› view larger imageCoronal jets are small-scale transient ejections of hot gases, or plasma, occurring in the solar atmosphere. During a typical event, about a million tons of matter are ejected at speeds reaching a million miles per hour over a few minutes' time. The jets are believed to contribute significantly to the mass flow constantly ejected by the Sun, known as the solar wind. Despite their relative simplicity, jets may serve as a paradigm for more complex and far larger events originating in the solar atmosphere, such as coronal mass ejections.
Until recently, all jet observations suffered from an inherent limitation: because they were observed from a single viewpoint, their complete geometry could not be determined unambiguously. This situation improved dramatically once the stunning images from the SECCHI instruments onboard the twin NASA/STEREO spacecraft became available in early 2007.
A unique polar coronal jet observation was made on June 7, 2007. Analysis of the images from the two distinct viewpoints of the STEREO spacecraft reveals an unmistakable helical structure in the jet. These pioneering, multi-viewpoint observations from STEREO provide the first conclusive evidence for the jet's helical geometry.
A recent theoretical model of jet initiation conjectures that the twisting of magnetic fields is the key element that explains the helical geometry of the jet. State-of-the-art numerical simulations have confirmed this prediction, and explained how the key observed features of the jets are formed. Highly twisted magnetic fields eventually become unstable, much like an overwound spring. When the writhing fields come into contact with nearby untwisted fields that extend into the solar wind, the twist is transferred to those very long field lines. The twist then rapidly leaves the Sun, pushing the plasma outward at high speed.
The combination of unprecedented STEREO observations and advanced numerical simulations allows us to meaningfully test physical mechanisms for coronal jets for the first time. The demonstration that magnetic twist is a key element in initiating ejections from the solar atmosphere deepens our understanding of the dynamic Sun-Earth relationship.