Coronal and heliospheric magnetic flux circulation and its relation to open solar flux evolution. Issue 6 (5th June 2017)

• Record Type: Journal Article
• Authors: Lockwood, Mike; Owens, Mathew J.; Imber, Suzanne M.; James, Matthew K.; Bunce, Emma J.; Yeoman, Timothy K.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 6(2017)
• Page Start: 5870
• Page End: 5894
• Publication Date: 2017-06-05
• Subjects: solar magnetic field -- open solar flux -- solar magnetic cycle -- meridional transport; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2016JA023644
• Languages: English
• ISSNs: 2169-9380

Abstract

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. Plain Language Summary: There is growing interest in being able to predict the evolution in solar conditions on a better basis than past experience, which is necessarily limited. Two of the key features of the solar magnetic cycle are that the polar fields reverse just after the peak of each sunspot cycle and that the polar field that has accumulated by the time of each sunspot minimum is a good indicator of the amplitude of the following cycle. Thus, understanding the evolution of the polar fields becomes crucial. We here use observations of the magnetic fields at the surface of the Sun and from satellites near Earth and Mercury, to identify how three unusually pronounced features of the most recent solar cycle have revealed that not all the magnetic flux emerging in sunspot regions progresses the evolution of the polar fields. The results have important implications for our understanding and prediction of the long‐term evolution of the Sun and the "space climate" it produces near Earth, which will influence the design and performance of several of humankind's operational systems such as spacecraft, long pipelines, and power grids. Key Points: The tilt of the heliospheric current sheet is shown to modulate the loss rate of open solar flux Not all magnetic flux emerged in active regions contributes to the solar cycle evolution of solar polar regions The rise in open flux in the declining phase of cycle 24 was due to reduction of the loss rate as well as increase in the production rate