Figure 6: Variation in the low-latitude coronal hole area and the sunspot number over solar cycle 23.
From: The solar magnetic activity band interaction and instabilities that shape quasi-periodic variability
(a) Using the CHARM21 automated SOHO/EIT and SDO/AIA coronal hole-detection algorithm, we show the variation of the 50-day running average in the total (black), northern (red) and southern (blue) hemispheric coronal hole areas below 55° latitude. For reference, the seasonal variation in the Sun’s axial tilt relative to the Sun–Earth line is shown (green—the dashed green line shows amplitude of the negative tilt). (b) Solar influences data center (SIDC). Total (black) and hemispheric (red—north; blue—south) daily sunspot numbers as in Fig. 3. Like the flare and CME timeseries shown above, the coronal hole areas peak after solar sunspot maximum (∼2004). A comparison of the hemispheric sunspot and coronal hole areas shows a systematic lag in the peaks of the latter (∼6 months). The 2004 peak in coronal hole area corresponds to the peaks in the Ap index and solar wind speed of Fig. 5. The panels of the figure show a set of dashed fine vertical lines that are 12 months apart and act as a timescale reference.
