Summary:

Precipitating electrons from field line curvature scattering mark the outer radiation belt’s outer boundary. Recent work has shown that the energetic particles precipitating from this boundary, observable from Low Earth Orbiting satellites, are associated with faint structured diffuse aurora in the nightside auroral regions. Studies on optical signatures of energetic particle precipitation have historically been rare due to their low-intensity emissions. However, these signatures are visible in ground-based all-sky cameras that can potentially image the ionospheric footprint of the outer radiation belt boundary and mark the magnetic fields associated with the auroral signature in the magnetosphere. This opens up an exciting possibility that the optical signatures combined with magnetic field models might allow us to estimate the dynamic extent and structure of the outer radiation belt boundary. Furthermore, these structures appear to occur adjacent to growth phase arcs and also before its formation and hence may provide insight into the dynamics that lead to the development of growth phase arcs. Therefore, we propose this project to ask: 1) What are the characteristics of the optical auroral signature of the outer boundary of the outer radiation belt? 2) How much energy is transferred from the magnetosphere to the ionosphere within this auroral signature? 3) To what degree can we improve magnetosphere models using this signature as a boundary condition?

We will address these questions by conducting a multi-event and statistical study to link physical processes in the outer radiation belt boundary to the optical auroral signatures observed by ground-based instruments. The study will use magnetically conjugate measurements from spacecraft in the magnetosphere (THEMIS, MMS, RBSP), Low Earth Orbit satellites (POES, FIREBIRD-II, ELFIN), and ground-based instrument suites (THEMIS GBO All Sky Cameras, Poker Flat Incoherent Scatter Radar and Meridian Scanning photometer, TREx All Sky Cameras).

We will quantify the signatures’ occurrence frequency, structure, scale, and dynamics by analyzing these measurements and will produce statistical evidence linking the signature with the outer radiation belt boundary. Finally, we will incorporate the optical auroral signature of the outer radiation belt boundary as a boundary condition in a radiation belt model (CIMI) coupled with a global MHD model (BATSRUS) and assess the degree to which this boundary condition can improve existing radiation belt models.

To our knowledge, this is the first work that attempts to investigate the optical auroral signature of precipitating energetic particles from the outer radiation belt. This project will contribute towards the Focused Science Topic 1, "Connecting Auroral Phenomena with Magnetospheric phenomena," and its following goals: (1) determine what processes in the magnetosphere are responsible for various types of aurora observed in the lower altitudes; (2) quantitatively assess the energy conversion processes associated with auroral forms and the impact these auroral processes have on the coupled magnetosphere, ionosphere, and thermosphere system; and (3) to improve existing mapping between the aurora and the source location in the magnetosphere (by identifying a new optical marker for the radiation belt boundary in the ionosphere).