[1] We performed three-dimensional Hall magnetohydrodynamic (MHD) simulations of magnetic reconnection with finite width along the direction perpendicular to the antiparallel magnetic field (i.e., the direction of the electric current). Previous similar simulations including the Hall term have shown that the localized reconnection region itself can broaden in the anticurrent direction when the initial current is carried only by electrons. However, there is still no clear understanding of the behavior of the reconnection region in the presence of the initial ion current as in the Earth's magnetotail plasma sheet since no simulations have been carried out under such situations. In this study, we performed a systematic parametric survey considering the cases in which the initial current is carried not only by electrons but also by ions and found that the speed and direction of the current-aligned broadening of the reconnection region are almost equal to those of background ion and electron flows that carry the current. This result means that location and size of the localized reconnection region vary with time, depending on plasma conditions in the background current sheet in Hall MHD regime. The rate of the localized reconnection can reach close to the value in the two-dimensional case, even when reconnection starts in an extremely narrow region with its current-aligned width equal to an ion inertial length. The localized reconnection process also produces the asymmetry of the current-aligned structure of the reconnection jet. These results can explain various observational features related to magnetic reconnection in the near-Earth magnetotail.Citation: Nakamura, T. K. M., R. Nakamura, A. Alexandrova, Y. Kubota, and T. Nagai (2012), Hall magnetohydrodynamic effects for three-dimensional magnetic reconnection with finite width along the direction of the current,

We investigated the temporal variations and saturation of the cross polar cap potential (CPCP) in the Bastille Day storm event (15 July 2000) by global magnetohydrodynamics (MHD) simulation. The CPCP is considered to depend on the electric field and dynamic pressure of the solar wind as well as on the ionospheric conductivity. Previous studies considered only the ionospheric conductivity due to solar extreme ultraviolet (EUV) variations. In this paper, we dealt with the changes in the CPCP attributable to auroral conductivity variations caused by pressure enhancement in the inner magnetosphere owing to energy injection from the magnetosphere because the energy injection is considerably enhanced in a severe magnetic storm event. Our simulation reveals that the auroral conductivity enhancement is significant for the CPCP variation in a severe magnetic storm event. The numerical results concerning the Bastille Day event show that the ionospheric conductivity averaged over the auroral oval is enhanced up to 18 mho in the case of Bz of less than −59 nT. On the other hand, the average conductivity without the auroral effect is almost 6 mho throughout the entire period. Resultantly, the saturated CPCP is about 240 kV in the former and 704 kV in the latter when Bz is −59 nT. This result indicates that the CPCP variations could be correctly reproduced when the time variation of auroral conductivity caused by pressure enhancement due to the energy injection from the magnetosphere is correctly considered in a severe magnetic storm event.

A sudden increase in the dynamic pressure of solar wind generates a prominent and transient change in ground-based magnetometer records worldwide, which is called a sudden commencement (SC). The magnetic field variation due to an SC at high latitudes shows a bipolar change, which consists of a preliminary impulse (PI) and main impulse (MI). The largest recorded SC had an amplitude of more than 200 nT with a spiky waveform at low latitudes, and the mechanism causing this super SC is unknown. Here, we investigate the cause of the super SC using a newly developed magnetosphere-ionosphere coupling simulation, which enables us to investigate the magnetospheric response to a large increase in the solar wind dynamic pressure. To simulate SCs, the dynamic pressure of the solar wind is increased to 2, 5, 10, and 16 larger than that under the stationary condition, and two different types of dynamic pressure increase are adopted by changing the solar wind density only or the solar wind speed only. It was found that the magnetic field variations of the PI and MI are several times larger and faster for a jump in the speed than for a jump in the density. It is inferred that a solar wind velocity of more than 2500 km/s in the downstream shock, which cannot be directly simulated in this study, would be consistent with the super SC.

We perform numerical modeling of the interchange cycle in the magnetosphere‐ionosphere convection system for oblique northward interplanetary magnetic field (IMF). The interchange cycle results from the coupling of IMF‐to‐lobe reconnection and lobe‐to‐closed reconnection. Using a global magnetohydrodynamic simulation code, for an IMF clock angle of 20° (measured from due north), we successfully reproduced the following features of the interchange cycle. (1) In the ionosphere, for each hemisphere, there appears a reverse cell circulating exclusively in the closed field line region (the reciprocal cell). (2) The topology transition of the magnetic field along a streamline near the equatorial plane precisely represents the magnetic flux reciprocation during the interchange cycle. (3) Field‐aligned electric fields on the interplanetary‐open separatrix and on the open‐closed separatrix are those that are consistent with IMF‐to‐lobe reconnection and lobe‐to‐closed reconnection, respectively. These three features prove the existence of the interchange cycle in the simulated magnetosphere‐ionosphere system. We conclude that the interchange cycle does exist in the real solar wind‐magnetosphere‐ionosphere system. In addition, the simulation revealed that the reciprocal cell described above is not a direct projection of the diffusion region as predicted by the “vacuum” model in which diffusion is added a priori to the vacuum magnetic topology. Instead, the reciprocal cell is a consequence of the plasma convection system coupled to the so‐called NBZ (“northward Bz”) field‐aligned current system.

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