Takashi Kikuchi

Penetration of magnetospheric electric fields to the equator during a geomagnetic storm

[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 (-0.3° 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 Rl 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.

Transmission line model for driving plasma convection in the inner magnetosphere

The plasma convection in the inner magnetosphere reacts quickly after a growth of the polar cap potential, as revealed by the quick onset of the development of the partial ring current as demonstrated by Hashimoto et al. [2002]. In order to explain the quick response of the inner magnetosphere, we apply the Earth-ionosphere waveguide (transmission line) model developed by Kikuchi and Araki [ 979b]. In this paper, we examine how the TMO mode is excited in the waveguide by the electric potential transported along with the field-aligned currents, and show that the electric potential is transmitted horizontally at the speed of light in the Earth-ionosphere waveguide. We then evaluate the attenuation of the TMO mode by calculating the Poynting flux transported upward from the Earth-ionosphere waveguide into the conducting ionosphere. A fraction of the Poynting flux is further transported into the magnetosphere when the Alfven conductance of the magnetosphere is not very small compared with the ionospheric conductance. The upward transportation of the Poynting flux causes a loss of the horizontally transmitting energy, resulting in the attenuation of the TMO mode, but it is not substantial compared to the geometrical attenuation due to the finite size of the polar electric field. We stress that the electric field associated with the ionospheric current is mapped upward into the magnetosphere by the Alfven mode, which drives the plasma convection in the inner magnetosphere. We note that the ionosphere is not a generator but constitutes a transmission line for the electromagnetic energy to be transported into the inner magnetosphere.

Quick response of the near-earth magnetotail to changes in the interplanetary magnetic field

We found that the magnetic field at the geosynchronous orbit started to change from the dipole- to tail-like configuration, i.e., plasmasheet thinning in the evening sector six minutes after the onset of development of the ionospheric convection as derived from the ground magnetometers, when the IMF turned southward. The ionospheric plasma convection as observed by SuperDARN started to change simultaneously, and completely changed its pattern from the four-cell to two-cell at the onset of the plasmasheet thinning. The plasmasheet thinning was followed by the development of the partial ring current (PRC) within a few minutes, which implies that the plasma convection was enhanced in the near-earth magnetotail. These results indicate that the convection electric field in the near-earth magnetotail develops concurrently with the large-scale two-cell convection in the ionosphere, suggesting a possible role of the ionosphere in driving the convection in the near-earth magnetotail.