Since 1937 balloon flights have been made in which instruments measuring cosmic rays were carried to high altitudes at high latitudes. The earlier flights involved only the years 1937, 1938, 1940, and 1946, but since 1951 only 1952 and 1953 have been missed. The instruments used were ionization chambers, which were compared with standards kept at Pasadena. Checks on absolute values of ionization at intervals indicate that experimental results over the 32‐year period can be closely compared. In addition to giving a summary of already published results, data from balloon flights at Thule for the years 1966 through 1969 are presented for the first time. In comparing cosmic rays at high altitudes during the 1930's with similar measurements in the 1960's, it is shown that the intensity of cosmic rays was at least 10% higher at 20 gm cm−2 during the earlier periods for similar solar activity. It is suggested that this long‐term modulation effect probably takes place where the solar wind encounters the interstellar medium and may be determined by the average solar activity of several preceding solar cycles.

During each of the two recent periods of special international coordination of scientific effort, the IGY (International Geophysical Year) and the IQSY (International Year of the Quiet Sun), we made a latitude survey of cosmic rays, using ionization chambers carried by balloons. All instruments were carefully compared with the same set of standards. In each case, simultaneous balloon flights were made from a high‐latitude base station. Such monitoring of the primary radiation allowed corrections to be made to the data from the mobile station. Ratios of the abundances of the different nuclei to that of protons in the primaries as found by other observers have been used to separate out the effect due only to protons. Two methods are used to find the differential spectrum of the protons for the different years. Above 1 bev, the areas under successive latitude difference curves are used. This method takes advantage of the earth's magnetic field as an analyzer. Below about 0.6 bev the ranges of protons in air and areas are used. The two methods give results that are consistent with each other. Where applicable the results are also in good agreement with the differential spectra of protons found by other observers using satellites and high‐altitude balloons. From the integral of the differential spectra so derived for the different years, it is found that the total number of primary protons increased by a factor of 3.1 between 1958 and 1965. From similar flights made during the previous solar minimum the change from 1954 to 1958 is found to be a factor of 4.2. This, as well as solar activity data, indicates that there was more residual modulation of the primary cosmic radiation during the 1965 minimum than was present in 1954.

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During the International Geophysical Year, two series of balloon flights were made. One of these sampled the ionization due to cosmic rays over the range of geomagnetic latitudes from 38°N to 87°N. The other covered the range from 13°N to 79°S. In each case similar balloon flights from a base station were made to make possible an allowance for changes in the primary radiation. The second series of nights is used in this paper to determine the location of the knee of the latitude curve at atmospheric depths from 10 to 220 g cm−2. The results indicate that at this time of the solar cycle the knee moved toward the pole with decreasing amount of air overhead, reaching a geomagnetic latitude of 55° at 10 g cm−2. This movement of the knee becomes very slow at the lower pressures, however, suggesting that even at much lower pressures, the knee would not move appreciably farther toward the pole.

Using a newly developed ionization chamber, which transmits its information by radio, simultaneous balloon flights were made from widely separated stations in the summer of 1951. Bismarck, North Dakota (geomagnetic latitude, X TO , 56°N) was used as a base station. Four flights were made from shipboard going north from Boston and five were made from Thule, Greenland (X m = 88°N), simultaneous with those at Bismarck. In all, 28 successful flights were made by the two expeditions. In seeking to determine the geomagnetic effects on the low energy primaries, considerable information was gathered on the radiation that fluctuates from day to day. The following are the chief experimental findings together with some of the conclusions that may be drawn.(1) The fluctuations in the primary radiation at 90 000 feet were as much as 10 percent in a few days. (2) These were simultaneous (except as noted in the text) and very close to the same amount at the two stations. (3) The magnitude of the fluctuations at high altitudes was considerably larger than the geomagnetic effect between Bismarck and Thule. (4) The radiation that fluctuated contained both high (>15 Bev/c) momentum and low (down to 1.5 Bev/c) momentum particles. (5) There was a good correlation between the fluctuations in the radiation at high altitudes and the fluctuations in the neutron and meson components at ground level. (6) The fact that no particles fluctuated at Thule that did not also fluctuate at Bismarck leads us to conclude that there are few, if any, low energy particles coming in at Thule that are not also present at Bismarck, otherwise they too would be expected to vary. (7) From the manner in which the fluctuating radiation is absorbed in the atmosphere, it is concluded that the fluctuations cannot be due to heavy primaries alone. Rather it appears that the particles that fluctuate are of the same nature as the other incoming particles but have somewhat less energy per particle. (8) Varying magnetic fields of the geomagnetic axially symmetrical type are discarded as being able to produce the kind of fluctuations observed. A more satisfactory mechanism appears to be varying electric fields. (9) There was a negative latitude effect in the total ionization at intermediate altitudes (30 000 to 50 000 feet) at high latitudes. This we attribute to the greater importance of ju-meson decay in the warmer air of the stratosphere which exists at the more northerly latitudes. The temperature coefficient arrived at is -0.19 percent °C*~1. (10) There was a positive latitude effect in the total ionization above 60 000 feet at high latitudes. Evidence is presented to show that this is not likely to be due either to atmospheric effects or to low energy particles admitted by the earth's magnetic field above 66°N. We attribute this increase to the shadow effect of the earth. (11) The absence of particles with momenta in the range 1.5 to 0.6 Bev/Zc (0.8 to 0.14 Bev for protons), shown by (a) a lack of increase of ionization at very high altitudes between geomagnetic la...

