Integrated performance of ionospheric indices in South America during the Saint Patrick’s Day geomagnetic storm

Abstract

This study presents a comparative analysis of ionospheric indices during the Saint Patrick’s Day geomagnetic storm. We selected three indices derived from Global Navigation Satellite Systems (GNSS) because they are susceptible to disturbances in the ionospheric plasma. The indices analyzed are: the Weather Index (W), the Rate of TEC Index (ROTI) and the Disturbance Ionosphere Index (DIX) in two versions, one that takes into account spatial gradients and temporal changes in VTEC, DIX1 and the other based on the modified classical form that measures the variability of vertical Total Electron Content (VTEC) concerning a reference value DIX2. To evaluate the effectiveness of each index during this extreme Space Weather event, we utilized GNSS observations from March 17–18, 2015, specifically around the time of the St. Patrick’s Day geomagnetic storm. We compared the temporal behavior of each index across different magnetic latitudes to assess their sensitivity to the various phases of the storm. The indices employ the geometry-free combination method instead of VTEC values, incorporating all available GNSS constellations and frequency combinations. Our results indicate that local time and geomagnetic latitude significantly influence the DIXs, ROTI, and W indices. This dependency relates to the evolution of electrodynamic processes occurring during the development of the magnetic storm. A novel outcome of this work is the interpretation of how the indices related to the phases of a storm vary, particularly in mid-latitudes. We observe an average change in the Rate of Change of TEC (ROT) that appears stable, even when analyzing temporal and spatial gradients (DIX). We can distinguish between the storm’s positive and negative phases. In the positive phase, the spatio-temporal gradient increases due to enhanced contributions from electric fields, thermospheric winds, and other factors, which accelerates the rise of VTEC. Conversely, during the negative phase, the overall effect of these processes on the ionosphere diminishes, leading to a deceleration in the growth of VTEC and, in some instances, even producing a decrease.