Magnetism defines the complex and dynamic solar corona. Twists and tangles incoronal magnetic fields build up energy and ultimately erupt, hurling plasmainto interplanetary space. These coronal mass ejections (CMEs) are transientriders on the ever-outflowing solar wind, which itself possesses athree-dimensional morphology shaped by the global coronal magnetic field.Coronal magnetism is thus at the heart of any understanding of the origins ofspace weather at the Earth. However, we have historically been limited by thedifficulty of directly measuring the magnetic fields of the corona, and haveturned to observations of coronal plasma to trace out magnetic structure. Thisapproach is complicated by the fact that plasma temperatures and densities varyamong coronal magnetic structures, so that looking at any one wavelength oflight only shows part of the picture. In fact, in some regimes it is the lackof plasma that is a significant indicator of the magnetic field. Such a case isthe coronal cavity: a dark, elliptical region in which strong and twistedmagnetism dwells. I will elucidate these enigmatic features by presentingobservations of coronal cavities in multiple wavelengths and from a variety ofobserving vantages, including unprecedented coronal magnetic field measurementsnow being obtained by the Coronal Multichannel Polarimeter (CoMP). Theseobservations demonstrate the presence of twisted magnetic fields withincavities, and also provide clues to how and why cavities ultimately erupt asCMEs.
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