journal contribution posted on 13.07.2017, 14:33 by Mike Lockwood, Mathew J. Owens, Suzanne M. Imber, Matthew K. James, Emma J. Bunce, Timothy K. Yeoman
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.
This work utilizes data from the National Solar Observatory Integrated Synoptic Program, which is operated by the Association of Universities for Research in Astronomy, under a cooperative agreement with the National Science Foundation and with additional financial support from the National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, and the United States Air Force. The GONG network of instruments is hosted by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofísica de Canarias, and Cerro Tololo Interamerican Observatory. (http://gong2.nso.edu/). The authors are also grateful to staff and funders of National Space Science Data Center for the provision of the OMNI2 data set (http://omniweb.gsfc.nasa.gov/html/ow_data.html). We also thank David Hathaway and the staff of the Solar Physics Group at NASA's Marshall Space Flight Center for maintaining the online database of SOON sunspot data (http://solarscience.msfc.nasa.gov/greenwch.shtml) and the staff of the Wilcox Solar Observatory (WSO) for the solar polar field data (http://http://wso.stanford.edu/Polar.html). The MESSENGER project is supported by the NASA Discovery Program under contracts NASW-00002 to the Carnegie Institution of Washington and NAS5-97271 to the Johns Hopkins Applied Physics Laboratory. The data used in this study are available from the Planetary Data Center (https://pds.nasa.gov/). The work at Reading has been funded by the UK Science and Technology Facilities Council (STFC) consolidated grant ST/M000885/1. The work at Leicester has been funded by STFC consolidated grant ST/N000749/1. S.I. is also funded by a Leverhulme Research Fellowship.
CitationJournal of Geophysical Research: Space Physics, 2017, 122(6), pp. 5870–5894
Author affiliation/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy
VersionVoR (Version of Record)
Published inJournal of Geophysical Research: Space Physics
PublisherAmerican Geophysical Union (AGU), Wiley