Abstract
AbstractForecasting space weather is an essential activity for increasing the resilience of modern technological infrastructure to hazards from the Sun. To provide an accurate forecast, space weather monitors positioned at L5 are proposed that carry in situ plasma detectors. Here we use data from the STEREO and ACE missions to investigate how well it is possible to predict the solar wind when there are two spacecraft located with the same longitudinal separation as from L5 to Earth. There are four intervals when this is the case: STEREO‐to‐STEREO both on the Earth's side and the far side of the Sun, STEREO‐B to ACE and ACE to STEREO‐A. We forecast the solar wind by mapping the observed solar wind at the first spacecraft to the second using a time delay calculated using the spacecraft's heliographic longitudinal separation and the difference in radial distance from the Sun, allowing for the solar wind speed. Using forecasting skill scores, we find that the predicted and observed solar wind data are, in general, in very good agreement with each of the four periods, including observed corotating interaction regions. However, there are some notable exceptions when corotating interaction regions have been missed by the forecast. The skill improves further for all time periods when removing coronal mass ejections, which cannot be predicted in this method. We suggest that an L5 monitor should be located at the same heliographic latitude as the Earth to optimize the forecasting ability of the monitor and to reduce the chance of missing important events.
Highlights
Space weather forecasting has been taken up by many global forecasting institutes, including the UK Met Office and NOAA in the United States
The main forms of solar activity that drive severe space weather and that are considered key to mitigating hazards at Earth are as follows: eruptions of plasma and magnetic field, known as coronal mass ejections (CMEs); compression regions caused by fast solar wind catching up to slower wind ahead of it, known as corotating interaction regions (CIRs); high-speed streams originating from coronal holes; and high-intensity bursts of radiation and energetic particles associated with solar flares and fast CMEs with strong interplanetary shocks (e.g., Klein & Dalla, 2017)
From the skill scores deduced from equations (3) and (4) and by analyzing the outputs for each heliospheric parameter during each time period, we can deduce how our forecasting method from the STEREO data compares to using data from one solar rotation previously from L1 data
Summary
Space weather forecasting has been taken up by many global forecasting institutes, including the UK Met Office and NOAA in the United States. The derived change in heliographic longitude is converted into the time taken for the solar wind to get from the leading spacecraft to the trailing using the angular solar rotation speed. The total time taken for solar wind from a given source to be observed at the first spacecraft and the second is these converted times plus the difference in time due to the heliographic longitudinal separation of the two spacecraft calculated by multiplying the angular separation of the two spacecraft, which changes by time, by the solar rotation rate, Ω. These are applied to the data from the first spacecraft to compare with the second This way we expect to observe long-lasting solar wind features, such as CIRs, in both data sets simultaneously (when including the time-delay) but not more local and impulsive events such as CMEs as these frequently only cross one of the two spacecraft. The cross-helicity allows us to investigate the presence of large-amplitude Alfvenwaves in the solar wind, which could act to scatter high-energy particles such as solar energetic particles and galactic cosmic rays (e.g., Lazarian, 2016)
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