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

Abstract

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 (similar to 19-2.5 hr) zone with e-fold decay times distributed in the range of similar to 0.5-similar to 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 similar to 190 m/s at 18 MLT to similar to 100 m/s near magnetic midnight, and the NmF2 is depleted as Vhor 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.