Secular variation of the Sun's magnetic flux

Abstract

Abstract. We present an extension of the model of that allows us to reconstruct the time evolution of both the total and the open magnetic flux at the solar surface since 1700. The flux emerging in large active regions is determined using the sunspot number as a proxy, while the flux emergence in small ephemeral regions is described by an extended cycle whose amplitude and length are related to the corresponding sunspot cycle.

Evolution of the solar magnetic flux on time scales of years to millenia

“…The models of Solanki et al (2000Solanki et al ( , 2002 of the evolution of the solar magnetic flux reproduce the long-term variation of the solar open flux reasonably well, but give a much smoother variation over the solar cycle than the observations. In addition, the computed open flux lags the observations by roughly 2−3 years.…”

“…Even for this period the total amount of magnetic flux emerging in small bipolar magnetic regions is uncertain due to the limited spatial resolution of the data (Krivova & Solanki 2004). In order to assess the solar magnetic flux budget on longer time scales semi-empirical models have been used (Krivova et al 2007;Solanki et al 2000Solanki et al , 2002, as well as flux transport computations (Baumann et al 2004;Mackay et al 2002;Schüssler & Baumann 2006;Wang et al 2002Wang et al , 2005. The former are based on the sunspot record and attempt to reconstruct the evolution of the total magnetic flux and the flux emerging in large and small bipolar regions as well as the evolution of large unipolar regions that give rise to the solar open flux.…”

The magnetic field in the solar atmosphere

“…If a sufficient amount of unipolar flux emerges and clusters together to form extended patches, the associated fields may be open. As a consequence, the open flux on global scales varies in the course of the solar cycle (its long-term variation was computed by Solanki et al 2000Solanki et al , 2002Jiang et al 2011). Since the interplanetary magnetic field is fed by the magnetically open regions on the Sun, a similar modulation over the course of a solar cycle was expected too.…”

Solar total irradiance in cycle 23

“…Among possible explanations he lists the uncertainty in the irradiance observations, a change in the global photospheric temperature or that the evolution of the weak magnetic flux might have been underrepresented in his model. The latter is indeed possible because the weak changes in the irradiance levels at minima are believed to be driven by the changes in the weak "background" magnetic flux (e.g., Solanki et al 2002;Krivova et al 2007). By setting the cut-off at 50 G, which almost eliminates this background flux, and by leaving out the latitudes above ±65 • , Steinhilber (2010) basically models the evolution of the TSI caused by active regions only.…”