Characteristics of geomagnetic sudden commencement observed in middle and low latitudes

Characteristics of geomagnetic sudden commencement (SC) observed in middle and low latitudes are discussed on the basis of data analyses of magnetic variations and a numerical calculation. From the data analyses, it is found that a negative impulse is usually superposed on the main impulse of SC in H component just after its onset, at the stations in middle to high latitudes in the local time range from the morning to the early afternoon. The superposition of the negative impulse causes the decrease of SC amplitude in H component with increasing latitude in this area. The occurrence of the negative impulse does not seem to be dependent on geomagnetic activity. Examining a case study and a numerical analysis, it is suggested that this negative impulse is a signature of the magnetic variation due to the polar-originated ionospheric current system responsible for the main impulse of SC (DPMI). The discussion includes the possible interpretation of PPI introduced by Kikuchi and Araki (1985) and the availability of D component magnetic data in middle latitude forarough estimationof the magnetosphere-ionosphere coupling process.