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
is 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 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 experiment 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.