Sudden commencement event of 17 April 2002: Aspects of the geomagnetic response at low latitudes

An analysis of a sudden commencement (SC) manifestation reveals a significant contribution of the polar current system which influences the long-term behavior of the H component at low-latitude ground stations located in the subsolar sector. A transition between “morning” and “afternoon” characteristics of the geomagnetic response occurs at ≈1300 MLT. “Morning” events are characterized by a positive then negative variation (P/N) preceding the longer-term variation and show a gentle latitudinal decrease of the asymptotic response; conversely, “afternoon” events, which are characterized by a N/P structure, show a steep latitudinal gradient. PPI and PRI structures are observed on opposite sides of the ≈1300 MLT boundary. The PPI consists of three dominant fluctuations at well-defined frequencies and the small differences in the signal manifestation at nearby stations can be imputed to the occurrence of local resonance phenomena. Large-amplitude pulsations appear soon after the SC occurrence at subauroral latitudes.