Secular changes in the cosmic ray equator

Abstract

Since the cutoff rigidity at the cosmic ray equator varies with longitude, the location of the minimum cosmic ray intensity obtained during a latitude survey depends on the direction by which the geographic equator is crossed. A method is developed to compare the location of the recorded minimum cosmic ray intensity with that of the predicted minimum along a route as calculated from cutoff rigidities. The data of several surveys, covering the period 1954–1986/1987, are used. Instead of the accurate but computer intensive trajectory‐tracing method, cutoff rigidities for every location and instant are interpolated from 5°×15° world grids. It is found that the positions of the minima in cosmic ray intensities obtained from a survey across the equator and those derived from the interpolated cutoff rigidities along the survey route are mutually consistent. The rms difference between the experimental and calculated positions of the cosmic ray minima for all the surveys amounts to only 0.32°. Major secular changes in cutoff rigidities occur in the Atlantic Ocean region. Whereas a fast northerly shift of the cosmic ray equator is evident between 320° and 345° longitude, the data points from different epochs within this longitude range show a negligible difference between the simulated and experimental positions of the route‐dependent cosmic ray equator. This indicates that the secular variation in cutoff rigidity is well accounted for by the grid values derived from the International Geomagnetic Reference Fields for 1955.0, 1965.0, and 1980.0, even when the values are extrapolated linearly beyond 1980.0 through to 1987 in equatorial regions.