Some initial ISEE‐1 results on the ring current composition and dynamics during the magnetic storm of December 11, 1977

The ISEE‐1 spacecraft carries a mass spectrometer for ions with E/Q ≤ 17 keV/e. On December 11, 1977, it traversed the equatorial magnetosphere in the prenoon and dawn sectors during the early recovery phase of a magnetic storm with peak D ST =−125γ. Along the dayside (inbound) leg O + was comparable to or exceeded H + in number density in the energy range 0.1 keV ≤ E ≤ 17 keV. The velocity distribution of O + showed a characteristic dispersion, consistent with the ions being transported to the dayside from the dusk to midnight sector. The high‐energy slope of the H + and O + distribution functions showed different radial dependence, with the slope of H + being consistent with a harder component carrying most of the energy density. Along the nightside (outbound) leg the O + density was very low outside of 5 R E , and most of the number denisty was due to the hard H + component.