THEMIS observations of a secondary magnetic island within the Hall electromagnetic field region at the magnetopause

Abstract

We present THEMIS observations of a small‐scale secondary magnetic island within the Hall electromagnetic field region at the dayside magnetopause. The reconnecting magnetic fields are nearly symmetric and anti‐parallel, whereas the ion plasma density is ∼8 times higher in the magnetosheath than in the magnetosphere. In the event, the Hall magnetic and electric fields are asymmetric and stronger on the magnetospheric side of the magnetopause. The small‐scale magnetic island was immersed in the out‐of‐plane Hall magnetic field on the magnetosheath side. We derive a map of the cross section of the magnetic island by use of magnetohydrostatic Grad‐Shafranov reconstruction under the force‐free assumption. The reconstruction shows a right‐handed magnetic island with a length of ∼200 km and a width of ∼100 km.