Cooperatives Roles of Dynamics and Topology in Generating the Magnetosphere‐Ionosphere Disturbances: Case of the Theta Aurora

Abstract

To understand the magnetosphere‐ionosphere (M‐I) disturbances, it is necessary to know the change in the null‐separator structure along with the development of convection. The present paper explains this proposition by taking the case of the theta aurora that occurs after the switch of interplanetary magnetic field By and reveals auroral development inside the polar cap. The interplanetary magnetic field change is followed by the reformation of the null‐separator structure to cause a convection transient and a successive deformation of separatrices inside the magnetosphere, where separatrices separate the space into different volumes that include different types of magnetic field lines, and intersections of separatrices form separators that connect nulls. By the reconnection on the separator, energy inflow occurs from the solar wind to the magnetosphere. Convection driven by this energy transports nulls. In the case of theta aurora, two new nulls on the dayside coexist with two old nulls carried toward the tail by convection. Each set of nulls generates two open‐closed boundaries (separatrices) bounded by separators. Thus, open‐closed boundaries are divide into four parts. Separatrices generated by different sets partially become side by side near the nightside stem limes. The theta aurora is generated at the area corresponding to the low‐latitude side with respect to both of two boundaries. Thus, the occurrence of the theta aurora is a natural consequence of a transition in the null‐separator structure. The present example indicates that the global structure of the magnetosphere is almost decided from the skeleton of nulls.

Magnetosphere‐Ionosphere Convection Under the Due Northward IMF

“…Thus, we can estimate how the numerical model is appropriately working by seeing the reproducibility of ionospheric disturbances. As ionospheric disturbances, REPPU code reproduces the quiet arc after the southward turning of the IMF (Tanaka, 2015;, the initial brightening at the substorm onset (Ebihara & Tanaka, 2015a;Tanaka, 2015;, equatorial counter electrojet after the substorm onset (Ebihara et al, 2014), the westward traveling surge (Ebihara & Tanaka, 2015b;Ebihara & Tanaka, 2017b), the Sun-aligned arc under the northward IMF (Tanaka, Obara, et al, 2017), and the theta aurora (Tanaka et al, 2004(Tanaka et al, , 2018 quite similarly to observations. In all cases, the reproducibility of the ionospheric features outperforms other global models.…”

“…The distribution of the Sun-aligned arc is not radially from the cusp but parallel to the 12-0 LT [Tanaka, Obara, et al, 2017;Tanaka et al, 2018]. The shape of the FAC in this part is similar to the Sun-aligned arc, but the direction is not similar.…”

“…Journal of Geophysical Research: Space Physics meridian [Tanaka, Obara, et al, 2017;Tanaka et al, 2018]. In the case of the Sun-aligned arc, distributions become parallel to the 12-0 LT meridian, because there is the void in the center of the polar cap.…”

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

“…For such comparison in the ionosphere, validity of the global simulation has been confirmed also under the northward interplanetary magnetic field (IMF), in reproducing the Sun‐aligned arc (Tanaka, Obara, et al, ), the theta aurora (Tanaka et al, , ), and the fan‐shaped arc (Tanaka, Obara, et al, ). These phenomena have been known for a long time, but consideration for their causes have been staying at the level of estimation.…”

Aurora in the Polar Cap: A Review

“…On the other hand, Tanaka et al's (2004) detailed examination of the magnetic field line convection after the B y sign change revealed, that a return flow blockage in the convection cells after an IMF B y sign change may be responsible for the pileup of closed flux at the location of the bifurcated plasma sheet. Whether this pile-up of closed flux is created by reconnection or is just a result of redistributing field lines is not addressed by Tanaka et al (2004) and remains to be shown (note that, just recently, a newer version of REPPU code was applied to the theta aurora case and the results are introduced in Tanaka et al 2018).…”