Bidirectional anisotropies in solar cosmic ray events: Evidence for magnetic bottles

Abstract

The occurrence of bidirectional anisotropies in low energy solar proton and electron events has been investigated in the period 1967–1973 by using data obtained from five satellites, viz., Explorer 34 and 41 and Vela 5B, 6A, and 6B. Some 16 observations of bidirectional distribution were detected in this time period, including the observation of bidirectionality in three solar electron events. The characteristics of two of these electron events are discussed in detail. The average duration over all events is 9 hours, implying a spatial scale of ≈0.13 AU, and the events show a strong correlation with the minimum of a Forbush decrease. The two maxima in the particle distributions are aligned with the interplanetary magnetic field. The origin of the bidirectional distributions is shown to be an interplanetary phenomenon rather than an effect of the bow shock. A magnetic configuration consisting of large‐scale loops in the interplanetary field (Gold bottle model) is suggested to account for the observations. The bidirectional flow can be produced by particle mirroring and by adiabatic expansion. The rarity of two‐way flow in solar electrons implies that special conditions must exist for its development. Our observations suggest that 1 out of 10 loop‐shaped coronal expulsions is detectable via two‐way cosmic ray particle flow near 1 AU. Bidirectional anisotropy observed in the event of August 4–5, 1972, in exact coincidence with the minimum of a striking Forbush decrease, suggests a shell1 of looped and well‐ordered field lines at the leading edge of the driver gas. The identification of this cavity, in which the galactic intensity is depressed, contradicts some aspects of published models of solar and galactic cosmic ray behavior.