How deep is a coronal hole?
Originally (in the Skylab era) we gave the name “coronal hole” to any exceptionally dark void in a solar soft X-ray image. Note that even before X-ray astronomy had been invented, noted Swiss solar physicist Max Waldmeier could refer to “koronale loecher,” which means much the same thing in German. Life has improved in several ways since then. The X-ray data let us see the corona projected against the solar disk, rather than just in projection (in terms of X-ray production, even the hellishly hot solar photosphere is jet-black). By correlation with the properties of the solar wind, the Skylab data showed us that in fact these coronal holes were the sources of high-speed solar wind. Thus they were dark because the field lines were open, so that the coronal gas could simply flow out into the interplanetary medium. In this science nugget we discuss how to observe coronal holes in detail; most of the work to date has been rather qualitative as regards the low corona one sees in soft X-rays
