A Statistical Analysis between Observed Coronal Holes during the Descending Phase of Solar Cycles 23 and 24 and the PFSS Model

We compare the coronal hole (CH) areas within the descending phases of solar cycles 23 and 24 using both observation and Potential-Field Source-Surface (PFSS) modeling. Candidate CHs were found using intensity thresholding in extreme ultraviolet (EUV) data, and were differentiated from filament channels by the skew of the magnetic field distribution. For the 28 Carrington rotations (CRs) analyzed for each cycle, it was found that when the highest spherical harmonic degree (lmax) was kept constant, the PFSS model was able to predict the total area satisfactorily, but could not reproduce the observed CH area distribution statistically. We devise a statistical method to determine the best-fit lmax for each CR. The method may be generalized to optimize other parameters in the model, or to calibrate the model against other observables (e.g., open flux and current sheet location). This study highlights the importance of choosing appropriate model parameters. A trend in the best fit lmax, if found, may shed light on the coronal evolution within a solar cycle.