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| Titel |
How to find magnetic null and construct field topology with MMS data? |
| VerfasserIn |
Huishan Fu, Andris Vaivads, Yuri Khotyaintsev, Vyacheslav Olshevsky, Mats André, Jinbin Cao, Shiyong Huang, Alessandro Retino, Jonathan Eastwood |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250103295
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| Publikation (Nr.) |
 EGU/EGU2015-2705.pdf |
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| Zusammenfassung |
| In this study, we apply a new method—Taylor expansion—to find magnetic null and
construct magnetic field topology, in order to use it with the data from the forth-coming MMS
mission. We compare this method with the previously used Poincare index (PI), and find that
they are generally consistent, except that the PI method can only find a null inside the
spacecraft (SC) tetrahedron, while the Taylor expansion can find a null both inside and
outside the tetrahedron and also deduce its drift velocity. Taylor expansion can also: (1)
avoid the limitations of PI method such as data resolution, instrument uncertainty
(Bz offset), and SC separation; (2) identify 3D null types (A, B, As, and Bs) and
determine whether these types can degenerate into 2D (X and O); (3) construct the
magnetic field topology. We quantitively test the accurateness of Taylor expansion in
positioning magnetic null and constructing field topology, by using the data from 3D
kinetic simulations. The influences of SC separation (from 0.05 to 1 di) and null-SC
distance (from 0 to 1 di) on the accurateness are both considered. We find that: (1) for
single null, the method is accurate when the SC separation is smaller than 1 di, and
the null-SC distance is smaller than 0.5 di (weakly chaotic reconnection) or 0.25
di (strongly chaotic reconnection); (2) for null pair, the accurateness is same as
the single-null situation, except at the null-null line, where the field is nonlinear.
We invent a parameter ξ ≈¡|(λ1 + λ2 + λ3)|/ |λ|max to quantify the quality of
the method—the smaller this parameter the better the results. Comparing to the
previously used one (η ≈¡|/ /
B|/ |/ x B |), this parameter is more relevant. Using the
new method, we construct the magnetic field topology around a radial-type null
and a spiral-type null, and find that the topologies are well consistent with those
predicted in theory. This means that our method is reliable. We therefore suggest using
this method to find magnetic null and construct field topology with the four-point
measurements, particularly the Cluster and forth-coming MMS measurements. |
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