Coronal holes were first spotted by NASA’s Skylab in the early 1970s. Scientists now know that their size and number varies in concert with the sun’s solar cycle, which reaches a maximum of activity approximately every 11 years. Every time the sun heads toward that maximum, currently expected for late 2013 or early 2014, the coronal holes move closer and closer to the sun’s poles.
This image of the sun from June 18, 2013, shows a coronal hole – that is, an area of cooler, less dense material in the sun’s atmosphere, the corona – in the upper left side, represented in blue. In this image from NASA’s Solar Dynamics Observatory, the blue color represents light in the 171 Angstrom wavelength, which shows the coolest material present in the image. Two other wavelengths of light are also shown in the picture: 211 Angstroms, in red, and 193 Angstroms in brown.