An Imaging Polarimeter for Hydrogen Lyman-α

With the exception of the radio and microwave wavebands, photon polarization is rarely exploited in astronomical or geophysical observations. We propose to develop instrumentation for imaging polarimetry of the hydrogen Lyman-α transition, with applications to observations of Hanlé depolarization in the solar corona, polarization as a diagnostic of shock physics and particle acceleration in coronal mass ejection (CME) driven collisionless shock waves, and polarization resulting from the scattering of solar Lyman-α by the Earth's hydrogen geocorona. Using facilities at the NRL Nanoscience Institute (NSI), this proposal is aimed at the development of polarizer structures built directly onto detector surfaces, reducing the size/mass of an instrument needed for monitoring of the geocorona, while also increasing the throughput to improve the spatio-temporal resolution of such observations. These would be the first steps along a path to a mission(s) to provide real-time monitoring observations. Each application will likely require different telescope/coronagraph optics. These are discussed in more detail below, though in each case the imaging polarimeter would be fundamentally the same. On-chip polarizers are well developed for the IR and visible light regimes because the longer wavelengths allow for larger polarizer structures. Our instrumentation extends "on-chip" polarization filters, already standard at optical wavelengths, into the far ultraviolet. Observations of the polarized light in Lyman-α are rare, e.g. observations of the solar chromosphere by the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP; Kano et al. 2012; Ishikawa et al. 2017) and the traditional approach as employed here requires substantial support structures for the filters and filter wheel mechanisms. These support structures reduce the throughput, while the additional mechanisms necessitate large instruments. While the particular characteristics of the solar chromosphere probably necessitate such an approach, there are other applications of Lyman-α polarization that do not require such complex instrumentation, and our instrument concept is designed with these in mind. In particular, our new applications in the solar corona do not require the same resolutions (spatial and spectral), largely due to the greatly simplified radiation transfer scenario. Our instrument concept is much less demanding of satellite resources, and could be deployed for example on a cubesat (or similar) in an advantageous location such as L5 for Space Weather applications. In summary, the major advantages of our approach are: • The "on-chip" polarization filters is a natural extension of already existing technology for optical wavelengths. • The necessity of support structures and extra mechanisms is avoided, with corresponding simplifications in specifying such instrumentation for flight in terms of compactness, vibration tolerance and throughput. • Stokes parameters I, Q and U are obtained simultaneously. • Use of onsite facilities, the NRL Nanoscience Institute. • Lyman-α imaging polarimetry opens up novel diagnostics of coronal magnetic fields, collisionless shock physics, and structure of the geocorona.

Support NASA's Heliophysics strategic science objectives to understand the Sun and its interactions with Earth and the solar system, including space weather. This will be achieved by developing/demonstrating instrumentation technology necessary to address the following science goals: Explore the physical processes in the space environment from the Sun to the Earth and throughout the solar system; Advance our understanding of the connections that link the Sun, the Earth, planetary space environments, and the outer reaches of our solar system; Develop the knowledge and capability to detect and predict extreme conditions in space to protect life and society and to safeguard human and robotic explorers beyond Earth.

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