Geomagnetic field variations at the equatorial electrojet station in Sri Lanka, Peredinia

Abstract. The paper discusses the variations of the horizontal (H), vertical (Z) and eastward (Y) components of the geomagnetic field at Peredinia (PRD), an electrojet station in Sri Lanka, with the time of the day, season, sudden commencement (SSC) and during geomagnetic storms. The daily variation of H showed a large peak around midday. The daily variation of Z appeared to be almost a time gradient curve of the daily variation of H, showing a maximum around 09:00 LT (75° EMT) when the H field was increasing fastest and not at noon when Δ H was the maximum. Storm time variation of H resembled the variation of the D_st index but that of Z showed a large minimum about 2-3h before the time of minimum D_st or at the time of maximum time gradient of D_st variation. These features are compared with corresponding variations at the equatorial stations Trivandrum (TRD) in India, and remarkable similarity in all observations is noticed at PRD and TRD. It is suggested that the observed abnormal features of Z variations at electrojet stations in India-Sri Lanka are due to (i) direct effect of the ionospheric electrojet current (ii) the induction effect of the image current by the average spatially extended conductivity region and (iii) the induction current in the local subsurface conductor. It is suggested that the conductor responsible for the observed features in Z in India and Sri Lanka has to have extended spatial domain to latitudes well south of India, rather than confined to narrow Palk Strait.