Shuttle Potential And Return Electron Experiment Research Articles

Beam‐induced electron modulations observed during TSS 1R

We report on modulations of electron fluxes at megahertz frequencies measured by the Shuttle Potential and Return Electron Experiment (SPREE) during fast pulsed electron gun (FPEG) beam experiments conducted after the tether break event of the Tethered Satellite System Reflight. Six intervals of sustained modulations were identified while FPEG emitted a 100 mA beam of 1 kev electrons. During five events the beam pitch angle αB was near 90° and the modulations were near even or odd half harmonics of the electron gyrofrequency ƒce. In the sixth event with 60° ≥ αB ≥ 45°, electron modulations were near estimated values of the electron plasma frequency ƒpe and 2ƒpe. Whenever SPREE detected beam electrons modulated at a given frequency, secondary electrons were also modulated at the same frequency over a broad range of energies. Occasionally, some secondary electrons were modulated simultaneously at a second frequency. Multiple frequencies were related as ratios of low integers. In one case the beam electrons were simultaneously modulated at 0.8 MHz and 1.25 kHz. SPREE measurements suggest that the beam electrons propagate in cylindrical shells whose inner edge is marked by steep spatial gradients in fluxes at 1 keV [Hardy et al., 1995]. Inside the shell, electron distribution functions have positive slopes ∂ƒ/∂v⊥ > 0 at velocities near that of the beam. Velocity space gradients act as free‐energy sources to drive cavity modes that alter the instantaneous guiding centers of electrons causing SPREE to sample alternating parts of the beam cylinder's inner edge. Associated time‐varying electric fields also modulated the fluxes of secondary electrons reaching SPREE. Other cavity modes may be excited through nonlinear processes [Calvert, 1982], With αB far from 90°, electrons in the beam cylinder evolved toward bump‐on‐tail distributions to excite large‐amplitude Langmuir modulations at ƒpe and its harmonics [Klimas, 1983]. Low‐frequency modulations are attributed to electron interactions with ion acoustic‐like waves generated as the beam moved across magnetic field lines in the ionosphere at supersonic speeds.

MHz and KHz modulations of particle fluxes during beam experiments of the tethered satellite system missions

Modulations of electron and ion fluxes in the frequency ranges 0 – 10 MHz and 0 – 10 kHz were monitored by the Shuttle Potential and Return Electron Experiment (SPREE) during electron beam experiments conducted on the Tethered Satellite System missions. Electron fluxes detected by SPREE were modulated at megahertz frequencies only when beams were emitted at large pitch angles with respect to the Earth's magnetic field. High frequency modulations fall into two classes close to: (1) even or odd half harmonics of the electron gyrofrequency if beam pitch angle α B ≈ 90°, and (2) harmonics of estimated plasma frequencies if 60° > α B > 45°. When SPREE intercepted beam electrons after one gyroturn, they were modulated at the same frequencies as secondary electrons. These measurements indicate that beam electrons propagate in cylindrical shells whose inner edge is marked by steep spatial gradients in fluxes of beam electrons. Inside the shell, electron distribution functions have positive velocity space slopes at energies near that of the beam. These gradients act as free energy sources to drive cavity modes that alter the instantaneous guiding centers of electrons causing SPREE to sample alternating parts of the beam cylinder's inner edge. Associated time-varying electric fields also modulated the fluxes of secondary electrons reaching SPREE. Low kilohertz modulations, detected at all beam pitch angles, correspond to no natural plasma mode and are interpreted as signatures of large-amplitude, ion acoustic-like waves excited through Cerenkov interactions between electron beams and ambient plasmas as the shuttle moved at supersonic speeds across the ionosphere. This hypothesis is confirmed by the simultaneous detection of electrons and ions modulated at the same frequencies when the shuttle was negatively charged. These results explain the sources of coherent whistler and ion acoustic-like waves detected during electron beam experiments of Spacelab 2.

Shuttle charging by fixed energy beam emissions

We present environmental responses observed by the Shuttle Potential and Return Electron Experiment during 13 prolonged electron beam emissions from the Fast Pulsed Electron Generator (FPEG) during the TSS 1R deployment. As the tether lengthened from 0.18 to 2.5 km, FPEG fired nine times at a nominal current of 100 mA at 1 keV. The motional potential induced across the system increased from 24 to 225 V. With a 15 Ω resistor connecting the tether to shuttle ground, current was higher and the degree of negative shuttle charging lower when FPEG was off than when it was on. While most FPEG firings left the shuttle uncharged, two created significant potentials: one to +70 V in a nighttime, equatorial plasma depletion and one to ‐60 V, with the tether ∼ 15 km.

