Corotating MeV/amu ion enhancements at ≤1 AU from 1978 to 1986

MeV/amu ion enhancements associated with corotating high‐speed solar wind streams in 1978–1986 during pre‐solar maximum to near solar minimum conditions are studied using ISEE 3/ICE, IMP 8, and Helios 1 data. Around 50% of corotating streams contain energetic ion increases. These increases extend to ∼25 MeV/amu, where they merge into the galactic cosmic ray background, and are most evident approaching solar minimum. Sunward ion streaming in the solar wind frame (first‐order anisotropy ∼20%) and positive radial intensity gradients (∼400%/AU) are consistent with acceleration in the outer heliosphere at corotating shocks followed by streaming into the inner heliosphere. The spectra and intensities show little solar cycle variation. The spectra of ions from protons to Fe at ∼2–20 MeV/amu are approximated equally well by exponentials in momentum dJ/dP ≈ exp ( −P/P 0 ), P 0 = 11–16 MeV c −1 amu −1 , or by distribution functions ƒ ≈ exp (−υ/υ 0 ), υ 0 = 0.18–0.25 (MeV/amu) 1/2 , with equivalent power law in energy slopes in the range ∼ −3 to −4. Ion abundances are correlated with the stream peak solar wind speed. In slower corotating streams (maximum solar wind speed <600 km/s), mean abundance ratios are protons/ 4 He = 43 ± 18; 4 He/O = 54 ± 23; C/O = 0.62 ± 0.06; Mg/O = 0.19 ± 0.03, and Fe/O = 0.14 ± 0.02. These show some similarity to the corresponding ratios for “solar energetic particles” (SEP) (protons/ 4 He = 70 ± 10; 4 He/O = 55 ± 3; C/O = 0.48 ± 0.02; Mg/O = 0.21 ± 0.01 and Fe/O = 0.16 ± 0.02) which are typically accelerated by shocks passing through slow solar wind. In corotating events in higher‐speed streams, these ratios become protons/ 4 He = 19 ± 5; 4 He/O = 130 ± 35; C/O = 0.89 ± 0.05; Mg/O = 0.14 ± 0.01, and Fe/O = 0.10 ± 0.01 and more closely resemble the corotating event abundance ratios measured in high‐speed streams during the mid‐1970s solar minimum (protons/ 4 He = 17 ± 7; 4 He/O ∼ 160 ± 50; C/O = 0.89 ± 0.1; Mg/O = 0.13 ± 0.03, and Fe/O = 0.096 ± 0.05). Solar wind plasma may also show similar variations in composition with solar wind speed (based on the limited solar wind composition measurements available) so that the energetic ion compositions are consistent with the acceleration of corotating event ions and SEPs from the solar wind. The ordering of corotating event and solar wind abundances by first ionization potential and their variation with solar wind speed suggest that conditions in the ion‐neutral fractionation region in the upper chromosphere determine the abundances and are associated in some way with regulation of the solar wind speed.