The evolution of solar storms in the inner heliosphere

The goal of this project is to enhance our understanding of how solar storms move and expand in the solar wind between the Sun and the Earth. This is directly related to our capability to forecast their potentially disastrous consequences at Earth in real time. These storms, known as coronal mass ejections (CMEs), are expelled from the Sun`s outermost layer with enormous speeds of up to 3000 kilometers per second, and may reach Earth in one to five days. They are the source of the strongest disturbances of the Earth`s magnetosphere, and a "super CME event" may pose a great threat to our modern technological infrastructure, in particular to satellites in Earth orbit, flight crews on polar routes, and power grids on the ground. Thus, a strong incentive exists to check the validity of the results provided by various CME models. However, there has been a lack of verification of these results with multi-point in situ data of CMEs. This project aims at filling this gap. We pursue one main goal: to understand the propagation and shape of coronal mass ejections in the inner heliosphere. To this end, we define two working packages (WPs): in WP1 (METHOD DEVELOPMENT), new techniques are developed to model the evolution of CMEs with NASA/STEREO images. In WP2 (CME PROPAGATION AND PREDICTION), we will evaluate these model results and their predictions with multi-point in situ data of CMEs to determine their physical characteristics, such as their global shape, 3D-orientation, and kinematics. We will use data provided by the heliospheric network of suitable space probes currently operating: MESSENGER at Mercury, Venus Express at Venus, ACE/Wind at Earth, and STEREO-A/B in the solar wind. The results will be powerful software packages and analyses on CME evolution and their global shape, published in international peer-reviewed journals, and they will provide inputs for efforts on real time space weather prediction. Importantly, newly developed methods and results will also contribute to our understanding of the influence of CMEs on the atmospheres and magnetospheres of the terrestrial planets. Furthermore, the Solar Orbiter and Solar Probe Plus missions are currently designed and will approach the Sun closer than ever before by the end of the decade. The know-how of Austrian research in these fields, fostered in this project, will form a basis for the involvement in these future, promising missions.

In this project we invented a new method for modeling and predicting solar storms, based on data from an instrument which can make images of the solar wind, a stream of particles and magnetic fields emitted by the Sun. We compared the performance of this new method to commonly used CME prediction tools and found that we were able to improve the prediction accuracy of the arrival time and especially of the arrival speed. In another study, the model was applied to multi-spacecraft observations in order to verify the CME frontal shape as well as its kinematics at various points in space. This method is now the state-of-the-art and will be used on data from future spacecraft missions, such as Parker Solar Probe, Solar Orbiter, and a possible mission to the Lagrange 5 point of the Sun-Earth system. Similarly to a problem in weather forecasting on Earth, we found that the small errors in calculating the direction, speed and shape of the solar storm near the Sun, are a mostly unrecognized but major source for the errors in predicting the solar storm arrival time. Another process we found concerned the eruption of solar storms. Instead of erupting in a direction pointing straight away from the Sun, we now know that they can propagate under special circumstances along an angle of up to 45 degree. Thus, a solar storm that seems to be coming straight towards Earth might completely miss us, or one thought to miss us may nevertheless lead to a significant impact. One of the biggest unsolved problems in our field concerns how the magnetic field inside a solar storm core, which consists of organized field rotations, can be predicted. To this end, we tested in two studies, each with a single solar storm event, how either a new way of modeling the solar storm core or an observation at Venus orbit could make this possible. The results were positive in both studies, however, some free parameters would need to be constrained if these ideas were to be used in real- time. Our results hint that a ring of small spacecraft circling the Sun may lead to a solution of the problem for predicting the solar storm core magnetic fields. This could lead in the future to precise forecasts that may allow to mitigate power blackouts and improves aurora viewing for people at high latitudes. The project has thus made a significant impact in this field internationally, and laid the foundation for both the future involvement in exciting new space missions and the making of an Austrian space weather prediction service.