A pseudo-magnetic flux rope observed by the THEMIS satellites in the Earth's magnetotail

Abstract: We investigate an extraordinary event showing all the typical magnetic flux rope (MFR) signatures, although it is not really a MFR structure. It occurred on 1 March 2008 in the Earth''s magnetotail and was observed by a major tail conjunction of Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. THEMIS B and C being located inside the central plasma sheet and almost symmetrically above and below the neutral sheet observed the same tailward retreating MFR-like structure: they indeed detected strong but oppositely directed cross-tail magnetic field excursions: positive “B y core” for TH-C and negative for TH-B; an apparent inconsistency. We finally categorize the case under study as a pseudo-MFR event and we doubt that the previously studied MFR-like structures were really rope structures. We suggest that the B y excursions are dictated by Ampere''s law; they are produced by filamentary field-aligned currents (FACs) created in front of the “akis structure”, as it is introduced by : In a locally thinned plasma sheet, the akis potentially causes charge separation due to non-adiabatic motion and stochastic scattering of ions. In turn, the newly tailward escaped ions drive field-aligned ionospheric currents in order to neutralize this region. We extensively discuss an additional and extremely rare phenomenon of “irregular MFR” cited in the literature and observed by the Cluster satellites; filamentary FACs suffice to reproduce all the observed magnetic field signatures, too. [Copyright &y& Elsevier]