title: Tail reconnection triggering substorm onset.
abstract: Magnetospheric substorms explosively release solar wind energy previously stored in Earth's magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at approximately 10 R(E) (R(E): Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at approximately 20 to 30 R(E). We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 R(E), at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.
authors.affiliation: Institute of Geophysics and Planetary Physics/ESS, University of California at Los Angeles, Los Angeles, CA 90095, USA. vassilis@ucla.edu
authors.forename: Vassilis
authors.initials: V
authors.lastname: Angelopoulos
authors.forename: James P
authors.initials: JP
authors.lastname: McFadden
authors.forename: Davin
authors.initials: D
authors.lastname: Larson
authors.forename: Charles W
authors.initials: CW
authors.lastname: Carlson
authors.forename: Stephen B
authors.initials: SB
authors.lastname: Mende
authors.forename: Harald
authors.initials: H
authors.lastname: Frey
authors.forename: Tai
authors.initials: T
authors.lastname: Phan
authors.forename: David G
authors.initials: DG
authors.lastname: Sibeck
authors.forename: Karl-Heinz
authors.initials: KH
authors.lastname: Glassmeier
authors.forename: Uli
authors.initials: U
authors.lastname: Auster
authors.forename: Eric
authors.initials: E
authors.lastname: Donovan
authors.forename: Ian R
authors.initials: IR
authors.lastname: Mann
authors.forename: I Jonathan
authors.initials: IJ
authors.lastname: Rae
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Russell
authors.forename: Andrei
authors.initials: A
authors.lastname: Runov
authors.forename: Xu-Zhi
authors.initials: XZ
authors.lastname: Zhou
authors.forename: Larry
authors.initials: L
authors.lastname: Kepko