Development of Magnetic Topology During the Growth Phase of the Substorm Inducing the Onset of the Near‐Earth Neutral Line

• Published in: Journal of geophysical research. Space physics 2019-07, Vol.124 (7), p.5158-5183
• Main Authors: Tanaka, T., Ebihara, Y., Watanabe, M., Den, M., Fujita, S., Kikuchi, T., Hashimoto, K.K., Kataoka, R.
• Format: Article
• Language: English
• Subjects: Bifurcations; Coils; core By; Curvature; Divergence; divergence flow; Interplanetary magnetic field; Ionosphere; Magnetic fields; Magnetic structure; Magnetism; Magnetopause; Magnetospheres; null‐separator structure; open‐closed boundary; Plasma sheet reconnection; Polar ionosphere; Topology
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2018JA026386

The transition of magnetic topology is analyzed during the substorm. During the growth phase in the polar ionosphere, magnetic fields consist of three categories: closed magnetic field, open magnetic field leading to the northward interplanetary magnetic field (IMF), and open field leading to the southward IMF. While the open magnetic field region leading to the southward IMF expands consistently during the growth phase, the ionospheric onset starts inside the closed field line region. There should be triple points where three kinds of magnetic field coexist in the ionosphere. Connected to the triple points, complex magnetic topologies appear in the magnetosphere. There occur definitive changes in the global magnetic topology such as retreat of cusp nulls generated under the northward IMF, generation of plasma sheet magnetic fields with reverse curvature, generation of bifurcation areas on the magnetopause, formation of new dayside nulls, development of null lines on the flank, and generation of bifurcation areas (group of nulls) on the magnetopause. During the late growth phase, divergence Vx flow is excited in the central plasma sheet associated with the appearance of reverse curvature. The plasma sheet reconnection starts 5 min before the ionospheric onset from a localized area at X = −17.5 Re due to the reduction of the normal component Bz caused by divergence Vx flow. It occurs in the residual magnetic structure formed under the northward IMF as a three‐dimensional reconnection with a guide field By. The resulting two‐turn coil of magnetic field forms a plasmoid, which is ejected downtail. Key Points The growth phase of the substorm proceeds not only with dynamical development but also with change in magnetic topology During the growth phase, magnetic field lines with reverse curvature appear in the plasma sheet in the topological structure caused by retreating nulls The plasma sheet reconnection starts at the interface between the dipole and the tail through the reduction of Bz caused by divergence Vx flow