Abstract
In the present work we have analyzed the spectra emitted by two quiet solar regions observed off the solar disk by the SUMER instrument on board the SOHO satellite. The two complete spectra were recorded when the SOHO north-south axis was rotated relative to the Sun north-south axis by 150° clockwise. As a result, the SUMER slit could be placed so that it is perpendicular to the solar limb in an intermediate orientation between the equator and the poles. The SUMER fields of view consisted of two 1'' wide radial strips of the solar corona from 1.02 to 1.34 R☉. The aim of the present work was to measure the physical properties of the emitting plasma, namely, the electron density and temperature, the plasma emission measure, and the nonthermal mass motions, as a function of the distance from the solar limb. The measurement of the plasma absolute element abundances is deferred to a future paper. In measuring the nonthermal velocities of both source regions, we have discovered a residual instrumental systematic effect to line widths. The plasma in the SUMER field of view is nearly isothermal. The measurements of electron density and temperature allow us to check the hydrostatic assumption commonly adopted in the literature and to find that the plasma is denser than predicted. The wide wavelength range covered by the SUMER instrument includes several Li-like ions, allowing us to investigate the relative contribution of the radiative and collisional excitation mechanisms in the Li-like resonance doublet formation. We confirm the earlier findings that a significant radiative excitation occurs for O VI and Ne VIII resonance lines even at low altitudes.
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