[1] Penetration of the magnetospheric electric field to the equatorial ionosphere was examined for the geomagnetic storm on 6 November 2001, by analyzing the difference in magnitude of the geomagnetic storm recorded at the dayside geomagnetic equator, Yap (À0.3°GML) and low latitude, Okinawa (14.47°GML). The penetrated electric field caused the DP2 currents at the equator, i.e., eastward currents during the main phase of the storm, while the overshielding currents, i.e., westward currents dominated during the recovery phase. It is shown that the ring current started to develop simultaneously with the onset of the equatorial DP2 within the temporal resolution of a few minutes. These results imply prompt transmission of the dawn-to-dusk convection electric field to the inner magnetosphere as well as to the equatorial ionosphere. It is found that the equatorial DP2 started to decrease one hour after the onset of the ring current development, indicating shielding effects becoming effective at the equator during the latter half of the storm main phase. The DP2 was then overwhelmed by the overshielding, which resulted in the counter electrojet (CEJ) in the beginning of the storm recovery phase. The IMAGE magnetometer chain data indicate that the westward auroral electrojet (AEJ) in the dawn sector was driven over midlatitude centered at 57°corrected geomagnetic latitude (CGML) during the main phase, while the AEJ shifted rapidly poleward to the auroral latitude centered at 67°CGML in the beginning of the recovery phase. The overshielding must be caused by the abrupt poleward shift of the R1 FACs as inferred from the poleward shift of the AEJ, in addition to the decrease in their magnitude due to the decrease in magnitude of the southward IMF. The geomagnetic storm at the dayside geomagnetic equator was enhanced in amplitude with the ratio of 2.7 as compared with the geomagnetic storm at low latitude. This amplification is a result of both effects of the DP2 currents and the CEJ associated with the main and recovery phases, respectively. It is suggested that the electric field associated with the DP2 currents contributed to the development of the ring current during the main phase, while the overshielding electric field may contribute to cease developing the ring current during the recovery phase.

[1] The substorm mechanism is investigated by analyzing the numerical results obtained through the use of the recently refined magnetohydrodynamic model. After showing the growth phase features, the numerical solution reproduces the observed signatures of a substorm onset, including the formation of a near-Earth neutral line (NENL), earthward directed flow in the plasma sheet, a dipolarization, a geosynchronous D deflection, the development of the nightside field aligned currents (FACs), and electrojets in the ionosphere. The onset is triggered by a sudden collapse of the plasma sheet and a successive formation of a high-pressure region in the inner magnetosphere. The energy source of this high-pressure region is the magnetic tension released from the NENL. The tail disturbance is primary transmitted to the ionosphere by the region 2 FAC accompanying the high-pressure region formed in the inner magnetosphere. The associated nightside region 1 FAC is not connected to the traditional current wedge but to the cusp region. The region 1 FAC path to the ionosphere develops from dayside to nightside, so as to construct a grand loop with the region 2 FAC from the partial ring current. In the grand loop, the region 1 FAC provides a short circuit in the ionosphere for the region 2 FAC. In the expansion phase, convection enhances to relax the distorted pressure distribution formed at the onset, accompanied by a further development of dipolarization and a thickening of the plasma sheet in the magnetosphere and increases in the westward and eastward electrojets in the ionosphere. The present model can explain the explosive growth phase and the thinning of the midtail plasma sheet just after the onset.

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Equatorial counterelectrojet (CEJ) events are analyzed in association with changes in the interplanetary magnetic field (IMF), polar cap potential (PCP), and electric field measured in the equatorial ionosphere. In one event on 16 July 1995, the equatorial CEJ was observed at the afternoon dip equator during the recovery phase of the substorm when the IMF turned northward. Rapid decreases in the PCP and in the auroral electrojet occurred simultaneously with the equatorial CEJ, suggesting instantaneous equatorward penetration of the rapid decrease in the electric field associated with the region 1 field‐aligned currents (R1 FACs) under the condition of a well‐developed shielding electric field due to the R2 FACs. In the other event on 8 April 1993, the equatorial CEJ associated with the northward turning of the IMF was directly related to a rapid decrease in the equatorial electric field measured by the Jicamarca incoherent scatter radar as well as to a decrease in the PCP. We confirm the scenario for the substorm‐associated equatorial CEJ as caused by the dominant R2 FACs when the R1 FACs decrease abruptly because of the northward turning of the IMF. We also suggest that the DP 1 current system is composed of the Hall currents surrounding the R2 FACs and the equatorial CEJ closing with the R2 FACs, which are superposed on the DP 2 currents caused by the R1 FACs, being dominant when the IMF turns northward. The coherent occurrence of the electric field in the F region with the electric current in the E region at the equator is explained by applying the Earth‐ionosphere waveguide model of Kikuchi and Araki [1979b] as a most promising transmission mechanism. All the conditions for the equatorial CEJ most likely occur during the substorm, but the northward turning of the IMF and the resultant decrease in the PCP play a crucial role under the condition of well‐developed R2 FACs.

A major neurotransmitter dopamine transmits signals via five different seven transmembrane G protein-coupled receptors termed D1-D5. It is now evident that dopamine is released from leukocytes and acts as autocrine or paracrine immune modulator. However, the role of dopamine for dendritic cells (DCs) and T(h) differentiation remains unclear. We herein demonstrate that human monocyte-derived dendritic cells (Mo-DCs) stored dopamine in the secretary vesicles. The storage of dopamine in Mo-DCs was enhanced by forskolin and dopamine D2-like receptor antagonists via increasing cyclic adenosine 3',5'-monophosphate (cAMP) formation. Antigen-specific interaction with naive CD4(+) T cells induced releasing dopamine-including vesicles from Mo-DCs. In naive CD4(+) T cells, dopamine dose dependently increased cAMP levels via D1-like receptors and shifts T-cell differentiation to T(h)2, in response to anti-CD3 plus anti-CD28 mAb. Furthermore, we demonstrated that dopamine D2-like receptor antagonists, such as sulpiride and nemonapride, induced a significant DC-mediated T(h)2 differentiation, using mixed lymphocyte reaction between human Mo-DCs and allogeneic naive CD4(+) T cells. When dopamine release from Mo-DCs is inhibited by colchicines (a microtubule depolymerizer), T-cell differentiation shifts toward T(h)1. These findings identify DCs as a new source of dopamine, which functions as a T(h)2-polarizing factor in DC-naive T-cell interface.