Takashi Kikuchi

Counter equatorial electrojet and overshielding after substorm onset: Global MHD simulation study

By performing a global magnetohydrodynamic (MHD) simulation, we have demonstrated for the first time that an electrojet at the dayside magnetic equator can be reversed and an overshielding condition can be established in the inner magnetosphere after substorm onset without northward turning of the interplanetary magnetic field. Near the substorm onset, the plasma pressure is highly enhanced in the inner magnetosphere on the nightside. The Region 2 field-aligned current diverges from the diamagnetic current on the surface of the dayside extension of the high-pressure region, which is connected to the ionosphere in the relatively low-conductivity region a few degrees equatorward of the main auroral oval that is formed as the projection of the plasma sheet. The separation of the equatorward boundary of the auroral region and the equatorward boundary of the Region 2 current results in dusk-dawn electric fields that generate a counter electrojet (CEJ) at the dayside magnetic equator. Poleward electric fields in a narrow latitudinal width, which may be regarded as subauroral ion drift and subauroral polarization stream, are simultaneously intensified. The dusk-dawn electric fields may propagate to the inner magnetosphere along a field line as shear Alfven waves. Then, the inner magnetosphere is completely constrained by the overshielding condition. The intensity and polarity of the CEJ depend largely on at least the ionospheric conductivity that is related to the plasma pressure (probably associated with diffuse aurora). This may explain the observational fact that overshielding does not always occur after onset.

Transmission line model for the near‐instantaneous transmission of the ionospheric electric field and currents to the equator

The simultaneous onset of the preliminary impulse (PI) of the geomagnetic sudden commencement at high latitude and dayside dip equator is explained by means of the TM0 mode waves propagating at the speed of light in the Earth-ionosphere waveguide (EIW) [Kikuchi et al., 1978]. A couple of issues remain to be addressed in the EIW model: (1) How is the TM0 mode wave excited by the field-aligned currents (FACs) in the polar region? (2) How are the quasi-steady ionospheric currents achieved by the TM0 mode waves? (3) How simultaneous or delayed are the onset and peak of the equatorial PI with respect to the high-latitude PI? To address these issues, we examine the TEM (TM0) mode wave propagation in the finite-length transmission lines replacing the pair of FACs (magnetosphere-ionosphere (MI) transmission line) and the Earth-ionosphere waveguide (ionosphere-ground (IG) transmission line). The issue (1) is addressed by showing that a fraction of the TEM mode wave is transmitted from the MI to IG transmission lines through the polar ionosphere. To address the issues (2) and (3), we examine the properties of the finite-length IG transmission line with finite ionospheric conductivity. It is shown that the ionospheric currents start to grow instantaneously and continue to grow gradually with time constants of 1–10u2009s depending on the ionospheric conductivity. The MIG transmission line enables us to explain the instantaneous onset and delayed peak time of the equatorial PI and quick electric field response of the low-latitude ionosphere and inner magnetosphere.

Selective Generation of Formamides through Photocatalytic CO2 Reduction Catalyzed by Ruthenium Carbonyl Compounds

The selective formation of dialkyl formamides through photochemical CO2 reduction was developed as a means of utilizing CO2 as a C1 building block. Photochemical CO2 reduction catalyzed by a [Ru(bpy)2(CO)2](2+) (bpy: 2,2-bipyridyl)/[Ru(bpy)3](2+)/Me2NH/Me2NH2(+) system in CH3CN selectively produced dimethylformamide. In this process a ruthenium carbamoyl complex ([Ru(bpy)2(CO)(CONMe2)](+)) formed by the nucleophilic attack of Me2NH on [Ru(bpy)2(CO)2](2+) worked as the precursor to DMF. Thus Me2NH acted as both the sacrificial electron donor and the substrate, while Me2NH2(+) functioned as the proton source. Similar photochemical CO2 reductions using R2NH and R2NH2(+) (R = Et, nPr, or nBu) also afforded the corresponding dialkyl formamides (R2NCHO) together with HCOOH as a by-product. The main product from the CO2 reduction transitioned from R2NCHO to HCOOH with increases in the alkyl chain length of the R2NH. The selectivity between R2NCHO and HCOOH was found to depend on the rate of [Ru(bpy)2(CO)(CONR2)](+) formation.

Transmission of the electric fields to the low latitude ionosphere in the magnetosphere-ionosphere current circuit

The solar wind energy is transmitted to low latitude ionosphere in a current circuit from a dynamo in the magnetosphere to the equatorial ionosphere via the polar ionosphere. During the substorm growth phase and storm main phase, the dawn-to-dusk convection electric field is intensified by the southward interplanetary magnetic field (IMF), driving the ionospheric DP2 currents composed of two-cell Hall current vortices in high latitudes and Pedersen currents amplified at the dayside equator (EEJ). The EEJ-Region-1 field-aligned current (R1 FAC) circuit is completed via the Pedersen currents in midlatitude. On the other hand, the shielding electric field and the Region-2 FACs develop in the inner magnetosphere, tending to cancel the convection electric field at the mid-equatorial latitudes. The shielding often causes overshielding when the convection electric field reduces substantially and the EEJ is overcome by the counter electrojet (CEJ), leading to that even the quasi-periodic DP2 fluctuations are contributed by the overshielding as being composed of the EEJ and CEJ. The overshielding develop significantly during substorms and storms, leading to that the mid and low latitude ionosphere is under strong influence of the overshielding as well as the convection electric fields. The electric fields on the day- and night sides are in opposite direction to each other, but the electric fields in the evening are anomalously enhanced in the same direction as in the day. The evening anomaly is a unique feature of the electric potential distribution in the global ionosphere. DP2-type electric field and currents develop during the transient/short-term geomagnetic disturbances like the geomagnetic sudden commencements (SC), which appear simultaneously at high latitude and equator within the temporal resolution of 10xa0s. Using the SC, we can confirm that the electric potential and currents are transmitted near-instantaneously to low latitude ionosphere on both day- and night sides, which is explained by means of the light speed propagation of the TM0 mode waves in the Earth-ionosphere waveguide.

