Evolution of chromospheric structures: How chromospheric structures contribute to the solar He II 30.4 nanometer irradiance and variability

The bright He II 30.4 nm solar emission is an important energy source for i onization and heating of the Earth's upper atmosphere. The analysis of the Solar and Heliospheric Observatory (SOHO) Extreme-Ultraviolet Imaging Teles cope (EIT) He II 30.4 nm images provides an improved understanding of how t he 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 qu iet-chromosphere intensity scale is stable to within 0.7% (1 sigma) over th e 2 yr data set. The plage, enhanced-network, active-network, and quiet-chr omosphere structures are then identified on each EIT He Ir image with an al gorithm that uses criteria of intensity, size, filling factor, and continui ty. This decomposition leads to time series of structure area and integrate d intensity, their spatial distribution on the solar disk, and their intens ity 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 irradianc e variability during solar minimum. Conversely, the quiet-chromosphere irra diance does not vary during this time period; thus we conclude that long-te rm He II 30.4 nm irradiance variations can be traced purely to magnetic act ivity during this time period. We also find that the plage, enhanced-networ k, active-network, and quiet-network intensity contrasts, relative to the q uiet 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.