Explorer 33

The Ames magnetometer experiment consisted of a boom-mounted triaxial fluxgate magnetometer and an electronics package. The sensors were orthogonally mounted with one sensor oriented along the spin axis of the spacecraft. A motor interchanged a sensor in the spin plane with the sensor along the spin axis every 24 hours, allowing inflight zero-level determination. The instrument package included a circuit for spin-demodulating the outputs from the sensors in the spin plane. The noise threshold was about 0.2 nT. The instrument had three ranges covering ± 20, 60, and 200 nT full scale for each vector component. The digitization accuracy for each range was 1% of the entire range covered. The magnetic field vector was measured instantaneously, and the instrument range was changed after each measurement. A period of 2.05-seconds elapsed between adjacent measurements and 6.14-seconds between measurements using the same range.[4]

Three EON type 6213 Geiger–Müller tubes (GM1, GM2, and GM3) and a silicon solid-state detector (SSD) provided measurements of solar X rays (Geiger–Müller (GM) tubes only, between 2 and 12 A) and of solar, galactic, and magnetospheric charged particles. The Geiger–Müller tubes measured electrons of energies greater than 45 to 50 keV and protons of energies greater than 730 to 830 keV. The SSD output was discriminated at four thresholds: (1) PN1, which detected protons between 0.31 and 10 MeV and alphas between 0.59 and 225 MeV, (2) PN2, which detected protons between 0.50 and 4 MeV and alphas between 0.78 and 98 MeV, (3) PN3, which detected protons between 0.82 and 1.9 MeV and alphas between 1.13 and 46 MeV, and (4) PN4, which detected alphas between 2.1 and 17 MeV. GM1 and the SSD were oriented parallel to the spin axis, and GM3 was oriented antiparallel to the spin axis. Data from GM1 and PN1 were divided into data from quadrants oriented with respect to the Sun (sectors I, II, III, and IV were centered 180°, 270°, 0° and 90° from the Sun, respectively). Data were read out in either 82-seconds or 164-seconds intervals. High temperatures adversely affected the SSD particle data during the periods from 16 September to 14 January and from 16 March to 14 July of each year following 16 September 1966. However, the alpha particle data are believed to be unaffected. On rare occasions (less than 10), a GM tube would produce a high, spurious count rate for a period of several hours. This effect apparently was produced only during periods of extremely high particle and X-ray fluxes. Accumulator failures occurred on 21 July 1967 and 24 September 1967.[5]

The instrumentation for this experiment consisted of a boom-mounted triaxial fluxgate magnetometer. Each of the three sensors had a range of ± 64 nT and a digitization resolution of ± 0.25 nT. Zero-level drift was checked by periodic reorientation of the sensors. Spacecraft fields at the sensors were not greater than the digitization uncertainty. One vector measurement was obtained each 5.12-seconds. The bandpass of the magnetometer was 0 to 5 Hz, with a 20-dB per decade decrease for higher frequencies. The detector functioned well between launch and 10 October 1968, when the DC power converter failed. No useful data were obtained after that date.[6]

This experiment consisted of a 10.2 cm (4.0 in), Neher-type ionization chamber and two Lionel type 205 HT Geiger–Müller tubes (GM). The ion chamber responded omnidirectionally to electrons above 0.7 MeV and protons above 12 MeV. Both GM tubes were mounted perpendicular to the spacecraft spin axis. GM tube A detected electrons above 45 keV which were scattered off a gold foil. The acceptance cone for these electrons had a full-angle of 61° and axis of symmetry which was perpendicular to the spacecraft spin axis. GM tube B responded to electrons and protons above 22 and 300 keV, respectively, in an acceptance cone of 45° full-angle with axis of symmetry perpendicular to the spacecraft spin axis. Both GM tubes responded omnidirectionally to electrons and protons of energies above 2.5 and 35 MeV, respectively. Pulses from the ion chamber and counts from each GM tube were accumulated for 39.72-seconds and read out every 40.96-seconds. The time between the first two ion-chamber pulses in an accumulation period was also telemetered. The ion chamber operated normally from launch through 2 September 1966. From 2 September 1966, the ion chamber operated at a lower threshold voltage.[7]

A wide-aperture, multi-grid potential analyzer was used to observe the intensity of the electron and ion components of the low-energy plasma in interplanetary space and near Earth. Integral spectra were obtained for both ions and electrons in the energy ranges from 0 to 45 eV (15 steps) and 0 to 15 eV (15 steps). Complete spectra for protons and electrons were obtained every 80-seconds. The experiment operated until 29 June 1967.[8]

A split-collector Faraday cup mounted on the spacecraft equator was used to study the directional intensity of solar wind ions and electrons. The following 25-seconds sequence was executed three times for ions and once for electrons each 328-seconds. Twenty-seven directional current samples from the two collectors were taken in the energy per charge (E/Q) window from 80 to 2850 eV. The currents in the two collectors were then sampled in eight E/Q windows between 50 and 5400 eV at the azimuth at which peak current appeared in the previous 27 measurements. Due to telemetry limitations, only the following data were returned to Earth every 328-seconds: for ions, the sums of currents measured on the two collector plates twice and the difference once, and for electrons, the sums once. The experiment worked well from launch until the final spacecraft data transmission (21 September 1971).[9]