On 9 October 2022 a huge stream of gamma rays, the strongest ever detected, was recorded by many satellites orbiting in interplanetary space and hit the Earth. At 13:21 Universal Time, ESA INTEGRAL's International Gamma-Ray Astrophysics Laboratory detected a very intense and long-lasting gamma-ray burst, lasting more than 800 seconds, which has been named "GRB221009A". The GRB was detected by the China Seismo-Electromagnetic Satellite (CSES-01), a collaboration between the Italian and Chinese Space Agencies which includes the participation of the Department of Physics of the University of Trento. About four minutes after the beginning of the phenomenon, CSES-01 recorded a strong electric field perturbation in the upper layer of the Earth's atmosphere, caused by a sudden and strong current. This has never been observed before in the upper ionosphere, the highest layer of Earth's atmosphere.
"When the GRB hit, the satellite was on the side of the planet that was lit up by the enormous flow of gamma rays. A stroke of luck," comments Roberto Battiston, UniTrento physicist, founder of the CSES project in Italy and among the first authors of the article that has just been published in Nature Communications. "The interaction with the ionosphere was so intense and long - more than 800 seconds - that disturbances in ionospheric radio transmissions were recorded for hours. The impact of the GRB was immediately detected by the particle detector on board the CSES. But the amount of energy that hit the planet in such a short time was so great that it immediately made us think that it could affect the entire ionosphere as never before. This result shows how, under certain conditions, the entire planet can become a sensor for the detection of astrophysical phenomena if there are sufficiently sensitive instruments capable of operating in space," continues Battiston.
The photon flux of GRB221009A, which originated from a galaxy almost two billion light-years away, still had enough energy to disrupt the ionosphere when it reached Earth. This type of phenomenon has always affected the lower layers of the ionosphere, causing disturbances to radio transmissions. GRB221009A has essentially "moved" the ionosphere downwards throughout its duration. A similar effect occurs during strong solar flares that cause radio blackouts.
"This type of result confirms the idea that a supernova taking place in our galaxy could have very serious consequences for our planet. There is a great scientific debate about the possible consequences of a galactic GRB, potentially billions of times more intense than this one. In the worst case, it could alter the atmospheric ozone layer that protects biological life from ultraviolet radiation generated by the Sun. Some argue that a similar event may be the possible cause of some of the mass extinctions that occurred in the past, but this however remains at the level of hypothesis," comments Battiston.
The CSES-01 satellite – The ionosphere is fundamental for the propagation of radio waves, that is, for low-frequency radio transmissions around the planet. However, its density is so low that satellites can orbit within it.
One of these satellites is CSES-01, which monitors the upper ionosphere and magnetosphere. Its mission is to identify disturbances that can be correlated to natural phenomena of terrestrial origin, such as earthquakes, tsunamis or volcanic eruptions, and of external origin such as disturbances due to solar storms.
The satellite has various instruments on board, including a particle detector created by INFN, an electric field detector developed in collaboration with INAF-IAPS in Rome, as well as a series of detectors including those for the measurement of the magnetic field and of the properties of ionosphere plasma created by the Chinese colleagues.
In the past only a few GRBs had been able to significantly affect the ionosphere, but only at low altitudes and at night, when the effect of solar illumination is not present. The impact of a GRB in the upper atmosphere where CSES-01 orbits and during the day had never been observed.
We were able to observe this phenomenon for the first time thanks to the extraordinary sensitivity of the electric field instrument. The effect recorded by the EFD detector on board CSES when GRB221009A hit is proof that the Earth's ionosphere had been ionized so intensely by high-energy gamma rays, that it generated a variation in conductivity such as to produce variations in the ionospheric electric field.
The collaboration with UniTrento - To exploit the potential of measurements made from space, the group of researchers of the Physics Department of the University of Trento has created an advanced version of the Gamma Ray Burst sensor that will be launched with the CSES-2 satellite in 2024. "The new sensor has a greater sensitivity to the effects of gamma ray bursts on our planet" explains Roberto Iuppa, the physicist of UniTrento who coordinated the development of the new instrument that will operate from space. "It will contribute to the worldwide network for the detection of these signals from the extreme Universe."
The results of this measurement were published in "First Evidence of Earth's top-side ionospheric electric field variation triggered by impulsive cosmic photons" by Piersanti M., Ubertini, P., Battiston, R., et al. [Nature Communication, Nature Communications, volume 14, Article number: 7013 (2023)].