The cause of two plasma-tail disconnection events in comet P/Halley during the ICE-Halley radial period

Abstract

We have examined ICE magnetometer and electron plasma data for possible causes of the plasma tail disconnection events (DE’s) which were observed in Halley’s comet on 1986 March 20–22 and April 11–12. We attribute the DE of March 20–22 to an interplanetary magnetic field polarity reversal, and the DE of April 11–12 to either a compression region in the solar wind, an interplanetary magnetic field polarity reversal, or a combination of the two. The measured average speeds of tail recession determined from pairs of successive photographs for both events vary between about 40 and 90 km s− 1, and the average acceleration determined from successive pairs of average velocity measurements is quite variable. The calculated times of disconnection for the two events are, in UT decimal dates, March 19.55 ± 0.16 and April 10.91 ± 0.36. Assuming that the two disconnection events are due to frontside reconnection after an interplanetary magnetic field reversal (the most likely explanation for the March 20–22 event, and one possible explanation for the April 11–12 event), we estimate that the time period between the onset of reconnection and the final disconnection of the tail is 0.1 ≦τ rec ≦ 0.6 day. This suggests that the average speed at which reconnection proceeds through the cometary magnetic field pileup region is 1 ≦ υ rec ≦ 6 km s −1, or several tenths the local Alfvén speed.