Kenro Nozaki

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.

Field‐aligned current effects on midlatitude geomagnetic sudden commencements

The geomagnetic sudden commencement (SC) on February 18, 1999, was preceded by a preliminary positive impulse (PPI) at noon (1146 LT) mid-latitudes (34.9° and 26.9° geomagnetic latitude (GML)), and by a preliminary reverse impulse (PRI) near the dip equator (−0.3° and 4.9° GML) in the same local-time sector. By assuming that the step-like SC at a lower latitude (14.5° GML) was entirely caused by the Chapman-Ferraro currents, we subtracted this magnetic field from the SC at midlatitudes and equatorial latitudes, to identify the magnetic fields caused by the field-aligned currents (FACs) and ionospheric currents. We found that the PPI was composed of a positive impulse (true-PPI) with a timescale of less than l min and a succeeding negative impulse (several minutes), with their amplitudes decreasing with decreasing latitudes. The true-PPI occurred simultaneously with the equatorial PRI, and the succeeding negative impulse occurred with the DP 2-type ionospheric current component of the main impulse (MI) of the SC (DP (MI)). Analysis of 46 well-defined PPI events showed that the afternoon PPIs occurred exclusively in winter, while there was no significant seasonal dependence in the morning PPIs. None of the afternoon PPIs could be explained by the conventional SC model based on the Chapman-Ferraro currents and the DP 2-type ionospheric currents. We apply the Biot-Savart law to a three-dimensional current circuit including FACs to interpret the afternoon PPIs. Model calculations assuming a seasonal asymmetry in the ionospheric conductivity indicate that the FACs played a predominant role at midlatitudes in the winter hemisphere, while the ionospheric currents played a predominant role in the summer hemisphere. It is concluded that the true-PPIs and succeeding negative impulses were dominated by the magnetic effects of the FACs that carry the electric fields responsible for the PRIs and DP (MIs), respectively.

Ionospheric height changes at two closely separated equatorial stations and implications in spread F onsets

Abstract Changes in the ionospheric height during post-sunset hours are compared for two stations near the magnetic equator: Cebu Island ( 123.9° E , 10.3° N ; 2.0° N MagLat at h=250 km ) and Manila ( 121.1° E , 14.7° N ; 7.1° N MagLat at h=250 km ), both in the Philippines. Ionograms were obtained every 5 min during the WestPac98 (Western Pacific) ionospheric campaign conducted in March 1998. The ionospheric height changes during evening hours at the two stations showed various features from day to day. We summarized them into three cases: (a) the heights at the two stations varied almost identically; (b) they varied similarly but the height at Cebu was always 20– 30 km higher than at Manila; and (c) they varied in a fairly complicated way. Equatorial spread F during evening hours was observed on two case-c days. The height changes on those days are analyzed in detail. The pre-reversal enhancement in upward E × B drift started simultaneously at both stations, but drift direction reversed 1 h earlier at Manila than at Cebu. The difference in drift direction between the two stations was presumed to be the difference at the two corresponding heights over the magnetic equator. One possible explanation for this complexity is the manifestation of a vortex-like convection of E × B drift. Although we only have observations for 2 days during the campaign, the spread F occurrence and the peculiar height changes seem to be related to each other.