Statistical and superposed epoch study of dipolarization events using data from Wind perigee passes

摘要

From 1995 to 2000, the Wind spacecraft spent over 500 h in the magnetotail, much of it within ～2 x 10~4 km of the predicted location of the neutral sheet. Wind passed through the near magnetotail at distances of -15 R_E < X GSM < -6 R_E on 35 occasions. Another 10 passes took place at distances of -30 R_E < X GSM < -15 R_E. We identified 65 dipolarization events in the Wind magnetic field data set between Y GSM～-16 and +16 R_E 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 100 km/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 20 min apart, were characterized by a decrease in B_x GSM and B_(TOTAL), and an increase in B_z GSM and the magnetic field inclination. Dipolarizations that occurred as part of a series of small dipolarizations spaced less than 20 min apart were characterized by a transient increase in B_z GSM and the magnetic field inclination, but no significant change in B_x GSM and B_(TOTAL). 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.