In situ spectroscopy of the solar corona

Context: Future spacecraft missions, such as the proposed Solar Probe mission, will venture close to the Sun, allowing spectrometers measuring emission from heavy ions or neutrals in the solar wind to have radial lines of sight (LOS) pointing away from the Sun, or indeed in any direction other than sunwards. Aims: We show that a radial LOS gives excellent solar wind diagnostics, with tight constraints on ion density, outflow velocity, and effective temperature parallel to the coronal magnetic field. In addition, we present the concept that a spectrometer onboard a spacecraft reaching the solar corona can yield measurements somewhat similar to an in situ sampling instrument, in that the 3D velocity distribution and density of the emitting ions can be measured. Methods: The well-studied O VI doublet at 1031.96 and 1037.6 Å and the H Ly-α line at 1215.67 Å are chosen as examples. Solar wind parameters obtained from a 2D three-fluid magnetohydrodynamic (MHD) model, and formulations for collisional and radiative emission along a radial LOS, are used to calculate spectral line profiles for these lines at various heights within a streamer and coronal hole. Results: For O VI, the collisional line profiles directly measure the ion velocity distribution in the radial direction, with the general Doppler shift of the profiles related to the bulk ion outflow velocity and the width of the line related to the effective ion temperature parallel to the magnetic field. An obvious skew in the collisional profiles is seen in regions with a high gradient in outflow velocity and/or temperature. The resonant (or radiative) line profiles behave very differently from those currently observed in 90° scattering. They are more closely related to the profile and distribution of the exciting chromospheric spectrum: the lines are narrow and are centered at wavelengths mirrored around the rest wavelength of the ion emission, allowing easy separation of the collisional and radiative components. Despite the Ly-α line being much more intense than the O VI lines, the large width and high intensity of the Ly-α radiative component in comparison to the collisional component is such that these two components cannot be separated. The Ly-α line is therefore less suitable for solar wind diagnostics. Conclusions: The prospect of coronal in situ spectral observations, combined with simultaneous in situ sampling measurements of the solar wind and magnetic field will give unsurpassed constraints on models of solar wind heating and acceleration.