Solar Magnetism in the Polar Regions

Abstract

This review describes observations of the polar magnetic fields, models for the cyclical formation and decay of these fields, and evidence of their great influence in the solar atmosphere. The polar field distribution dominates the global structure of the corona over most of the solar cycle, supplies the bulk of the interplanetary magnetic field via the polar coronal holes, and is believed to provide the seed for the creation of the activity cycle that follows. A broad observational knowledge and theoretical understanding of the polar fields is therefore an essential step towards a global view of solar and heliospheric magnetic fields. Analyses of both high-resolution and long-term synoptic observations of the polar fields are summarized. Models of global flux transport are reviewed, from the initial phenomenological and kinematic models of Babcock and Leighton to present-day attempts to produce time-dependent maps of the surface magnetic field and to explain polar field variations, including the weakness of the cycle 23 polar fields. The relevance of the polar fields to solar physics extends far beyond the surface layers from which the magnetic field measurements usually derive. As well as discussing the polar fields' role in the interior as seed fields for new solar cycles, the review follows their influence outward to the corona and heliosphere. The global coronal magnetic structure is determined by the surface magnetic flux distribution, and is dominated on large scales by the polar fields. We discuss the observed effects of the polar fields on the coronal hole structure, and the solar wind and ejections that travel through the atmosphere. The review concludes by identifying gaps in our knowledge, and by pointing out possible future sources of improved observational information and theoretical understanding of these fields. Article RevisionsLiving Reviews supports two ways of keeping its articles up-to-date:Fast-track revision. A fast-track revision provides the author with the opportunity to add short notices of current research results, trends and developments, or important publications to the article. A fast-track revision is refereed by the responsible subject editor. If an article has undergone a fast-track revision, a summary of changes will be listed here.Major update. A major update will include substantial changes and additions and is subject to full external refereeing. It is published with a new publication number.For detailed documentation of an article's evolution, please refer to the history document of the article's online version at http://dx

Structure and evolution of the photospheric magnetic field in 2010–2017: comparison of SOLIS/VSM vector field and BLOS potential field

“…However, other studies found that the polar field is approximately radial (see, e.g., Petrie 2015, and references therein). We note that the conclusions of Petrie (2015) do not take into account the Ulrich & Tran (2013) results. The presence of a systematic meridional inclination of the photospheric magnetic field would also affect the derivation of the pseudo-radial field.…”

“…Polar fields are difficult to observe due to the partial invisibility of poles, but they are highly important for coronal and heliospheric models (Bertello et al 2014;Virtanen & Mursula 2016), and for dynamo theory (see, e.g., Petrie 2015;Muñoz-Jaramillo et al 2013, and references therein). Polar fields are in general rather weak, and the LOS projection of the radial field approaches zero at high latitudes.…”

Improvement of solar-cycle prediction: Plateau of solar axial dipole moment

“…Because a significant amount of the sunspots appear during the plateau, we need explanations for such small cross-equatorial flux transport during the period. The anomalously weak polar field and the extended minimum at the end of Cycle 23 has been studied by various authors (see also a review by Petrie 2015). One explanation of the small polar field at Cycle 23/24 minimum is that the emergence of the active region with the small tilt angle or the opposite polarity causes the source of the small cross-equatorial flux transport (Jiang et al 2015).…”

Photospheric and coronal magnetic fields in six magnetographs

“…As we already pointed out, polar fields in the KP data are erroneous especially in the 1980s. Petrie (2015) shows an improved version of the KP magnetic butterfly diagram, where obvious errors have been filtered out. Also, filtered polar field observations of SOLIS are available at NSO site 7 .…”