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IMP 8 observations of traveling compression regions in the mid‐tail near substorm expansion phase onset
Author(s) -
Taguchi S.,
Slavin J. A.,
Lepping R. P.
Publication year - 1997
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 - 10.1029/97gl00121
Subject(s) - substorm , plasma sheet , plasmoid , physics , astrophysics , compression (physics) , phase (matter) , lobe , field (mathematics) , magnetic field , magnetic reconnection , magnetosphere , anatomy , biology , mathematics , quantum mechanics , pure mathematics , thermodynamics
The large data set returned by the IMP 8 magnetic field investigation has been examined to understand the characteristics of traveling compression regions (TCRs). Using 15 years of the AL index, we identified 565 isolated substorm events with well‐developed expansion phase for which IMP 8 was in the mid‐tail lobes and providing magnetic field measurements. From this data set, 17 substorms were found to produce the bipolar B Z TCRs frequently observed farther down the tail. However, another 14 cases have field compressions during which the field tilts north‐then‐south, but the maximum B Z does not reach a positive value, i.e., the B Z variation is not bipolar. These “negative B Z ” compression regions (NCRs) are examined in some detail and found to occur at larger |Z| values. The usual bipolar B Z TCRs, in contrast, are found at smaller |Z| values closer to lobe‐plasma sheet interface. We interpret an NCR as a TCR which is observed at large |Z| where the growth phase flaring is too strong for the lobe field draping about the plasmoid to produce a positive B Z between the leading edge and the center of the compression region. Hence, at IMP 8 distances TCRs are often not accompanied by true bipolar B Z signatures; rather only a north‐then‐south tilting of the field relative to flared lobe field is observed.