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

Abstract

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 as a prediction tool for the ambient wind, and for benchmarking of solar wind models. We show that 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 to 2011, and replacing these with solar wind from a previous synodic rotation, persistence forecasts improve (relative to a baseline): skill scores for Bz, a crucial parameter for determining solar wind geoeffectiveness, improve by 7.7 percentage points when using a proton temperature‐based indicator with good operational potential. We also show that 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 to 2013 to create such a reduced lead time persistence forecast, we show that 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 (in spring 2015) of the normal (ACE‐based) and reduced lead time (STEREO‐based) persistence forecasts in the Met Office's Space Weather Operations Centre, as well as plans for future improvements.

Near‐Earth Solar Wind Forecasting Using Corotation From L5: The Error Introduced By Heliographic Latitude Offset

“…Near‐Earth solar wind can be forecast from in situ L5 observations by assuming the solar wind is steady state over the 4.5 days it takes structures to rotate between the two positions (Kohutova et al, ; Miyake et al, ; Thomas et al, ). Such L5‐corotation forecasting will undoubtedly be a useful additional tool in space weather prediction.…”

“…During these few months of data during a particularly deep solar minimum, L5 corotation was shown to be superior to a 27‐day recurrence forecast from near‐Earth data (Kohutova et al, ). The improvement, however, was fairly modest, with only ∼20% skill gain in solar wind speed forecast relative to 27‐day recurrence (Kohutova et al, ; Thomas et al, ). As the STEREO spacecraft approached the far side of the Sun (from Earth's viewpoint), they again achieved a brief 60° separation in December 2013.…”

“…The STEREO mission Kaiser (), with two spacecraft in Earth‐like orbits but drifting ahead and behind Earth at a rate of 22.5° per year, provided a unique opportunity to test a corotation forecast from L5 (Simunac et al, ; Turner & Li, ); the STEREO spacecraft were separated from each other by 60° longitude in early 2008, and from Earth by 60° near the end of 2009. During these few months of data during a particularly deep solar minimum, L5 corotation was shown to be superior to a 27‐day recurrence forecast from near‐Earth data (Kohutova et al, ). The improvement, however, was fairly modest, with only ∼20% skill gain in solar wind speed forecast relative to 27‐day recurrence (Kohutova et al, ; Thomas et al, ).…”

Evaluating the Skill of Forecasts of the Near‐Earth Solar Wind Using a Space Weather Monitor at L5

“…A second cause for the differences between the time periods shown in Table is that the plasma instruments on STEREO and ACE may become less well calibrated in time (Kohutova et al, ). If the sensitivity of different anodes within the instrument changes with time, and this is not corrected for by some kind of onboard calibration, one can very quickly observe serious errors the direction of the velocity vector.…”

Cost‐Loss Analysis of Ensemble Solar Wind Forecasting: Space Weather Use of Terrestrial Weather Tools

“…For instance, both the NOAA Space Weather Prediction Center and the Met Office Space Weather Operations Centre (MOSWOC) mainly use the ENLIL MHD model (Odstrcil et al, ) for solar wind forecasting, based on the need to predict the arrival times for coronal mass ejections (CMEs). MOSWOC also uses a simpler persistence forecast model (Kohutova et al, ) useful especially at solar minimum, when transient effects like CMEs are less important than repetitive effects from coronal holes. In their simplest formulations, however, these forecast models are deterministic.…”