The Variation of Counter-Electrojet Current at the Southeast Asian Sector during Different Solar Activity Levels

Abstract

Studies on counter-electrojet currents (CEJ) using ground data revealed that this current could occur simultaneously among locations that are less than 30° longitude apart. In our work, the symmetricity of CEJ variation between the west and east of Southeast Asia, separated by ~25°, was preliminarily examined according to its types: morning (MCEJ) and afternoon (ACEJ). Since most of the past studies had overlooked the occurrence after dusk, the monitoring period was also extended from 18:00 to 21:00 LT, namely, the post-sunset depletion (PSD). The magnetometer station in Davao, Philippines (DAV) and Langkawi, Malaysia (LKW) were chosen to represent the east and west parts. The EEJ index (i.e., EUEL) over the periods of the solar cycle 24 (2008–2018) was utilized specifically during magnetically quiet days (Kp < 3). As the result, both parts symmetrically showed that MCEJ and ACEJ were positively and negatively correlated with the F10.7 index. Contrarily, MCEJ and ACEJ were asymmetrically prominent in the east and west. CEJ types also varied symmetrically with the season, especially for MCEJ and ACEJ (at high level), prominent during Equinox and J-solstice. Post-sunset depletion (PSD) in both parts was symmetrically solar activity independent, as no correlation with the F10.7 index was observed in the extended observation. PSD that varied symmetrically with season was also solar activity independent, except in the east during Equinox, where it was negatively correlated with the F10.7 index. Our finding also revealed that PSD was prominent during Equinox, except for the high level in the west part.