ISEE 3 observations of low‐energy proton bidirectional events and their relation to isolated interplanetary magnetic structures

Abstract

Bidirectional, field‐aligned flows of low‐energy particles in interplanetary space have been proposed as a possible signature of large‐scale, looplike magnetic structures. We present the results of a survey of bidirectional anisotropy observations using data from the low‐energy proton and magnetometer experiments on board ISEE 3 covering a 45‐month period corresponding to the last solar maximum. During this period, 66 bidirectional flow (BDF) events have been identified, of which 48 could positively be associated with isolated magnetic field structures. We suggest that these structures are an interplanetary manifestation of coronal mass ejection (CME) events, and are able to group the BDF events into five classes according to the field signature of the related magnetic structure and the association or otherwise with an interplanetary shock. We conclude that the BDF events are an interplanetary signature not only of energetic, flare‐associated transients but also low energy, nonflare CMEs. From a comparison of anisotropy signatures at 35 and 620 keV we conclude that our observations are most consistent with the transient magnetic structures being detached bubbles comprising closed loops or tightly wound cylindrical helices rather than extended tonguelike loops attached to the Sun at the time of observation.