The objective of this experiment was to contribute to a study of the chemical,
dynamic, and energetic processes that control the structure of the thermosphere by providing
direct, in situ measurements of both major and minor neutral atmospheric constituents having
masses in the range from 1 to 48 atomic mass units (u). A double-focusing Mattauch-Herzog
magnetic deflection mass spectrometer with an impact ion source was flown. Two ion collectors were
included to measure ions differing in mass by a factor of 8, i.e., the two mass ranges covered were
1 to 6 u and 6 to 48 u. In the ion source the neutral species were ionized by means of electron
impact. At altitudes greater than 380 km, ion currents were measured with an electron multiplier
counting individual ions. Counts were accumulated for 1/20 s before automatically switching to a
different mass number. While complete mass spectra could be swept, in the common mode of operation
peak stepping was employed, with readings on the principal peaks in the mass spectrum being repeated
approximately every 0.5 s and on other species less frequently. Data below 380 km were measured using
an electrometer. In addition to the peak stepping mode, there were several other operating modes which
were selected by ground command. In the fly-through mode, the ion source voltages were adjusted so
that there was no electric field to draw ions out of the electron beam when they were formed.
Ambient particles striking the ion source retained energies less than 0.1 eV, which was not high
enough to overcome the negative space charge potential holding the ions in the beam. Those
ambient particles that did not strike the ion source retained their incoming energy of several
eV after ionization and escaped into the accelerating region of the analyzer. The electron accelerating
potential was 75 eV in normal mode operation and was 25 eV in the fly-through mode. In another
operating mode, the instrument switched automatically to a sequence of masses of particular interest
such as, e.g., between masses 16 and 32 or between masses 28 and 32. More details can be found in
A. O. Nier et al., Radio Sci., v. 8, n. 4, p. 271, 1973. NSSDC has all the useful data that exist
from this investigation.