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Reduction in field‐aligned currents preceding and local to auroral substorm onset
Author(s) -
Murphy Kyle R.,
Mann Ian R.,
Rae I. Jonathan,
Waters Colin L.,
Anderson Brian J.,
Milling David K.,
Singer Howard J.,
Korth Haje
Publication year - 2012
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2012gl052798
Subject(s) - substorm , ionosphere , magnetosphere , geophysics , physics , field line , field (mathematics) , coupling (piping) , atmospheric sciences , magnetic field , mathematics , quantum mechanics , pure mathematics , mechanical engineering , engineering
We examine the global field‐aligned current (FAC) topology associated with a clear substorm on the 16 February 2010. We show that for this particular substorm there is a clear and localised reduction in the FACs observed by AMPERE at least 6 minutes prior to auroral onset. A new auroral arc forms in the region of reduced FAC and on closed field lines which subsequently brightens and expands poleward, signifying the start of the substorm expansion phase. We argue that the change in FACs observed prior to onset is the result of a change in the magnetosphere‐ionosphere (M‐I) coupling in a region local to the subsequent auroral onset. Such a change implies an important role for M‐I coupling in destabilising the near‐Earth tail during magnetospheric substorms and perhaps more importantly in selecting the location in the ionosphere where auroral onset begins.