Scientific Design Studies for a Polarizing Heliospheric Imager, 15-R8309
Principal Investigators
Craig DeForest
Tim Howard
Inclusive Dates: 04/30/12 – 8/30/12
Background — SwRI researchers are advancing the technology of "heliospheric imaging" of tenuous clouds of free electrons and ions in interplanetary space. The clouds, while visible, are extremely faint compared to other objects in the night sky. The technology will aid understanding of the solar wind and coronal mass ejections (CMEs), two phenomena that link activity in the Sun's atmosphere with the Earth and other planets. The clouds are visible because sunlight scatters off of individual free electrons in interplanetary space. The physics of the scatter gives rise to a polarization signal, which can potentially be exploited to measure the 3-D location of features in the solar wind. This concept had never been fully developed, and is a key step to building a next-generation heliospheric imaging instrument.
Approach — SwRI researchers developed an analytic theory of small feature polarization given the known physics of Thomson scattering (the process that renders plasma clouds visible), and a simulation framework to demonstrate the effect on realistic images of a simulated CME. A numerical inversion scheme was also developed to extract 3-D location of propagating CMEs, and it was demonstrated using data from the simulation framework.
Accomplishments — The development goals were met and the results were published in two papers submitted to the Astrophysical Journal. SwRI researchers were able to demonstrate that 3-D imaging of solar wind features and CMEs is feasible, given an instrument with similar imaging quality to existing instruments and also polarization capability. The illustration shows the fundamental result, indicating inferred location of 15 modeled small features as they propagate away from the Sun. For example, the green shape represents a feature leaving the Sun 20° from the plane of the sky, as it crosses various "elongations" (angles from the Sun). The feature is inferred to be traveling along its correct trajectory (horizontal sheaf of curves) or along a physically unfeasible "ghost trajectory" that can be ruled out by dynamics considerations (angled green line). Each shape in the sheaf represents a different feature brightness in the instrument focal plane. Similar plots in the companion paper show the feasibility of similar analysis for large features.
