The Outer Atmosphere

The outer atmosphere of the Sun begins at the temperature minimum region (TMR) and extends outward through the chromosphere, transition region and corona. Figure 5.5 shows the distribution of temperature with height and column mass for the solar VAL3C chromospheric model [V4]. Travelling outwards from the photosphere (height = 0 marks rstd = 1 in the continuum, the photospheric 'boundary'), the temperature first decreases through the TMR where the energy balance still reflects radiative equilibrium. At the height where magnetic (non-radiative) heating becomes important in the overall energy balance, the temperature reverses its decline and begins to rise, reaching a plateau at ^7,000 K (the chromosphere) before rising abruptly through the transition region and levelling out at ~106 K in the corona (not shown in the figure). VAL3C is a semi-empirical model, found by postulating an atmospheric structure, computing the emergent brightness temperatures in numerous diagnostics, comparing these with observations, and changing the structure until a good match is obtained between the model and the data. The locations of formation for the principal diagnostic lines in the solar chromosphere - Ha, Ca II K, and Mg II k - are indicated on the figure. Chromospheric models for AD Leo are superposed on the solar model, illustrating the compressed nature of the chromosphere in the lower mass, higher gravity M dwarfs.

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