|
|
 |
|
|
Ann. Geophys., 26, 1617-1639, 2008
www.ann-geophys.net/26/1617/2008/
© European Geosciences Union 2008
A physical mechanism producing suprathermal populations and initiating substorms in the Earth's magnetotail
D. V. Sarafopoulos
Department of Electrical and Computer Engineering, Democritus University of Thrace, Xanthi, Greece
Abstract. We suggest a candidate physical mechanism, combining there dimensional
structure and temporal development, which is potentially able to produce
suprathermal populations and cross-tail current disruptions in the Earth's
plasma sheet. At the core of the proposed process is the "akis" structure;
in a thin current sheet (TCS) the stretched (tail-like)
magnetic field lines locally terminate into a sharp tip around the tail
midplane. At this sharp tip of the TCS, ions become non-adiabatic, while a
percentage of electrons are accumulated and trapped: The strong and
transient electrostatic electric fields established along the
magnetic field lines produce suprathermal populations. In
parallel, the tip structure is associated with field aligned and mutually
attracted parallel filamentary currents which progressively become more
intense and inevitably the structure collapses, and so does the local TCS.
The mechanism is observationally based on elementary, almost autonomous and
spatiotemporal entities that correspond each to a local
thinning/dipolarization pair having duration of ~1 min.
Energetic proton and electron populations do not occur
simultaneously, and we infer that they are separately accelerated at local
thinnings and dipolarizations, respectively. In one example energetic
particles are accelerated without any dB/dt variation and before the
substorm expansion phase onset. A particular effort is undertaken
demonstrating that the proposed acceleration mechanism may explain
the plasma sheet ratio Ti/Te≈7. All
our inferences are checked by the highest resolution datasets obtained by
the Geotail Energetic Particles and Ion Composition (EPIC)
instrument. The energetic particles are used as the best diagnostics for
the accelerating source. Near Earth (X≈10 RE) selected
events support our basic concept. The proposed mechanism seems to
reveal a fundamental building block of the substorm phenomenon and may be
the basic process/structure, which is now missing, that might help explain
the persistent, outstanding deficiencies in our physical description of
magnetospheric substorms. The mechanism is tested, checked, and found
consistent with substorm associated observations performed ~30 and 60 RE away from Earth.
Full Article in PDF (38466 KB) |
|
|
