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Ann. Geophys., 22, 2107-2113, 2004
www.ann-geophys.net/22/2107/2004/
© European Geosciences Union 2004
Average characteristics of the midtail plasma sheet in different dynamic regimes of the magnetosphere
N. P. Dmitrieva, V. A. Sergeev, and M. A. Shukhtina
V. A. Fock Institute of Physics, St.-Petersburg State University, St.-Petersburg, Russia
Abstract. We study average characteristics of plasma sheet convection in the middle
tail during different magnetospheric states (Steady Magnetospheric
Convection, SMC, and substorms) using simultaneous magnetotail (Geotail,
15-35 RE downtail) and solar wind (Wind spacecraft) observations during
3.5 years. (1) A large data set allowed us to obtain the average values of the
plasma sheet magnetic flux transfer rate (Ey and directly compare it
with the dayside transfer rate (Emod for different magnetospheric
states. The results confirm the magnetic flux imbalance model suggested by
Russell and McPherron (1973), namely: during SMC periods the day-to-night
flux transport rate equals the global Earthward plasma sheet convection;
during the substorm growth phase the plasma sheet convection is suppressed
on the average by 40%, whereas during the substorm expansion phase it twice
exceeds the day-to-night global flux transfer rate. (2) Different types of
substorms were revealed. About 1/3 of all substorms considered displayed
very weak growth in the tail lobe magnetic field before the onset. For these
events the plasma sheet transport was found to be in a balance with the
day-to-night flux transfer, as in the SMC events. However, the lobe
magnetic field value in these cases was as large as that in the substorms
with a classic growth phase just before the onset (both values exceed the
average level of the lobe field during the SMC). Also, in both groups similar
configurational changes (magnetic field stretching and plasma sheet
thinning) were observed before the substorm onset. (3) Superimposed epoch
analysis showed that the plasma sheet during the late substorm recovery phase
has the characteristics similar to those found during SMC events, the SMC
could be a natural magnetospheric state following the substorm.
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