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

摘要

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 F1(h) and F2(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 dEM(h, T)/dT, which includes also a temperature dependence of density scale heights λn(T) = qλλT and allows us to quantify deviations (qλ ≠ 1) from hydrostatic equilibrium (qλ = 1). This parametrization facilitates a proper line-of-sight integration and relates the widely used filter ratio temperature TFR to the peak of the DEM distribution. A direct consequence of the multi-scale height atmosphere is that the filter ratio temperature TFR(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 qλ 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.