Future possibilities for doppler and magnetic field measurements in the extended solar atmosphere: Dissecting the transition region

For the first time, a vacuum ultraviolet (VUV) telescope can be built to rapidly observe the magnetic fields, plasma flows, and heating events in the Sun's atmosphere. These observations can provide key data for space weather models. The vacuum ultraviolet region allows remote sensing of the upper levels of the solar atmosphere where the magnetic field dominates the physics. A VUV Fabry-Pérot interferometer (FPI) will allow us to observe the magnetic field, flows, and heating events in the mid-transition region (between the chromosphere and corona). Observations of this region are needed to directly probe the magnetic structure and activity at the base of the corona where the magnetic field is approximately force-free, i.e., where gas pressures are very small. This is a key element in developing accurate models of the Sun's dynamics for space weather. The specific region of interest is the 100 km thick transition region, between the chromosphere and the much hotter corona, which strongly emits at 155 nm from triply ionized carbon (C3+) at 100,000 K. This is best observed by an imaging interferometer that combines the best attributes of a spectrograph and an imager. We present the latest results from the NASA Marshall Space Flight Center (MSFC) FPI. The major elements of the tunable Civ VUV FPI are the 35 mm MgF2 etalon plates with a plate finesse of F>25 at 155 nm, the π-dielectric coatings, a Hansen mechanical mount in a pressurize canister, and the piezoelectric control system. The control system for the etalon is a capacitance-stabilized Hovemere Ltd. standard system. The special Cascade Optical Corporation reflectance coatings are 25 pi-multilayers of high-low refractive layers paired in phase. This Civ interferometer, when flown above Earth's atmosphere, will obtain narrow-passband images, magnetograms, and Dopplergrams of the transition region in the Civ 155 nm line at a rapid cadence. We recently measured the MSFC VUV FPI using the University of Toronto's fluoride excimer laser as a proxy for Civ 155 nm. The test demonstrated the first tunable interferometer with the passband required for a VUV filter magnetograph. The measured values have a full-width half-maximum (FWHM) passband of 10 pm, a free-spectral range (FSR) of 61 pm, and a transmittance of 58% at 157 nm. The resulting VUV interferometer finesse is 5.9. With this success, we are developing an instrument suitable for a flight on an orbiting solar observatory. A description of the interferometer for this mission is described.