The Evolution of Polar Coronal Holes Observed by IBEX

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 a full solar cycle of high-time resolution ENA flux measurements from IBEX. The ENA spectral slope over the poles shows a periodic evolution over the solar cycle. 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. In addition, the higher-cadence ENA data at the highest latitudes show a rapid evolution of the PCHs. These results are also compared qualitatively with the evolution of polar ENA spectral slope from simulations using a simple time-dependent heliospheric flow model.