Tracking the Rapid Opening and Closing of Polar Coronal Holes through IBEX ENA Observations

Abstract

Fast solar wind (SW) flows outward from polar coronal holes (PCHs). The latitudinal extent of the fast SW varies during different phases of the solar cycle. The fast SW in the inner heliosheath produces a flatter proton spectrum than the slow SW that can be observed through energetic neutral atoms (ENAs) by the Interstellar Boundary Explorer (IBEX). In this study, we investigate the evolution of PCHs as reflected in the high-time resolution ENA flux measurements from IBEX-Hi, where the PCHs are identified by ENA spectral indices <1.8. The ENA spectral index over the poles shows a periodic evolution over the solar cycle 24. The surface area with flatter ENA spectra (<1.8) around the ecliptic south pole increases slightly from 2009–2011 and then decreased gradually from 2012–2014. The PCH completely disappears in 2016 and then starts to appear again starting in 2017, gradually growing until 2019. 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 ENA spectrum near the south pole in 2014 and 2017. The rapid evolution in 2014 is related to a rapid closing of PCHs in 2012 and that in 2017 is related to a rapid opening of PCHs in late 2014. These results also agree qualitatively with the evolution of the ENA spectral index from simulations using a simple time-dependent heliospheric flow model.

Influence of the interstellar magnetic field and 11-year cycle of solar activity on the heliopause nose location

“…In some, the nose is identified with the stagnation point of the interstellar medium flow (e.g., Drake et al 2010;Desai et al 2015;Dayeh et al 2019;McComas et al 2020;Shrestha et al 2023). Many articles refer to the HP nose as 'the nose region of the heliosheath', 'the nose of the heliosphere' (e.g., McComas & Schwadron 2006;Lee et al 2009;Fisk & Gloeckler 2009, 2014Galli et al 2019;Kornbleuth et al 2020Kornbleuth et al , 2021aOpher et al 2017;Shrestha et al 2023), 'the nose direction' (e.g., Müller et al 2008;Opher & Drake 2013;Opher et al 2015Opher et al , 2017Opher et al , 2021Zirnstein et al 2020) or 'the upwind (nose) direction' (e.g., McComas et al 2020;Kornbleuth et al 2021b). However, relatively little attention has been paid to the consideration of the location of the heliosphere nose, more precisely the location of the noses of the TS, HP, and BS and the displacement of the nose.…”