluxes of up to ∼2 × 108 cm−2 s−1 sr−1 were recorded by an onboard energetic particle detector throughout much of the flight. Electron density measurements were made by the bistatic transmitter-receiver set high-frequency exciter (HEX) and receiver for exciter (REX) using a special technique based on the characteristics of the Z mode of propagation. This electron density information was used to identify the frequency limits of propagation of various emissions in the 0-5 MHz bandwidth of the receiver REX which was configured for passive detection of rf current induced in the tether. On two occasions in the flight, sharply delimited depletions of ambient density correlated closely with enhancements of radio noise, notably in the whistler mode at frequencies below 800 kHz. These depletions were located on the flanks of “inverted-V” structures in the precipitating electron fluxes. The whistler mode emissions were very weak compared with what can be observed at greater heights in the auroral topside ionosphere. However, their detection apparently was enhanced by a resonance condition between the tether length and the wavelength of sheath waves. By ray tracing analysis, depletions were found incapable of trapping the whistler mode Waves, which exhibited levels 8-10 dB higher than those outside the depletion, or of rendering them more detectable by the tether than unguided waves. The tether resonance may compensate for the tether's relative inefficiency as a receiving antenna because of its orientation along the magnetic field direction. It is inferred that the waves are generated close to the payload by suprathermal electrons.

The tethered satellite system (TSS) project was developed, starting from 1984, as a unique system to make active experiments in space plasmas. This paper aims at providing a thorough description of the basic electrodynamic aspects of a TSS orbiting in the ionosphere. As will be seen, a wide variety of plasma physics phenomena are associated with the interaction of a conducting tether with the ionospheric plasma and, as a consequence, numerous basic science objectives can be addressed with a TSS system. Besides reviewing the various electrodynamic phenomena and our present knowledge on them, we provide also a description of the scientific payload and the nominal science objectives of the first tethered satellite project (TSS-1). A brief account, as well as a first evaluation, are also given of the mission itself, which was flown with the shuttle Atlantis launched on July 31, 1992.

We study plasma responses to a high potential satellite in the electromagnetic tether system with the aid of two-dimensional electromagnetic particle computer experiments. In the computer experiments, one conducting body representing the satellite is placed in the center of the simulation plane in the vehicle frame. The satellite potential [phi][sub s] is fixed to be higher than the space potential of the surrounding plasma. The transient process of the plasma response to the satellite and the spatial profiles of plasma density and of plasma current are examined. Due to the high potential, an electron sheath and an ion cavity, are formed around the satellite. The ion cavity is extended downstream by the ambient plasma flow. In the direction perpendicular to B[sub o], the density profile of the plasma flow, shows asymmetric distortion caused by the deceleration of the ambient plasma owing to the intense E [times] B effect. An intense transient current is observed followed by an exponentially decreasing current. The current flows along and across B[sub o]. The cross-field current is mainly produced by the electron E [times] B drift motion around the satellite. The current-voltage characteristics of the satellite in the steady state are examined by changing themore » satellite potential up to e[phi][sub s]/[kappa][sub B]T[sub e] [approx equal] 10[sup 3] and comparing to the classical theories. The average current obtained by the computer experiments is between that predicted by the Langmuir theory and that predicted by the single particle theory. From e[phi][sub s]/[kappa][sub B]T[sub e] [approx equal] 10[sup 1] to 10[sup 3], the theory based on the constant density model provides a good estimate of the upper bound of the current flowing from the satellite. 35 refs., 12 figs.« less

[1] Magnetic excitation of whistlers by a square array of electrodynamic tethers is discussed. The array is made of perpendicular rows of tethers that carry equal, uniform, and time-modulated currents at equal frequency with a 90° phase shift. The array would fly vertical in the orbital equatorial plane, which is perpendicular to the geomagnetic field B0 when its tilt is ignored. The array radiates a whistler wave along B0. A parametric instability due to pumping by the background magnetic field through the radiated wave gives rise to two unstable coupled whistler perturbations. The growth rate is maximum for perturbations with wave vector at angles 38.36° and 75.93° from B0. For an experiment involving a wavefront that moves with the orbiting array, which might serve to study nonlinear wave interactions and turbulence in space plasmas, characteristic values of growth rate and parameters, such as the number of tethers and their dimensions and distances in the array, are discussed for low Earth orbit ambient conditions.

Alfven waves are important in natural-space phenomena, and studies of these waves have been conducted by performing active experiments in space plasma. One method to study these waves involves the use of an electrodynamic tether. The electrodynamic tether consists of a subsatellite with a conducting outer surface connected to the Space Shuttle by an insulator-clad, conducting wire about 20 kilometers in length. The emf generated by the motion of the tether across the geomagnetic field will act to drive a current through the tethered system. It is pointed out that this current in the tether acts as a source for exciting the shear-Alfven wave. In the present investigation it is shown that fringing effects play a dominant role in determining the radiation pattern for currently envisioned deployments of the tether. The current paper is the first of three papers concerned with the excitation of low-frequency waves arising from a current source moving through a cold, magnetized plasma.