Abstract
Solar cycle 24 is notable for three features that can be found in previous cycles but which have been unusually prominent: (1) sunspot activity was considerably greater in the northern/southern hemisphere during the rising/declining phase; (2) accumulation of open solar flux (OSF) during the rising phase was modest, but rapid in the early declining phase; (3) the heliospheric current sheet (HCS) tilt showed large fluctuations. We show that these features had a major influence on the progression of the cycle. All flux emergence causes a rise then a fall in OSF, but only OSF with foot points in opposing hemispheres progresses the solar cycle via the evolution of the polar fields. Emergence in one hemisphere, or symmetric emergence without some form of foot point exchange across the heliographic equator, causes poleward migrating fields of both polarities in one or both (respectively) hemispheres which temporarily enhance OSF but do not advance the polar field cycle. The heliospheric field observed near Mercury and Earth reflects the asymmetries in emergence. Using magnetograms, we find evidence that the poleward magnetic flux transport (of both polarities) is modulated by the HCS tilt, revealing an effect on OSF loss rate. The declining phase rise in OSF was caused by strong emergence in the southern hemisphere with an anomalously low HCS tilt. This implies the recent fall in the southern polar field will be sustained and that the peak OSF has limited implications for the polar field at the next sunspot minimum and hence for the amplitude of cycle 25.
Highlights
The importance of flux transport in the solar photosphere and corona was first recognized by Leighton [1964], as discussed in a review of the development of the concepts by Sheeley [2005]
Solar cycle 24 is notable for three features that can be found in previous cycles but which have been unusually prominent: (1) sunspot activity was considerably greater in the northern/southern hemisphere during the rising/declining phase; (2) accumulation of open solar flux (OSF) during the rising phase was modest, but rapid in the early declining phase; (3) the heliospheric current sheet (HCS) tilt showed large fluctuations
Evolution of photospheric flux is predicted by numerical models which follow magnetic flux tubes that have emerged through the photosphere in “bipolar magnetic regions” (BMRs) [Harvey and Zwaan, 1993] and evolve under the combined effects of differential rotation, meridional flow, and diffusion resulting from quasi-stochastic granular and supergranular motions
Summary
The importance of flux transport in the solar photosphere and corona was first recognized by Leighton [1964], as discussed in a review of the development of the concepts by Sheeley [2005]. These transport mechanisms lead to the accumulation of magnetic flux of the polarity of the trailing spots at high solar latitudes, eventually reversing the polarity of the polar fields which have the right sense to be the “seed field” for the solar cycle While this evolution from BMRs to poloidal polar flux is ongoing, the loops rise through the solar corona, emerge through an arbitrarily defined surface at the top of the corona, and enter the heliosphere, where they are “frozen-in” in the supersonic and super-Alfvénic solar wind and so dragged away from the Sun. The surface is called the “coronal source surface” and is often taken to be a heliocentric sphere of radius r = 2.5 R⊙, where R⊙ is a mean solar radius: in the PFSS modeling, this is where the field is assumed to be radial.
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