Response of ionospheric electric fields at mid‐low latitudes during sudden commencements

Using in situ observations from the Republic of China Satellite-1 spacecraft, we investigated the time response and local time dependence of the ionospheric electric field at mid-low latitudes associated with geomagnetic sudden commencements (SCs) that occurred from 1999 to 2004. We found that the ionospheric electric field variation associated with SCs instantaneously responds to the preliminary impulse (PI) signature on the ground regardless of spacecraft local time. Our statistical analysis also supports the global instant transmission of electric field from the polar region. In contrast, the peak time detected in the ionospheric electric field is earlier than that of the equatorial geomagnetic field (~20u2009s before in the PI phase). Based on the ground-ionosphere waveguide model, this time lag can be attributed to the latitudinal difference of ionospheric conductivity. However, the local time distribution of the initial excursion of ionospheric electric field shows that dusk-to-dawn ionospheric electric fields develop during the PI phase. Moreover, the westward electric field in the ionosphere, which produces the preliminary reverse impulse of the geomagnetic field on the dayside feature, appears at 18–22u2009h LT where the ionospheric conductivity beyond the duskside terminator (18u2009h LT) is lower than on the dayside. The result of a magnetohydrodynamic simulation for an ideal SC shows that the electric potential distribution is asymmetric with respect to the noon-midnight meridian. This produces the local time distribution of ionospheric electric fields similar to the observed result, which can be explained by the divergence of the Hall current under nonuniform ionospheric conductivity.

Evolution of the current system during solar wind pressure pulses based on aurora and magnetometer observations

We investigated evolution of ionospheric currents during sudden commencements using a ground magnetometer network in conjunction with an all-sky imager, which has the advantage of locating field-aligned currents much more accurately than ground magnetometers. Preliminary (PI) and main (MI) impulse currents showed two-cell patterns propagating antisunward, particularly during a southward interplanetary magnetic field (IMF). Although this overall pattern is consistent with the Araki (solar wind sources of magnetospheric ultra-low-frequency waves. Geophysical monograph series, vol 81. AGU, Washington, DC, pp 183–200, 1994. doi:10.1029/GM081p0183) model, we found several interesting features. The PI and MI currents in some events were highly asymmetric with respect to the noon–midnight meridian; the post-noon sector did not show any notable PI signal, but only had an MI starting earlier than the pre-noon MI. Not only equivalent currents but also aurora and equatorial magnetometer data supported the much weaker PI response. We suggest that interplanetary shocks impacting away from the subsolar point caused the asymmetric current pattern. Additionally, even when PI currents form in both pre- and post-noon sectors, they can initiate and disappear at different timings. The PI currents did not immediately disappear but coexisted with the MI currents for the first few minutes of the MI. During a southward IMF, the MI currents formed equatorward of a preexisting DP-2, indicating that the MI currents are a separate structure from a preexisting DP-2. In contrast, the MI currents under a northward IMF were essentially an intensification of a preexisting DP-2. The magnetometer and imager combination has been shown to be a powerful means for tracing evolution of ionospheric currents, and we showed various types of ionospheric responses under different upstream conditions.

Propagation and evolution of electric fields associated with solar wind pressure pulses based on spacecraft and ground‐based observations

We investigate spatial and temporal evolution of large-scale electric fields in the magnetosphere and ionosphere associated with sudden commencements (SCs) using multi-point equatorial magnetospheric (THEMIS, RBSP, GOES) and ionospheric (C/NOFS) satellites with radars (SuperDARN). A distinct SC event on March 17, 2013 shows that the magnetospheric electric field in the equatorial plane propagates from dayside toward nightside as a fast mode wave. The ionospheric electric field responds ~41 s after the onset of dayside magnetospheric electric field, which can be explained by the propagation of the Alfven wave along magnetic field lines. Poynting fluxes toward the ionosphere support these propagations. From a statistical analysis of response time, tailward propagation speed is estimated at about 1000–1100 km/s. We also statistically derive a spatial distribution and time evolution of the magnetospheric electric field in the dawn-dusk direction (Ey). Our result shows that negative Ey (dawnward) propagates from noon toward the magnetotail, followed by positive Ey (duskward). The propagation characteristics of electric fields in the equatorial plane depend on magnetic local time. At noon, negative Ey lasts for about 1 min, and positive Ey becomes dominant about 2 min after the SC onset. Negative Ey soon attenuates in the nightside region, while the positive Ey propagates fairly well to the pre-midnight or post-midnight regions while maintaining a certain amplitude. The enhancement of positive Ey is due to the enhancement of magnetospheric convection associated with the main impulse of SCs.