Mass-Energy Spectrometer for Space Plasmas

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Abstract
A satellite‐borne instrument for the study of the interplanetary medium is described in detail. This plasma experiment illustrates a whole class of such measurements that can now be done on small unmanned scientific satellites. Plasma ions are resolved in both energy and mass per unit charge in order to determine hydrogen‐helium abundance ratio, ion temperatures in the ``solar wind'' and transition region, and the angular distributions and fluctuations of ion flux. Protons and alpha particles with energy per unit charge between 300 and 5000 eV are resolved to better than 5 in 103 and their energy spectrum determined. Instead of current measurements, we employ single ion counting with an efficiency of 75% at rates up to 3×107 sec−1. A histogram of counts against angular rotation is formed with a quantization of 23° and its area, variance and the azimuthal position of the peak derived by a small computer for transmission to the earth over a data link with a capacity of approximately 6 bits sec−1. Provision is made for a preliminary study of variations in the ion flux with periods greater than a few seconds. Two companion papers present the details of developments which proved necessary for this experiment and are of wider interest as well.

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