Solar flare effects in the ionosphere

Substantial increases in columnar electron content of the ionosphere were observed at four or at five stations, simultaneously with the onset of solar flares on May 21 and 23, 1967. The observations are most readily explained by a large, but brief, enhancement of the solar EUV flux on two occasions. An explanation based on X-ray enhancement only does not appear attractive. Time-correlated values of visual brightness at the center of the Ha line, radio noise, radio wave absorption, and several other phenomena are shown.