PROBLEMS OF GEOCOSMOS

Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)
THEMIS results
Satellite segment of THEMIS system consist of 5 spacecraft on the equatorial orbits synchronized so that
every 4 days they form a radial configuration in their apogees. Their apogees are: for most distant spacecraft
B it is ~28 RE, for spacecraft C ~18 RE, the apogees of three nearby spacecrafts A, D and E are about 10 RE.
One of THEMIS purposes is a studying of the large-to-medium scale dynamical structures in the
magnetosphere, their radial configurations are especially interesting to specify the geometry of kink-like
flapping folds in the tail current sheet. Here we use magnetic measurements from the FGM instrument with
the 3 sec time resolution.
a) B
D
E
C
A
b)
c)
d)
e)
f)
Figure 2. Flapping event on March 5, 2008 registered by the THEMIS spacecrafts. Lines #1, #2, #3, #4, #6,
#7 indicate the variations for which the calculated MVA normals are shown in Table 1. a) the approximate
shape of the structure and relative locations of spacecrafts consistent with observations, Lx and Ly —
approximate sizes of scale-sizes of flapping structrure, dark blue arrows show the expected MVA normals; b)
true spacecraft locations in XY plane; c) Bz-component of magnetic field of spacecrafts B, C, E colored
according to the legend; d) Bx-component of magnetic field at spacecraft B, C, E e) Bx-component of
magnetic field at spacecraft D (the dark blue line) and A (the violet line); f) Bx-component of a magnetic field
of spacecraft D (the dark blue line) with a time shift-8 minutes and A (the violet line) with a time shift of +8
minutes.
We present here a remarkable event on March, 5th, 2008, between 9-11 hours (fig.2), when the
spacecraft were in a radial configuration (have been placed approximately in one line along X axis, fig.2b).
The distance between spacecrafts E and B is here about 6 RE (fig.2b). The event is remarkable because the
spacecrafts B, C, E simultaneously observed similar large-amplitude variations of magnetic field Bxcomponent
(fig.2с) with almost no time shift. Although only the most distant spacecraft B actually crossed
the neutral sheet (all others probed the variations in the negative Bx range), the structure looks similar to the
previously described Y-kink flapping structures (Runov et al., 2005, Sergeev et al., 2006). (In the following
we confirm this conclusion by analyzing the geometry of the perturbations using the MVA analyses.) In a
striking difference, even if the Bx variations are synchronous (fig.2c) over the distance > 6 RE in Xdirection(B,
C, E), the similarity is lost between the nearby spacecrafts E, D and A, over much shorter
distances about ~1Re but across the tail. This immediately suggests (fig 2a) that the measured structure is a
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