Helmet streamers are bright loop-like structures which develop over active regions on the Sun. They are closed magnetic loops which connect regions of opposite magnetic polarity. Electrons are captured in these loops, and cause them to glow very brightly. The solar wind elongates these loops to pointy tips. They far extend above most prominences into the corona, and can be readily observed during a solar eclipse. Helmet streamers are usually confined to the "streamer belt" in the mid latitudes, and their distribution follows the movement of active regions during the solar cycle. Small blobs of plasma, or "plasmoids" are sometimes released from the tips of helmet streamers, and this is one source of the slow component of the solar wind. In contrast, formations with open magnetic field lines are called coronal holes, and these are darker and are a source of the fast solar wind. Helmet streamers can also create coronal mass ejections if a large volume of plasma becomes disconnected near the tip of the streamer.
Helmet streamers are composed of three main features: a dome of high electron density, a cavity of low electron density beneath this dome, and a quiescent prominence within this cavity.
The white-light glow of helmet streamers is due to their relatively high number of electrons compared with the surrounding corona. Light from the photosphere is Thompson scattered off of these electrons with the intensity of scattered light depending on the number of electrons along the observer's line of sight.
Upon the eruption of a coronal mass ejection (CME), the overlying helmet streamer deforms becoming the CME's leading edge. Similarly, the helmet streamer's cavity becomes the CME's cavity and the helmet streamer's prominence becomes the CME's core.