Cosmic ray electrons from 0.2 to 8 Mev: Pioneer 8 and 9 measurements of their spectrum, time variations, and interplanetary radial gradient

Abstract

Using data from the University of New Hampshire cosmic ray experiment on the Pioneer 8 and 9 space probes at radial distances from 0.75 to 1.10 AU from the sun, we have made a detailed examination of the spectrum, time variations, and interplanetary gradients of cosmic ray electrons from ~0.3 to 8 Mev. These data cover the time period from December 1967 to April 1969. Obvious solar flare electron increases have been removed from the data, so that the remaining data should represent quiet time electron behavior. We observe a strong and direct correlation between changes in electron intensity over the entire 0.3-to 8-Mev range on Pioneers 8 and 9 and those for 3-to 12-Mev electrons observed simultaneously on the Imp satellites. We find that, when an intensity change occurs, the electron spectrum simply moves bodily up and down, maintaining very nearly the same spectral exponent; in effect, the electron modulation is found to be essentially rigidity independent throughout the 0.3-to 8-Mev range. This behavior appears to be an average property of quiet time electron changes during the times we have studied them and extends as well to the quiet time electron increases that , we also Observe in direct coniunction with those reported from Imp. The average quiet time day to day changes in electron intensity show very little correlation with day to day changes in higher-energy nuclei, however, except during Førbush decreases when the observed modulation of electrons is found to be directly co.rrelated with, but ~2.5 X larger than the •60-Mev nuclei changes. The average electron spectrum we measure is d J/dE --170/E •'7 particles cm •' ster sec Mev between 0.2 and 8 Mev. The average quiet time interplanetary electron gradient is observed to be -]-25 ..+__ 20%/AU from Pioneer 8 data in 1968 and -[-3% -,-5'%/AU from Pioneer 9 data in 1969 over the radial range from 0.75 to 1.10 AU. The study of the low-energy cosmic ray electron component and its time variations is of particular interest in terms of the information it might yield on the transport of cosmic rays in the solar system and indeed on the origin of these electrons (e.g., galactic or solar). To study these electrons, detector systems carried on satellites and space probes outside the earth's atmosphere are neces•ry. At the present time the bulk of the information on cosmic ray electrons with E ( 10 Mev and their time variations comes from studies of the Goddard Cosmic Ray Group [Simnett and McDonald, 1969; McDonald et at., 1972] made on the Imp series of satellites. A spectrum of cosmic ray electrons from 1.5 to 9 .Mev has also been , reported by Cline and Porreca [1970]. At somewhat higher energies (10-100 Mev) the electron spectrum and its changes with time have been followed on both balloons and satellites, and the picture that has emerged is one in which the •eosmic ray electron variations closely parallel the variations of high-energy nuclei and the changes in neutron monitor intensity (e.g., P. Meyer, unpublished manuscript, 1971). These changes seem to be exp...

Implications of the reported low energy electron gradients

“…The recently reported observations of small positive gradients for these low energy electrons by Webber et al (1973a) further supports the contention of their galactic origin. These measurements were made by comparing the electron flux measured aboard the Pioneer 8 and 9 spaceprobes with that measured simultaneously aboard Imp 4 located near the Earth.…”

“…The fact that the electron flux measured during quiet time increases was included as a subset of the data used to determine the low energy gradients confirms the galactic origin for these quiet time increases as well as for the quiescent electron flux (Webber et al, 1973a). The fact that the electron flux measured during quiet time increases was included as a subset of the data used to determine the low energy gradients confirms the galactic origin for these quiet time increases as well as for the quiescent electron flux (Webber et al, 1973a).…”

“…In addition to the weak long term modulation of the low energy electron flux is the curious phenomena of quiet time increases first observed in the 3-12 MeV energy range by McDonald et al (1972) and also observed by Webber et al (1973a). During a quiet time increase the low energy electron spectrum maintains the same spectral exponent as during quiet periods but increases in magnitude by a factor of 3-5 over a period of a few days.…”

“…It should be emphasized however that on a day-to-day basis a virtual complete lack of correlation exists between the low energy electron flux and the higher energy nuclei flux (Webber et al, 1973a) except during times of Forbush decreases. Careful consideration of quiescent time periods yields a long term decrease of the low energy electron intensity of about a factor of 2 from solar minimum to solar maximum in correlation with the long term variation observed for the higher energy nuclei .…”

Propagation of cosmic rays in the galaxy

“…Recently these measurements have been extended down to energies about a few tens of keV (Lin, et al, 1972;Webber, et al, 1973d;Hurford, et al, 1973a) and from the observed temporal variations, electrons above 100 keV are considered to be of galactic origin. One notices from the inset of .18), one is inclined to consider these low energy electrons to be of secondary origin; the present situation therefore doesn't warrant any new mechanism to be invoked (Ramaty, et al, 1972) to understand the observations.…”