Abnormal Ionization in the Lower Ionosphere Associated with Cosmic-Ray Flux Enhancements

Abnormal ionization in the lower ionosphere associated with cosmic-ray flux enhancements is discussed mostly in terms of the great solar event of February 23, 1956. Two kinds of abnormality were recognized: early effects observable in the dark hemisphere at the time of the sudden cosmic-ray enhancement, and late effects which began gradually and reached a maximum a few hours after the cosmic-ray enhancement. The late effects died away over a period of several days in geomagnetic latitudes above 70°, but more rapidly in lower latitudes. Both effects were more intense in higher geomagnetic latitudes. Early effects, though much the less intense of the two, appear to have been observable in lower geomagnetic latitudes than the late effects. The early effects are explained as a consequence of a plausible difference in composition between streams of solar particles of cosmicray energies, and ordinary cosmic rays. The late effects are explained in terms of ionization produced in the lower ionosphere (the range of height from 30 to 110 km is studied) by the passage or stopping of solar particles-mostly protons. Such particles are shown, like the more energetic solar cosmic rays, to be arriving essentially isotropically after the maximum effects occur. A plausible extension of the observed magnetic-rigidity spectrum of the solar cosmic rays downward in rigidity by about a power of 10 would provide the protons necessary to account for the observations. The absence of auroral and significant magnetic activity accompanying the abnormal ionization is consistent with the explanations offered.