Authors: Bishwas L. Shrestha (Princeton University), Eric J. Zirnstein (Princeton University), Jamey R. Szalay (Princeton University), David J. McComas (Princeton University)

Fast solar wind (SW) is emitted radially outward from Polar Coronal Holes (PCHs). The latitudinal extent of the fast SW varies during different phases of solar cycle. The advected fast SW in the inner heliosheath (IHS) results in a less steep proton spectrum that can be observed through energetic neutral atoms (ENAs) by the Interstellar Boundary Explorer (IBEX). In this study, we investigate the evolution of PCHs using high-time resolution ENA flux measurements from IBEX-Hi. The ENA spectral slope over the poles shows a periodic evolution over the solar cycle 24: the area with a less steep ENA spectrum decreased gradually from 2009 to 2014 and then increased again starting 2017. In addition, the higher-cadence ENA data at the highest latitudes show a rapid evolution of the PCHs in the south pole related to their “closing and opening”. This evolution shows a clear correlation with the change in the PCH area observed at the Sun once the delay in the ENA observation time is included. These results also agree qualitatively with the evolution of ENA spectral slope from simulations using a simple time-dependent heliospheric flow model.