Small‐Scale Irregularities Within Polarization Jet/SAID During Geomagnetic Activity.

We study the spatial structure of a polarization jet/Sub‐Auroral Ion Drift (PJ/SAID) based on data from the NorSat‐1 and Swarm satellites during a geomagnetic storm. Observations of plasma parameters inside the PJ/SAID are obtained with NorSat‐1 using a system of Langmuir probes with a nominal sampling rate of up to 1 kHz, which allowed measurements with such a high temporal resolution for the first time. A comparative analysis of plasma parameters and electron density spectra inside PJ according to the data from both satellites is presented. Our results show that fluctuations of plasma parameters inside the PJ increase at all scales with increasing geomagnetic activity. Small‐scale irregularities in the PJ are measured in situ down to hundreds of meters. The role of large‐scale effects in the PJ increases in comparison with the small‐scale ones during high geomagnetic activity. The PJ consists of structures ∼0.2° latitude in size within which small‐scale irregularities are present. Plain Language Summary: Polarization jet (PJ), also known as Sub‐Auroral Ion Drift (SAID), events are fast westward plasma drifts with a narrow latitudinal extent, occurring at subauroral latitudes in the Earth's ionosphere. The decrease in the density of the ionospheric plasma inside PJ/SAID significantly affects the conditions for the propagation of shortwave radio waves, which indicates the practical importance of studying this phenomenon. Despite the importance of using a variety of ground‐based observation facilities for studying and analyzing PJ/SAID properties, as well as developing analytical models and numerical modeling, in situ observations are the most valuable. Such in situ observations can be obtained only with satellites flying through a developing PJ/SAID. Large‐scale features of PJ/SAID are currently well understood, but small‐scale processes within PJ/SAID are practically not studied, and many open questions remain. In this work, we study the small‐scale structures in PJ/SAID during a geomagnetic storm of 20 April 2018, using multi‐instrumental approach involving low‐Earth orbit. Key Points: Fluctuations of plasma parameters inside the polarization jet (PJ) increase at all scales during higher geomagnetic activitySmall‐scale irregularities inside the PJ are measured in situ down to hundreds of metersThe role of large‐scale effects in the PJ increases in comparison with small‐scale ones with geomagnetic activity [ABSTRACT FROM AUTHOR]