Solar storms are huge explosions in the Sun’s atmosphere which eject large amounts of solar matter into space. When these storms reach Earth, they create beautiful auroral displays, but can also endanger vital technological systems, such as telecommunication satellites or power networks. Being able to forecast the conditions in space around Earth, the “space weather”, just like we forecast atmospheric weather, is crucial to protect these infrastructures.
Accurate space weather forecasting requires a detailed understanding of how solar storms interact with near-Earth space. The surroundings of our planet are not just empty space: the Earth’s magnetic field stretches out hundreds of thousands of kilometres above the atmosphere, forming a protective bubble around the Earth. Extending ahead of this magnetic bubble, the foreshock is the first region of near-Earth space that solar storms encounter when journeying towards Earth. It is home to intense electromagnetic waves which can then propagate down to the Earth’s surface. A new study reveals for the first time that solar storms modify profoundly these waves, shifting them to higher frequencies, but also increasing their variability and disrupting their global structuring. When the frequencies of these magnetic waves are transformed into audible signals (see the video below), they give rise to an uncanny song that might recall more the sound effects of a science fiction movie than a natural phenomenon.
The Vlasiator computer model, developed at the University of Helsinki, was instrumental in the discovery. Vlasiator provides global simulations of near-Earth space with unprecedented detail, including the small-scale processes triggering foreshock waves. A first analysis of Vlasiator runs, published in 2018 in the Journal of Geophysical Research, had suggested that foreshock waves could become more intricate during solar storms. The runs corresponded however to idealised conditions, and what happened during an actual solar storm still remained to be confirmed.
The Cluster mission offered a perfect opportunity to look for the decisive observational evidence. Launched in 2000 by the European Space Agency, Cluster comprises four identical spacecraft flying in formation, a configuration that allows to determine foreshock wave properties with great accuracy. Despite the many constraints of the study, several events with suitable spacecraft observations were identified.
“When I started looking at Cluster's data, I first thought something was wrong!” says Lucile Turc, lead author of the study. “The waves were so variable, so different from what we're used to see in the foreshock, that I thought it was another type of waves. Together with my colleagues, we performed two independent analyses of the waves. Both investigations confirmed that these were the waves we were looking for.”
The study also investigates a possible source for the increased complexity of the waves, which appears to be linked with the properties of particles reflected off the Earth’s bow shock, to which the foreshock is intimately connected. How particles are accelerated at shocks is a key question in space physics, and the results presented in this new paper suggest that solar storms may alter these processes as well when reaching Earth.
The major part of this work was carried out in the framework of the Marie Sklodowska-Curie Individual Fellowship held by Lucile Turc until March 2019, which aimed at characterising the role played by the foreshock in the interaction of solar storms with near-Earth space. This project also contributed to further our understanding of waves in near-Earth space, one of the key science goals of the Finnish Centre of Excellence in Research of Sustainable Space.
First observations of the disruption of the Earth’s foreshock wave field during magnetic clouds: L. Turc, O. W. Roberts, M. O. Archer, M. Palmroth, M. Battarbee, T. Brito, U. Ganse, M. Grandin, Y. Pfau-Kempf, C. P. Escoubet, I. Dandouras, Geophysical Research Letters, 46, https://doi.org/10.1029/2019GL084437, 2019