Chapter 2 - Energetic neutral atom imaging of the terrestrial global magnetosphere

Abstract

Energetic neutral atom (ENA) imaging of the terrestrial magnetosphere is reviewed. The history from the first measurements of ENAs to the realization that their detection can be used as a remote imaging diagnostic is presented and the basic production mechanism of ENAs are described. The measurement principle is laid out and categorized for high (≥few keV) versus low (<few keV) energies. This chapter also focuses on the scientific results from missions including the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) and the Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) missions, including observations of the ring current, plasmasheet, and ion outflow. A constrained linear inversion technique for retrieving the parent ion distributions from ENA images is described and an example of a retrieved ring current proton distribution from a geomagnetic storm recovery phase is compared to simultaneous in-situ proton measurements by the Cluster mission, providing a validation for the technique. The chapter concludes with a brief overview of future directions of ENA imaging, prioritizing improved angular resolution at energies less than about 10keV for enabling imaging of bursty bulk flows (BBFs) and dipolarization fronts. The importance of polar and equatorial imaging vantage points is emphasized, as offered by the Magnetosphere Energetics, Dynamics, and Ionospheric Coupling Mission (MEDICI) science mission concept recommended in the 2013 Solar and Space Physics Decadal Survey.