I work on how the magnetic flux is generated in the solar interior (solar dynamo) and how it finds its way to the surface (flux emergence). Here are some results below (backward in time).
To better understand brightness variations of solar-type stars, it is instructive to view the Sun as a star. After all, we know in detail what is going on over its surface. In this study led by Nina Nèmec as part of her PhD thesis, we set up numerical simulations of the Sun's brightness variations (its light curve), as seen from different perspectives with respect to its rotation axis, and through the sensitivity functions of space telescopes such as Kepler, Gaia, and TESS. It is generally assumed that solar brightness variations in Kepler and Gaia directly represent the solar irradiance variations (the total solar irradiance, TSI). Here we have shown quantitatively that this is a reasonable assumption. To conclude, we have shown that TSI is a good measure of solar variability, when considering the Sun among its stellar siblings. We also simulated (see the figure above) a decade-long light curve of the Sun for various inclinations of its rotation axis with respect to the line of sight, from 90˚ (Sun seen equator-on) to 0˚ (pole-on).
Reference
Nèmec, N.-E., Işık, E., Shapiro, A.I., Solanki, S.K., Krivova, N.A., Unruh, Y. 2020, Astron. & Astrophys. 638, A56
The average tilt angle of sunspot groups emerging throughout the solar cycle determines the net magnetic flux crossing the equator, which is correlated with the strength of the subsequent cycle. It has recently been discovered by Dasi-Espuig et al. (2010) that the cycle-averaged sunspot group tilt angle is inversely correlated with the cycle strength. I suggest that a deep-seated, non-local process can account for the observed cycle-dependent changes in the average tilt angle. Motivated by helioseismic observations indicating cycle-scale variations in the sound speed near the base of the convection zone, I determined the effect of a thermally perturbed overshoot region on the stability of flux tubes and on the tilt angles of emerging flux loops. I found that 5-20 K of temperature perturbation (cooling) near the base of the solar convection zone is sufficient for emerging flux loops to reproduce the reported amplitude of cycle-averaged tilt angle variations, suggesting that it is a plausible effect responsible for the nonlinearity of the solar activity cycle. The figure shows the tilt angle as a function of emergence latitude (so-called Joy’s law for sunspot group tilt angles) of toroidal flux tubes from numerical simulations carried out using thermally perturbed stratification models. In stronger cycles, the local cooling is enhanced, leading to a more stable stratification, which is responsible for stronger flux tubes becoming unstable, leading to lower tilt angles.
Reference
Işık, E. 2015, Astrophys. J. Lett., 813, L13
We have discovered (as a group led by R. Cameron from MPS) that near-equatorial emergence of BMRs (bipolar magnetic regions) with extraordinary properties can trigger unprecedented and substantial changes in the magnetic flux budget of each solar hemisphere. The impact can be quite strong if a large BMR emerges with a negative tilt angle, or when each polarity emerges at opposite hemispheres. Such events are quite rare and occur unpredictably. Our simple calculations demonstrate that local surface flows efficiently transport unsigned magnetic fluxes of the order of 10^{22} Mx poleward in some cases. Such flux plumes can lead to a reduction or enhancement of the polar fields that build up. Since flux transport dynamo calculations indicate that polar fields during an activity minimum are correlated with the toroidal fields of the subsequent cycle (see previous entry above), this means that the ‘next‘ cycle is highly dependent on Poisson statistics of rare events.
Reference
Cameron, R H, Dasi-Espuig, M, Jiang, J, Işık, E., Schmitt, D, Schüssler, M, 2013, *Astron. & Astrophys.* 557, A141
Motivated by the results of the paper in 2012 (see below), we have run our flux transport dynamo model to simulate solar cycles for nearly a century. We have used the observed sunspot group areas and tilt angles to determine the average source magnetic field as a function of latitude at the surface. When the radial source field at the surface is fed by the observations, we have seen that the Babcock-Leighton-type dynamo model successfully reproduces the observed correlations between (a) the polar field at solar minima (second panel from top) and the strength of the subsequent cycle, and (b) the toroidal field near the base of the convection zone (bottom panel) and the strength of the subsequent cycle. The coloured time-latitude diagrams show the radial field strength at the surface (top) and the toroidal (azimuthal) field strength at the base of the convection zone.
Reference
Jiang, J, Cameron, R H, Schmitt, D, Işık, E. 2013, *Astron. & Astrophys.* 553, A128