The Solar Mass Ejection Imager and Its Heliospheric Imaging Legacy

The Solar Mass Ejection Imager (SMEI) was the first of a new class of heliospheric and astronomical white-light imager. A heliospheric imager operates in a fashion similar to coronagraphs, in that it observes solar photospheric white light that has been Thomson scattered by free electrons in the solar wind plasma. Compared with traditional coronagraphs, this imager differs in that it observes at much larger angles from the Sun. This in turn requires a much higher sensitivity and wider dynamic range for the measured intensity. SMEI was launched on the Coriolis spacecraft in January 2003 and was deactivated in September 2011, thus operating almost continuously for nearly nine years. Its primary objective was the observation of interplanetary transients, typically coronal mass ejections (CMEs), and tracking them continuously throughout the inner heliosphere. Towards this goal it was immediately effective, observing and tracking several CMEs in the first month of mission operations, with some 400 detections to follow. Along with this primary science objective, SMEI also contributed to many and varied scientific fields, including studies of corotating interaction regions (CIRs), the high-altitude aurora, zodiacal light, Gegenschein, comet tail disconnections and motions, and variable stars. It was also able to detect and track Earth-orbiting satellites and space debris. Along with its scientific advancements, SMEI also demonstrated a significantly improved accuracy of space weather prediction, thereby establishing the feasibility and usefulness of operational heliospheric imagers. In this paper we review the scientific and operational achievements of SMEI, discuss lessons learned, and present our view of potential next steps in future heliospheric imaging.