Context space weather

Solar flares and coronal mass ejections (CMEs) are the most energetic events in the solar system, influencing a panorama of physical systems from the solar surface, through the inner heliosphere and onwards into geo-space. These events are the main contributor of adverse space weather at Earth, and can, from time to time, significantly affect the performance of Earth- and space-based technological systems, such as terrestrial communications links, power grids, and satellite operations. Although significant progress has been made in understanding the fundamental physics of solar eruptions, accurate forecasting of these enigmatic events remains elusive.

Short warning window after flaring event calls for prediction

The main drivers of adverse space weather at Earth are solar flares and CMEs. Advances in the prediction of both are therefore central to protecting ground- and space-based assets. For flares, there is is no early warning for high-energy photons (X- and γ-rays), with a little early warning for flare-accelerated energetic particles, in the worst-case scenario. When first observed, electromagnetic radiation emitted by the flare is already contributing to adverse conditions at Earth: radio bursts can directly interfere with global navigation satellite signals operating at microwave frequencies or jam ground-based radars; UV and X-ray photons cause the ionosphere to expand, increasing drag on low-orbit satellites; X- and !-ray photons deliver high- dose exposures to astronauts during extravehicular activity. These effects can soon be followed by flare-accelerated solar energetic particles, within tens of minutes.

Longer warning window for CMEs

In contrast, for CMEs there exists an early-warning window of typically 20 hours to 3.5 days, due to their lower speed of propagation through the heliosphere. However, the challenge with CMEs is a successful, timely case-by-case prediction of (i) the arrival time of the interplanetary CME at Earth and (ii) the degree of geoeffectiveness (i.e., the strength of the CME interaction with the terrestrial magnetosphere). Given the timescales involved, the most formidable of these prediction problems is clearly that of solar flares. This said, flares and CMEs often erupt from the same locations at the same time, so a suitable, robust method of flare prediction offers clues on whether a CME may occur.

Conditions leading to flaring

Flares typically occur in active regions and their occurrence is related to the size and complexity of the magnetic patterns comprising these regions. It is now known that sunspots are formed by the convective action of sub-surface fluid motions pushing magnetic flux tubes through the photosphere. These turbulent photospheric and sub-photospheric motions jostle these flux tubes around and, when the conditions are right, a flare is produced. Currently, the exact conditions that are necessary for or lead to flaring are not known. This is partly due to limitations in our observational capabilities and the existing active region characterization algorithms. FLARECAST directly addressed the limitations of the latter.