A HIGH-RESOLUTION IMAGING SPECTROMETER FOR VISIBLE CORONAL EMISSION LINES

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The Earth orbits through a sea of magnetised plasma flowing from the Sun called the solar wind. Travelling at high speed through the solar wind, Coronal Mass Ejections (CME) are enormous clouds of high-density plasma associated with eruptions and flares at the Sun. The Earth's magnetic field acts as a shield to prevent damage from CMEs. Society, however, depends on satellites and large CMEs under certain conditions may cause widespread damage. The ability to predict the occurrence of a CME, and to predict their subsequent evolution, depends critically on understanding the dominant physical processes which occur in the corona, and on understanding the medium through which CMEs propagate - the solar wind. In understanding the solar wind and CMEs, the extended inner corona is a critical region. This complex region starts at the solar surface and extends to where the coronal plasma starts to expand radially and becomes supersonic. Current observations of this region are very limited. There are a handful of broadband visible light coronagraphs which provide rather noisy measurements of the Thomson-scattered light from coronal electrons such as the COR1/STEREO coronagraphs, or the MLSO MKIV coronameters (to be upgraded to COSMO soon). Current EUV imagers and spectrometers provide very high-quality data, but their field of view is typically limited to a few tenths of solar radii from the photosphere at best. There is no spectrometer currently in use which can observe the corona above a few tenths of a solar radius from the limb, severely restricting efforts to understand the source region of CMEs and the solar wind.The goal of the proposed spectrometer is to capitalize on the diagnostic properties of coronal forbidden emission lines in the visible and near-IR, to infer electron temperatures, ion densities, elemental abundances and charge states, bulk flow speeds, non-thermal heating (i.e. wave heating) and indirectly the properties of the coronal magnetic field. The scientific advantage of observing these lines stems primarily from the strength of their radiatively excited component, which enables the emission to extend out to large heliocentric distances. It is surprising therefore that observations of forbidden coronal emission lines in the visible have been seriously neglected in current solar missions. The proposed spectrometer directly addresses this shortcoming and will provide unique constraints on the physical processes that heat the solar atmosphere to over one million degrees and accelerate the solar wind.