THE ESCAPE OF MAGNETIC-FLUX FROM THE SUN

.Analysis of heliospheric magnetic fields at 1 AU shows that 10(24) Mx of net azimuthal flux is ejected by the Sun per solar cycle. This rate is identified with the rate of toroidal flux generation. It is compared with indicators of flux ejection from the solar atmosphere, including coronal mass ejections (CMEs), filament eruptions, and active region loop expansion. The rate is consistent with estimates of flux escaping in these phenomena. The toroidal flux escape rate is compared with the apparent rate of flux emergence at the solar surface, and it is concluded that escaping toroids will remove at least 20% of the emerging flux, and probably remove 100% of emerging flux, since multiple eruptions occur on the toroids. The data imply that flux escapes the Sun with an efficiency far exceeding Parker's upper limit estimate of 3%. Toroidal flux escape is almost certainly the source of the observed overwinding of the interplanetary magnetic field spiral. Two mechanisms to facilitate net flux escape are discussed: helicity charging to push open the fields and flux transport with reconnection to close them off. We estimate the Sun will shed similar to 2 x 10(45) Mx(2) of magnetic helicity per solar cycle, leading to a mean helicity density of 100 Mx(2) cm(-3) at 1 AU, which agrees well with observations. Helicity shedding and flux escape are seen as essential to the cyclic renewal of the solar dynamo. It is argued that because left-handed and right-handed helical fields accumulate in the northern and southern hemispheres, separately, conservation of magnetic helicity requires that the dynamo-generated fields be expelled. The mean lifetime of magnetic flux on the solar surface is 3-6 months. The mechanisms described here should also enable Sun-like stars to shed dynamo-generated fields.