Measurements of the Cosmic-Ray Positron Fraction from 1 to 50 G[CLC]e[/CLC]V

Two measurements of the cosmic-ray positron fraction as a function of energyhave been made using the High Energy Antimatter Telescope (HEAT) balloon-borneinstrument. The first flight took place from Ft. Sumner, New Mexico in 1994,and yielded results above the geomagnetic cutoff energy of 4.5 GeV. The secondflight from Lynn Lake, Manitoba in 1995 permitted measurements over a largerenergy interval, from 1 GeV to 50 GeV. In this letter we present results on thepositron fraction based on data from the Lynn Lake flight, and compare thesewith the previously published results from the Ft. Sumner flight. The resultsconfirm that the positron fraction does not increase with energy above ~10 GeV,although a small excess above purely secondary production cannot be ruled out.At low energies the positron fraction is slightly larger than that reportedfrom measurements made in the 1960's. This effect could possibly be aconsequence of charge dependence in the level of solar modulation.Comment: 7 pages, LaTeX, two eps figures, aas2pp4.sty and epsf.tex needed, accepted by Ap.J. Lett. (March 27, 1997