According to results released by NASA in 2015, most of Mars’ atmosphere was likely stripped away by the solar wind. In 2001 and 2005, this solar wind is also thought to have caused the loss of several telecommunications satellites. In 1989, Canada suffered a massive blackout when a solar storm knocked out its electrical power grid. In most cases, such storms disturb Earth’s magnetic field and ionosphere, causing GPS signals received by aircraft to be delayed or dropped. Understanding the cycle and intensity of such solar outbursts—which would help us to anticipate and mitigate their effects—is the goal of space weather, a field of research that came into being in the 1990s. “We know as much about space weather now as we did about Earth’s weather systems 100 years ago,” says Iannis Dandouras, a research director at the French national scientific research centre CNRS and principal investigator for the Cluster Ion Spectrometry instrument (CIS) on Cluster.
Solar wind slows in front of magnetic field
In this simulation, the magnetosphere (dark blue) is on the left and the Earth is the dot at its centre. The bright thin line encircling it is the magnetopause, itself encased in a magnetosheath delimited by an arc called the bow shock. Where the solar wind hits the bow shock, a zone of disturbances forms outside the magnetosheath, as shown by the swirling pattern.
Credits: Vlasiator team, University of Helsinki
Since its launch in 2000, the Cluster mission has been observing the interactions between our magnetosphere and the solar wind. Made up of ions and electrons, this stream of plasma flows out from the Sun at an average speed of 400 km/s, reaching Earth in less than three days. During a solar storm, our roiling star ejects this hot matter in abundance, strengthening the solar wind’s magnetic field and increasing the density of its high-energy particles.
While such storms are fairly frequent, Earth is shielded from them by its own magnetic field. Storm waves arriving in the vicinity of our planet are slowed well before they reach the magnetosphere, which influences its neighbourhood. It’s in this zone ahead of the magnetosphere, magnetopause, magnetosheath and bow shock—which acts like a kind of border guard—that the Cluster satellites observed the effects of four solar storms between 2001 and 2004.
Shocked ions reveal a chain reaction
To characterize the ion particles in this region where the solar wind slows, the IRAP astrophysics and planetology research institute in Toulouse developed a spectrometer with support from CNES. Called CIS, this spectrometer is on the four satellites composing the Cluster mission along with 10 other instruments. “It tells us about the chemical make-up of ions, their density, velocity and temperature... and how they evolve with variations in solar activity,” explains Iannis Dandouras. “We showed that during a solar storm, the solar wind was denser, the characteristics of ion populations varied greatly and the waves generated in this region got more complex. The result is the first ever observation confirming that solar storms have a big impact on the space environment ahead of the magnetosphere.”
Left: simulation of Earth’s foreshock during calm solar weather conditions. Right: simulation of Earth’s foreshock during solar storm conditions. Credits: Vlasiator team, University of Helsinki (link to the video)
Behind the bow shock, Earth’s magnetic field resonates with the complex waves generated by the plasma storm. While these disturbances occur more than 90,000 km (14 Earth radii) away, the energy they generate takes less than 10 minutes to reach Earth. We need to learn more about these chain physical reactions not only to mitigate their effects on the ground, but also to keep crews safe on future space exploration missions. Understanding how these storms propagate outside the near-Earth space environment is precisely the goal of the European Solar Orbiter mission scheduled for launch in March 2020.
L. Turc et al, First observations of the disruption of the Earth’s foreshock wave field during magnetic clouds, Geophysical Research Letters
CIS Principal Investigator at the IRAP astrophysics and planetology research institute, Toulouse.
Tel.: +33 (0)5 61 55 83 20
E-mail: idandouras at irap.omp.eu