Sinuous coronal loops are commonly observed in Yohkoh soft X-ray telescope data. The S-shape of these loops is a manifestation of the helical structure of the coronal magnetic fields and, hence, follows the hemispheric helicity rule established for quiescent filaments and photospheric magnetic fields. Sigmoids are often associated with the CMEs; they exist prior to eruption and disappear after. In addition, active regions that exhibit sinuous loops are more likely to be eruptive than non-sigmoidal regions. Once erupted, sigmoids tend to produce stronger geomagnetic storms, and often the orientation of magnetic field in interplanetary disturbance can be directly linked to the coronal field of the sigmoid. We review the observational properties of sigmoids, current theoretical models, and application of sinuous loops to space weather forecasting.
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