Global "Sawtooth" activity  in the April 2002 geomagnetic storm

G. D. Reeves, M. F. Thomsen, M. G. Henderson, R. M. Skoug, J. E.
Borovsky, J.-M. Jahn, C. J. Pollock, D. G. Mitchell, P. C. Brandt,
and S. B. Mende


One of the most interesting aspects of the April 2002 geomagnetic
storm was the quasi-periodic geomagnetic activity that we refer to as
"Sawtooth" injections. Sawtooth injection events were first
identified as an intriguing signature in the LANL geosynchronous
energetic particle fluxes showing sharp increases in particle fluxes
followed by gradual decreases which repeat roughly every 2-3 hours.
The variations are similar to the standard dropout and injection  of
energetic particles see in isolated substorms. However, unlike
isolated substorms,  after each injection a new dropout  over a broad
range of energies begins immediately producing the characteristic
sawtooth profile. Furthermore the sawtooth signatures are  seen at
all local times not just the near-midnight sector. Space magnetometer
data shows that the dropouts and injections are produced by
exceptionally strong stretching and dipolarization of the magnetic
field and confirms that the stretching begins immediately after the
preceding injection without an intervening recovery phase. However,
ENA observations show that these events are true injections of fresh
particles and not an adiabatic re-arrangement in response to the
field changes. Auroral observations likewise show clear auroral
onsets with each sawtooth injection but, those onsets can be embedded
in ongoing auroral activity rather than distinct episodes separated
by quiet intervals. It is not surprising that sawtooth activity is
associated with decreases in the Dst index and are therefore a subset
of geomagnetic storm activity. The quasi-periodicity and clear
separation of each onset is not, however, characteristic of most
geomagnetic storms and provides a unique opportunity to deconvolve
the effects of quasi-steady and impulsive responses of the
magnetosphere during sawtooth intervals such as those observed during
the April 2002 storm. In addition to discussing the magnetospheric
response and solar wind driving that produces these interesting
sawtooth signatures we discuss the implications for the
interpretation of the broad range of geomagnetic responses to the
April 2002 storm interval.

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Fall 2002 Meeting of the American Geophysical Union
San Francisco, CA, USA, 6-10 December 2002