Field‐aligned current effects on midlatitude geomagnetic sudden commencements

<jats:p>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 <jats:italic>DP</jats:italic> 2‐type ionospheric current component of the main impulse (MI) of the SC (<jats:italic>DP</jats:italic> (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 <jats:italic>DP</jats:italic> 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 <jats:italic>DP</jats:italic> (MIs), respectively.</jats:p>