Direct measurements of geomagnetic cutoffs for cosmic-ray particles in the latitude range 45° to 70° using balloons and satellites

Abstract

The quiet-time cutoff rigidities for cosmic-ray particles have been measured directly in 1963–65 at six locations in North America with geomagnetic latitudes (λ) between 45° and 70° using balloon-borne Cerenkov-scintillation counters. An identical counter with 0.1 of the area has been carried aloft in a polar-orbiting satellite from which it has been possible to determine the cutoffs for protons in four intervals between L = 3 and L = 5. The measured cutoffs are consistently below those expected on the basis of detailed orbit calculations based on the surface field of the earth (Shea and Smart 1967). These differences amount to a [Formula: see text] reduction at λ = 45°, increasing to [Formula: see text] at 60°. At 70° the cutoff is apparently <12% of the expected internal field value at 190 MV at 2100 hours local time.A limit to the amount of reduction produced by a ring current within the magnetosphere can be set from the data at latitudes <60°. The most reasonable value for the magnetic moment of this ring current is 0.08 Me. The situation at latitudes >60° is characterized by large diurnal changes in the cutoffs.