Bidirectional solar wind electron heat flux events

Abstract

Normally the ≳ 80‐eV electrons which carry the solar wind electron heat flux are collimated along the interplanetary magnetic field (IMF) in the direction pointing outward away from the sun. Occasionally, however, collimated fluxes of ≳ 80‐eV electrons are observed traveling both parallel and antiparallel to the IMF. Here we present the results of a survey of such bidirectional electron heat flux events as observed with the plasma and magnetic field experiments aboard ISEE 3 at times when the spacecraft was not magnetically connected to the earth's bow shock. The onset of a bidirectional electron heat flux at ISEE 3 usually signals spacecraft entry into a distinct solar wind plasma and field entity, most often characterized by anomalously low proton and electron temperatures, a strong, smoothly varying magnetic field, a low plasma beta, and a high total pressure. Significant field rotations often occur at the beginning and/or end of bidirectional heat flux events, and, at times, the large field rotations characteristic of “magnetic clouds” are present. Approximately half of all bidirectional heat flux events are associated with and follow interplanetary shocks, while the other events have no obvious shock associations. When shock associated, the delay from shock passage typically is ∼13 hours, corresponding to a radial separation of ∼0.16 AU. Independent of any shock association, bidirectional heat flux events typically are ∼0.13 AU thick in the radial direction, although considerable variability is evident from one event to another. Near solar activity maximum, bidirectional heat flux events occurred at a rate of ∼3 per month, and the solar wind electron heat flux was bidirectional ∼5% of the time. Bidirectional heat flux events often contain strong out‐of‐the‐ecliptic field components and thus can be effective in producing geomagnetic disturbances. This is particularly true for shock‐associated events where the intrinsically strong fields in the leading portions of the events are amplified by compression in transit from the sun and where strong out‐of‐the‐ecliptic field components resulting from compression and draping of the ambient field are often present within the shocked plasma immediately ahead. Consistent with previous work we interpret the bidirectional heat flux as evidence for a closed field topology in interplanetary space. Further, we suggest that these events are one of the more prominent signatures of coronal mass ejection events in the solar wind at 1 AU.