A-CHAIM: Near-Real-Time Data Assimilation of the High Latitude
Ionosphere with a Particle Filter
Abstract
The Assimilative Canadian High Arctic Ionospheric Model (A-CHAIM) is an
operational ionospheric data assimilation model that provides a 3D
representation of the high latitude ionosphere in Near-Real-Time (NRT).
A-CHAIM uses low-latency observations slant Total Electron Content
(sTEC) from ground-based Global Navigation Satellite System (GNSS)
receivers, ionosondes, and vertical TEC from the JASON-3 altimeter
satellite to produce an updated electron density model above $45^o$
geomagnetic latitude. A-CHAIM is the first operational use of a particle
filter data assimilation for space environment modeling, to account for
the nonlinear nature of sTEC observations. The large number
(>10^4) of simultaneous observations creates significant
problems with particle weight degeneracy, which is addressed by
combining measurements to form new composite observables. The
performance of A-CHAIM is assessed by comparing the model outputs to
unassimilated ionosonde observations, as well as to in-situ electron
density observations from the SWARM and DMSP satellites. During
moderately disturbed conditions from September 21st, 2021 through
September 29th, 2021, A-CHAIM demonstrates a 40% to 50% reduction in
error relative to the background model in the F2-layer critical
frequency (foF2) at midlatitude and auroral reference stations, and
little change at higher latitudes. The height of the F2-layer (hmF2)
shows a small 5% to 15% improvement at all latitudes. In the topside,
A-CHAIM demonstrates a 15% to 20% reduction in error for the Swarm
satellites, and a 23% to 28% reduction in error for the DMSP
satellites. The reduction in error is distributed evenly over the
assimilation region, including in data-sparse regions.