Occurrence of Ionospheric Scintillation During Geomagnetic Storms in Indonesia (2003–2024) Using Superposed Epoch Analysis

Ionospheric scintillation refers to rapid radio signal amplitude and phase fluctuations due to small-scale irregularities in the ionosphere. Primarily at equatorial and low latitudes, scintillation is linked to Equatorial Plasma Bubbles (EPBs), regions of depleted plasma density that form after sunset. Ionospheric scintillation typically occurs during post-sunset hours until midnight. Post-sunset EPBs can be enhanced or suppressed during geomagnetic storms depending on local sunset timing concerning the storm's main or recovery phases. This study analyzes ionospheric scintillation in Indonesia, located at low geomagnetic and geographic latitudes, during geomagnetic events from 2003 to 2024. Using the S4 index, scintillation was examined with data from seven observation stations during geomagnetic storm events. Geomagnetic activity was evaluated using Dst, SYM-H, and AE indices, employing Superposed Epoch Analysis (SEA) to assess scintillation occurrence linked to minimum SYM-H, defined as epoch 0 to represent the storm peak or the onset of recovery phase in each event. The analysis categorized storms into weak-moderate (-30 nT ≤ min. Dst ≤ -100 nT) and strong (min. Dst < -100 nT) geomagnetic storms and examined their dependence on the local time of minimum SYM-H. Results indicate that scintillation first appears ~6 hours after epoch 0 in weak-moderate geomagnetic storms and ~12 hours after epoch 0 in strong geomagnetic storms. The average AE index returns to its baseline value (quiet condition) ~6 and ~12 hours after epoch 0 for weak-moderate and strong geomagnetic storms, respectively. Further analysis based on the classification of the local time of epoch 0 shows that scintillation occurrence is not observed in post-sunset hours when epoch 0 falls between 16:00 and 19:00 LT for weak-moderate geomagnetic storms. In strong geomagnetic storms, scintillation occurrence during post-sunset hours is absent when epoch 0 is between 10:00 and 19:00 LT. Notably, when the minimum SYM-H (epoch 0) nearly coincides with local sunset, scintillation activity occurs around sunset in both weak-moderate and strong geomagnetic storms. Furthermore, when epoch 0 falls within midnight until early morning, scintillation can be generated in the post-sunset hours before epoch 0. Still, post-midnight scintillation is not observed in the equatorial region during recovery phase both weak-moderate and strong storm events. We discussed our findings: When sunset falls before or coincide with epoch 0, the likelihood of post-sunset EPB and scintillation increases due to the prompt-penetration electric field (PPEF) in the main phase of storm. The disturbance dynamo electric field (DDEF) in the recovery phase driven by equatorward winds from auroral Joule heating operates for at least 6- and 12-hours post-epoch 0 in the cases of weak-moderate and strong geomagnetic storms, respectively. When the local sunset falls within these operational DDEF periods, post-sunset EPBs will likely be suppressed, inhibiting ionospheric scintillation during post-sunset hours. Finally, this study provides essential information for developing more accurate ionospheric scintillation prediction models in space weather services in equatorial regions.