Impulsive emission of ∼40-kev electrons from the Sun

Using data from a system of three thin-window Geiger tubes and one thin silicon surface barrier detector on the Mars-bound spacecraft Mariner 4, we have observed three interplanetary particle events, on May 25–28, June 5–7, and June 13–14, 1965. The particles are identified conclusively as electrons of energy Ee ≳ 40 kev with steeply falling energy spectrums and with approximately isotropic angular distributions. The maximum unidirectional intensities are 80, 58, and 5 cm−2 sec−1 ster−1, respectively. The time profiles of intensity exhibit abrupt onsets, increases to maximum values in times of the order of several hours, and gradual declines over periods of the order of a day. It appears that these bursts of interplanetary electrons are emitted impulsively from the sun at approximately the same time as bursts of radio noise and X rays observed by others with terrestrial equipment. The interplanetary diffusion of electrons is discussed, and it is estimated that about 1034 electrons Ee > 40 kev were emitted on May 25 and on June 5 and about 1033 on June 13. The solar emission of nonthermal electrons has been suggested previously but not observed so clearly. Such electrons constitute a new tool for study of the structure of the interplanetary magnetic field and for discussion of the physics of solar flares.