Flow-line Wandering in the Turbulent Solar Wind and Space Environment Forecasts

Abstract

Abstract Space environment forecasts are based on ab initio modeling of the solar wind (SW) wherein solar magnetic fields and plasmas are propagated from an initial/boundary model source surface in the lower solar corona out to 1 au. Testing of these space environment forecasts relative to in situ measurements at 1 au typically shows uncertainties in the arrival times of fast SW streams of the order of a day, with broad distributions, means/medians of the order of half a day, and large variations between models but no definite winner. Here the effect of flow-line (FL) wandering due to the higher frequency velocity fluctuations within the turbulent SW on the arrival-time statistics of parcels of SW plasma transiting through the inner heliosphere out to 1 au is evaluated for a range of cutoff timescales in the velocity fluctuations. Used for this evaluation are in situ SW velocity measurements onboard Wind at 1 au, detailed spectral analysis of these measurements, WKB extrapolations to the inner heliosphere and simple application of a newly extended theoretical calculation of the mean SWFL cross-flow and “flow-aligned” displacements from the measured spectra. It is found that the velocity fluctuations near 1 au have little effect on the arrival times. The effect of the velocity fluctuations increases sunward, however, to a level sufficient to explain the large and broadly distributed “uncertainties” found in the testing of the forecasts.