Determination of the chromospheric quiet network element area index and its variation during 2008-2011

TL;DR

This study quantifies the area of chromospheric network elements during solar cycle 24's minimum and ascending phase, revealing a 7% decrease during low activity, which helps understand their role in solar irradiance variations.

Contribution

It introduces a method to separate and measure chromospheric network elements from Ca II K images, quantifying their area index and its variation during a solar cycle.

Findings

Network elements occupy about 30% of the solar disk.

Network area index decreases by about 7% during solar minimum.

Plages occupy less than 1% of the solar disk.

Abstract

Generally it has been considered that the plages and sunspots are the main contributors to the solar irradiance. There are small scale structures on the sun with intermediate magnetic fields that could also contribute to the solar irradiance. It has not yet been quantified how much of these small scale structures contribute to the solar irradiance and how much it varies over the solar cycle. In this paper, we used Ca II K images obtained from the telescope installed at Kodaikanal observatory. We report a method to separate the network elements from the background structure and plage regions. We compute the changes in the network element area index during the minimum phase of solar cycle and part of the ascending phase of cycle 24. The measured area occupied by the network elements is about 30% and plages less than 1% of the solar disk during the observation period from February 2008-2011. During the extended period of minimum activity it is observed that the network element area index decreases by about 7% compared to the area occupied by the network elements in 2008. A long term study of network element area index is required to understand the variations over the solar cycle.