Measurements of the cosmic-ray intensity have now been made for most of the past solar cycle at balloon altitudes near the north geomagnetic pole. Such experiments have been conducted during the summer months, but these data are thought to represent a relatively true measure of cosmic rays in the vicinity of the earth for that particular phase of the solar cycle, since large solar flare activity was absent when these flights were made. It is found (1) that the energy density of cosmic rays in the vicinity of the earth was 40 per cent less in 1958-1959 than in 1954; (2) that the new particles present in 1954 that were not present in 1951 were almost entirely those absorbed in 200 g cm -• of the atmosphere; (3) that after the cosmic-ray maximum of 1954 the first particles to disappear wore very absorbable, having an absorption length of about 60 g cm-•; (4) that as the solar cycle progressed the particles removed became increasingly energetic, until those that disappeared between 1957 and 1958 were absorbed about like primaries of mean rigidity 9 bv; (5) that the particles now reappearing after the cosmic-ray minimum of 1958-1959 have close to the same rigidity as those removed before the cosmic-ray minimum was reached, when the intensity was approximately the same; (6) that, if the general solar activity can be measured by the Zurich sunspot numbers, there appears to be a 9-to 12-month lag of cosmic-ray change from being anticorrelated with solar activity; and (7) that sunspot numbers averaged over periods like a quarter of a year appear to be a better measure of solar activity as it relates to cosmic-ray intensity than similarly averaged planetary magnetic character figures.

Cosmic-ray electrons enter the atmosphere only in a definitely limited and relatively narrow energy range. The maximum of this energy distribution curve lies at about 6 billion e-volts. At 1 billion e-volts the curve has fallen to about a third of its maximum value, and on the other side of the maximum at some 17 billion e-volts the incoming energy also has about a third of its maximum value. The observed banded structure may possibly be partly due to the blocking effect of the sun's magnetic field on electrons trying to pass through it to the earth. The total cosmicray energy brought in by electrons of energy above 17 billion e-volts plus all that brought in by photons of all

The experiments herein described support and extend the relationship found by Forbush, namely, that there is an inverse relationship between solar activity and cosmic-ray intensity. During the summer of 1954 the sun was at a minimum of activity. Ionization chambers set up in the Arctic as well as at intermediate latitudes showed a considerable increase in intensity, during the period, compared with 1951.Atmospheric absorption combined with magnetic rigidity requirements indicate that protons with energies down to at least 150 Mev were coming to the earth in the summer of 1954. We conclude that there was no cutoff of primary particles, i.e. , no "knee, " at least for protons down to this energy. Using the increased areas under the ionization-depth curves, combined with geomagnetic calculations, an estimate may be made of the numbers of primary particles coming in near the poles in 1937 (solar maximum), 1951 and 1954 (solar minimum). These values are 0.10, 0.14, and 0.24 cm s sec ' sterad ', respectively. The Quctuations in 1954 were also small compared with those measured in 1937, 1938, and 1951.The evidence points to low intensity and large fluctuations of cosmic rays when the sun is active and vice versa. These changes may be understood qualitatively by assuming a modulating mechanism in the form of clouds ejected from the sun and varying with the solar cycle.