ASI Precipitation Inversion for Swarm Over Poker 2023 Events: Searching for Hall Current Divergence and Atypical RGB Spectra

Three-color narrowband auroral imagery, like that from the Poker Flat Digital All-Sky Camera (DASC), is a valuable tool for studying auroral precipitation over a large spatial extent with relatively high spatial resolution. When the shape of the precipitation spectrum is assumed, a transport model such as GLOW can be used to generate lookup tables for varying energy fluxes and characteristic energies. The blue (428 nm) line is strongly controlled by energy flux, while the red/green ratio (630 nm, 558 nm) is strongly controlled by characteristic energy, allowing the inversion of both parameters at every image pixel. We have coded this inversion routine - provided by Don Hampton (UAF/GI) and with software organization/distribution contributions from Leslie Lamarche (SRI) - in the Python language, and made it available for generic use with calibrated, three-color imagers at github.com/almule12/asispectralinversion. We have also added error estimation/propagation and image preprocessing and denoising to the routine, which we now use to explore both expected cases and deviations from the status quo of auroral morphology and current closure.

First, we note that though GLOW-generated inversion tables vary widely with background ionospheric conditions, the region of RGB space that they occupy is relatively compact. This opens the possibility of identifying unusual auroral structures by searching for regions of sky that do not fall into the inversion solution space typically filled by GLOW. These include the enhanced aurora, which has a signature absence of blue emissions, but could potentially include some normal auroral emissions driven by unusual spectra as well. Second, we note that the ratio of Hall to Pedersen conductance is influenced by characteristic energy, with high energy precipitation penetrating deeply and strongly enhancing Hall conductance. Because of this, a high-energy event with any along-arc background field would likely demonstrate strong diverging Hall current, and therefore either strong Hall closure to FAC (low Cowling efficiency), strong polarization E-fields (high Cowling efficiency), or some combination of those. A Swarm crossing of the Poker DASC site for such events provides reliable magnetometer and cross-track TII measurements in the context of inverted imagery, while a Poker Flat Incoherent Scatter Radar (PFISR) measurement estimates the vector background electric field, allowing us to search for these signatures of diverging Hall current.
We use the Swarm over Poker 2023 Database, a set of conjunctions of Swarm, DASC, and PFISR data over the months of February and March of 2023, to explore both arc morphology and current closure, and to enhance the database with higher-level analysis of these.