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Reconnection layer bounded by switch‐off shocks: Dayside magnetopause crossing by THEMIS D
Author(s) -
Sonnerup Bengt,
Paschmann Götz,
Haaland Stein,
Phan Tai,
Eriksson Stefan
Publication year - 2016
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2016ja022362
Subject(s) - magnetosheath , magnetopause , physics , magnetosphere , geophysics , magnetic reconnection , solar wind , magnetohydrodynamics , bow shock (aerodynamics) , ionosphere , computational physics , mechanics , plasma , astrophysics , shock wave , quantum mechanics
We discuss observations of reconnection, obtained by Time History of Events and Macroscale Interactions during Substorms (THEMIS) D during an outward bound traversal of the low‐latitude dayside magnetopause. The reconnection signatures include high magnetic shear, a southward directed Alfvénic jet, bounded by slow‐mode shocks near the switch‐off limit (as in the symmetric Petschek geometry), a small, sunward directed normal magnetic field and plasma inflow into the jet from both sides. We conclude that cold, unmeasured ionospheric ions helped establish the symmetry. The effective ion mass, estimated from the switch‐off condition, was 2.39 amu on the magnetospheric side, where the number density was inferred from the spacecraft potential, and 1.09 amu on the magnetosheath side. After a modest pressure correction in the magnetospheric shock, the MHD jump conditions for density, pressure, temperature, and entropy were well satisfied. The shock jumps were much larger on the magnetosphere side than on the magnetosheath side; we show this to be a plasma β effect. The main dissipation mechanism appears to be irreversible transfer between thermal motion parallel and perpendicular to the field, such that both shocks bring about approximate downstream temperature isotropy. Hall currents and electric fields were present, albeit in a strongly asymmetric configuration. The magnetospheric shock had longer duration than the magnetosheath one, possibly as a result of a nonconstant magnetopause speed. We infer an average earthward magnetopause speed (14 km/s), corresponding nominal shock thicknesses (12 and 6 λ i ), dimensionless reconnection rates (0.061–0.085), and reconnection wedge angles (5° between shocks; 13° between separatrices).