BIDIRECTIONAL SIMILAR-TO-1 MEV AMU(-1) ION INTERVALS IN 1973-1991 OBSERVED BY THE GODDARD-SPACE-FLIGHT-CENTER INSTRUMENTS ON IMP-8 AND ISEE-3/ICE

A search has been made for bidirectional energetic ion flows (BIFs) in the solar wind at 1 AU during 1973-1991 using approximately 1 MeV amu-1 ion data from the Goddard Space Flight Center instruments on ISEE3/ICE and IMP8. Some 4000 intervals have been identified. These range in duration from approximately 15 minutes to over 60 hr (mean = 3 hr), corresponding to scale sizes from approximately 0.003 to >0.7 AU. The occurrence rate falls off exponentially with an e-folding duration of approximately 4 hr (approximately 0.05 AU). BIFs are observed more frequently around solar maximum, when they are observed approximately 12% of the time compared with approximately 5% at solar minimum. Intervals with durations greater than 4 hr are observed on average approximately every 3-4 days at solar maximum and every 2 weeks at solar minimum, with approximately 33% of these intervals following within 2 days of an interplanetary shock. Around 80% of previously reported bidirectional >35 keV ion flows, bidirectional solar wind electron heat fluxes, magnetic clouds and He enhancements, signatures attributed to the passage of coronal mass ejections (CMEs), show evidence of bidirectional >1 MeV amu-1 BIFs. Of the other 20%, around half are associated with low energetic ion fluxes, and the remainder with solar particle events and shock-accelerated ions which do not show bidirectional streaming. These various CME signatures and the >1 MeV amu-1 BIFs usually do not coincide exactly, however, and additional bidirectional ion events are identified. A one-to-one association between individual BIFs and CMEs does not occur. In particular, intermittent episodes of ion bidirectionality may occur during the passage of clear magnetic cloud signatures. Hence CME boundaries and durations may not be identified unambiguously from BIF observations.