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Coronal mass ejections (CMEs) cause various disturbances of the spaceenvironment; therefore, forecasting their arrival time is very important.However, forecasting accuracy is hindered by limited CME observations ininterplanetary space. This study investigates the accuracy of CME arrival timesat the Earth forecasted by three-dimensional (3D) magnetohydrodynamic (MHD)simulations based on interplanetary scintillation (IPS) observations. In thissystem, CMEs are approximated as spheromaks with various initial speeds. TenMHD simulations with different CME initial speed are tested, and the densitydistributions derived from each simulation run are compared with IPS dataobserved by the Institute for Space-Earth Environmental Research (ISEE), NagoyaUniversity. The CME arrival time of the simulation run that most closely agreeswith the IPS data is selected as the forecasted time. We then validate theaccuracy of this forecast using 12 halo CME events. The average absolutearrival-time error of the IPS-based MHD forecast is approximately 5.0 h, whichis one of the most accurate predictions that ever been validated, whereas thatof MHD simulations without IPS data, in which the initial CME speed is derivedfrom white-light coronagraph images, is approximately 6.7 h. This suggests thatthe assimilation of IPS data into MHD simulations can improve the accuracy ofCME arrival-time forecasts. The average predicted arrival times are earlierthan the actual arrival times. These early predictions may be due tooverestimation of the magnetic field included in the spheromak and/orunderestimation of the drag force from the background solar wind, the latter ofwhich could be related to underestimation of CME size or background solar winddensity.

Validation of Coronal Mass Ejection Arrival-Time Forecasts by Magnetohydrodynamic Simulations based on Interplanetary Scintillation Observations