Mission Overview

The objectives of this satellite were to perform solar physics experiments above the atmosphere during a complete solar cycle and to map the entire celestial sphere for direction and intensity of UV light, X-ray, and gamma radiation. The OSO 4 platform consisted of a sail section, which pointed two experiments continuously toward the sun, and a wheel section, which spun about an axis perpendicular to the pointing direction of the sail and carried seven experiments. Attitude adjustment was performed by gas jets and a magnetic torquing coil. A pointing control system permitted the pointed experiments to scan the region of the sun in a 40- by 40-arc-min raster pattern. Data were simultaneously recorded on tape and transmitted by PCM/PM telemetry. A command system provided for 140 ground-based commands. The spacecraft performed normally until the second tape recorder failed in May 1968. The spacecraft, which was put in standby condition in November 1969, would be turned on only for recording special events in real time. Such an event occurred on March 7, 1970, when OSO 4 recorded data during the solar eclipse. For more information, see A. W. L. Ball, Spaceflight, v. 12, p. 244, 1970.

Launch Date: 1967-10-18 at 16:04:00 UTC
Launch Vehicle: Delta
Launch Site: Cape Canaveral, United States
Decay Date: 1982-06-15
Mass: 605.0 kg

Type: Orbiter
Central Body: Earth
Epoch start: 1967-10-18 16:04:00 UTC

Orbital Parameters
Periapsis: 546 km
Apoapsis: 466 km
Period: 93.6 minutes
Inclination 32.9°
Eccentricity 0.00227

Instrumentation

Broadband Solar X-Ray Emission Measurement

Study soft solar x-ray region, 1.2-3.6A, 3-9A, 6-18A (proportional counters and 8-channel differential analyzers) and 44-55A and 44-70A (Geiger counters with gas tight windows). NHB 8030.2B. The energy scale accuracy was plus or minus 2 percent, and the absolute photon flux should be accurate to plus or minus 15 percent.

Cosmic X-Ray Measurements

This experiment was designed (1) to make a survey of the directional intensity of nonsolar cosmic x-rays, (2) to make a thorough survey of their spectral composition between 0.1 and 10 A, (3) to distinguish between the stellar and the synchrontron components, (4) to correlate regions of strong intensity with optical and radio objects of special interest, and (5) to study auroral x-rays. For the 0.1 - 1.0 A range, a thin CSI crystal and two photomultipliers were used; and for the 1.0 - 10 A range, SRF2 on top of a plastic scintillator. The experiment failed soon after launch, and no data from it exist.

Solar Helium II Resonance Emission

Study the solar he2 line (303.8A) to determine the absolute value of the quiet sun component of the solar flux and the short-term enhancement due to localized solar activity. Will use a photomultiplier with 30,000 line/inch grating at grazing incidence and a 2 degree angular field of view. NHB 8030.2B

Proton Electron Detector

This experiment was designed to investigate the energy spectra and angular distributions of protons and electrons in the earth's magnetosphere. The instrument consisted of a single scintillator-photomultiplier assembly having a look direction normal to the satellite spin axis. Particle identification was accomplished by pulse-shape discrimination. Spin-integrated differential proton spectra in eight intervals between 1.73 and 36.7 MeV and differential electron spectra in eight intervals between 80 keV and 5 MeV were obtained by pulse-height discrimination. Energy-integrated angular distributions were obtained in 16 intervals of 22.5 deg each. Eight data registers and subcommutation techniques were used in the transmission of one full set of data every 15.36 s. The instrument provided good data from launch to December 1968. However, only real-time data were obtained after May 12, 1968, when the onboard tape recorder failed.

Geocoronal Lyman Alpha Telescope

This experiment was designed to observe the hydrogen lyman-alpha radiation (1040 to 1350 A) scattered from the hydrogen geocorona and the celestial background and to determine the ratio of sky glow to albedo, as well as the flux variation from horizon through the antisolar point at all hours of local time. The precession of the satellite's orbit allowed the determination of the latitudinal variation of lyman-alpha flux. The lyman-alpha telescope consisted of a collimator, an electrometer, a filter window, and an ion chamber. The ion chamber was filled with nitric oxide gas to provide a long wave cutoff of 1350 A, while the shortwave cutoff of 1040 A was provided by the LIF filter window. The telescope, mounted on the rotating wheel section of the OSO 4 spacecraft, scanned the sky from a circular orbit of approximately 560-km altitude and at an elevation of 18 deg above the satellite wheel. Because the wheel rotated in a plane intersecting the sun, the detector could not view the sun. It scanned down across the earth, observing radiation from near-earth hydrogen. As the wheel continued to rotate, the detector scanned the sky and measured the emission rate from high-altitude hydrogen. Scans near zenith and nadir were obtained when the sun was in the orbital plane. Observations from this experiment combined with similar measurements from OGO 3 and 4 (after correction for the celestial background) yielded typical nadir-zenith intensity ratios of 0.84 and 1.20 for solar zenith angles of 180 deg and 10 deg, respectively, in the autumn 1967 epoch. Scattering calculations indicated that the role of high-altitude hydrogen as a medium of radiation transport through the geocorona is much more importnat than previously thought. The epxeriment produced data unitl July 15, 1971, when it became inoperable.

