Methods

HUXt

HUXt is an open source reduced-physics solar-wind model [1]. It is designed to provide similar results to 3D-MHD models at a fraction of the computational cost [2]. This greatly reduces forecast latency and enables large ensembles to be run in order to better estimate forecast uncertainty. For these forecasts, HUXt uses an inner boundary distance of 21.5 solar radii and an outer boundary distance of 230 solar radii. The ambient solar wind speed inner boundary condition is taken from either WSA, CorTom, or BRaVDA. The Cone CME parameters are taken from cone files produced by NASA M2M forecasters or from ACME.

Ensemble Modelling

We use ensemble forecasting methods to estimate the uncertainty in a forecast and to enable probabilistic forecasting. More specifically, we use perturbed parameter ensembles to quantify the forecast uncertainty resulting from the uncertainty in the boundary conditions of HUXt; this includes both uncertainty in the ambient solar wind structure, and in the Cone CME parameters. The output of the HUXt ensembles are time-series of the solar wind speed at Earth, from which we compute a probabilistic forecast of CME arrival times and speeds.

Data Assimilation

Data assimilation methods can be used to both improve the accuracy and reduce the uncertainty of a forecast. BRaVDA is a variational data assimilation scheme that assimilates in-situ solar wind speed observations and produces an updated estimate of the solar wind speed at the inner-boundary of HUXt [3]. This typically provides a more representative estimate of the ambient solar wind structure than models such as WSA, although this depends on the availability and quality of the in-situ solar wind observations.