ESA’s Space Probe Vigil to Warn of Solar Storms

ESA’s spacecraft Vigil will significantly reduce the warning time for hazardous solar storms. One of the mission's instruments is being developed at MPS.

To the point:

• Current decision: The ESA Ministerial Council has now decided on the further roadmap for the Vigil space mission. The space probe is expected to detect and potentially forecast hazardous space weather from 2031 onwards.
• Knowledge advantage: Vigil observes the Sun from a previously unused observation position. This extends the warning time for solar storms by up to five days.
• Hardware from Göttingen: Vigil's Photospheric Magnetic Field Imager (PMI) is being developed and built under the leadership of the Max Planck Institute for Solar System Research (MPS) in Germany. Work on the instrument has already begun.
• Scientific data: PMI provides detailed maps of the magnetic field on the visible surface of the Sun almost continuously and in real time. This is also valuable for scientific purposes. 

Several telescopes on the Earth’s surface and space probes in near-Earth space constantly observe the Sun. If our star is brewing up a potentially dangerous solar eruption, they are the first to notice – however with a fairly short warning time of no more than three days. This is because, just like the planets, the Sun also rotates around its own axis. The regions that determine future space weather on Earth only gradually rotate into our field of view, meaning that the first signs of a hazardous solar storm remain hidden from us. A more effective early warning system therefore requires, first and foremost, a change of perspective.

ESA’s space probe Vigil is expected to provide just that. It is scheduled to launch in 2031. At a distance of around 150 million kilometers, the probe will follow the Earth on its orbit around the Sun. Vigil will thus look at the side of the Sun that will only turn towards the Earth four to five days later. The targeted observation post, known to experts as L5, is one of the so-called Lagrange points. There are a total of five positions in the Sun-Earth system where a space probe can orbit the Sun in sync with the Earth. The Lagrange point L5 has not yet been used in space exploration.

An instrument and its predecessor

Four scientific instruments on board Vigil will look into different layers of the Sun: from its surface to its inner and outer atmosphere. In addition, the probe will use two instruments to measure the “local” solar wind that flows around the space probe. Vigil’s Photospheric Magnetic Field Imager (PMI), which is currently being developed at the Max Planck Institute for Solar System Research (MPS), will observe the solar surface and determine the strength and direction of the magnetic field there. When the magnetic field locally restructures and releases energy, this can trigger a solar eruption.

Antetype for PMI is the Photospheric and Helioseismic Imager (PHI), which has been orbiting the Sun aboard ESA’s Solar Orbiter spacecraft since 2020. PHI was also developed and built under the leadership of MPS. “The experience we have gained in recent years in building PHI is very valuable for our contribution to Vigil,” says PMI project manager Jan Staub from MPS. However, the new instrument is not an exact copy of PHI. Unlike PHI, PMI is designed to provide the basis for space weather forecasts in near real-time and continuously.

Scientific applications

The uninterrupted data stream from a unique perspective is not only in demand for predicting solar storms, but also among researchers. New opportunities are emerging, particularly in collaboration with other space probes:

• Views of the Sun's far side: Using helioseismology methods, Vigil could further increase its knowledge advantage. Sound waves travel through the Sun, connecting its far side and its near side. Since such waves propagate faster under the influence of strong magnetic fields, it is possible to infer regions of high magnetic activity on the far side of the Sun by observing the sound waves on the near side. Large and complex active regions are often potential sites for future solar eruptions. “The perspective from the L5 Lagrange point provided by Vigil and PMI will allow us to detect active regions on the Sun’s far side with improved accuracy and reliability,” says Laurent Gizon, Managing Director of the MPS.
• Magnetic field measurements: The Sun's magnetic field drives many processes on our star – and is key to understanding it. However, until now, only the magnetic field component in the line of sight could be measured clearly and routinely from the Earth perspective. A stereoscopic view of the Sun from two perspectives can remedy this. This allows the magnetic field component perpendicular to the line of sight to be determined with certainty, as researchers at MPS have been able to demonstrate. To do this, they evaluated observational data recorded simultaneously by ESA’s Solar Orbiter and NASA's Earth-orbiting Solar Dynamics Observatory (SDO). Vigil will, for the first time, make such data available on a continuous basis.
• Brightness of solar faculae: In addition to dark sunspots, solar faculae are of crucial importance for estimating the Sun’s overall brightness and its fluctuations. Faculae are particularly bright areas on the Sun's surface with high magnetic field strengths. Key properties of solar faculae cannot be measured adequately when they are located at the limb of the Sun as seen from Earth. “With a side-view at these regions, we can better determine the magnetic fields associated with the faculae and learn more about their brightness,” explains Kinga Albert from MPS, who has conducted corresponding studies using data from ESA’s Solar Orbiter and SDO.