Recently research advances on the polar cap patches

The polar cap patch is a common ionospheric structure. It often appears in the F region of the ionosphere over the polar caps; these patches are usually characterized by electron densities that can be even twice greater than that of the surrounding area. Untangling the formation and evolution of the polar cap patch can unveil the transportation of energy and momentum through magnetosphere-ionosphere-thermosphere couplings. However, owing to their wide horizontal reach (~100～1000 km), these high electron density structures greatly obstruct radio wave propagation over the polar cap, particularly at their edges, seriously impeding efforts tied to communication, navigation, and positioning. Therefore, the study of polar cap patches is not only of significance within space physics but also of great value within the context of space weather monitoring and prediction. In this paper, recent progress in polar cap patch research is summarily reviewed. The focus of our review includes the possible mechanisms underlying the formation of the dominant dayside reconnection; the recently proposed sunward return flows, likely produced by the nightside reconnection or other processes; and the newly defined patch-polar cap hot patch and its dependencies on various factors (e.g., solar & geomagnetic activities, local plasma transport, and particle precipitation). We will also delve into patch occurrences, as they relate to spatiotemporal dynamics as well as the interplanetary magnetic field conditions. We will also comprehensively review the evolutionary process tied to the Dungey convection cycle, as it moves from the dayside to the nightside and finally as it exits the polar caps, initially modulated by the pulsed nightside reconnection and then sunward transportation by the return flow. Lastly, we will detail the effects of ion upflow and ionospheric scintillation associated with the polar cap patch. For each subject, we will provide a detailed account of progress that has been made and its corresponding prospects.