BIFURCATION IN THE LOW CHROMOSPHERE

Bright chromospheric faculae of diameter dc covering an area of the solar disk fc < 1 yield a predictable level of limb brightening. For a resolution diameter dr, the combination fc and dc leads, also, to a prediction of the fraction, φ0, of observing pixels (resolution areas) that are free of facular emission. Data from the first flight of the High Resolution Telescope and Spectrograph of the US Naval Research Laboratory with dr ≈1″ are used to measure both the level of limb brightening and φ0 in EUV emission lines of O I, O I, and Fe II. The results favor fc ≈ 0.9 and dc ≳ 2″.5. We conclude that the chromospheric temperature rise is present over ∼90% of the solar surface.