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MAPPING THE DISTRIBUTION OF ELECTRON TEMPERATURE AND Fe CHARGE STATES IN THE CORONA WITH TOTAL SOLAR ECLIPSE OBSERVATIONS

The inference of electron temperature from the ratio of the intensities of emission lines in the solar corona is valid only when the plasma is collisional. Once collisionless, thermodynamic ionization equilibrium no longer holds, and the inference of an electron temperature and its gradient from such measurements is no longer valid. At the heliocentric distance where the transition from a collision-dominated to a collisionless plasma occurs, the charge states of different elements are established, or frozen-in. These are the charge states which are subsequently measured in interplanetary space. We show in this study how the 2006 March 29 and 2008 August 1 eclipse observations of a number of Fe emission lines yield an empirical value for a distance, which we call Rt, where the emission changes from being collisionally to radiatively dominated. Rt ranges from 1.1 to 2.0 R☉, depending on the charge state and the underlying coronal density structures. Beyond that distance, the intensity of the emission reflects the distribution of the corresponding Fe ion charge states. These observations thus yield the two-dimensional distribution of electron temperature and charge state measurements in the corona for the first time. The presence of the Fe x 637.4 nm and Fe xi 789.2 nm emission in open magnetic field regions below Rt, such as in coronal holes and the boundaries of streamers, and the absence of Fe xiii 1074.7 nm and Fe xiv 530.3 nm emission there indicate that the sources of the solar wind lie in regions where the electron temperature is less than 1.2 × 106 K. Beyond Rt, the extent of the Fe x [Fe9+] and Fe xi emission [Fe10+], in comparison with Fe xiii [Fe12+] and Fe xiv [Fe13+], matches the dominance of the Fe10+ charge states measured by the Solar Wind Ion Composition Spectrometer, SWICS, on Ulysses, at −43° latitude at 4 AU, in March–April 2006, and Fe9+ and Fe10+ charge states measured by SWICS on the Advanced Composition Explorer, ACE, in the ecliptic plane at 1 AU, at the time of both eclipses. The remarkable correspondence between these two measurements establishes the first direct link between the distribution of charge states in the corona and in interplanetary space.