The latest in the Argon Release for Controlled Studies (ARCS) series of argon plasma gun experiments on sounding rockets, ARCS-4, was launched from the Poker Flat Research Range (PFRR), Alaska in 1990 (Moore et al., 1991). The purpose of the experiment was to study particles and waves resulting from the interaction of the argon ion beam with the space environment. This was accomplished both in the vicinity of the beam and at a distance by having a payload separate into a beam emitting platform and a main platform, each with appropriate instrumentation to study the beam interactions. Results of the interactions have been reported at the separation distance of up to 100s m (Arnoldy et al., 1990; Moore et al., 1990; Vago and Kintner, 1990) and close to the argon gun (Cahill et al., 1993). The 100 eV to 200 eV ions emitted produced waves with multiple harmonics in the 10-1000 Hz range with amplitudes of up to 0.1 V/m close to the argon emitter. The authors speculate that the waves may be ion-ion hybrid waves generated by the observed field aligned electron fluxes neutralizing the beam emitting platform.
Active electron beam experiments have been more numerous than ion beam studies. Just prior to the period covered by this report the most comprehensively instrumented payload to use electron beams to probe the magnetosphere, ECHO-7 (descriptive of the expected electron beam reflection process) was launched (Winckler et al., 1989). In 1990, the most recent of a series of sounding rocket payload payloads called Several Compatible Experiments using a rocket-borne accelerator (SCEX) was launched and named SCEX-3 (Goerke et al., 1992, 1993). The very high frequency wave data taken from the SCEX-3 experiment together with luminosity data is used as evidence of the Beam Plasma Discharge (BPD) occurring in space as opposed to other reports of the phenomenon in ground-based space simulation chambers (Goerke et al., 1992). The SCEX payload separated into four elements in flight, two Throw-Away Detectors (TADs), a forward payload and an aft payload. The separation of the TADs in directions perpendicular to the geomagnetic field allowed the detection of plasma waves propagating up to 270 m perpendicular to the geomagnetic field. These waves showed little attenuation, and their frequencies indicated that they had emanated from diffuse resonance emission from the electron beam (Goerke et al., 1993).
Another experiment in which wave generation by electron beams
was a prime objective was the most recent experiment in the
Cooperative High Altitude Rocket Gun Experiments (CHARGE) series of
sounding rockets. CHARGE-2B was launched in 1992 from PFRR, and
successfully emitted a modulated electron beam of 
2 amps and
3 keV energy (Raitt et al., 1994a). The payload was composed of
three parts: a mother section containing the electron gun, a
tethered daughter section, and a free flyer. Wave receivers were
on the free flyer which separated from the main payload downwards
at about 5 m/s at an angle of 10
to the geomagnetic
field. This resulted in the maximum separation reaching 
2 km
by the end of the flight. Wave receivers were also included in the
tethered payload which separated from the beam platform to a
distance of up to 400 m perpendicular to the geomagnetic field.
Wave emissions from the electron beam which was modulated at
selected very low frequencies to act as a virtual antenna were
detected by the tethered payload receivers and by the free flyer
receivers. However there was no detectable evidence of wave energy
associated with the beam emission at down-range ground sites
(Fraser-Smith, 1992).
Experiments to study major perturbations to the space
environment formed the basis of the third Excitation by Electron
Deposition (EXCEDE-3) sounding rocket experiment. EXCEDE-3
contained electron guns which emitted a total beam current of 18
amps at 2.5 keV into the atmosphere up to an apogee of 
120
km. The high beam current and relatively dense atmosphere at the
working altitude resulted in very high levels of local ionization
causing glows around the vehicle and along the path of the emitted
beam. The altitude of the experiment was such that it effectively
produced in situ artificial aurorae (Rieder et al., 1993). The
payload was instrumented to study the atmospheric optical response
to very high electron fluxes, and provided a wealth of data on the
optical signatures of the high electron dosage (Paulsen and Murphy,
1992, 1993; Duff et al., 1993).
In addition to the several sounding rocket particle beam experiments in the US program, there were two orbital electron beam experiments in the reporting period. In each case the space shuttle was used as the orbital platform.
The ATLAS-1 mission which was flown in 1992 included the Space
Experiments with Particle ACelerators (SEPAC) experiment,
referenced earlier in relation to neutral material emissions.
SEPAC contained an electron gun capable of emitting an electron
beam up to a current of 1.2 amps when the beam energy was 6.3 keV
(Burch et al., 1993). The principal objective of the SEPAC
electron beam experiment was to emit the electron beam towards the
earth and observe the production of controlled artificial aurorae
(Torr, 1993). This objective was successfully achieved, and low
light level television imaging clearly showed the image of the
light produced as the emitted beam passed through the atmosphere
until it was ultimately absorbed at an estimated altitude of
110 km. The size of the light emission region on the imager
was consistent with emission all along the beam path. The emission
near the space shuttle resulted in an enhanced signal over the
maximum absorption region due to the effect of the inverse square
law on the received light intensity (Mende et al., 1993).
Experiments were also carried out on beam interactions with the
ambient space shuttle gas environment, and with enhanced densities
produced by the emission of xenon from the plasma contactor. In
addition the SEPAC electron beam was used to perform wave
experiments by modulating it at 50 Hz and 7 kHz over ground
observing sites to see if propagation of these frequencies to the
ground could be observed. At present this experiment has, like
CHARGE-2B, produced a negative result with respect to detection of
the waves on the ground (Taylor, 1993).
Electron beam experiments were an integral part of the electrodynamic aspects of the first Tethered Satellite System (TSS-1) tethered satellite mission to be described later. However, in addition to using an electron gun in conjunction with deployed tether operations, several stand-alone experiments with the electron beam and associated diagnostic instruments were planned and executed during the mission using the electron gun forming part of the Shuttle Electrodynamic Tether System (SETS) experiment (Aguero et al., 1994). The configuration of the particle detector of the Shuttle Potential and Return Electron Experiment (SPREE) and the Fast Pulse Electron Gun (FPEG) was such that in the correct orientation to the geomagnetic field, the 1 keV beam of the FPEG could have direct access to the SPREE aperture after one revolution about the magnetic field direction. The three-dimensional scanning of SPREE allowed the changes to the beam after this initial interaction with the environment to be measured (Hardy et al., 1994). In addition the SPREE instrument was able to study the positive charging of the space shuttle as a result of electron emission by FPEG as a function of the ambient conditions along the orbital path (Oberhardt et al., 1993).