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Average Characteristics of Triggered and Nontriggered SubstormsMagnetic field data from ground stations, geosynchronous orbit, and magnetotail are examined to study the response to substorm activity with and without apparent interplanetary magnetic field (IMF) perturbations. Global substorms are identified using a sudden, persistent decrease in the AL index. The onset of this global expansion is taken to be the time of the Pi2 burst nearest to the beginning of the AL decrease. IMF triggers were identified subjectively through visual scanning of the data. Both northward turnings of the IMF B, and decreases in the amplitude of the By component were considered as possible triggers. Two different solar wind monitors were used in the investigation: IMP 8 in a circular orbit with a distance between approx.12 and approx.35 R(sub E) from the Earth-Sun line and ISEE 2 in an elliptical orbit with a distance of only approx.5- 10 R(sub E) from the Earth-Sun line. The results of superposed epoch analysis show that the temporal response from ground stations, geosynchronous orbit, and magnetotail are nearly identical for triggered (with apparent IMF perturbation) and nontriggered (without apparent IMF perturbation) substorms. It is therefore concluded that the nontriggered substorms are not a different form of activity than triggered substorms. However, we demonstrate that the magnitude of the response is different for the two types of substo&. By every measure considered, triggered substorm are systematically larger than nontriggered substorms. We interpret the fact that nearly 40% of all substorms cannot be associated with an IMF trigger as evidence that substorms are caused by an internal instability. However, the fact that so many appear to be triggered suggests that this internal instability is susceptible to external perturbations by the IMF. The fact that triggered substorms are larger than nontriggered substorms is counterintuitive, and we have no explanation for the observation.
Document ID
20060011203
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Hsu, Tung-Shin
(California Univ. Los Angeles, CA, United States)
McPherron, Robert L.
(California Univ. Los Angeles, CA, United States)
Date Acquired
August 23, 2013
Publication Date
July 17, 2004
Publication Information
Publication: Journal of Geophysical Research
Publisher: American Geophysical Union
Volume: 109
ISSN: 0148-0227
Subject Category
Solar Physics
Funding Number(s)
CONTRACT_GRANT: NSF ATM-99-72069
CONTRACT_GRANT: NAG5-11898
Distribution Limits
Public
Copyright
Other

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