  We present the development of SIR-HUXt, the integration of a sequential
importance resampling (SIR) data assimilation scheme with the HUXt solar wind
model. SIR-HUXt is designed to assimilate the time-elongation profiles of CME
fronts in the low heliosphere, such as those typically extracted from
heliospheric imager data returned by the STEREO, Parker Solar Probe, and Solar
Orbiter missions. We use Observing System Simulation Experiments to explore the
performance of SIR-HUXt for a simple synthetic CME scenario of a fully Earth
directed CME flowing through a uniform ambient solar wind, where the CME is
initialised with the average observed CME speed and width. These experiments
are performed for a range of observer locations, from 20 deg to 90 deg behind
Earth, spanning the L5 point where ESA's future Vigil space weather monitor
will return heliospheric imager data for operational space weather forecasting.
  We show that SIR-HUXt performs well at constraining the CME speed, and has
some success at constraining the CME longitude. The CME width is largely
unconstrained by the SIR-HUXt assimilations, and more experiments are required
to determine if this is due to this specific CME scenario, or is a general
feature of assimilating time-elongation profiles. Rank-histograms suggest that
the SIR-HUXt ensembles are well calibrated, with no clear indications of bias
or under/over dispersion. Improved constraints on the initial CME speed lead
directly to improvements in the CME transit time to Earth and arrival speed.
For an observer in the L5 region, SIR-HUXt returned a 69% reduction in the CME
transit time uncertainty, and a 63% reduction in the arrival speed uncertainty.
This suggests SIR-HUXt has potential to improve the real-world representivity
of HUXt simulations, and therefore has potential to reduce the uncertainty of
CME arrival time hindcasts and forecasts.
