Dynasonde Observations of Ionospheric Polar Holes Under Quiet Geomagnetic Conditions at Jang Bogo Station, Antarctica

Noticeable F-region electron density (NmF2) depletions were observed in the winter-nighttime polar cap ionosphere during solar minimum from the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) with Dynasonde analysis at Jang Bogo Station (JBS) in Antarctica. We focus on the F-region density depletion events (known as polar holes) following a steady quiet condition that is defined with Kp values ≤ 1+ during 6 hr. Forty-five polar holes were identified by JBS VIPIR/Dynasonde (JVD) in 2019. All of the events started over a wide range of nightside magnetic local time (22-05 MLT) with a peak occurrence at 01–03 MLT. JVD measured exponential NmF2 decrease in the nightside MLT (∼19–2.5 hr) zone with e-fold decay times distributed in the range of ∼0.5–∼3.5 hr before the onset of a polar hole. The e-folding times decrease along the longitude from dusk toward midnight. The horizontal ion drift velocity (V_hor) estimated from JVD monotonically goes down from ∼190 m/s at 18 MLT to ∼100 m/s near magnetic midnight, and the NmF2 is depleted as V_hor decreases prior to the polar hole formation. The observations of the exponential NmF2 decrease and the positive correlation between NmF2 and V_hor prior to the polar holes are discussed in light of possible formation mechanisms of polar holes, including temporal variations and spatial structure of the polar ionosphere.