Evolution of the Sun's large-scale magnetic field since the Maunder minimum

Abstract

The most striking feature of the Sun's magnetic field is its cyclic behaviour. The number of sunspots, which are dark regions of strong magnetic field on the Sun's surface, varies with a period of about 11 years. Superposed on this cycle are secular changes that occur on timescales of centuries and events like the Maunder minimum in the second half of the seventeenth century, when there were very few sunspots. A part of the Sun's magnetic field reaches out from the surface into interplanetary space, and it was recently discovered that the average strength of this interplanetary field has doubled in the past 100 years. There has hitherto been no clear explanation for this doubling. Here we present a model describing the long-term evolution of the Sun's large-scale magnetic field, which reproduces the doubling of the interplanetary field. The model indicates that there is a direct connection between the length of the sunspot cycle and the secular variations.

An evaluation of the correlation between open solar flux and total solar irradiance

“…The more detailed similarities between the two, however, imply that the open magnetic flux in some way quantifies both the spectrum of flux tube dimensions and the total flux of magnetic flux tubes threading the photosphere. Solanki et al (2000) obtained a good match to both the open flux variation derived by Lockwood et al (1999a) and to the cosmic ray flux variation inferred from the 10 Be cosmogenic isotope (Beer et al 2000;Lockwood 2001;McCracken & McDonald 2001) using a simple model of open flux emergence in active regions and its subsequent decay. Recently, Solanki et al (2001) have extended this modelling to include flux emergence in ephemeral regions and to estimate the total photospheric flux.…”

“…The correlation is expected to be poorer on shorter scales because individual photospheric magnetic features, such as for example a sunspot group, can grow and fade on timescales equivalent to, or shorter than, the 1-month averaging intervals used here and so produce rapid variations in I TS . On the other hand, open flux variations change on longer time scales associated with open flux emergence and decay (Solanki et al 2000). Significant anticorrelations are found with cosmic rays for the full range of geomagnetic rigidity cut-offs (2.4-13 GV).…”

Tests of Sunspot Number Sequences: 1. Using Ionosonde Data

“…It must always be remembered that sunspot numbers have applications only because they are an approximate proxy indicator of the total magnetic flux threading the photosphere and hence can be used to estimate and reconstruct terrestrial influences such as the received shortwave Total Solar Irradiance (TSI) and UV irradiance (Krivova, Balmaceda, and Solanki, 2007;Krivova et al, 2009, respectively), the open solar magnetic flux (Solanki, Schüssler, and Fligge, 2000;Lockwood and Owens, 2014a), and hence also the near-Earth solar-wind speed (Lockwood and Owens, 2014b), mass flux (Webb and Howard, 1994), and interplanetary magnetic-field strength (Lockwood and Owens, 2014a). Sunspot numbers also provide an indication of the occurrence frequency of transient events, in particular coronal mass ejections (Webb and Howard, 1994;Owens and Lockwood, 2012), and the phase of the decadal-scale sunspot cycle is used to quantify the tilt of the heliospheric current sheet (Altschuler and Newkirk, 1969;Owens and Lockwood, 2012) and hence the occurrence of fast solar-wind streams and co-rotating interaction regions (Smith and Wolf, 1976;Gazis, 1996).…”

Assessment of different sunspot number series using the cosmogenic isotope 44 Ti in meteorites

“…In brief, the OSF is modelled as a continuity equation (Solanki, Schüssler & Fligge 2000), with the source term, S, set by an empirical relation to SN using the data for the recent decades. The loss term, L, varies with the heliospheric current sheet inclination, which is assumed to be invariant between solar cycles (Asvestari & Usoskin 2016).…”