High Altitude Neutron Intensity Diurnal Variation

Two balloon flights in which boron-trifluoride neutron counters were carried aloft were launched from Brownwood, Texas, during September, 1958. The flights attained altitudes of 86 000 and 79 000 feet at a conventional geomagnetic latitude of 41°N. They showed that the slow-neutron intensities in the atmosphere had decreased by about 12% since the time of minimum solar activity in 1954. They also show that this decrease was mainly in the low-energy end of the spectrum, as the mean free path for absorption had increased from 180±25 g/${\mathrm{cm}}^2$ to 240±30 g/${\mathrm{cm}}^2$. A high-altitude decrease apparently associated with the geomagnetic storm of September 25, 1958 was also detected. After achieving altitude, the balloons floated at a constant elevation through sunset. A sharp peak in the intensity which occurs just before sunset at balloon altitudes was detected on both flights. The origin of this phenomenon, which results in a doubling of the intensity for about 25 minutes, is unexplained, although some possible mechanisms are discussed.