The Escape of Magnetic Flux from the Sun

Abstract

Analysis of heliospheric magnetic fields at 1 AU shows that\r1024 Mx of net azimuthal flux is ejected by the Sun per\rsolar cycle. This rate is identified with the rate of toroidal flux\rgeneration. It is compared with indicators of flux ejection from the\rsolar atmosphere, including coronal mass ejections (CM Es), filament\reruptions, and active region loop expansion. The rate is consistent\rwith estimates of flux escaping in these phenomena. The toroidal flux\rescape rate is compared with the apparent rate of flux emergence at\rthe solar surface, and it is concluded that escaping toroids will\rremove at least 20% of the emerging flux, and probably remove 100% of\remerging flux, since multiple eruptions occur on the toroids. The data\rimply that flux escapes the Sun with an efficiency far exceeding\rParker's upper limit estimate of 3 %. Toroidal flux escape is almost\rcertainly the source of the observed overwinding of the interplanetary\rmagnetic field spiral. Two mechanisms to facilitate net flux escape\rare discussed: helicity charging to push open the fields and flux\rtransport with reconnection to close them off. We estimate the Sun\rwill shed ˜2 × 1045 Mx2 of magnetic\rhelicity per solar cycle, leading to a mean helicity density of 100\rMx2 cm-3 at 1 AU, which agrees well with\robservations. Helicity shedding and flux escape are seen as essential\rto the cyclic renewal of the solar dynamo. It is argued that because\rlefthanded and right-handed helical fields accumulate in the northern\rand southern hemispheres, separately, conservation of magnetic\rhelicity requires that the dynamo-generated fields be expelled. The\rmean lifetime of magnetic flux on the solar surface is 3-6 months. The\rmechanisms described here should also enable Sun-like stars to shed\rdynamo-generated fields.