Doubly Helical Coronal Ejections from Dynamos and Their Role in Sustaining the Solar Cycle

Abstract

Two questions about the solar magnetic field might be answered together once their connection is identified. The first is important for large scale dynamo theory: what prevents the magnetic backreaction forces from shutting down the dynamo cycle? The second question is: what determines the handedness of twist and writhe in magnetized coronal ejecta? Magnetic helicity conservation is important for answering both questions. Conservation implies that dynamo generation of large scale writhed structures is accompanied by the oppositely signed twist along these structures. The latter is associated with the backreaction force. We suggest that coronal mass ejections (CME's) simultaneously liberate small scale twist and large scale writhe of opposite sign, helping to prevent the cycle from quenching and enabling a net magnetic flux change in each hemisphere. Observations and helicity spectrum measurements from a simulation of a rising flux ribbon support this idea. We show a new pictorial of dynamo flux generation that includes the backreaction and magnetic helicity conservation by characterizing the field as a 2-D ribbon rather than a 1-D line.