Quasi‐stationary auroral patches observed at the South Pole Station

We present quasi‐stationary auroral patches (QSAPs) observed by an all‐sky imager at the South Pole Station (−74.3° CGLAT) on the dayside between ∼0900 and ∼1400 MLT. QSAPs appeared in a closed field line region where I (557.7 nm) ≫ I (630.0 nm) and each patch tends to preserve its form, luminosity, and location for up to several hours. The quasi‐stationary nature of the QSAPs cannot be explained by the traditional role of the corotation electric field because the South Pole is located on the axis of Earth's rotation. The most plausible scenario is that a corotation electric potential, which dominates the convection potential, results in a trapping region of the magnetospheric cold plasmas. On the analogy of the formation of the plasmasphere, the trapping region would capture cold plasmas originated from the ionosphere and result in a locally enhanced plasma density (miniplasmasphere) that may lead to electron scattering through cyclotron wave‐particle interactions. This circumstance may occur when the South Pole is located on a closed field line and the local electric potential is dominated by the corotation potential. QSAPs were also accompanied with periodic fluctuations in the Pc 5 range, which coincide closely with ground magnetic fluctuations. Assuming that the pulsation is associated with field line resonances, we could estimate the equatorial mass density of thermal plasmas, which gradually increased from 0.13 to 0.44 amu cm −3 . The increase in the thermal plasma density would support the hypothesis that a miniplasmasphere exists, though further investigation is necessary to confirm it.