During periods in 1969-1971 when interplanetary flow conditions were quiet, 17 determinations of the solar wind Fe and Si abundances and 7 of the O abundance were made. On the average (N(Fe)/N(H))= 5.3 • 10 5, (N(Si)/N(H))=7.6 • 10 -5 and (N(O)/N(H)) = 5.2 • 10 -4. Variations from the averages over a total range of factors of ~4 for O, ~4 for Si, and ~ 10 for Fe have have observed. Although Fe and Si abundance variations appear to be correlated, no other element correlation pairs are unambiguously discernible from the present data set.

A study of the lower corona thermal properties was made using the best examples of solar wind heavy ion spectra obtained with Vela 5 and 6 plasma analyzers at times of quiet solar wind (low speed, low temperature). The multiple Si and Fe ion species peaks in the spectra were fit with solutions of the ionization equilibrium equations to determine 'freezing in' temperatures for the various species over a range of heliocentric distances r. Assuming a power law electron temperature model, T= = T| (Re/r)% spherical symmetry, and mass conservation, the following results for the quiet corona were obtained: (1) The average freezing in temperatures ranged near 1.5 x 106 K at r_ ~ 2.4 to 3.9 R e.(2) Values for T e ranged between 1.7 x 106 K and 2.5 • 106 K with an average of ]l .84 :[: 0.13 x 106 K.(3) The temperature gradient parameter ~ lay between 0.20 and 0.41 with an average value of 0.29 ~: :L0.06. This is consistent with the predicted value a = 2!7 derived from conduction dominated spherically symmetric models of the corona. (4) The 0 and N lines which freeze in at a distance of r-~l.5 R e indicate temperatures of ~ 2.l x 166 K. Temperatures higher at 1.5 R e than at 3 R e, in agreement with extrapolations of the power law model, suggest that coronal heating in regions of open field geometry is not important beyond r -~ 1.5 R e.

An upper limit for the steady state solar wind He+ concentration is estimated from four Vela 5 and 6 heavy ion spectrums. The upper limit value for N(He+)/N(H) of 2.5 × 10−6 so obtained is at least an order of magnitude less than that predicted from presently accepted models of the local interstellar medium. The most likely explanation of the discrepancy is that wave‐particle interactions are not capable of fully assimilating the initially non‐Maxwellian high‐temperature He+ particles of interstellar origin into the solar wind flow. Occasional flow conditions resulting in complete assimilation cannot be ruled out.

The production of knock-on electrons and of electron pairs by muons of mean energy >50 BeV has been measured from cloud-chamber photographs of soft showers produced in lead plates. The cloud chamber was operated at a depth of 8.42 X10 4 g/cm 2 underground. About half the data were taken while the cloud chamber was tilted 66° in order to favor observation of the high-average-energy muons that come in at large zenith angle. The energies of the electrons that initiated the showers was obtained from an experimental calibration (to be published). The shower energies that were studied extended from 85 MeV to about 10 4 MeV. The observed frequency is compared with the expected frequency calculated from the results of Bhabha for knockon electrons and those of Zapolski and of Murota, Ueda, and Tanaka for electron pairs. The agreement is satisfactory except for knock-on electrons in the region of 10 9 eV, where the predicted frequency appears to be significantly lower than the observed frequency.

Observations of solar particles in the energy region above approximately 75 GeV provide good evidence for particle acceleration during the initial phase of a solar flare.