Intense interplanetary magnetic fields observed by geocentric spacecraft during 1963–1975

Ninety‐two percent of the intense magnetic fields (>13 γ) observed at 1 AU during solar cycle 20 (1963–1975) were associated with shocks, stream interfaces, or cold magnetic enhancements (CME's). Most (52%) of the magnetic field intensity enhancements occurred at stream interfaces; 27% occurred behind shocks without interfaces; and 11% occurred in CME's. The most intense fields (25–37 γ) followed shocks. Magnetic field intensities at interfaces did not exceed 25 γ, suggesting that some mechanism limits the intensity ahead of streams. The intensities in CME's did not exceed 20 γ. Intense magnetic fields persisted longest behind shocks. During the period 1973–1975, interface‐associated enhancements occurred 3.5 times as frequently as shock‐associated enhancements, and during an earlier period (1967–1969) when the sun was more active, shock‐associated enhancements occurred 1.2 times as frequently as interface‐associated enhancements. The frequency of CME's was the same in both periods. The absolute occurrence frequency of interface‐associated enhancements increased by a factor of 2.4 in going from the early period to the later period, while the absolute occurrence frequency of shock‐associated enhancements decreased by a factor of 1.7 in going from the early to the late period.