Asymmetry in the Earth\u2019s magnetotail neutral sheet rotation due to IMF B_y sign?

G. Siung Chong,Anita Kullen,Alexandre De Spiegeleer,Timo Pitkänen,Anita T. Aikio,Maria Hamrin,Heikki Vanhamäki,Quanqi Shi,Lei Cai,Jong-Sun Park

doi:10.1186/s40562-020-00171-7

Abstract

Evidence suggests that a non-zero dawn–dusk interplanetary magnetic field (IMF B_y) can cause a rotation of the cross-tail current sheet/neutral sheet around its axis aligned with the Sun–Earth line in Earth’s magnetotail. We use Geotail, THEMIS and Cluster data to statistically investigate how the rotation of the neutral sheet depends on the sign and magnitude of IMF B_y. In our dataset, we find that in the tail range of -30< XGSM <-15R_{mathrm{E}}, the degree of the neutral sheet rotation is clearly smaller, there appears no significant rotation or even, the rotation is clearly to an unexpected direction for negative IMF B_y, compared to positive IMF B_y. Comparison to a model by Tsyganenko et al. (2015, doi:10.5194/angeo-33-1-2015) suggests that this asymmetry in the neutral sheet rotation between positive and negative IMF B_y conditions is too large to be explained only by the currently known factors. The possible cause of the asymmetry remains unclear.

Highlights

The two magnetic hemispheres in the Earth’s magnetotail are separated by a dawn-to-dusk-directed cross-tail current sheet
Shown are the positions of the neutral sheet crossings in the geocentric solar magnetospheric (GSM) YZ plane for each IMF category
We have fitted a line of best fit of linear least squares sense to the data points to statistically estimate the neutral sheet rotation

Summary

Introduction

The two magnetic hemispheres in the Earth’s magnetotail are separated by a dawn-to-dusk-directed cross-tail current sheet. The boundary between the magnetic hemispheres is usually defined as a surface at which the X component (along the Sun– Earth line) of the tail magnetic field reverses ( Bx = 0). It is called the neutral sheet (Ness 1965). The hinging effect shifts the neutral sheet northward and southward with respect to the Sun–Earth line when the geomagnetic dipole tilt angle is positive (generally northern hemisphere summer) and negative (northern hemisphere winter), respectively. IMF By > 0 , the neutral sheet is rotated counter-clockwise in the tail cross-sectional plane when looking from Earth toward the tail. IMF By < 0 , the rotation is clockwise

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Discussion

Conclusion