Mariner II carried two experiments designed to measure the charged-particle radiation in space, including galactic cosmic rays and streams of high-energy particles which are released intermittently from the sun. Virtually continuous measurements of the particle fluxes in space were made by the instruments throughout the 109-day journey to Venus and during the passage near the planet on December 14, and additional data have been received for approximately 10 hours per day since that time.
One experiment, for observing the higher-energy particles (protons above 10 million electron volts (Mev) and electrons above 0.5 Mev in energy) was designed by Dr. H. R. Anderson of the Jet Propulsion Laboratory and Dr. H. V. Neher of the California Institute of Technology. Somewhat lower-energy particles (protons above 0.5 Mev or electrons above 0.04 Mev) are detected by the experiment of L. A. Frank and Dr. J. A. Van Allen of the State University of Iowa. Preliminary results of the two experiments were reported at the Stanford meeting by Dr. Anderson and Frank, respectively.
The instrumentation for the high-energy experiment consisted of a large spherical ionization chamber and two matched Geiger counters. The ionization chamber, which was invented by Dr. Neher, has been widely used by him and by other investigators for several years as a standard instrument for surveying the absolute intensity of the cosmic rays.
In addition to its use in almost countless balloon flights, airplane flights, and ground-based experiments, this type of chamber was also carried on the earth satellite Explorer VI and on this country's only previous successful interplanetary probe, Pioneer V. The two Geiger counters are matched to count the same kind of particles which are registered by the ionization chamber.
The detector for the lower-energy particles is a cigarette-sized Geiger counter, the Anton 213, which was used in several of the early Explorer and Pioneer satellites for investigating the Van Allen radiation belts around the earth and also in numerous more recent satellites.
These experiments have three principal scientific objectives, all of which were reported on at the Stanford meeting.
Objective 1: To detect, if possible, the presence of magnetically-trapped particle belts about Venus. For this purpose, the Anton 213 counter was the most sensitive indicator. At 20,000 miles from the earth it is known to have a counting rate of several thousand per second, but during the closest approach to Venus it detected an average count of only one particle per second, in agreement with the rate observed during most of the month of November.
The absence of additional particles near the planet was confirmed also by the other radiation detectors. Near the earth, the number of trapped particles observed decreases very sharply with distance near the boundary between the earth's magnetic field and the interplanetary field.
Thus the absence of particles near Venus indicates that the planet's magnetic field does not extend as far out as the trajectory of Mariner. This fact was confirmed by the magnetometer on board. The small intensity and extent of the field is believed to be explained by the very slow rate of rotation of the planet.
Objective 2: To measure the intensity of the galactic cosmic rays far away from the perturbing effect of any planet, and to look for variations in this intensity in different parts of the solar system. Years of earth-based research have shown that the flux of relatively low-energy galactic cosmic rays (5000 Mev and below) have a systematic variation with a period of about eleven years which is somehow connected with the solar activity cycle (sunspot cycle).
It is hoped that cosmic-ray measurements made simultaneously in widely separated parts of the solar system will elucidate the nature of the mechanism responsible for this variation. For this purpose, the ionization chamber is best suited. It measured a rate of ionization near 670 ion pairs per cubic centimeter per atmosphere of air. The value did not change significantly during the flight, and furthermore is in agreement with measurements in high-altitude balloons made last summer at Thule, Greenland, by Dr. Neher.
The Geiger counter on Mariner indicated a cosmic-ray flux of approximately 3.0 particles per square centimeter per second throughout the flight. The constancy of the cosmic-ray intensity over the very great distance traveled by Mariner is a new and significant piece of information, but its real meaning will not become clear until we have repeated the experiment several times on space vehicles going out away from the sun as well as in toward it.
Objective 3: To study the number and the nature of the high-energy changed particles emitted by the sun. (Another Mariner experiment investigated the very low-energy solar particles also.)
The presence of these particles is indicated by sudden increases above the cosmic-ray background reading of the various particle detectors. Some idea of their composition can be obtained from a comparison of the response of different detectors. The Mariner results were that high-energy solar particles, such as could be detected by the JPL-Caltech experiment, were generally absent except for a single event which began on October 23. The Iowa counter, on the other hand, detected not only this event but at least eight others, which must therefore have been produced by radiation or particles of very low penetrating power. Its exact nature is still in doubt at this time.
The nature of the solar-particle event of October 23 was described in detail by Dr. Anderson. A solar flare of a type which has frequently produced streams of charged particles was observed between 9:42 A.M. and 10:45 A.M., and the reading of the ionization chamber began to increase even before the flare had disappeared. Its reading rose rapidly from a background of 670 to a peak of above 18,000, underwent several oscillations, and remained above 10,000 for about six hours before declining gradually over the next few days. The flux of particles detected by the Geiger counters rose from a background of 3 to a peak of 16 particles per square centimeter per second. The fact that the ionization increased much more than did the number of particles indicates that the solar particles had much lower average energies than the galactic cosmic rays, and it is calculated that a typical energy in this event was about 25 Mev. The details of the time and energy variations will be further studied in the hope of learning more about how the particles were produced in the photosphere of the sun and how they may have been trapped in the magnetic fields around the sun before being released to the region where Mariner was waiting to detect them.
The problem of solar flares and their production of high-energy charged particles is a particularly important one for interplanetary space research because the very largest solar particle streams may contain particles in such numbers and of such high energies as to constitute a significant hazard to manned space missions. No such events have been observed by Mariner, however.
The total radiation dose seen by the ionization chamber in the October 23 event was only about 0.24 roentgen inside its 0.01-inch thick steel wall, and the radiation was so non-penetrating that a moderate increase in the wall thickness would have excluded the particles almost entirely. For comparison, the radiation dose recorded during the entire flight to Venus was about 3 roentgens, and much of this radiation was extremely penetrating.
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