Comparisons of Supergranule Characteristics During the Solar Minima of Cycles 22/23 and 23/24

TL;DR

This study compares supergranule characteristics during the solar minima of cycles 22/23 and 23/24, analyzing velocity, size, and lifetime to understand differences in solar convection and magnetic field behavior.

Contribution

It provides a comparative analysis of supergranule properties during two recent solar minima, highlighting subtle differences in size and flow velocity.

Findings

Lifetimes of supergranules are similar (~18 hours) in both minima.

Supergranules were slightly larger in 1996 than in 2008.

Horizontal flow velocities showed minimal differences between the two minima.

Abstract

Supergranulation is a component of solar convection that manifests itself on the photosphere as a cellular network of around 35 Mm across, with a turnover lifetime of 1-2 days. It is strongly linked to the structure of the magnetic field. The horizontal, divergent flows within supergranule cells carry local field lines to the cell boundaries, while the rotational properties of supergranule upflows may contribute to the restoration of the poloidal field as part of the dynamo mechanism that controls the solar cycle. The solar minimum at the transition from cycle 23 to 24 was notable for its low level of activity and its extended length. It is of interest to study whether the convective phenomena that influences the solar magnetic field during this time differed in character to periods of previous minima. This study investigates three characteristics (velocity components, sizes and lifetimes) of solar supergranulation. Comparisons of these characteristics are made between the minima of cycles 22/23 and 23/24 using MDI Doppler data from 1996 and 2008, respectively. It is found that whereas the lifetimes are equal during both epochs (around 18 h), the sizes are larger in 1996 (35.9 +/- 0.3 Mm) than in 2008 (35.0 +/- 0.3 Mm), while the dominant horizontal velocity flows are weaker (139 +/- 1 m/s in 1996; 141 +/- 1 m/s in 2008). Although numerical differences are seen, they are not conclusive proof of the most recent minimum being inherently unusual.