Diurnal and seasonal variation in Pc4 occurrence at low latitudes in India

A study has been carried out for Pc4 geomagnetic pulsations (in 6.7 - 22.2 mHz frequency range) recorded on ground at three very low latitude stations, viz. Hanle, Nagpur and Pondicherry, in India employing three axis fluxgate magnetometers, established and operated by Indian Institute of Geomagnetism (IIG), Navi Mumbai. Digital dynamic spectra for north-south (X), east-west (Y) and vertical (Z) components of the recorded data were constructed for each day for one year (1 January 2005 to 31 December 2005). The X- and Y-component of these dynamic spectra were investigated for carrying out statistical study of diurnal and seasonal variation of occurrence of Pc4 events. The main peaks of Pc4 occurrence were detected between 1500 hrs UT (2030 hrs 1ST) and 2000 hrs UT (0130 hrs 1ST). The monthly diurnal variation provided Pc4 peak occurrence between 1700 and 1900 hrs UT (2230 - 0030 hrs 1ST). The seasonal variation of hourly Pc4 occurrence in both local winter and autumn was detected at 1800 - 1900 hrs UT (2330 - 0030 hrs 1ST) that occurred one hour later than the main peak of occurrence for the total year determined at 1700 - 1800 hrs UT (2230 - 2330 hrs 1ST). However, the peak in the seasonal variation of hourly Pc4 occurrence for both local autumn and summer was detected at 1700 - 1800 hrs UT (2230 - 2330 hrs 1ST). The observed diurnal characteristics suggest that Pc4 pulsations detected on the nightside actually originated on the dayside by an extended origin of ULF waves in the bow shock.