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Figure 5.5. The VAL3C semi-empirical model of the solar chromosphere is the solid line. Two chromospheric models for AD Leo are shown: a semi-empirical model from Mauas and Falchi [M6] (dashed line), and a schematic model from Hawley and Fisher [H5] (dotted line).

If the solar atmosphere was in strict radiative equilibrium with the energy diffusing outward from the hot core, there would be no temperature rise outside of the photosphere, and the temperature would slowly approach interstellar values as the atmospheric density and radiative flux decreased. The thermal bifurcation model for the solar atmospheric proposed by Ayres and collaborators asserts that radiative equilibrium does occur in the quiet regions of the photosphere, and that only in the active regions (within magnetic flux tubes) is there a chromospheric temperature rise [A6], [A7]. The solar model shown in Figure 5.5 is then misleading, representing a global average over disparate regions and matching the physical properties of neither. Unfortunately, we are not yet able to resolve surface features on M dwarfs, and must use globally averaged values in our analysis.3 In the following section, we refer to the outer atmosphere as though it is uniform and ubiquitous over the surface, keeping in mind that lateral inhomogeneity of considerable complexity is almost certainly present.

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