“The easiest way to explain our work is to compare it with Chinese shadows: instead of observing the phenomenon directly, we look at the shadow, the imprint that is left in the atmosphere by the sudden fluctuation of a fraction of stellar radiation. And we do it by using satellite navigation systems, also known as GPS.” So explains Manuel Hernández-Pajares -a professor from the Department of Mathematics of the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) and the head of the Ionospheric determination and navigation based on Satellite And Terrestrial systems research group (IONSAT) −the method that he has developed in collaboration with David Moreno-Borràs −a recent graduate in Informatics Engineering from the Barcelona School of Informatics (FIB) who is now a researcher at the Institute of Space Studies of Catalonia (IEEC)− to detect extra-solar stellar flares just using general measurements from global navigation satellite systems (GNSS) such as the global positioning system (GPS). The technique is described in the paper ‘Real-time detection, location and measurement of geoeffective stellar flares from Global Navigation Satellite System data: new technique and case studies’, published in the scientific journal Space Weather of the American Geophysical Union.

Stellar flares are sudden increases in star radiation, i.e. sudden electromagnetic emissions in certain areas of a star’s surface that release large amounts of energy. The ion release produces a sudden overionisation in the Earth’s upper atmosphere (ionosphere). So far, flares were detected by space probes such as SOHO (Solar and Heliospheric Observatory) - a spacecraft built on a joint mission between ESA and NASA to study the Sun that contains a specific telescope to detect solar flares from increased photon flux - and telescopes such as Swift or Fermi for extrasolar stellar flares. The researcher, who is linked to the Barcelona School of Telecommunications Engineering (ETSETB) and a member of the Institute of Space Studies of Catalonia (IEEC), explains that they “use these instruments to observe the phenomenon directly.”

Measuring increased solar fluxes as if they were Chinese shadows
The method developed by the UPC researchers involves “looking at the imprint left by stellar flares in the Earth’s upper atmosphere, the so-called ionosphere” as if watching a Chinese shadow puppetry show.

Global navigation satellite systems (GNSS) use data from more than 24 constellation satellites (GPS, GLONASS, Galileo and BeiDou) orbiting the Earth; each satellite emits signals as electromagnetic waves, similar to those used by mobile phones. GPS receivers use the waves to time how long it apparently takes for the signals to travel from the satellite to the receiver and transform it into a pseudorange (the basic GNSS observable) by multiplying it by the speed of light in vacuum.

When a solar flare occurs, additional free electrons are suddenly released in the ionosphere, at 100-1,000 kilometres altitude. The electromagnetic waves emitted by GNSS satellites make free electrons “dance” and emit a very similar electromagnetic signal, which overlaps with the original signal and thus modifies the speed of propagation and produces a pseudorange error. Therefore, GNSS satellites emit a second signal that, in combination with the first one, eliminates the effect of overionisation and the error generated. In fact, this system that eliminates the margin of error caused by the ionosphere also allows to isolate the effect of very intense stellar flares and thus to detect them. The GNSS system thus becomes a system for detecting and locating stellar flares.

First test: with the Sun in 2012
This finding has been possible owing to the refinement of the technique developed previously by the researcher Hernández-Pajares to detect and measure high-, medium- and low-intensity solar flares. The real-time technique is already implemented in an ESA project for the Space Situational Awareness programme. The algorithm developed, which the authors call Blind GNSS Search of Extraterrestrial EUV Sources (BGEES), has been verified with two stellar flares that were more distant and therefore more difficult to detect: Proxima Centauri (18 March 2016) and NGTS J121939.5-355557 (1 February 2016). The estimates obtained with the BGEES algorithm have been contrasted with those of studies that have analysed the stellar flares with conventional astronomical techniques.

“It is most likely the first reported detection of extrasolar stellar flares with GPS”, explains the researcher. The technique allows not only to detect and measure flares, but also to estimate approximately the position of the extrasolar source in the celestial sphere with a small error of up to a few degrees. It opens the door for a new kind of astronomy to detect and study this phenomenon; a new method that uses free, open and real-time data thanks to globally distributed GNSS receivers.