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

Abstract

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.

Formation and Release of the Harang Reversal Relating With the Substorm Onset Process

“…Consequently, there appear two flank areas where the magnetic field lines bifurcate to many types of magnetic field lines. Tanaka et al (2019a) have already reported these structures. Figure 12 shows magnetic field lines that are further inside of those shown in Figure 11.…”

“…The downtail side of the null line leads to the retreating null formed under the northward IMF. Magnetic field lines in the green region were shown in Figure 7 of Tanaka et al (2019a). Figure 11 shows a magnetic field line that connects the triple point in the ionosphere, the retreating null, and the IMF.…”

Reproduction of Ground Magnetic Variations During the SC and the Substorm From the Global Simulation and Biot‐Savart's Law

“…Associated with the onset, path of return convection in the tail is changed from the lobe‐plasma sheet boundary to the plasma sheet center (Tanaka, Ebihara, et al, ). This change occurs in synchronous with the transition of the tail topology (Tanaka, Ebihara, et al, ). These processes are often understood by a simplified picture that the NENL is formed to release flow jet from it.…”

“…The main auroral events during the substorm will be the quiet arc during the growth phase, the initial brightening at the onset, and the WTS during the expansion phase. Recent global simulations have reproduced the occurrences of these auroras as a series of events (Tanaka, ; Ebihara & Tanaka, , , ; Tanaka et al, , Tanaka, Ebihara, et al, ; Tanaka, Ebihara, et al, ). Another phenomenon that characterize the substorm, as well as the aurora, may be the global variation of geomagnetic field.…”

“…The global simulation reproduces a series of auroral phenomena during the substorm, such as the quiet arc (Tanaka, ; Tanaka, Ebihara, et al, ) and the preonset N‐S arc (Ebihara & Tanaka, ) during the growth phase, the initial brightening at the onset (Ebihara & Tanaka, ; Tanaka, Ebihara, et al, ; Tanaka, Ebihara, et al, ), and the westward traveling surge (WTS) during the expansion phase (Ebihara & Tanaka, ). The global simulation is verified by comparing these results with observations.…”