Discussion of a paper by D. J. Hofmann and J. R. Winckler, ‘Simultaneous balloon observations at Fort Churchill and Minneapolis during the solar cosmic ray events of July 1961’

Hofmann and Winckler [1963] have presented data from several high-latitude balloon flights made during the solar cosmic ray events of July 18 and 20, 1961. It is the purpose of this note to call attention to several questionable interpretations of the data. Particularly, we shall discuss (a) the power law decay, and (b) the suitability of the diffusion model. Power law decay. In their Figure 8b the authors present, on log-log coordinates, counter excess counts per second versus time from 0930 UT, July 18 to test the suitability of a power law decay, I(P > P0 = I0(P > P0)t−χ with t defined zero at the instant of generation of particles (solar X-ray burst, flare ‘flash phase’ or centimeter-wave radio burst might be used alternatively). Initially the exponent (χ) of the July 18 event is 2.1, but it steepens after 30 hours and is 5.8 just before the July 20 event for which the indicated decay is χ = 11.5. The decay law exponent deduced for the July 20 event is, however, in error. Each event must be considered separately and a unique starting time assigned; the starting time for the July 18 event cannot be employed for the July 20 event as well! This has resulted in an error of about 54 hours in the starting time of the second cosmic ray increase and a consequent steepening of the decay rate. A reasonable starting time for the second event is 1545 UT, July 20. It is not possible to scale data from Hofmann and Winckler's Figure 8a; in order to redetermine the power law exponent because of the errors inherent in scaling from a small reproduction. It is hoped that they will report on a corrected value for the decay rate. A recent study of 7 ground-level and satellite cosmic ray increases [Bennett, 1964] resulted in a mean power law decay exponent of 2.03 with a variance of 0.012.