Authors: Manolis K. Georgoulis, Barry J. LaBonte, David M. Rust
Affiliation: The Johns Hopkins University, Applied Physics Laboratory
Coronal mass ejections (CMEs) are known to supply the heliosphere with magnetic helicity. This is a generally accepted fact, irrespective of the nature of the erupted structure in the solar atmosphere. Because of the unknown magnetic fields above the solar photosphere, magnetic helicity is usually calculated indirectly by monitoring its variations on the photospheric boundary. This calculation, based on line-of-sight magnetic fields and local correlation tracking (LCT) velocities, is both uncertain and incomplete, while its efficiency in explaining or predicting solar eruptions and CMEs is yet to be conclusively proven. Instead of the helicity variations, we calculate the total relative magnetic helicity in solar active regions by means of a simple idea based on force-free extrapolations of the photospheric magnetic fields. Using extrapolated fields is also a compromise which, however, allows a much more complete calculation of the magnetic helicity. Moreover, our calculation is based on vector magnetic field measurements and does not require the LCT velocities. We present an example in which the unsigned total magnetic helicity decreases precipitously in a X3 flare and a subsequent CME and remains low until the end of our observing interval. The extrapolated magnetic field before the event shows a sigmoidal structure which clearly expands and finally exits our calculation box, thus accounting for the decrease of the magnetic helicity.