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

Abstract

With the interplanetary magnetic field (IMF) that turns from northward to southward, the global simulation successively reproduces the growth phase, the onset, and the expansion phase of the substorm. The calculated ionospheric convection for the growth phase reproduces the development of the Harang reversal (HR), the upward field‐aligned current (FAC), and the upcoming onset point as observed. Magnetic field lines traced from the center of the nightside upward FAC are open, while the magnetic field line traced from the onset point is closed. These open magnetic field lines map to flow shear just outside the O/C boundary. Seen from the magnetic configuration, the growth phase proceeds as the replacement process of nulls. Two new nulls appear on the dayside under the southward IMF, while two old nulls under the northward IMF retreat tailward forming two bifurcation regions on the dawn and dusk flanks. Flow shear around the O/C boundary forms by magnetospheric convection that returns to the dayside via bifurcation regions. The expansion phase proceeds through a topological change by the near‐earth neutral line (NENL). The NENL occurs inside the thinned structure of the northward IMF remnant, on the low‐latitude side of the flow shear, and projects down to the low‐latitude edge of the upward FAC. Associated with the NENL, the convection return path changes to the center of the plasma sheet and reveals in the ionosphere as the release of the HR. By the shrinkage of projecting magnetic field line, the O/C boundary migrates poleward in the ionosphere.

Development of the Substorm as a Manifestation of Convection Transient

“…From the AU/AL magnitude, the numerically reproduced substorm is a medium-scale one. Above appearances of the simulated substorm have already been reported by and Tanaka et al (2020Tanaka et al ( , 2021.…”

“…Movie M2 in the Supporting Information shows the development of pressure in the noon-midnight meridian plane. Change of magnetic topology in the magnetosphere from the growth phase to the onset was reported by and Tanaka et al (2010Tanaka et al ( , 2021. We can see null structure affecting the substorm processes.…”

“…The credibility for the present solution of numerically generated substorm can be confirmed more directly, by comparing numerical solutions with ionospheric observations such as the AU/AL variations (Tanaka et al, 2020) and auroral disturbances estimated from the FAC (Ebihara & Tanaka, 2015a;Tanaka et al, 2021). The simulation reproduces overall features of ionosphere disturbances that have been known for a long age.…”

“…In their model, the origin of the FAC is the non-parallelism of contour lines of P and |B| in the plasma sheet. In the global simulation by and Tanaka et al (2021), the same perspective of relating the quiet arc to the FAC appears also. In this case, however, the upward FAC is associated with shear at the lobe/plasma sheet boundary.…”

“…The HR is explained as the deflection of plasma sheet convection (Ohtani et al, 2016). Among the global simulations, only Tanaka et al (2021) reproduced both the quiet arc and the HR simultaneously as a part of global convection. It is a large difference between the conventional interpretation and the global simulation that the high-latitude side of the quiet arc is on a close magnetic field in the former but this part is on an open magnetic field in the latter.…”