Using RISR‐N to Resolve Variations in Dayside and Nightside Plasma Density Spatial‐Scales.

To provide new insights into plasma density scale‐sizes in the polar cap, irregularity spectra are developed and tracked relative to magnetic local time (MLT) and solar zenith angle (SZA). A novel Incoherent Scatter Radar (ISR) technique is applied to develop spectra between 20 and 300 km using 2016 to 2018 imaginglp mode data from Resolute Bay ISR‐North. This technique leverages: (a) volumetric plasma density measurements from Advanced Modular ISRs, (b) the slow F‐region cross‐field plasma diffusion at scales greater than 10 km, and (c) that high‐latitude geomagnetic field lines are nearly vertical. The results of this work find that the largest spectral features within periodograms that use sunlit or dayside plasma densities are predominately above 100 km, indicating that structures that are above 100 km are more common than structures below 100 km in dayside/sunlit plasma. However, the opposite is true when plasma is in the dark or on the nightside, where the largest spectral features are predominately below 100 km. This contrast between the dayside and nightside is symptomatic of photoionization generating structures larger than 100 km, highlighting the role of photoionization or E‐region shorting in removing structures less than 100 km or driving larger scale‐structures more strongly, and the role of other mechanisms (such as flows, recombination, precipitation, and instabilities) in generating small‐scale structures. This paper will discuss these findings in detail, as well as discuss forthcoming works. Plain Language Summary: Plasma density variations in the polar cap are challenging to characterize, and it is difficult to discern what mechanisms create what sizes of "structures". This paper is part of a series of papers that use data from the Resolute Bay Incoherent Scatter Radar‐North to examine the drivers of polar cap variations that are between 20 and 300 km. Focusing on the local time and the angle of the sun, this paper finds that plasma density structures above 100 km are more prevalent in the daylight polar cap ionosphere than structures below 100 km. Meanwhile, plasma density structures below 100 km are more prevalent at night than structures above 100 km. This contrast between day and night indicates the effects of mechanisms that generate structuring only on the dayside and mechanisms that generate structuring anywhere in the polar cap. Key Points: Ionospheric polar cap irregularity spectra between 20 and 300 km are calculated using magnetic field mapping and incoherent scatter radarsThe largest dayside spectral features are typically over 100 km due to photoionization and E‐region shorting removing smaller irregularitiesNightside spectral features suggest important role of flows, recombination, precipitation, and instabilities in breaking down structures [ABSTRACT FROM AUTHOR]