The Response of High Altitude lonization Chambers during the 19541965 Solar Cycle

The response of an integrating ionization chamber at 10 g/cm •' depth in the atmosphere to particles of various rigidities is evaluated by using the change of ionization with latitude. This procedure yields the differential response curves at solar minimum and solar maximum and also the mean rigidity of response at any given latitude. For high latitude and Minneapolis, the mean responses are 2.5 bv and 3.2 by, respectively, at solar minimum and 3.6 and 3.8 at solar maximum. The solar cycle effect at 10 g/cm = is evaluated using 250 balloon flights at Minneapolis and at high latitude. Total ionization minimum lags about 18 months behind sunspot maximum. The high altitude ionization when compared with neutron monitors forms a single correlation curve over the solar cycle with significant deviations occurring only for a few months in 1957. We conclude that the relative rigidity effects between 3.6 and 15 by are very similar during the decreasing and increasing phases of the solar cycle. Comparison of data from ionization chambers of Pioneer 5 and Mariner 2 with data from detectors on earth shows no definite gradient effects. The larger intensity changes show a correlation between earth and deep space, but real fluctuations of smaller amplitude are frequently not correlated. Introduction. Integrating-type ionization chambers have long been used for studying various aspects of energetic particles and other radiation both in the high atmosphere on balloons and, more recently, in free space on satellites and space probes [Winckler, 1960; Neher and Anderson, 1962; Arnoldy et al., 1964]. If the general nature of the radiation is known, then this instrument can give the mean energy by measuring the loss rate by ionization integrated over the incident spectrum. This information has been of use in the study of aurora.1 and solar X rays and solar cosmic ray protons at high altitudes and in space [Winckler, 1962; Mosley et al., 1962; Winckler, 1963]. If the ionization chamber is supplemented by a Geiger counter having similar stopping power for the radiation under investigation, then the average ionization per count is a significant, quantity for estimating the mean energy of the radiation and, in some cases, its nature [Ho[mann and Winckler, 1963]. Because such ionization chambers are capable of long-term accurate calibration and standardization, they are useful for studies of primary cosmic ray time variations • NASA Argentine Space Research Committee Fellow on leave from the University of Tucuman. such as the solar cycle and Forbush-type modulations. Also, attempts have been made to study the space gradient of primary intensity in the solar system near the earth's orbit using ionization chambers on space probes [Neher and Anderson., 1964; Arnoldy et al., 1964]. Since many months of flight data are required, reliability of ion chambers is a great. asset here. If the ionization chamber is exposed to a complex radiation environment, a detailed interpretation usually becomes impossible. For example, attempts to determine the electron flux in some parts of the Van Allen radiation belts have failed because of the mixture of electrons, X rays, and protons [Arnoldy et al., 1962]. Another case is that of an ionization chamber on a balloon at high altitude responding to galactic cosmic rays. The response is due partly to direct primaries but also to secondary particles produced in the atmospheric transition of the primaries, and possibly to re-entrant albedo particles. Attempts to unravel these. effects and obtain any detailed information about the primary particles from a single total ionization rate measured at high altitudes from a balloon seem useless. However, if high altitude ionization measurements are made over a range of geomagnetic