Tail modeling (Diversion of cross-tail current)
In the field of magnetic substorm process, one of the outstanding problems concerns the
role of near earth region (6-15 RE, called the inner tail region) during the expansion
phase. Observations of the aurorae show that at the onset of the substorm expansion, the
equatorward-most arc brightens and moves poleward. In the equatorial plane, fast flows
and dipolarization of the magnetic field are observed (Baumjohann et al. 1999). As
already discussed earlier, according to near-geosynchronous onset (NGO) models, the arc
brightening is caused by the disruption of the near-Earth cross-tail current sheet by as yet
to be identified plasma instability processes (Lui, 1996). These models suggest that the
current disruption causes a tailward propagating rarefaction wave which thins the current
sheet and subsequently initiates reconnection in the magnetotail. The poleward motion of
the arc is identified with the tailward propagating rarefaction waves.
The near-earth neutral line (NENL) models (Baker et al. 1996) on the other hand, suggest
that reconnection begins in the 20-30 RE region of the magnetotail initially slowly but
explosively when lobe field lines begin to participate in the reconnection. Reconnection
launches flow bursts both tailwards and earthwards which have speeds of several hundred
km/s. When the flow impacts the inner magnetosphere, it is braked by the strong field of
the inner magnetosphere resulting in flux pile-up (dipolarization). A substorm current
wedge forms where the currents is diverted from the magnetosphere into the ionosphere
by the inertial currents and by the sustained build up of north-south gradient of plasma
pressure (Birn et al. 1999). As more flow arrives, the magnetic field dipolarization front
moves tailward. The poleward moving arc is associated with the outward motion of this
dipolarization front. In the NENL model, reconnection and flows precede current
disruption whereas in the NGO models, reconnection follows current disruption in the
near-Earth region. Unfortunately, the expected time difference between the two processes
is of the order of a few minutes and a sustained effort from researchers has not yielded an
unambiguous answer (Baker et al. 2002, Miyashita et al. 2000).
Fortunately, Themis because of its multi-spacecraft nature and enhanced ground
instrument capabilities is poised to address this problem comprehensively during the
extended phase. The Themis probes P3, P4 and P5 would be placed in the near-earth
magnetotail region with apogees ranging between 10-12 RE during the period 2010-2012
and will have inter-spacecraft spacings between a few hundred kilometers to ~ 1 RE (at
apogee) during this time. The spacecraft will monitor flows, dipolarization front and
current sheet thinning in the magnetosphere during this phase while the ground
instrumentation would look for substorm initiations and monitor their progress. The
spacecraft configurations have been optimized to yield the best results for strong currents
and gradients observed in this region. Because, the expected gradients of magnetic field
components are large in the Z direction near the current sheet, we will keep the spacecraft
separations in the Z direction at half or less than the separations in the R direction.
Theoretically (and optimally), in order to determine all nine of the spatial gradients and
the background field at a point, simultaneous measurements from four spacecraft in a
tetrad formation are required. However, in practice it is often found that the magnetic
field gradients are negligible in certain directions (because currents tend to flow in
sheets) and by assuming constancy in those directions measurements from fewer
spacecraft can be gainfully used to correctly infer the currents. For example, to measure
the strength of the ring current in the magnetosphere, one can use the approximation that
the azimuthal gradients induced by a ring current are infinitesimal. Similarly, in the
regions of field-aligned currents that link the substorm wedge region with the ionosphere,
a sheet approximation of the current structures can be used to justify the neglect of field
gradients in the field-aligned direction. Thus by using the measurements judiciously from
three spacecraft we expect to overcome the lack of fourth spacecraft for the electric
current measurements.
In addition, simultaneous measurements of flows, rarefaction waves and the
dipolarization fronts from three spacecraft will allow us to easily and unambiguously test
the predictions of NENL and NGO models in space for the first time.
References
Baker, D. N., T. I. Pulkkinen, V. Angelopoulos, W. Baumjohann, and R. L. McPherron
(1996), Neutral line model of substorms: Past results and present view, J. Geophys.
Res., 101, 12,975– 13,010.
Baker, D. N., Peterson, W. K., Eriksson, S., Li, X., Blake, J. B. and Burch, J. L.: 2002,
‘Timing of magnetic reconnection initiation during a global magnetospheric
substorm onset’, Geophys. Res. Lett., 29(24), 2190, 10.1029/2002GL015539.
Baumjohann, W., M. Hesse, S. Kokubun, T. Mukai, T. Nagai, and A. A. Petrukovich
(1999), Substorm dipolarization and recovery, J. Geophys. Res., 104, 24,995–
25,000.
Birn, J., M. Hesse, G. Haerendel, W. Baumjohann, and K. Shiokawa (1999), Flow
braking and the substorm current wedge, J. Geophys. Res., 104, 19,895– 19,904.
Lui, A. T. Y. (1996), Current disruption in the Earth’s magnetosphere: Observations and
models, J. Geophys. Res., 101, 13,067– 13,088.
Miyashita, Y., Machida, S., Mukai, T., Saito, Y., Tsuruda, K. and Hayakawa, H.: 2000,
‘A statistical study of variations in the near and middistant magnetotail associated
with substorm onsets: GEOTAIL observations’, J. Geophys. Res., 105, 15913–
15930.