Magnetic Flux Pileup and Magnetic Field Dipolarization During Substorm

Zhang, H., Pu, Z., Cao, X., Fu, S., Xiao, C., Zong, Q., Liu, Z., Cao, J., 
Korth, A., Frazen, M., Carr, C., Reme, H., Glassmeier, K.


During the period from July to October in 2004, the orbit apogee of Double Star
TC-1 of 13Re located in the magnetotail, which made it possible to survey some
basic features of substorm processes in the magnetotail. In the present paper we
make a case study of substorm occurring on Sep. 17, 2004. At about 0117 UT FUV
WIC on board IMAGE observed an aurora breakup. Almost at the same time the
LANL-01A and 90-085 at the geosynchronous altitude detected dispersionless
injection of energetic electrons. About 2-3 minutes later a weak dipolarization
was seen by NOAA geosynchronous satellite GOES 12. Interestingly, ten minutes
before substorm onset, Cluster observed an earthward flow at (-15.00, 2.05,
3.50) Re(GSM). At 0116 UT (one min before onset) TC-1 saw a beginning of
magnetic flux pile-Up at its position (-10.26, -1.36, 1.01) REGSM, characterized
by an increasing of Bz component with almost constant Bx and thermal pressure.
At about 0126 UT (9 minutes after onset) TC-1 observed a local dipolarization of
the magnetic field which was characterized by a rapid decrease of Bx component
and a sharp jump in the thermal pressure, together with a continuous increase of
Bz. In the literature, some authors treated the flux pile-Up and dipolarization
in the tail as a single process named 'dipolarization'. However, as TC-1
measurements show in this event, flux pile-Up and dipolarization are
characterized by completely different signatures. The same feature holds for
many other events. Detailed inspections of TC-1 data in this event show that the
beginning of flux pile-Up was about 10 minutes preceding the dipolarization.
Besides, the latter lasted only for about two minutes, whereas the former kept
for almost one hour. All these aforementioned differences imply that flux
pile-Up and dipolarization are two different dynamical processes. Nevertheless,
further studies indicate that they are 'cause-effect' related: The magnetic flux
pileup may lead the field lines with 'good' curvature to change to the 'bad'
ones, further compresses the magnetic field and yields disturbances. These can
set up appropriate conditions for development of instabilities and then trigger
substorm expansion onset. In summary, this case study seems consistent with the
synthesis substorm model of near-Earth neutral line (NENL) and near-Earth
current disruption (NECD).

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Fall Meeting, American Geophysical Union, San Francisco, U.S.A., 
5-9 December 2005