When solar storms erupt from the surface of the Sun, space weather forecasters leap into action to determine whether these dangerous blasts of charged particles and radiation are headed toward Earth and when they will arrive. Most storms take a few days to travel the 150 million kilometers from the Sun to Earth, but the fastest and most powerful can take less than 24 hours. Since these storms threaten Earth’s communications networks and power grids, it’s important to understand this variation in travel time—but there are a couple of different possible strategies to deal with it.
Weather on Earth is notoriously hard to predict. True to the famous butterfly effect, small changes to the ambient conditions in the atmosphere—changes too small and numerous to measure—can cause big differences in the outcome. For this reason, the predicted path of a hurricane diverges quickly in computer models, requiring forecasters to run ensembles of dozens of simulations to investigate the range of possible trajectories.
However, not all phenomena on Earth are like that. Look at tsunamis: When an underwater earthquake strikes, the waves of energy spread out across the ocean in a very predictable way that models can reliably capture. Simple physics dictates the way the energy bends around islands and obstacles; the local ocean conditions have almost no effect. Therefore, instead of running simulations every time an earthquake strikes, scientists can run them in advance over a range of realistic situations. Then, when the ground rumbles and coastlines are at risk, they can simply consult a table to find out when the tsunami will arrive, saving crucial time that can give people quicker warning.
Are solar storms—which transit the interplanetary space between the Sun and Earth—more like hurricanes or tsunamis? To find out, Pizzo et al. ran ensembles of simulations with the model currently in use at NOAA’s Space Weather Prediction Center, paying attention to when it predicted the storm would arrive. They found that although the forecast transit time varied on the basis of the mass of the storm and its initial speed, that variation was predictable—there was no hint of the sort of chaotic behavior that hurricanes are known for. In particular, ambient conditions had the least effect on the arrival times of the fastest, most dangerous storms. This suggests that a forecasting strategy similar to that of tsunamis may be effective, giving the planet more advance warning.
The authors say their study lays the theoretical groundwork for applying an ensemble approach to solar storm forecasting in the future. The next step is to verify their predictions by comparing their results to the archives of solar storm observations that NASA and European Space Agency satellites have amassed. Although the study does not address the issue of the intensity of the storms at Earth, that topic will be the focus of further work. (Space Weather, doi:10.1002/2015SW001221, 2015)
—Mark Zastrow, Freelance Writer
Citation: Zastrow, M. (2016), Solar storms are more predictable than hurricanes, Eos, 97, doi:10.1029/2016EO046311. Published on 22 February 2016.