Multi‐Instrument Investigation of the Polar Holes

During the solar minimum the F‐region plasma density can become extremely low, and in some exceptional cases the F‐layer can completely disappear. These F‐region density depletions are particularly deep during geomagnetically quiet nights, when slow plasma convection creates polar holes, extending 100–1000 km in size, where the electron density is several orders of magnitude lower than the background values. Polar holes are believed to be formed during the periods of the very slow anti‐sunward convection, when the plasma is trapped just poleward of the auroral oval in the absence of any ionization sources. At the same time, fast convection with rapid vertical plasma transport has also been associated with the polar hole formation. Combining the electron density measurements from the Resolute Bay Incoherent Scatter Radar (RISR) and Poker Flat ISR (PFISR), the state of art variational data assimilation tool Ionospheric Data Assimilation Four‐Dimensional (IDA4D), and the convection data from the Super Dual Auroral Radar Network (SuperDARN) array, the polar hole formation and evolution are experimentally investigated. The F‐region plasma is traced back in time prior to the polar hole formation to analyze the favorable conditions for the formation of the polar holes. The role of plasma convection in the formation of the polar holes is experimentally investigated. The results show that the polar holes can be formed in the twilight condition and without being trapped in the closed convection loops. The formation of the polar holes were equally possible during slow and strong convection flows, predominantly with a standard two‐cell convection configuration. Key Points The role of plasma convection in the formation of the polar holes is investigated Polar holes has larger latitudinal extent than previously thought Plasma does not have to be trapped in the closed convection loops is full darkness to become depleted