Megahertz electron modulations during TSS 1R

Experiments were conducted during the Tethered Satellite System Reflight in which a 1 keV, 100 mA electron beam was emitted from the shuttle at pitch angles near 90°. Rapid plasma responses measured by the Shuttle Potential and Return Electron Experiment (SPREE) show time‐modulated electron fluxes within the beam flux tube. Megahertz modulations fall into two classes: (1) narrow‐bands close to harmonics of the electron gyrofrequency fce and (2) broad‐bands at interharmonic frequencies in which electrons of different energies had different modulation frequencies. When SPREE intercepted beam electrons after a single gyroturn they, too, were modulated at similar frequencies. Data suggest that beam electrons were modulated by strong plasma interactions near the emission aperture, generating time‐varying electric fields that modulated other electrons near the beam cylinder. This is analogous to the electron cyclotron maser responsible for auroral kilometric radiation.

SPREE measurements of wave-particle interactions generated by the electron guns on TSS-1 and TSS-1R

The Shuttle Potential and Return Electron Experiment (SPREE) was flown in the orbiter bay during TSS-1 and TSS-1R flights. Measurements presented here correspond to times of DC electron firings of the Shuttle Electrodynamic Tether System (SETS) fast pulsed electron generator (FPEG). SPREE included particle correlators measuring particle modulations in the 0–10kHz and 0–10MHz frequency ranges. Wave Particle Interactions, WPI, were observed on TSS-1 & TSS-1R in both frequency ranges: around a few MHz whenever the emitted beams were close to 90° pitch angle; and at a few kHz at many other beam pitch angles. On both missions problems with the tether deployment lead to more of the mission timeline being dedicated to the SPREE/FPEG active plasma studies reported here. The strong MHz waves occur when the 90° FPEG beam has the same low parallel velocity component, V par values as the thermal electrons. Then both the beam source and the thermal sink can interact with the same waves. The strong kHz waves are generated directly by the passage of the FPEG beam through the ionospheric plasma at the orbiter velocity with a period determined by the geometry of the disturbance.

Heating and low‐frequency modulation of electrons observed during electron beam operations on TSS 1

We have studied electron responses measured by two electrostatic analyzers (ESA A and B) that comprise the Shuttle Potential and Return Electron Experiment (SPREE) to 60 prolonged beam emissions by the fast pulsed electron generator (FPEG) during the first flight of the Tethered Satellite System (TSS 1). When the beam cleanly escaped into space, responses depended on whether the pitch angle of the beam, αB, was less than or greater than 90°. Beam‐like structures were detected by SPREE when αB < 90°, but not when αB > 90°. Secondary electron fluxes measured by SPREE peaked at pitch angles a between 65° and 75° when αB < 90°, and at α ≈ 90° when αB > 90°. At other pitch angles the distributions of electrons returning to the shuttle had repeatable thermal and power law shapes. The distinctive distribution functions are attributed qualitatively to the different regions in and near the beam traversed by electrons reaching SPREE under the two αB conditions. A large fraction of the trajectories of electrons reaching SPREE ESA A with αB < (>)90° lie inside (outside) beam flux tubes. Measurements by a particle correlator in the SPREE data processor show that in 25 cases some of the returning‐electron distributions fe were modulated at frequencies in the low kilohertz range. The modulations appeared in portions of the distributions where ∂ƒe/∂υ < 0 and at frequencies that correspond to none of the plasma's normal modes. In light of previously reported wave measurements taken near the shuttle during electron beam emissions, we suggest that the modulated electrons were bunched by large‐amplitude, ion acoustic waves propagating nearly perpendicular to the Earth's magnetic field. The waves were generated as plasma responses to negative space charges in the electron‐beam flux tubes moving at orbital speed across the ionosphere.

Negative pickup ions detected during the TSS 1 mission

We report the occasional detection of negative pickup ions by the Shuttle Potential and Return Electron Experiment (SPREE) in the shuttle payload bay during the first Tethered Satellite System (TSS 1) mission. These ions appear as arch‐like structures of enhanced fluxes in energy versus time spectrograms for the SPREE electron zone looking in the plane perpendicular to the Earth's magnetic field. Maximum energies of ∼56 eV were measured when the sensor looked closest to the shuttle ram direction. Empirically, the energy of associated spectral peaks decreases as cos2 ξ, where ξ is the angle between the sensor look direction and the projection of the shuttle's velocity onto the plane perpendicular to the magnetic field. These signatures may be found under day or night conditions and always in conjunction with positively charged pickup ions generated as a result of firings by the L5D and/or R5D vernier thrusters. We show that positive ion contamination and accelerated electrons cannot be the responsible agents. Observed spectral characteristics are most easily explained as either NO2− or MMH− ions created near the shuttle and accelerated by the motional electric field to energies detectable by SPREE.

