Correlation of core field polarity of magnetotail flux ropes with the IMF B_y: Reconnection guide field dependency

Abstract

The relationship between the core field and the interplanetary magnetic field (IMF) B y has been addressed by spacecraft observations in the magnetotail, but it is not yet fully clear since observations by Slavin et al. (2003) and Borg et al. (2012) show controversial results. In this study, we examine 13 flux ropes from the Cluster observations to show for the first time that the correlation between the core field B core and the IMF B y depends on the guide field B g . For large guide fields (> 20% of the reconnecting field), we show that the B core is found to correlate with the IMF B y . However, for weak guide fields (< 10% of the reconnecting field) we show that the core fields have either a positive or negative polarity, irrespective of the IMF B y . This result indicates that for weak guide field reconnection the core field generation of the magnetotail flux rope is not governed by the external IMF B y . We can explain the previous controversy in terms of the guide field dependency. We also confirm earlier suggestions that for weak guide field reconnection the flux ropes can have a significant core field whose polarity agrees with the ambient quadrupole Hall field. In addition, the Grad-Shafranov reconstruction results suggest that the axis of the flux rope may have been kinked. Our findings are crucial and can help advance theoretical simulations of the core field generation for weak guide reconnection. Discussions of the possible core field generation for weak guide field will be given in terms of 3-D reconnection.

Evidence for the core field polarity of magnetic flux ropes against the reconnection guide field

“…Results from Teh et al . [] are thus different from Lui et al . [] for which the change of the core field polarity occurred within the complex flux rope structure.…”

“…Recently, Teh et al . [] showed that the correlation between the core field polarity of the magnetotail flux ropes and the IMF B y is guide‐field dependent: for strong guide fields (>20% of the reconnecting field), the core field polarity was correlated with the IMF B y , while for weak guide fields (<10% of the reconnecting field) the core fields have either a positive or negative polarity, irrespective of the IMF B y . However, due to the small guide field strength (<2.0 nT) and the dynamic motion of the current sheet, they cannot unambiguously determine whether the polarity of the guide field changed during the event, when considering the uncertainty of the estimate of the X‐line orientation.…”

“…It should be noted that only a single core field peak within the flux rope was considered in the study by Teh et al . [] to exclude the complex flux rope structures, e.g., multiple core field peaks [ Lui et al ., ; Borg et al ., ]. Results from Teh et al .…”

Dipolarization fronts in the near-Earth space and substorm dynamics

“…The Grad-Shafranov (GS) (Hau and Sonnerup, 1999;Hu and Sonnerup, 2002;Sonnerup et al, 2006;Möstl et al, 2009;Isavnin et al, 2011;Teh et al, 2014) reconstruction method is applied to recover the two-dimensional magnetic field maps of the events. The technique assumes that the analyzed magnetic structure is in magnetohydrostatic equilibrium ( p = j × B), and is 2 1/2-dimensional, i.e., the structure has translational symmetry along its invariant axis ( ∂ ∂z = 0).…”

Enhancement of oxygen in the magnetic island associated with dipolarization fronts

“…The magnetic island is characterized by a bundle of circular magnetic field lines without a strong core field in the “cross‐tail” direction [ Borg et al ., ; Kiehas et al ., ; Huang et al ., ]; it is typically a signature/product of multipoint reconnection in the magnetotail [ Deng et al ., ; Eastwood et al ., ; Huang et al ., ; Fu et al ., ]. The magnetic island with strong core field may be related to guide‐field reconnection called the flux rope [ Teh et al ., ; Huang et al ., ], while the magnetic island without core field is usually called magnetic island [ Fu et al ., ] or plasmoid [ Zong et al ., ]. In 2‐D [e.g., Markidis et al ., ] and 3‐D [e.g., Daughton et al ., ] simulations, the magnetic‐island (flux‐rope) structures can be well recognized, as well as in spacecraft in situ measurements [ Slavin et al ., ].…”