Ring current local time dependence during geomagnetic storms using equatorial Dst-proxies

Abstract In this paper, we calculate Local Disturbance indices (LDi) using data from two equatorial observatories (Ascension ASC and Fúquene FUQ) to use them as Disturbance Storm-time (Dst) index proxies. We find that the LDi response to geomagnetic storms is different depending on the observatory’s local time at the storm onset. In order to explore this local time influence on the measurements on the ground at low latitudes, we build new proxies using two observatories located at approximately the same longitude, in order to balance measurements in the north and south averaging meridional and measuring only zonal variations. The average of the longitude pairs and Dst-index proxies from single observatories exhibit strong correlation to the Dst index ( $$\ge $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≥</mml:mo> </mml:math> 0.88) during active periods and a moderate correlation ( $$\le $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>≤</mml:mo> </mml:math> 0.5) during quiet periods. We find that the storm intensity is associated with local time. We confirm that the fastest variation in the geomagnetic field during the storm is recorded between dusk and midnight, while the region between dawn and noon records more moderate variations, sometimes missing the storm effects altogether. Our results show an azimuthal asymmetry of the magnetospheric ring current, becoming most intense on the night side of the dusk terminator during active periods. We propose a new configuration for local time Dst proxies including the use of equatorial observatories. This will get insights of the evolution of storms in an area where there are limited geomagnetic observatories. Graphical abstract