Beam arc distributions of shuttle pickup ions and their instabilities

Data from the Shuttle Potential and Return Electron Experiment (SPREE) flown as part of the Tethered Satellite System (TSS 1) are used to determine the detailed characteristics of beam arc distributions of pickup ions due to molecules outgassed or ejected from the shuttle. These ion distributions are only detected near the plane perpendicular to the magnetic field direction in an angular range of ±45° about the minimum angle to ram. Their flux is largest when the angle between this plane and the shuttle ram direction is smallest. Generally, ion spectra peak in the range 19 to 25 eV at the minimum angle between the perpendicular plane and the ram direction. The peak energy decreases smoothly as this angle increases. Weak fluxes are measured above the peak, to energies as high as 150 eV. Within the SPREE energy range, two‐dimensional distribution functions of beam arc ions in the perpendicular plane have teardrop shapes, symmetric about the minimum angle to shuttle ram with deep minima in the centers. Variations in the peak energies of differential number fluxes agree with collisionless trajectory analysis, assuming that the ions are H2O+ and allowing for different initial velocities before charge exchange. The lowest densities for beam arc ions occur during periods of purely residual outgassing from the shuttle. Ion densities increase by a factor of 5 during waste water dumps. The highest densities occur during operation of the flash evaporator system when the pickup ion densities in daylight can exceed 105 ions cm−3, about 30% of the estimated ambient plasma density. We also present a nonlinear numerical analysis to study the stability of beam arc generated plasma waves and explain electrostatic spectra measured during previous shuttle flights.

Thruster effects on the shuttle potential during TSS 1

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.

Correlator measurements of megahertz wave‐particle interactions during electron beam operations on STS

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 (< 3×106 m s−1 ) where beam‐generated waves are in resonance with suprathermal electrons.

Observations of electron beam propagation perpendicular to the Earth's magnetic field during the TSS 1 mission

We report on measurements by the Shuttle Potential and Return Electron Experiment (SPREE), acquired during a period of the Electrodynamic Tethered Satellite mission when the fast pulsed electron generator (FPEG) injected a 1‐keV electron beam nearly perpendicular to the Earth's magnetic field. Using multiangular electrostatic analyzers mounted on rotary tables, SPREE was capable of determining the flux of electrons and ions in the energy range from 10 eV to 10 keV and over a solid angle of 2π sr. SPREE was located in the shuttle bay where it could observe beam electrons after they had completed ∼1 gyrocycle when fired nearly perpendicular to the local magnetic field. For the case presented here, the beam's intensity decreased from ∼100 mA cm−2 at FPEG's aperture to ∼0.18 nA cm−2 at the location of SPREE. The spectrum of the return electrons displays a sharp peak at the beam energy with an intensity at the peak of approximately 2×1010 electrons cm−2 s−1 sr−1. The distribution of the electrons around the peak has a half width of several hundred eV, with observed energies as high as 1850 eV. For energies between 10 and a few hundred eV, intense fluxes of electrons are seen at all look angles. For angles where the beam is observed the spectrum in this energy range has a power law shape. At angles away from the direction of beam return, the spectrum in this energy range can display a more thermal shape with a peak at energies up to 50 eV. In general, the flux intensity in the lower‐energy portion of the spectrum is isotropic with an average integral flux of 0.5 to 2×1012 electrons cm−2 s−1 sr−1. Integrating over energy and pitch angle gives number densities of ∼ 5×104 electrons cm−3. The return current density of 0.5 to 2µA cm−2 s−1 sr−1 carried by this isotropic component is sufficient to balance that emitted by FPEG and keep the shuttle at a low potential. We find that both scattering and spreading of the beam near FPEG are necessary for primary electrons to reach the locations of the SPREE detectors.

Energy distributions of thruster pickup ions detected by the Shuttle Potential and Return Electron Experiment during TSS 1

Instrumentation to monitor the shuttle environment during the Tethered Satellite System (TSS 1) mission included the Shuttle Potential and Return Electron Experiment (SPREE) and the Quadrupole Ion‐Neutral Mass Spectrometer (QINMS). SPREE measured fluxes of electrons and ions with energies between 10 eV and 10 keV; QINMS monitored neutral and ion species close to the shuttle. We report on energy distributions of pickup ions detected during and after thruster emissions while the shuttle's velocity vector was nearly perpendicular to the Earth's magnetic field. With SPREE looking in the ram direction and almost perpendicular to the magnetic field, fluxes >1011 ions cm−2 s−1 sr−1 were detected in the 10‐to‐60‐eV energy range. Both prompt and prolonged ion flux enhancements were recorded. The prolonged fluxes lasted many seconds after the initiating thruster turned off. Only when thrusters fired close to the magnetic field direction were no pickup‐ion flux enhancements detected. The SPREE measurements are compared with the predictions of a simple two‐dimensional model of collisionless pickup‐ion trajectories. Using the distribution of ion species recorded by QINMS, the model explains the main features of the ion energy spectra measured by SPREE. Comparison of the model with data also indicates that strong scattering of ejected materials must occur almost immediately after thruster emission and later between the time of pickup‐ion creation and detection of the increased ion flux by SPREE.

