Ulysses observation of a noncoronal mass ejection flux rope: Evidence of interplanetary magnetic reconnection

Abstract

A well‐defined, small‐scale (≈0.05 AU) magnetic flux rope was observed by Ulysses at about 5 AU in close proximity to a heat flux dropout (HFD) at the heliospheric current sheet (HCS). This magnetic flux rope is characterized by a rotation of the field in the plane approximately perpendicular to the ecliptic (and containing the Sun and the spacecraft) and with a magnetic field maximum centered near the inflection point of the bipolar signature. The edges of the flux rope are well defined by diamagnetic field minima. A bidirectional electron heat flux signature is coincident with the magnetic flux rope structure. The event occurred during a time of slightly increasing solar wind speed, suggesting that the field and plasma were locally compressed. Unlike most coronal mass ejections/magnetic clouds, this event is characterized by high proton temperatures and densities, high plasma beta, no significant alpha particle abundance increase, and a small radial size. We interpret these observations in terms of multiple magnetic reconnection of previously open field lines in interplanetary space at the HCS. Such reconnection produces a U‐shaped structure entirely disconnected from the Sun (which we associate with the HFD), a closed magnetic flux rope (which we associate with the counterstreaming electron event), and a closed magnetic loop or tongue connected back to the Sun at both ends. These observations suggest that magnetic reconnection, and its changes to magnetic field topology, can occur well beyond the solar corona in interplanetary space.