In recent years, neutral releases have also been used specifically to study the CIV effect which has been postulated to occur when the component of the relative velocity between a gas and a background plasma which is perpendicular to the local magnetic field exceeds the speed corresponding to the kinetic energy of the gas molecules being equal to the ionization potential of the gas. This inequality can be satisfied at sounding rocket speed for heavy gases or at orbital speeds for lighter gases, or the gas can be ejected from a container at high speeds as in shaped charge experiments.
Several active experiments involving the release of gas have
occurred in the last four years. The Combined Release and
Radiation Effects Satellite (CRRES) released quantities of
vaporized barium, strontium and calcium into the magnetosphere at
geocentric distances of between 2.0 and 6.2 earth radii and also
into the upper atmosphere at 
450 km altitude at low latitudes
in 1991 (Huba et al., 1992a, 1992b). The CRRES studies have added
considerably to our knowledge of the interaction of plasma with the
space environment at high and low altitudes. The formation of
magnetic cavities has been observed at high altitudes, and also the
polarization field is sufficient for cross-field motion of the
plasma for a few seconds until the density drops to a low value and
the field weakens. Associated with the dynamical behavior of the
plasma a variety of instabilities have been observed both remotely
and in situ.
A more complex active experiment including both ground-based and sounding rocket experiments with a CRRES chemical release was devised for the El Coqui campaign staged in and over Puerto Rico in 1992. In this experiment the ionosphere was modified by a chemical release from CRRES, then a high power transmitter was used to heat the ionospheric electrons, and the resulting modifications to the ionosphere were monitored in situ by the rocket payloads, and remotely by the Arecibo incoherent scatter radar and optical sensors (Romero and Bernhardt, 1993). This study enabled direct measurements to be made of multi-ion expansion of plasma both parallel and perpendicular to the local geomagnetic field (Szuszczewicz et al., 1993).
Orbital experiments of gas releases were performed during the
first Atmospheric Laboratory for Applications and Science (ATLAS-1)
mission in 1992 (Torr and Sullivan; 1992; Torr, 1993). Experiments
on the CIV effect were made with neutral xenon releases from one of
the instruments associated with the Space Experiments and Particle
Accelerators (SEPAC) experiment carried on the ATLAS-1 payload
(Marshall et al., 1993). In this experiment the xenon was released
at a low velocity of 
30 m/s relative to the space shuttle,
and the fact that the orbital speed of 7.7 km/s exceeds the
threshold for critical ionization of 4.2 km/s was used to test the
CIV theory. A factor of 60 increase in plasma density was observed
by a Langmuir probe mounted in the payload bay supporting the
expectation that the released xenon should be partially ionized by
the CIV effect. However, there is still some doubt due to the
location of the plasma enhancement which modeling showed should
have been at least 12 m away from the xenon source, not in the
payload bay.
A sounding rocket program of two payloads called CRIT-1 and CRIT-2 (abbreviation of critical ionization), the latter flown in 1989, has been used to look for evidence of critical ionization. Vaporized barium was expelled from the payloads in flight by using a shaped charge which produced a velocity distribution with speeds up to 13 km/sec (Torbert et al., 1992). Particle, wave and ground-based optical measurements during the CRIT-2 flight showed results expected for critical ionization, and this was followed by ionization of the released barium up to yields of 1-2%. Although the CRIT launches were earlier than the reporting period, analysis and interpretation has been reported in the last four years (Stenbaek-Nielsen 1990; Swenson et al., 1990; Kelley et al., 1991, Brenning et al., 1991; Papadopoulos, 1992). While the indications point to an observation of critical ionization, there are still inconsistencies in the data which prevent a definite identification of the phenomenon to be made.
There has been one neutral beam experiment in the reporting
period. The Beam Experiments Aboard Rockets (BEAR) sounding rocket
experiment was launched from White Sands Missile Range in 1989.
The core of this experiment was an accelerator which produced a
neutral beam of hydrogen atoms with an energy of 
1 MeV and a
flux equivalent to a current of 10 mA. The beam was pulsed at 5
Hz, each pulse having a duration of 50 mS. In addition to the
accelerator, the payload contained diagnostics to image the beam,
monitor the electrical potential of the platform, and determine the
characteristics of the emitted beam.