Diagnostics of Polar Field Reversal in Solar Cycle 23 Using a Flux Transport Dynamo Model

Abstract

Motivated by observed anomalous features in cycle 23, as inferred from records of photospheric magnetic flux, we develop a flux transport dynamo-based scheme in order to investigate the physical cause of such anomalies. In this first study we focus on understanding anomalies occurring in the polar field evolutionary pattern in cycle 23, namely, why the polar reversal in cycle 23 was slow, why after reversal the buildup of the polar field was slow, and why the south pole reversed approximately a year after the north pole did. We construct a calibrated flux transport dynamo model that operates with dynamo ingredients such as differential rotation, meridional circulation, and large-scale poloidal field source derived from observations. A few other dynamo ingredients, such as diffusivity and quenching pattern, for which direct observations are not possible, are fixed by using theoretical guidance. By showing that this calibrated model can reproduce major longitude-averaged solar cycle features, we initialize the model at the beginning of cycle 22 and operate by incorporating the observed variations in meridional circulation and large-scale surface magnetic field sources to simulate the polar field evolution in cycle 23. We show that a 10%-20% weakening in photospheric magnetic flux in cycle 23 with respect to that in cycle 22 is the primary reason for a $1 yr slowdown in polar reversal in cycle 23. Weakening in this flux is also the reason for slow buildup of polar field after reversal, whereas the observed north-south asymmetry in meridional circulation in the form of a larger decrease in flow speed in the northern hemisphere than that in the southern hemisphere during 1996-2002 and the appearance of a reverse, high-latitude flow cell in the northern hemisphere during 1998-2001 caused the north polar field to reverse before the south polar field.

The Dynamics of the Solar Magnetic Field: Polarity Reversals, Butterfly Diagram, and Quasi-Biennial Oscillations

“…We underline that B merid and B EW can provide information on the dynamo-generated poloidal and toroidal components, although they are just a distant reflection and that there is no way to further resolve these two components without a true vector magnetograph (Ulrich et al 2002;Hoeksema & Scherrer 2010). In particular, the EW component (superficial toroidal field) is found to be confined primarily in the range [−35 • , +35 • ] and shows an antisymmetric behavior with respect to the equator, as also happens for the theoretical toroidal field as derived from dynamo models (e.g., Dikpati et al 2004) at the tachocline level (∼0.7 R ).…”

“…In particular, the antisymmetric behavior with respect to the equator, found in the photospheric field, can be reproduced by an α-effect localized near the bottom of the convection zone (Ossendrijver 2003). The most advanced dynamo models include the contribution of two types of α-effects, i.e., a Babcock-Leighton poloidal source term in a thin layer near the surface and a deep seated α-effect (e.g., Dikpati & Gilman 2001;Dikpati et al 2004).…”

The Role of Diffusivity Quenching in Flux-Transport Dynamo Models

“…We note that, despite several attempts have been made in flux-transport dynamo models to include more than one convective cell (Bonnano et al 2006;Jouve & Brun 2007), no inferred magnetic field distribution agrees better with the observations than the one that is obtained by considering only one cell pattern (Dikpati et al 2004;Guerrero & de Gouveia Dal Pino 2007b). Note, however, that the depth of penetration, as well as the magnitude of the flow in the deeper layers are still uncertain in these models.…”

“…5 Estimating the Babcock-Leighton α-effect by computing the buoyant eruption of magnetic flux tubes followed by their decay is beyond the scope of this paper. So, in order to capture the properties of the BMRs described above, we simply use the following Babcock-Leighton α-effect profile (Dikpati et al 2004):…”

The solar magnetic field since 1700

“…3, we expect that for η R = 0 the correlation between polar field and strength of the next cycle would be even stronger. Such a correlation is potentially relevant in connection with flux transport dynamo models (e.g., Chatterjee et al 2004;Dikpati et al 2004). Figure 7 shows the relationship between the total unsigned flux and the sunspot number.…”