Masakazu Watanabe

Synthesis models of dayside field‐aligned currents for strong interplanetary magnetic field By

Using particle and magnetic field data acquired with DMSP-F6 and DMSP-F7 satellites, we have investigated interplanetary magnetic field (IMF) By dependence of the global pattern of plasma regime and field-aligned currents (FACs) on dayside high latitudes during strong IMF By (averaged |By| > 3.7 nT) and geomagnetically disturbed (mainly IMF Bz < 0) periods. From particle data we have identified five plasma regimes: inner plasma sheet, outer plasma sheet, cleft, cusp, and mantle. All the plasma domains except the inner plasma sheet show By dependence in spatial distribution. Region 1 and “traditional cusp” currents appear in cusp/mantle domains, which we call midday region 1 and region 0 currents, respectively, in this paper. These currents perfectly reverse their flow directions depending on IMF By polarity. Traditional region 1 currents occurring in cleft and outer plasma sheet almost always flow into the ionosphere in the prenoon sector and flow away from the ionosphere in the postnoon sector regardless of By polarity. Thus the midday region 1 and region 0 current system that appears at local noon is not a simple continuation of flankside region 1/region 2 current system. Midday region 1 and region 0 currents are not necessarily balanced in intensity; region 0 current intensity occasionally exceeds midday region 1 current intensity. Furthermore, intensity imbalance also appears in cleft-associated region 1 currents; that is, region 1 current in the farside cleft from the reconnection site (“downstreamside” cleft) is larger than region 1 current in the nearside cleft (“upstreamside” cleft). On the basis of these observational facts we discuss the source mechanisms of the dayside FAC system: (1) directly coupled generation of region 0 and midday region 1 current in the cusp/mantle domains around noon and (2) generation of extra region 0 current in the tail magnetopause which is connected to the extra downstreamside cleft-associated region 1 current.

Silica precipitation behavior in a flow field with negative temperature gradients

To clarify the precipitation behaviour of silica from a hydrothermal solution flowing with decreasing temperature through rock, the effects of temperature, temperature gradient, pH, flow velocity, and solid surface area/fluid mass ratio (A/M ratio) were analyzed by experimentation. Although these effects were not evaluated precisely, we found “threshold conditions” where the solution remains in equilibrium with amorphous silica in a flow field with a negative temperature gradient. The equilibrium conditions are: the mean pore velocity is less than 100 m yr−1, when A/M ratio is more than 700 m2 kg−1, temperature is 80°–120°C, temperature gradient is <50°C m−1, and pH is 6.5 to 8. For the same A/M ratio, temperature gradient, and pH, the mean pore velocity must be less than 5 m yr−1 to keep the solution in equilibrium with the amorphous silica in a flow field at 80°–25°C. These threshold conditions are probably satisfied in many natural geothermal systems and near a repository for high-level radioactive waste (HLW), suggesting that once groundwater becomes saturated with amorphous silica along the flow path from a geothermal source or HLW repository, it would flow in equilibrium with amorphous silica.

Global MHD modeling of ionospheric convection and field‐aligned currents associated with IMF By triggered theta auroras

Using numerical magnetohydrodynamic simulations, we investigate the evolution of ionospheric convection and field-aligned currents (FACs) when θ auroras are formed in response to interplanetary magnetic field (IMF) By transitions. When the polarity of IMF By switches abruptly during northward IMF periods, the crossbar of the θ aurora is isolated from the flankside auroral oval and drifts into the polar cap. This drift motion is involved in a large round cell associated with new IMF By, with sunward convection residing only on the dayside tip of the crossbar. There exists an IMF By-controlled large-scale FAC system on the crossbar. When the θ aurora is drifting duskward (dawnward), the FACs are located on the dawnside (duskside) boundary of the crossbar adjacent to the “new” lobe. In contrast, the magnetospheric source region of the crossbar FAC system is located on the duskside (dawnside) boundary of the protruded plasma sheet adjacent to the “old” lobe. In the source region, plasma thermal pressure feeds the electromagnetic energy of FACs, and these processes can be interpreted as coupling of slow mode and Alfven mode disturbances. In the ionosphere, the crossbar-associated FACs close with part of the region 1 currents associated with the new crescent cell. The magnetospheric source of that part of the region 1 FACs is located on the plasma sheet boundary and the magnetopause both adjacent to the new lobe. Dynamo processes in the old-lobe side and the new-lobe side work together to drive the ionospheric drift motion of the crossbar.

