Interplanetary magnetic field connection to the Sun during electron heat flux dropouts in the solar wind

Abstract

Gosling (1975) and MacQueen (1980) pointed out that coronal mass ejections (CMEs) add significant magnetic flux which is connected to the Sun, to the interplanetary medium. Since the interplanetary magnetic field is observed to stay at a roughly constant level, some compensating removal of flux is necessary. McComas et al. (1989) suggested that magnetic reconnection in current sheets near the Sun removes flux by producing U‐shaped magnetic structures in the interplanetary medium which are disconnected from the Sun at both ends. Since solar wind halo electrons normally carry heat flux outward from the Sun's hot corona to the cool interplanetary medium, McComas et al. (1989) interpreted observations of dropouts of the solar wind electron heat flux as evidence for such disconnections. Here we use 2‐ to 8.5‐keV solar electrons observed by ISEE 3 as tracers of the magnetic topology for these heat flux dropout (HFD) events. These electrons are superior to the ∼0.1‐ to 1‐keV halo electrons as test particles since they have longer mean free paths, they are less affected by electric fields, and their transit times from the Sun to 1 AU are only ∼1–2 hours compared to 3–10 hours for the halo electrons. Furthermore, ≥2‐keV electrons are often accelerated in impulsive events, which are sometimes associated with solar flares, and they also generate type III solar radio bursts, which can be tracked from the Sun to 1 AU by the ISEE 3 radio experiment. In at least eight of the 25 HFDs we find strong streaming of the ≥2‐keV electrons outward from the Sun. In one HFD an impulsive solar electron event was observed with an associated type III radio burst which could be tracked from the Sun to ∼1 AU. We conclude that in many HFDs the interplanetary field is still connected to the Sun, and that some energy‐dependent process may produce HFDs without significantly perturbing electrons of higher energies. For two of the 25 HFDs, however, the ≥2‐keV electron observations exhibit all the characteristics of real magnetic disconnection events, including a depletion in the total ≥2‐keV fluxes.