Evolution of Chromospheric Structures: How Chromospheric Structures Contribute to the Solar Heii30.4 Nanometer Irradiance and Variability

Abstract

The bright He II 30.4 nm solar emission is an important energy source for ionization and heating of the Earth's upper atmosphere. The analysis of the Solar and Heliospheric Observatory (SOHO) Extreme-Ultraviolet Imaging Telescope (EIT) He II 30.4 nm images provides an improved understanding of how the solar surface structures, i.e., plage, enhanced network (plage remnants), active network, and the quiet chromosphere, contribute to the solar He II 30.4 nm irradiance and its variability. We first normalize the intensities of each image to the background quiet-chromosphere intensity with a global fit that preferentially weights network cell intensities. The resulting quiet-chromosphere intensity scale is stable to within 0.7% (1 σ) over the 2 yr data set. The plage, enhanced-network, active-network, and quiet-chromosphere structures are then identified on each EIT He II image with an algorithm that uses criteria of intensity, size, filling factor, and continuity. This decomposition leads to time series of structure area and integrated intensity, their spatial distribution on the solar disk, and their intensity contrast relative to the quiet-chromosphere intensity; thus, these time series show how the solar surface structures contribute to the He II 30.4 nm irradiance. For example, we find that the active network contributes as much as the plage and enhanced network to the solar He II 30.4 nm irradiance variability during solar minimum. Conversely, the quiet-chromosphere irradiance does not vary during this time period; thus we conclude that long-term He II 30.4 nm irradiance variations can be traced purely to magnetic activity during this time period. We also find that the plage, enhanced-network, active-network, and quiet-network intensity contrasts, relative to the quiet chromosphere and averaged over the full area of each structure, are 4.8, 3.3, 2.1, and 1.6, respectively, and these contrasts remain essentially constant with time.