Role of observable nonlinearities in solar cycle modulation

Abstract

Context. Two candidate mechanisms have recently been considered for the nonlinear modulation of solar cycle amplitudes. Tilt quenching (TQ) is a negative feedback between cycle amplitude and the mean tilt angle of bipolar active regions relative to the azimuthal direction; latitude quenching (LQ) consists in a positive correlation between cycle amplitude and average emergence latitude of active regions. Aims. Here we explore the relative importance of and the determining factors behind the LQ and TQ effects. Methods. The degree of nonlinearity induced by TQ, LQ and their combination is systematically probed in a grid of surface flux transport (SFT) models. The role of TQ and LQ is also explored in the successful 2x2D dynamo model optimized to reproduce the statistical behaviour of real solar cycles. Results. The relative importance of LQ vs TQ is found to correlate with the ratio u 0 /{\eta} in the SFT model grid, where u 0 is the meridional flow amplitude and {\eta} is diffusivity. An analytical interpretation of this result is given, further showing that the main underlying parameter is the dynamo effectivity range {\lambda} R which in turn is determined by the ratio of equatorial flow divergence to diffusivity. The relative importance of LQ vs TQ is shown to scale as C 1 +C 2 /{\lambda} 2 R . The presence of a latitude quenching is demonstrated in the 2x2D dynamo, contributing to the nonlinear modulation by an amount comparable to TQ. For other dynamo and SFT models considered in the literature the contribution of LQ to the modulation covers a broad range from being insignificant to being the dominant form of feedback. On the other hand, the contribution of a TQ effect (with the usually assumed amplitude) is never negligible.