Understanding the Causes that Led to the Long and Deep Minimum of Solar Cycle 23

The unusual minimum of solar cycle 23 was characterized by very weak polar field strength and a largenumber of sunspot-less days that were unprecedented in the space age. This has had significant consequences in the heliospheric space environment in terms of record-high cosmic-ray flux and low levels of solar irradiance - which are primary natural drivers of the climate system.

Taking advantage of the improvements performed by us on kinematic dynamo models (see below), we were able to obtain the first consistent explanation of the defining magnetic characteristics of this unusual minimum (large amount of days without sunspots and weak polar fields) based on variations in the solar meridional plasma flows.

Improvement of Kinematic Models of the Solar Magnetic Cycle

Kinematic dynamo models are among the best tools we have for understanding the solar cycle and solar magnetic variability. In this type of models, a set of prescribed plasma flows, turbulent magnetic diffusivity, and magnetic sources are used to study the time evolution of the solar magnetic field.

During the last decade, these models have seen a continuous evolution and have become increasingly successful at reproducing solar cycle characteristics. However, their ingredients largely remained poorly constrained which allowed modelers to obtain solar-like solutions by “tuning” the input parameters (leading to controversy regarding which parameter set is more appropriate).

In this project I revisited each of the dynamo ingredients, using observational data and theoretical considerations to constrain them better and improve the way they are modeled.

This project had three parts:

- Using helioseismic data to improve the way differential rotation and meridional flow are characterized inside the model.
- Reconciling commonly used diffusivity profiles with theoretical estimates.
- Introducing a new and better way of modeling active region (sunspot-pair) emergence and decay.