Generation of field-aligned current (FAC) and convection through the formation of pressure regimes: Correction for the concept of Dungey's convection

In this paper, we try to elucidate the generation mechanism of the field-aligned current (FAC) and coexisting convection. From the comparison between the theoretical prediction and the state of numerical solution from the high-resolution global simulation, we obtain the following conclusions about the distribution of dynamo, the magnetic field structure along the flow path that diverges Poynting flux, and energy conversion promoting the generation of electromagnetic energy. The dynamo for the region-1 FAC, which is in the high-latitude-side cusp-mantle region, has a structure in which magnetic field is compressed along the convection path by the slow mode motion. The dynamo for the region-2 FAC is in the ring current region at the inner edge of the plasma sheet, and has a structure in which magnetic field is curved outward along the convection path. Under these structures, electromagnetic energy is generated from the work done by pressure gradient force, in both dynamos for the region-1 and region-2 FACs. In these generation processes of the FACs, the excitation of convection and the formation of pressure regimes occur as interdependent processes. This structure leads to a modification in the way of understanding the Dungeys convection. Generation of the FAC through the formation of pressure regimes is essential even for the case of substorm onset.

Formation of the sun-aligned arc region and the void (polar slot) under the null-separator structure

From the global magnetosphere-ionosphere (M-I) coupling simulation, we examined the formation of the sun-aligned arc region and the void (polar slot) under the northward interplanetary magnetic field (IMF) with negative By condition. In the magnetospheric null-separator structure, the separatrices generated from two null points and two separators divide the entire space into four types of magnetic regions, i.e. the IMF, the northern open magnetic field, the southern open magnetic field, and the closed magnetic field. In the ionosphere, the sun-aligned arc region and the void are reproduced in the distributions of simulated plasma pressure and field-aligned current (FAC). The outermost closed magnetic field lines on the boundary (separatrix) between the northern open magnetic field and the closed magnetic field are projected to the northern ionosphere at the boundary between the sun-aligned arc region and the void, both on the morning and evening sides. The magnetic field lines at the plasma sheet inner edge are projected to the equatorward boundary of the oval. Therefore, the sun-aligned arc region is on the closed magnetic field lines of the plasma sheet. In the plasma sheet, an inflated structure (bulge) is generated at the junction of the tilted plasma sheet in the far-to-mid tail and non-tilted plasma sheet in the ring current region. In the northern hemisphere, the bulge is on the evening side wrapped by the outermost closed magnetic field lines that are connected to the northern evening ionosphere. This inflated structure (bulge) is associated with shear flows that cause the sun-aligned arc.

Global simulation study for the time sequence of events leading to the substorm onset

We have developed a global simulation code which gives numerical solutions having an extremely high resolution. The substorm solution obtained from this simulation code reproduces the precise features of the substorm onset in the ionosphere. It can reproduce the onset that starts from the equatorward side of the quiet arc, two step development of the onset, and the westward traveling surge (WTS) that starts two minutes after the initial brightening. Then, we investigated the counter structures in the magnetosphere that correspond to each event in the ionosphere. The structure in the magnetosphere promoting the onset is the near-earth dynamo in the inner magnetospheric region away from the equatorial plane. The near-earth dynamo is driven by the field-aligned pressure increase due to the parallel flow associated with the squeezing, combined with equatorward field-perpendicular flow induced by the near-earth neutral line (NENL). The dipolarization front is launched from the NENL associated with the convection transient from the growth phase to the expansion phase, but neither the launch nor the arrival of the dipolarization front coincides with the onset timing. The arrival of flow to the equatorial plane of the inner magnetosphere occurs two minutes after the onset, when the WTS starts to develop toward the west. The expansion phase is further developed by this flow. Looking at the present result that the onset sequence induced by the near-earth dynamo reproduces the details of observation quite well, we cannot avoid to conclude that the current wedge (CW) is a misleading concept.

Observation of a unipolar field-aligned current system associated with IMF By-triggered theta auroras

We investigate the existence of a specific field-aligned current (FAC) system predicted by numerical magnetohydrodynamic simulations in a past study. The FAC system is expected to occur when a drifting θ aurora is formed in response to a stepwise transition of interplanetary magnetic field (IMF) By during strongly northward IMF periods. When the IMF By changes from positive to negative, a crossbar forms in the Northern Hemisphere that moves dawnward, while in the Southern Hemisphere the crossbar moves in the opposite direction. The crossbar motion reverses when the IMF By changes from negative to positive. The FAC system appears on the trailing side of the drifting crossbar of the θ aurora as it moves either dawnward or duskward. When the θ aurora is drifting dawnward, the FACs flow into the ionosphere. The FAC polarity reverses when the θ aurora is drifting duskward. Using low-altitude satellite data, we confirmed the real existence of the above model-predicted FAC system.