Electromagnetic field stimulation of the upper atmospheric environment has primarily been by the use of electric fields. These fields have been generated by differentially charging two parts of a payload by an internal high voltage power supply as in the Space Power Experiments Aboard Rockets (SPEAR) program, or by the use of motionally induced electromotive force (emf) as in the TSS-1 and a combination of both in the Plasma Motor-Generator (PMG) projects. In all cases the prime objectives were to study the collection of current from the ionosphere by highly biased collectors, and to study the effect of contaminants released in the vicinity of the collectors.
The TSS-1 project was to have generated up to 4800 volts to be applied to collectors 20 km apart by the use of an insulated, conducting tether of that length. At one end of the tether the current was collected by an instrumented 1.6 m diameter spherical, conducting satellite, while at the other end the current flow was controlled by a variety of means including inserting series resistors and allowing passive current collection by the orbiter engine bells; using an internally powered electron beam to aid in charge emission; or using an electron beam accelerated directly by the induced emf in the tether (Papadopoulos et al., 1992; Dobrowolny and Stone, 1994). The key issue for the electrodynamic TSS missions is to gain better understanding of the current closure path external to the orbiting system. In preparation for the analysis of flight data this topic has been studied theoretically (Chang et al., 1994) and in the laboratory (Urrutia and Stenzel, 1990). Unfortunately, during the mission in 1992, the tether deployment system became jammed and the deployment length was limited to 268 m. Although this precluded meeting any of the prime science objectives, the limited dynamics and electrodynamics achieved were as or better than expected, promising a valuable and successful reflight of the system as TSS-1R planned for 1996.
Another electrodynamic tether project which flew in 1993 was the PMG experiment which flew as a secondary payload on a Delta mission in 1993 and was inserted into a 200 km x 900 km elliptical orbit. The inclusion of plasma contactors at each end of a 500 m conducting tether and supplementation of the induced emf by an internal power supply provided the possibility of quite high current flow in the system ( Olsen et al., 1993; McCoy et al., 1993). The induced emf was sufficient to demonstrate the power generation capabilities of the system, while the power supply allowed the tether current to be reversed to demonstrate the motor function of the system. Current was found to be limited when the plasma contactors were required to function in the electron collection mode.
The response of the ionosphere to considerably higher electric
potentials was studied by the SPEAR-3 experiment flown on a
sounding rocket launched from Wallops Island, VA in 1993 (Raitt et
al., 1993, 1994b). A differential potential of 10 kV was applied
from a charged capacitor between a deployed 20 cm diameter sphere
and the rocket body. The polarity of the applied voltage was to
make the sphere positive relative to the rocket body. The sphere
was 
3 m from the rocket body and a graded boom was used for
the last 0.5 m of the sphere support boom to control the electric
field near the sphere mounting point. The application of the high
voltage was alternately ramped up in 
30 mS and switched
directly from a high voltage relay. The capacitor then discharged
to provide a sweep in the applied voltage. The purpose was to
assess several grounding schemes to reduce the negative potential
to which the rocket body was driven as the voltage was applied.
Diagnostic instruments were included in the payload to monitor the
effectiveness of the grounding schemes and to help study the way in
which they functioned both under steady-state conditions and with
microsecond time resolution (Rhee et al., 1993; Thompson et al.,
1993). Two different electron emission devices were only effective
in grounding the payload at lower ionospheric plasma densities
(Peterson et al., 1993), while the neutral gas release consistently
reduced the vehicle negative potential from >1.5 kV to 
250
V (Berg et al., 1993). A plasma contactor in the payload was not
effective at grounding the vehicle because of low electron emission
thought to be due to a poisoned cathode.