Stepping Into an Equatorial Plasma Bubble With a Swarm Overfly

ESA's Swarm constellation entered in a "overfly" configuration in the period between September and October 2021, when the longitudinal distance between the lower pair and the upper satellite was at its minimum since the launch of the spacecrafts. In addition, the local time of the nighttime tracks was favorable to detect and study the morphology of post-sunset equatorial plasma bubbles (EPBs). In this study, we focus on the Swarm overfly occurring between 00:41 UT and 00:59 UT on 30 September 2021, which covered one of the most densely instrumented regions for the study of the ionospheric irregularities embedded in the EPBs: the South American sector. By exploiting the use of ground-based receivers of Global Navigation Satellite System (GNSS) signals in combination with the Swarm plasma density measurements, we study the irregularities in the EPB formed at similar to 60 degrees W and investigate the different scales of the irregularities and the cascading processes along the magnetic flux tubes. We also highlight how diffusion along the magnetic field lines occurs simultaneously with the plasma uplift, contributing then to the correct interpretation of the EPB evolution and decay process. The precious overfly conditions also allow the introduction of ionosphere-related quantities, evaluated across the tracks at satellite altitudes enlarging the possibilities given by the same quantities already available along the tracks. Such opportunity envisages the possibility to proxy the impact of EPBs on GNSS signals with Low-Earth Orbit satellite data provided by future missions specifically dedicated to the characterization of the near-Earth environment and ionospheric studies.Plain Language Summary Ionospheric Equatorial Plasma Bubbles (EPBs) are the most dangerous threat to radio-wave propagation in the frequencies from HF, used for telecommunications, to L-band, emitted by Global Navigation Satellite Systems (GNSS). Such bubbles form in the post-sunset hours at equatorial latitudes and their occurrence is still a challenge for the scientific community, in terms of capability to model their triggering mechanisms, formation, growth and decay. On 30 September 2021, a precious occasion occurred. The 3 satellites constituting the ESA's Swarm constellation-a set of three satellites, two orbiting at 440 km and flying in pairs and one at 510 km-entered in a "overfly" configuration over Brazil. In other words, the lower pair and the upper satellite flew in a narrow longitudinal band and in one of the most densely instrumented regions in terms of ground-based GNSS receiver. This translated into the possibility to step into a single EPB. We exploited this configuration and the ground-based measurements to provide an unprecedented reconstruction of a plasma bubble and to introduce new measurements that can be used by future satellite missions to provide insights on the most dangerous natural hazard to radio waves.