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Power spectra of segmentation-cell length (a dominant length scale of EUVemission in the transition region) from full-disk HeII extreme ultraviolet(EUV) images observed by the Extreme ultraviolet Imaging Telescope (EIT)onboard the Solar and Heliospheric Observatory (SOHO) and the AtmosphericImaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) duringperiods of quiet Sun conditions for a time interval from 1996 to 2015 wereanalyzed. The spatial power as a function of the spatial frequency from about0.04 to 0.27 (EIT) or up to 0.48 (AIA) 1/Mm depends on the distribution of theobserved segmentation-cell dimensions, -- a structure of the solar EUV network.The temporal variations of the spatial power reported by Didkovsky and Gurman(Solar Phys., 289, 153) were suggested as decreases at the mid-spatialfrequencies for the compared spectra when the power curves at the highestspatial frequencies of 0.5 1/pix were adjusted to match each other. Thatapproach has been extended in this work to compare spectral ratios at highspatial frequencies expressed in the solar spatial frequency units of 1/Mm.Based on this approach we represent these ratio changes as a long-term networktransformation which may be interpreted as a continuous dissipation of mid-sizenetwork structures to the smaller-size structures in the transition region. Incontrast to expected cycling of the segmentation-cell dimension structures andassociated spatial power in the spectra with the solar cycle, the spectrademonstrate a significant and steady change of the EUV network. The temporaltrend across these structural spectra is not critically sensitive to anylong-term instrumental changes, e.g. degradation of sensitivity, but to thechange of the segmentation-cell dimensions of the EUV network structure.

A Long-Term Decrease of the Mid-Size Segmentation Lengths Observed in the HeII 30.4 nm Solar EUV Emission