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

The relationship between the core field and the interplanetary magnetic field (IMF) By 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 Bcore and the IMF By depends on the guide field Bg. For large guide fields (> 20% of the reconnecting field), we show that the Bcore is found to correlate with the IMF By. 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 By. 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 By. 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. Key Points Correlation between core field and IMF By is guide field dependent Reconnection guide field dependency can explain previous controversial results Results are crucial to studying core field generation for weak guide field