 |
| Titel |
First results from ideal 2-D MHD reconstruction: magnetopause reconnection event seen by Cluster |
| VerfasserIn |
W.-L. Teh, B. U. Ö. Sonnerup |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
0992-7689
|
| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 26, no. 9 ; Nr. 26, no. 9 (2008-09-12), S.2673-2684 |
| Datensatznummer |
250016218
|
| Publikation (Nr.) |
 copernicus.org/angeo-26-2673-2008.pdf |
|
|
|
|
|
| Zusammenfassung |
| We have applied a new reconstruction method (Sonnerup and
Teh, 2008), based on the ideal single-fluid MHD equations in a steady-state,
two-dimensional geometry, to a reconnection event observed by the Cluster-3
(C3) spacecraft on 5 July 2001, 06:23 UT, at the dawn-side
Northern-Hemisphere magnetopause. The event has been previously studied by
use of Grad-Shafranov (GS) reconstruction, performed in the
deHoffmann-Teller frame, and using the assumption that the flow effects were
either negligible or the flow was aligned with the magnetic field. Our new
method allows the reconstruction to be performed in the frame of reference
moving with the reconnection site (the X-line). In the event studied, this
motion is tailward/equatorward at 140 km/s. The principal result of the
study is that the new method functions well, generating a magnetic field map
that is qualitatively similar to those obtained in the earlier GS-based
reconstructions but now includes the reconnection site itself. In comparison
with the earlier map by Hasegawa et al. (2004), our new map has a slightly
improved ability (cc=0.979 versus cc=0.975) to predict the fields measured
by the other three Cluster spacecraft, at distances from C3 ranging from
2132 km (C1) to 2646 km (C4). The new field map indicates the presence of a
magnetic X-point, located some 5300 km tailward/equatorward of C3 at the
time of its traversal of the magnetopause. In the immediate vicinity of the
X-point, the ideal-MHD assumption breaks down, i.e. resistive and/or other
effects should be included. We have circumvented this problem by an ad-hoc
procedure in which we allow the axial part of convection electric field to
be non-constant near the reconnection site. The new reconstruction method
also provides a map of the velocity field, in which the inflow into the
wedge of reconnected field lines and the plasma jet within it can be seen,
and maps of the electric potential and of the electric current distribution.
Even though the velocity map is expected to be inaccurate near the X-point,
it provides high-quality predictions (cc=0.969) of the velocity components
at points along the path of C1, some of which are close to the X-point; the
predictions of density and pressure are less good. Except near the
reconnection site, the new reconstruction provides a complete
characterization, in unprecedented detail, of the entire dynamic plasma and
field equilibrium, reconstructed from the C3 data. It represents our best
prediction to date of what the actual configuration was like. But, since
substantial time variations were present in the event, the recovered
structure by necessity includes considerable time aliasing. The invariant
direction used in the reconstruction, is found to agree, within 6°,
with a recent theoretical prediction of the X-line orientation by Swisdak
and Drake (2007). |
| |
|
| Teil von |
|
|
|
|
|
|
|
|