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Extending High Sensitivity Velocity Resolved Measurements of Solar System Dynamics into the EUV. Advancement of the Spatial Heterodyne EUV Interferometer (SHEUVI)

Sponsored by NASA Headquarters

$133.1K Funding
2 People

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The extreme ultraviolet (EUV) spectral region (10<λ<100 nm) contains ion and neutral transitions that track energetic processes in the solar system at scales from planetary upper atmospheres to the entire heliosphere. EUV signatures are typically associated with transition effects such as shock boundaries, energy deposition sites (aurorae & ionospheres), magnetic field aligned accelerations and gryo-motion (in magnetospheres or the solar corona), plasma pickup processes (e.g. the Io plasma torus, comet ion tails, planetary exospheres) and interactions between different particle populations (e.g. resonant charge exchange). A shared characteristic of these processes is that their individual EUV emission signatures have Doppler signatures that reveal acceleration, multimodal velocity components, and thermal & non-thermal forcing effects that provide unique insight into the underlying processes giving rise to them. The perceived importance of EUV remote sensing of these processes for solar system studies is demonstrated by the frequent presence of imaging and spectroscopic instruments on remote probes (e.g. Voyager, BepiColombo, Cassini, New Horizons) and their inclusion in Earth-orbiting observatories (e.g. HUT, IMAGE, EUVE, SOHO). It is therefore unfortunate that the existing state of the art in EUV instrumentation is unable to resolve the very Doppler features that contain the most important information in the emission features under study. The goal of this proposed effort is to leverage off a previous NASA PICASSO (NNX16AJ31G) project, advancing the TRL of the Spatial Heterodyne Extreme Ultra-Violet Interferometer (SHUEVI) to address the ‘resolution gap’ in EUV spectroscopy. SHEUVI is a form of an all-reflective spatial heterodyne spectrometer (ARCSHS). An ARCSHS is a Fourier transform spectroscopic (FTS) sensor that samples spectral features by splitting incoming light from a target into two beams that are then interfere as they counter circulate a common optical path. SHS instruments have considerable advantages over dispersive instruments such as an Echelle spectrometer for observations of diffuse emission features, because they are able to sample a much larger field of view (FOV=Ω) at high resolving power (e.g. R~105). Moreover, they can be made compact enough to fit within the mass and volume limitations of space probes while still retaining an étendue (E = A Ω, where A = light collecting area) 100-1000 times greater than instruments at large telescopes (e.g. HST-STIS). SHEUVI will be a first for both high spectral resolving power and interference spectroscopy at λ <90 nm. Astronomical interferometry has not been implemented in the EUV, because the optical surface quality and mechanical accuracy requirements of the most common astrophysical implementations have been and remain beyond practical reach. Several convergent factors have contributed to enable its development with SHS. Both theory and testing have shown the interferometric performance of ARCSHS to be largely immune to key surface quality restrictions of other FTS designs, a finding that was demonstrated first for narrow bandpass applications in the visible, then in the far UV at 121 nm and now, through this effort, into the EUV. The self-compensating optical path of the instrument also enhances its stability against mechanical and thermal shift. Additional advances in the suppression of side-orders for coarse ruling, high-order gratings have eliminated a previous limitation that prevented ARCSHS designs intended to cover multiple pass bands simultaneously. Finally, advancements in optical fabrication techniques targeting industrial applications have improved optical surface quality, diffraction grating efficiency, and multilayer coating reflectance across the EUV spectral range.