All three Apollo 17 astronauts sketched a “lunar horizon glow,” seen from orbit above the Moon. It is shown that the shape of the glow is compatible with scattering of sunlight off of gas or dust at high altitudes above the Moon. Our mathematical modeling best simulates the glow with submicron dust grains whose spatial density varies with altitude above the Moon as exp(‐H/Hs), where Hs is in the range of 5 to 20 km. These dust grains are probably electrically charged and ejected above the lunar surface by local electric fields.
Abslract. A column of enhanced density plasma,exceeding the density expected from ionization by primary beam electrons, has been observed in a large vacuum system at low magnetic fields (1 to 1.5 G) and low ambient pressures (10 -6 to 10 -s torr). The peak luminosity of the discharge is about 10 times that of the beam alone, with a radius increase by a factor of 3. In the absence of the discharge, RF emission is observed at 1.1 to 1.2 fc-A strong band of RF noise with upper frequency cutoff at about fc is observed in the discharge mode, along with higher frequency noise at or near the plasma frequency.The onset of the plasma discharge is critically dependent on beam current.The present results agree with observations made at much higher densities and magnetic fields in fusion research studies. There has been difficulty in explaining many of the observational results obtained in rocket flights with modest and high current electron accelerators (Winckler, 1976, 1977). In particular, these include 1) the apparent neutralization of the vehicle under conditions where the return electron flux from the ambient plasma or the beam-produced ionization (based on the classical ionization rate of the local neutral gas) to the vehicle should have been much less than the emitted flux, and 2) the presence of a plasma cloud with increased Te and density surrounding the vehicle (Cartwright et el., 1977). We have recently concluded a series of electron beam experiments in the very large vacuum chamber at Johnson Space Center which can provide a plausible explanation for the flight observations. The experimental configuration is basically similar to that described by Bernstein et al. (1975, 1977) and is shown in fig. 1. The experimental conditions were as follows: 1. A tungsten cathode, convergent flow electron gun was used for most measurements; although the gun perveance is % 1.4x10 -6, the maximum beam currents and voltages employed were 100 ma and 2 kV respectively. The gun was operated DC. A pulsed electron gun was also operated for a short period of time. Although the guns could be isolated electrically, the present measurements were made with both the gun and collector grounded to the chamber walls. 2. A set of three coils have been added around the chamber periphery; the total variation in field strength along the beam path was % 15%. Most measurements were made at total mean field strengths ranging from 1.0-1.45 G. Typical beam injection pitch angles were <20 o . The path length between gun and collector was % 20 m. 3. The base pressure in the system was lx10 -6 torr, consisting primarily of water vapor (30%) and N 2. Increases in pressure to lx10 -s torr were accomplished with the addition of dry N 2.This pressure range corresponds to the altitude range 120-180 km; although rocket-borne accelerators have been flown at higher altitudes it is probable that the rockets are always surrounded by gas clouds of similar density which are produced by outgassing and residual motor exhaust. As shown in fig.
Measurements of high‐energy solar‐wind electrons have been made from a low orbit around the moon. Solar‐wind electrons can be identified up to energies of ∼3000 ev, at which an electron population of entirely different characteristics becomes dominant. The solar‐wind cavity on the moon's antisolar side shows evidence of being filled by plasma coming from the downstream direction. When the direction of the interplanetary field corresponds to solar ecliptic azimuth angles of about 90°, a partial solar‐wind cavity extends across most of the eastern sunlitside of the moon within ∼20° of the noon meridian. There are localized increases in the ∼500‐ev electron flux over much of the sunlit hemisphere. These increases are highly persistent and stable in their location over a 2½‐day period and hence are not due to intrinsic variations in the solar wind. They are usually associated with disturbances in the magnetic field. These increases are interpreted to be the result of an interaction between the solar wind and the moon that deflects some of the solar‐wind flow and results in limb shocks.
The plasma motor generator (PMG) experiment, launched June 26, 1993, was a tethered system of two identical plasma contactors connected via a 500‐m conducting tether. The experiment was designed to demonstrate the ability of plasma contactors to provide a low‐impedance connection between a spacecraft and the ionosphere for both the electron emission and collection. The flight data indicate that plasma contactors enhance electron collection and emission by both neutralizing the electron space charge and scattering electrons across the geomagnetic field lines. Up to a 0.3 A steady current flowed along the tether in a circuit completed through the ionosphere. An analytical model for plasma contactor interaction with a background plasma which incorporates electron scattering by plasma waves is compared with the flight data. Good agreement between the model and the data is achieved for an effective scattering frequency equal to one twentieth of the local plasma contactor plasma frequency.
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