Improving solar wind persistence forecasts : removing transient space weather events, and using observations away from the Sun-Earth line

This study demonstrates two significant ways of improving persistence forecasts of the solar wind, which exploit the relatively unchanging nature of the ambient solar wind to provide 27 day forecasts, when using data from the Lagrangian L1 point. Such forecasts are useful both as a prediction tool for the ambient wind, but also for benchmarking of solar wind models. We show solar wind persistence forecasts can be improved by removing transient solar wind features such as coronal mass ejections (CMEs). Using CME indicators to automatically identify CME-contaminated periods in ACE data from 1998-2011, and replacing these with solar wind from a previous synodic rotation, persistence forecasts improve (relative to a baseline): skill scores for the southward IMF component Bz, a crucial parameter for determining solar wind geoeffectiveness, improve by 7.7 percentage points when using a commonly-available indicator, based on the proton temperature. We also show persistence forecasts can be improved by using measurements away from L1, to reduce the requirement on coronal stability for an entire synodic period, at the cost of reduced lead time. Using STEREO-B data from 2007-2013 to create such a reduced lead time persistence forecast, we show Bz skill scores improve by 17.1 percentage points relative to ACE. Finally, we report on implications for persistence forecasts from any future missions to the L5 Lagrangian point, and on the successful operational implementation of the normal (ACE-based) and reduced lead time (STEREO-based) persistence forecasts in the Met Office’s Space Weather Operations Centre (MOSWOC), where they have been routinely used by forecasters since spring 2015, as well as plans for future improvements