Solar X-Ray Detectors

Solar x-rays in the wavelength bands 8-16 A, 2-8 A, 0.5-3 A, and 0.1-1.6 A were monitored by x-ray ion chamber photometers. This experiment had some electronic problems that made the data unusable at times of flares. The range-changing electrometers were not automatic, and commands could not be sent to make proper on-scale readings of x-ray flux.

Solar EUV Spectrometer

The objective of the experiment was to map solar EUV radiation intensities in the 300- to 1400-A region. A scanning spectrometer was used in two modes of operation. In the wavelength scan mode of operation, the instrument was pointed toward the center of the solar disk, and the spectrum from 300 to 1400 A for an area 1 sq arc-min was obtained. One complete scan required 31.5 min and consisted of approximately 11,000 discrete 0.1-A steps of the ruled grating. A visible-light, zero-order detector was used to indicate one particular position in the wavelength scan. In addition, a mechanical microswitch operating directly off the grating case provided a redundant wavelength reference indicator. Counts were recorded for 80 ms as a function of step number following the optical or mechanical reference position. In the raster mode, the grating was positioned at a selected wavelength to an accuracy of 0.5 A, and the pointed section of the spacecraft was commanded to make repeated raster scans. Each scan required about 5 min, and the count rate from a 1.0-sq arc-min field was recorded in a 40- by 48-element matrix. The complete matrix covered a 36.5 sq arc-min area in the center of the solar disk. The instrument provided a spectral resolution of approximately 1.6 A. The experiment started operating on October 25, 1967, and produced more than 100 wavelength scans and over 4000 spectroheliograms (raster scans) in 52 wavelengths. A failure in the high voltage power supply occurred during orbit 637 on November 29, 1967, and the experiment was turned off during orbit 646 on November 30, 1967. The sensitivity of the instrument varied with time and was wavelength dependent. Forty-five wavelengths distributed throughout the entire spectral range were used to follow the time-dependent changes. For additional information, see E. M. Reeves and W. H. Parkinson, App. Opt., v. 9, p. 1201, 1970; E. M. Reeves and W. H. Parkinson, Ap. J. Supp., v. 181, p. 1, 1970.

Solar X-Ray Telescope

This experiment was designed to obtain X-ray spectroheliograms of good spatial resolution (1 arc-min) in four wavelength bands (3 to 13 A, 3 to 21 A, 3 to 20 A, and 44 to 70 A) over periods of solar quiescence and solar activity. The instrument consisted of a two-mirror, image-forming telescope with a two-position aperture wheel and a four-position filter wheel. The detector consisted of a photocathode from which photoelectrons were focused and accelerated by an electrostatic lens onto an anthracene crystal scintillation detector. A complete raster scan, producing a 48- by 40-word array of count-rate values, was performed every 307 s. Except for slight degradation of data caused by internal noise, the experiment performed well during the life of the onboard tape recorders.

X-Ray Spectrometer

This experiment was designed to investigate the X-ray spectra of solar flares using Bragg crystal spectrometers that measured line and continuum emission spectra in the 1- to 8-A region. The measurements permitted a distinction to be made between emissions from a thermally excited coronal plasma (thermal process) and the emissions produced by fast electrons flowing into a relatively cool corona (nonthermal process). This distinction was of great importance in determining the mechanism underlying solar flare X-ray emission phenomena. The instrumentation, mounted in the stable pointed section of the OSO 4 spacecraft, was directed toward the sun and was arranged to scan the wavelength bands 0.63 to 3.83 A and 1.38 to 8.38 A simultaneously, once every 2 min. This instrumentation consisted of two separate Bragg crystal spectrometers that were positioned one above the other on a common axis of rotation and were driven by the same motor. The detectors in each spectrometer had mica windows (1.5 mg/sq cm) and were filled with argon gas (2.6 mg/sq cm). A filter composed of Mylar (0.00062-cm thick) and coated with a thin film of aluminum (1000 A on each side) was placed over the entrance aperture to reduce solar heating inside the instrument enclosure and to reflect any UV radiation to which the detector might respond. The reflecting crystals were made of LiF (lithium fluoride) (0.63- to 3.83-A band) and EDDT (ethylenediamine-d-tartrate) (1.38- to 3.38-A band). The experiment produced good quality data until December 7, 1971, when it was placed in an operational off mode. For more information, see J. F. Meekins et al., Science, v. 162, p. 981, 1968.

Science

Summary