Observations of ionosphere heating in the TSS‐1 subsatellite presheath

The first flight of the Tethered Satellite System (TSS‐1) was to investigate the mechanical and electrical dynamics of a conducting satellite deployed from the orbiter via a tether whose core was a conducting wire [Dobrowolny and Melchioni, 1993; Dobrowolny, 1987; Dobrowolny and Stone, 1994). In the TSS‐1 system the satellite deployed from the orbiter radially away from the Earth. The relative motion between the tether and Earth's magnetic field generated an electromotive force (EMF) that is the product of orbiter velocity, Earth's magnetic field, and the length of the deployed tether. This EMF drove a current through the tether. Electrons were collected on the satellite's electrically conductive skin and traveled through the tether to the orbiter, where they either went to orbiter structural ground or were emitted into the ionosphere via active electron emission. During TSS‐1 this electron emission was accomplished mainly by the 100 mA, 1‐keV fast pulsed electron gun (FPEG) of the shuttle electrodynamic tether system (SETS) (Williamson et al., 1988; Banks et al., 1994; V. M. Aguero et al., manuscript in preparation, 1994]. The FPEG electron emission was much higher than either ambient ion collection at the orbiter end or electron collection at the satellite. Potentials of the orbiter with respect to the ambient plasma were obtained from measurements from the Shuttle Potential and Return Electron Experiment (SPREE) (Oberhardt et al., 1993a, b, 1994), the SETS tether current voltage monitor (Thompson et al., 1993), and the Agenzia Spaziale Italiana deployer and satellite core equipment (Bonifazi et al., 1988, 1994). Despite the limited tether deployment length of 268 m the TSS‐1 system proved capable, during certain events, of generating satellite potentials sufficient to illuminate a previously unexplored aspect of plasma physics: that of an ion repelling, electron attracting, moving probe in a magnetoplasma. During such events the satellite boom‐mounted Langmuir probe flown as part of the Research on Electrodynamic Tethers experiment (Dobrowolny et al., 1994) measured an increase in the electron plasma temperature in the quasi‐neutral ionospheric region beyond the satellite sheath. This observed heating of the presheath electrons was distinctly different from the acceleration of the electrons in the sheath, which was also observed when the sheath expanded such that the probe was completely in the sheath. We show that the observed elevated electron temperatures are consistent with the formation of a Bohm stable electron collecting sheath.

The Shuttle Potential and return electron experiment (SPREE)

The Shuttle Potential and Return Electron Experiment (SPREE) was designed and fabricated for flight as part of the joint NASA/Agenzia Spaziale Italiana Tethered Satellite System (TSS-1) mission. The SPREE is a complex instrument package designed to measure ion and electron particle flux and wave-particle interactions. The SPREE flight hardware consists of two multiangular electrostatic analyzer units, two rotary tables, a data processing unit, a particle correlator experiment, and two data recording units. The electrostatic analyzers measure both electrons and ions, in an energy range from 10 eV to 10 keV and simultaneously over an angular fan of (100×10) degrees. These units are mounted on the rotary tables to provide a 2π steradian field of view out of the Orbiter's payload bay. To assess negative charging of the Orbiter with respect to the ambient plasma, ion data from the analyzers are processed real time by an on-board algorithm operating within the data processing unit. The particle correlator experiment determines wave-particle interactions in the frequency range 0–10 MHz for electrons and from 0–10 kHz for ions. SPREE operated successfully throughout the TSS-1 mission. Examples of the data returned by the SPREE are shown.

Space instrument neural network for real-time data analysis

A simple software implementation of an artificial neural network (ANN) was used to analyze up to 200 autocorrelation functions (ACFs) per second within the Shuttle Potential and Return Electron Experiment (SPREE) flown on the Shuttle STS46 mission, July 31, 1992. As all ACF data are stored onboard until postmission, this facility provided ground-based experimenters with their only access to ACF data in real time for optimum instrument control. ACFs contain data either as waveforms or as radar echoes. Operating directly on the ACF, the neural network identifies the type of data, ascertains the wave frequency or radar peak separation, and provides a score or measure of significance of its decision. An effective 16:1 data reduction is achieved and the data interpretation performance is comparable to that achieved by an expert data analyst. Erroneous analysis accounts for less than 1% of data analyzed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>