Observing Evolution in the Supergranular Length Scale During Periods of Low Solar Activity

TL;DR

This study observes and compares supergranular length-scale variations during solar minima, revealing a ~0.5Mm difference between cycles 22/23 and 23/24, providing insights into quiet solar activity and its influence on the heliosphere.

Contribution

It demonstrates the variation of supergranular length-scale across solar minima using multiple datasets and diagnostic measures, advancing understanding of quiet Sun dynamics.

Findings

Supergranular length-scale varies by ~0.5Mm between solar minima.

Multiple instruments confirm the scale variation.

Variation correlates with solar activity levels.

Abstract

We present the initial results of an observational study into the variation of the dominant length-scale of quiet solar emission: supergranulation. This length-scale reflects the radiative energy in the plasma of the upper solar chromosphere and transition region at the magnetic network boundaries forming as a result of the relentless interaction of magnetic fields and convective motions of the Sun's interior. We demonstrate that a net difference of ~0.5Mm in the supergranular emission length-scale occurs when comparing observations cycle 22/23 and cycle 23/24 minima. This variation in scale is reproduced in the datasets of multiple space- and ground-based instruments and using different diagnostic measures. By means of extension, we consider the variation of the supergranular length-scale over multiple solar minima by analyzing a subset of the Mt Wilson Solar Observatory (MWO) Ca II K image record. The observations and analysis presented provide a tantalizing look at solar activity in the absence of large-scale flux emergence, offering insight into times of "extreme" solar minimum and general behavior such as the phasing and cross-dependence of different components of the spectral irradiance. Given that the modulation of the supergranular scale imprints itself in variations of the Sun's spectral irradiance, as well as in the mass and energy transport into the entire outer atmosphere, this preliminary investigation is an important step in understanding the impact of the quiet sun on the heliospheric system.