Statistical and superposed epoch study of dipolarization events using data from Wind perigee passes
- 1Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, 52242 USA
- 2Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
- 3Space Sciences Laboratory, University of California, Berkeley, California, USA
- 4The Catholic University of America, Washington, D.C., 20064, USA
- 5NOAA Space Environment Center, Boulder, Colorado, 80305 USA
Abstract. From 1995 to 2000, the Wind spacecraft spent over 500h in the magnetotail, much of it within ~2x104km of the predicted location of the neutral sheet. Wind passed through the near magnetotail at distances of -15 RE<X GSM<-6 RE on 35 occasions. Another 10 passes took place at distances of -30 RE<X GSM<-15 RE. We identified 65 dipolarization events in the Wind magnetic field data set between Y GSM~-16 and +16 RE based upon our requirements that the magnetic field inclination had to change by more than 15°, the maximum inclination angle had to be greater than 20°, and the inclination angle had to increase by a factor of at least 1.5. Most of the dipolarization events occurred in the pre-midnight region of the magnetotail and were accompanied by earthward flows with speeds greater than 100km/s. The properties of the dipolarization events did not depend upon the Y GSM position. However, they did vary with the distance to the neutral sheet. Isolated dipolarization events, defined as occurring more than 20min apart, were characterized by a decrease in Bx GSM and BTOTAL, and an increase in Bz GSM and the magnetic field inclination. Dipolarizations that occurred as part of a series of small dipolarizations spaced less than 20min apart were characterized by a transient increase in Bz GSM and the magnetic field inclination, but no significant change in Bx GSM and BTOTAL. The events consisting of a series of small dipolarizations occurred predominantly near midnight. We interpret these results in terms of two different modes of magnetotail convection: 1) a classical substorm pattern featuring storage of magnetic energy in the tail lobes which is explosively released at onset, and 2) a directly driven process.