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We reproduced the substorm by global simulation and analyzed the development of convection, shear, the dynamo, and the field-aligned current (FAC) at the final stage of the growth phase. From these investigations, we show that the substorm is a manifestation of reconfiguration in global flow dynamics. Ionospheric convection can be understood from two aspects. One understanding is as the potential field generated by the FAC, and the other is as the projection of magnetospheric convection. In order for the two to coincide, the FAC must be transmitted together with the motion. As a consequence, the resulting convection must be continuous from the magnetosphere to the ionosphere. We see this connection from the drawing of shear to recognize that the substorm is the projection process of transient convection. In the growth phase, convective shear that causes the quiet arc and the Harang reversal (HR) occurs on the open/closed boundary along the surface of the plasma sheet with a continuous flow structure from the magnetosphere to the ionosphere. The onset starts from a more local flow that is induced by the near-earth neutral line (NENL). A narrow shear commences from the NENL in the mid-tail and extends to the ionosphere to replace growth phase shear. Along a closed magnetic field line connected to the NENL, direct penetration flow, squeezing flow, the near-earth dynamo, release of the HR, and the onset FAC occur successively and consequently cause the ground onset. Such onset mechanism is quite different from the model adopting the current wedge.

To investigate the energy involved by a substorm, we performed global magnetohydrodynamics simulation for different solar wind conditions. The intensity of the auroral electrojet increases with the southward component of interplanetary magnetic field (IMF) and the solar wind velocity, which is consistent with observations. To evaluate the energy that enters the magnetosphere, we first defined a solar wind effective cross‐sectional area in which all the Poynting fluxes entering the magnetosphere pass through. We found that the solar wind magnetic energy is insufficient to provide the magnetic energy entering the magnetosphere (intake magnetic energy) for southward IMF. About 33–88% of the intake magnetic energy is converted from the solar wind kinetic energy. About 2–7% of the solar wind kinetic energy (passing through the effective area) is converted to the magnetic energy that enters the magnetosphere. Significant contribution from the solar wind kinetic energy makes the energy coupling function complicated. The effective area also depends on the solar wind parameters, also making it complicated. An interesting point is that the rates of energy stored and released in the lobe also depend on the solar wind parameters. The ionospheric Joule heating rate is well correlated with the intake magnetic energy at onset, and during the substorm expansion. The correlation coefficients between them are better than that of the ε parameter. These results imply that both the directly driven process (manifested by intake magnetic energy) and loading‐unloading process (manifested by stored/released energy in the lobe) are largely regulated by the solar wind condition.

Major solar eruptions occasionally direct interplanetary coronal mass ejections (ICMEs) to Earth and cause significant geomagnetic storms and low-latitude aurorae. While individual extreme storms are significant threats to modern civilization, storms occasionally appear in sequence, acting synergistically, and cause "perfect storms"on Earth. The stormy interval in 1938 January was one of such cases. Here, we analyze the contemporary records to reveal its time series on their source active regions, solar eruptions, ICMEs, geomagnetic storms, low-latitude aurorae, and cosmic-ray (CR) variations. Geomagnetic records show that three storms occurred successively on January 17/18 (Dcx ≈ -171 nT), January 21/22 (Dcx ≈ -328 nT), and January 25/26 (Dcx ≈ -336 nT). The amplitudes of the CR variations and storm sudden commencements (SSCs) show the impact of the first ICME as the largest (≈6% decrease in CR and 72 nT in SSC) and the ICMEs associated with the storms that followed as more moderate (≈3% decrease in CR and 63 nT in SSC; ≈2% decrease in CR and 63 nT in SSC). Interestingly, a significant solar proton event occurred on January 16/17 and the Cheltenham ionization chamber showed a possible ground-level enhancement. During the first storm, aurorae were less visible at midlatitudes, whereas, during the second and third storms, the equatorward boundaries of the auroral oval were extended down to 40.3 and 40.0 in invariant latitude. This contrast shows that the initial ICME was probably faster, with a higher total magnitude but a smaller southward component. © 2021. The American Astronomical Society. All rights reserved..

In this paper, currents causing the sudden commencement (SC), the AU/AL indices, and the positive bay during the substorm are identified from the global simulation and Biot-Savart's law. Candidate currents assumed as causes of these ground magnetic variations are the ionospheric Hall current, the ionospheric Pedersen current, the field-aligned current (FAC), and other magnetospheric currents than the FAC. In general, FAC effect and Pedersen current effect cancel out each other under the restriction of Fukushima's theorem. During the SC, for instance, the midlatitude preliminary positive impulse appears in the prenoon and midlatitude preliminary reverse impulse (PRI) appears in the postnoon, due to the remaining effect of the Hall current. However, violations of the Fukushima's theorem are also common such as in the cases of the equatorial PRI, the auroral electrojet, and the positive bay. The equatorial PRI caused by the Pedersen current appears both in the prenoon and postnoon regions. In the auroral region, the Hall current effect prevails over other currents so much and determines the AU/AL indices only from it regardless other currents. The midlatitude positive bay on the nightside is generated by the effect of the FAC. From these diverse reproduction of ground magnetic variations, a further verification is given for the global simulation in reproductions of the magnetosphere-ionosphere coupling process.

Isolated proton aurora (IPA) is a manifestation of the wave‐particle interaction visible at subauroral latitudes, with activity on many timescales. We herein present the first observational evidence of rapid luminous modulation of IPA correlated with simultaneously observed Pc1 waves observed on the ground, which are equivalent to the electromagnetic ion cyclotron (EMIC) waves in the magnetosphere. The fastest luminous modulation of IPA was observed in the 1 Hz frequency range, which was twice the frequency of the related Pc1 waves. The time lag between variations of Pc1 wave power and the IPA luminosity suggests that the source regions of IPA are distributed near the magnetic equator, suggesting an EMIC wave‐energetic (a few tens of keV) proton or relativistic (MeV or sub‐MeV) electron interaction. The generation mechanism of this 1 Hz luminous modulation remains an open issue, but this study supports the importance of nonlinear pitch angle scattering via wave‐particle interactions.