Evidence of transmission of polar electric fields to the low latitude at times of geomagnetic sudden commencements

Abstract

The HF Doppler frequency deviation associated with the geomagnetic sudden commencement (sc), called SCF, consists of the negative preliminary frequency deviation (PFD) and the succeeding positive main frequency deviation (MFD) when observed during the night at geomagnetic low latitude (geomagnetic latitude = 25°). This type of HF Doppler frequency deviation, SCF(∓), corresponds to the sc(+) at low latitude (Kakioka, Japan) and the sc(∓) (=sc*) at high latitude (Husafell, Iceland). The SCF(∓) starts earlier than the low latitude sc by 50 s, while it occurs simultaneously with the high latitude sc* within a time accuracy of 10 s. This result implies that the negative PFD of the nighttime SCF(∓) is caused by a dusk‐to‐dawn polar electric field responsible for the preliminary reverse impulse (PRI) of sc*, the occurrence of which is earlier than the occurrence of the world‐wide geomagnetic increase. Consequently, the magnetospheric electric field generated at the onset of sc is transmitted to the low‐latitude ionosphere by way of the polar ionosphere, earlier than the direct transmission through the magnetosphere by compressional hydromagnetic waves. It is also shown that the daytime SCF(∓) starts simultaneously with the low‐latitude sc(+), suggesting that the positive PFD of the SCF(±) is caused by the westward electric field transmitted directly from the magnetosphere by the compressional hydromagnetic wave.