Astronaut Scientific Debriefings, 1962-1963

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115
Meteor ionization in the E-region
the micrometeorite and the subsequent ionization by the freed particle_ Experi­
mental information on collisions of neutral particles and the resulting excitation
and ionization is very limited. Much of the available information is contained in
MASSEY and BURHOP (1952).
Although the ionization efficiency is much greater for electrons than for heavy
ions, as long as the ion energy is greater than the threshold value for ionization,
the possibility of ionization exists. For example. a recent technique for obtaining
velocities for neutral particles near the range of meteor velocities is the shock-tube
method of RESLER et al. (1952). It was found that the extent of ionization as a .
function of Mach number was rather large. In the case of argon at Mach 18,
argon at 1 cm of
g pressu,re was 50 per cent ionized. Mach 18 corresponds to a
linear velocity of 18 X 0·350 = 6 km/sec, somewhat less than meteor velocities.
For this low velo ·ty there also was a highly luminous region associated with the
shock fronts in b h argon and air.
BERRY et al. (1942) investigated the ionization of gases by collisions of their
own accelerated
olecules. They found that the onset energies observed for such
ionization were roughly only three times the ionization potentials of the atoms.
They also concluded that in the range of speeds for argon between 48 e V to 1,000 e V,
relatively little change had occurred in the kinetic energy delivered to the newly
formed argon ion, and therefore it seemed unlikely that the mechanism of ionization
was one involving a transfer of kinetic energy. Also at energies of about 2,000 eV
the cross-section for ionization of argon was greater than for N 2 by only a factor
of 2. In fact the arrangement of the cross-sections for ionization in decreasing
order was found to be A, N 2, H 2, and He.
An estimate of the order of magnitude may be determined (for a few of the
reactions) from MASSEY and BuRHOP. Although most experiments have been
made with positive ions, it seems that the cross-section for ionization in the case
of neutral atoms is at least the same order, but usually somewhat higher.
The secondary emission coefficient for surfaces, y, is the :q.umber of ejected
electrons per incident positive ion. Some ob ervers have found a higher value of
y for surfaces which oxidize readily, implying that higher values arise from the
oxide layer. PAE'.1.'0W and WALCHER {1938) reason tha.t since the electron emission
cannot depend much on the work function of the adsorbed atoms, in th case of ·a
monolayer of ox~gen on caesium, it would follow that the extra electron emission
came from the bsorbed layer itself. The value of y for the low-energy range in
the case of mi rometeorites and based on positive ion bombardment would
probably be in
e range from 0·02 to 0·5. Allowing for an energy absorption of
10 e V for a secondary electron, these coefficients are in themselves sufficient to
give a ratio ofkinetic energy absorbed in ionization for 200 eV of better than 10 3 : 1.
Negative ions have been found to result from the impact of positive ions on
surfaces. ARNOT and MILLIGAN (1936) have estimated that for incident Hg+ ions
of about 200 e V energy, about 10- 3 Hg- ions were formed per incident ion. For
neutral oxygen atoms and molecules which also form negative ions, this ratio
could very well be higher. Positive ions incident; on surfaces may be reflected
without neutralization, although it is generally felt that an ion on striking a wall
become, neutralized. For rare gas ions incident on nickel, the reflection coefficient