The Evolution of a Polar Cap Patch Into a Blob in the Nightside Ionosphere: Direct Observations of Multiple Blob Formation Mechanisms Acting Simultaneously on the Same Plasma

Abstract A polar cap patch was observed to exit the polar cap to become a blob as it entered the auroral oval on the nightside of the Earth under moderately quiet geomagnetic conditions ( K p = 3−). Auroral particle precipitation led to an increase in the electron density of the blob. Blobs can be formed by multiple formation mechanisms, including transportation effects and auroral precipitation. This blob exhibited characteristics which fit within the definitions of both boundary and auroral blobs. This study therefore presents direct observations of multiple blob formation mechanisms acting simultaneously on the same plasma. These observations were made using the European Incoherent SCATter (EISCAT) radars. The polar cap patch was first observed in the EISCAT Svalbard Radar (ESR) and is later observed as a blob by the EISCAT VHF and UHF radars as the plasma moves antisunward from the polar cap and into the auroral oval. The Super Dual Auroral Radar Network radars showed that the plasma was moving antisunward and data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment provided supporting evidence for the location of the auroral boundary. In addition, Global Navigation Satellite Systems (GNSS) receivers monitored the scintillation effects that were associated with large‐scale structures. Overall, very low scintillation was observed by these receivers, suggesting that small‐scale structures (<365 m for GNSS L1 frequencies) did not develop significantly during this period.