Periodic magnetospheric substorms: Multiple space-based and ground-based
instrumental observations

Chao-Song Huang, J.C. Foster, G.D. Reeves, G. Le, H.U. Frey, C.J. Pollock, 
J.-M. Jahn


Quasi-periodic, sawtooth-like variations of energetic plasma particle fluxes are
often measured at geosynchronous orbit during magnetic storms. The outstanding
problems are whether the sawtooth flux variations represent particle injections
from the tail to the inner magnetosphere, whether the sawtooth variations
correspond to substorm onsets, and what mechanism is responsible for the
generation of periodic particle injections (or periodic substorms). In this
paper, we present the measurements of multiple space-based and ground-based
instruments in the magnetosphere and ionosphere during two magnetic storms. The
Geotail satellite was located in the near tail between X GSM = -20 and -30 R E
and detected periodic southward turnings of the magnetospheric magnetic field.
The southward turnings of B z are interpreted as the signature of periodic
near-tail reconnection and plasmoid formations. Geosynchronous satellites
measured periodic gradual dropouts and rapid increases of energetic charged
fluxes and magnetic field dipolarization. The IMAGE satellite measured enhanced
emissions of energetic neutral atoms in the ring current and auroral
brightenings. The Canadian Auroral Network for the OPEN Program Unified Study's
photometers measured intensifications and latitudinal motion of auroral
emissions. The auroral electrojet index showed periodic increases. These
magnetospheric and ionospheric phenomena have the same periodicity and indicate
the occurrence of periodic substorms. There is an excellent correspondence among
the substorm signatures from all the measurements. The results show that
periodic magnetospheric substorms can indeed occur during continuous southward
interplanetary magnetic field, and the mean period in these two cases is ~2.7
hours. The sawtooth variations of energetic plasma fluxes at geosynchronous
orbit represent true particle injections from the tail to the inner
magnetosphere. The sawtooth injections are related to the near-tail reconnection
and correspond to the onsets of periodic substorms. 

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Journal of Geophysical Research, 108 (A11), 1411, doi:10.1029/2003JA009992, 2003