Tempurature Tomography of the Soft X‐Ray Corona: Measurements of Electron Densities, Tempuratures, and Differential Emission Measure Distributions above the Limb

We analyze long-exposure and off-pointing Yohkoh /SXT data of the solar corona observed on 1992 August 26. We develop a new (temperature) tomography method that is based on a forward-fitting method of a four-parameter model to the observed soft X-ray fluxes F 1 ( h ) and F 2 ( h ) of two SXT wavelength filters as a function of height h . The model is defined in terms of a differential emission measure (DEM) distribution d EM( h , T )/ dT , which includes also a temperature dependence of density scale heights λ n ( T ) = q λ λ T and allows us to quantify deviations ( q λ ≠ 1) from hydrostatic equilibrium ( q λ = 1). This parametrization facilitates a proper line-of-sight integration and relates the widely used filter ratio temperature T FR to the peak of the DEM distribution. A direct consequence of the multi-scale height atmosphere is that the filter ratio temperature T FR ( h ) is predicted to increase with height, even if all magnetic field lines are isothermal. Our model fitting reveals that coronal holes and quiet-Sun regions are in perfect hydrostatic equilibrium but that coronal streamers have a scale height that exceeds the hydrostatic scale height by a factor of up to q λ ≲ 2.3, which underscores the dynamic nature of coronal streamers. Our density measurements in coronal holes are slightly lower than most of the white-light polarized brightness inversions and seem to come closer to the requirements of solar wind models. Our DEM model provides also a physical framework for the semiempirical Baumbach-Allen formula and quantifies the temperature ranges and degree of hydrostaticity of the K, L, and F coronae.