The Sun shines brightly in visible light, but what does it look like in higher energies of electromagnetic radiation? The gamma-ray image of the Sun is a lethal portrait, fortunately hidden by Earth's atmosphere and only visible from space. Each photon carries a billion times more energy than its ultraviolet relative. How does the regular emission of gamma rays from the Sun vary over time? And is it possible to relate it to the periods of violent events we see on the surface of our star?
Um study, published today in the scientific journal The Astrophysical Journal, produced a compact film of fourteen years of observing the Sun in gamma rays, a visualization tool that revealed that, contrary to the expected uniform distribution of these high-energy photons, the solar disk may be brighter in the polar regions.
This tendency for the Sun's gamma-ray brightness to be dominant at higher latitudes is evident during peak solar activity, as seen in June 2014.
The study, led by Bruno Arsioli. Institute of Astrophysics and Space Science (IA) and the Faculty of Sciences of the University of Lisbon (Ciências ULisboa), can contribute to the understanding of the still unknown process that makes the Sun shine ten times brighter in gamma rays than physicists expected. It can also inform space meteorology forecasts.
Solar gamma rays are produced in our star's halo and in solar flares, but are also released from its surface. The latter were the object of this study.
“The Sun is bombarded by particles at almost the speed of light, coming from outside our galaxy and in all directions”, says Bruno Arsioli. "These so-called cosmic rays have an electrical charge and are deflected by the Sun's magnetic fields. Those that interact with the solar atmosphere generate a shower of gamma rays."
Scientists thought these showers were equally likely to be seen anywhere on the sun's disk. What this work suggests is that cosmic rays may interact with the Sun's magnetic field and thus produce a distribution of gamma rays that is not uniform across all latitudes of our star.
“We also detected a difference in energy between the poles”, adds Bruno Arsioli. “At the south pole, there is an excess of higher energy emissions, of photons with 20 to 150 gigaelectron volts, while most of the less energetic photons come from the north pole.” Scientists still do not have an explanation for this asymmetry.
During the maximum of the solar activity cycle, it becomes evident that gamma rays are radiated more frequently at higher latitudes. In June 2014, when the magnetic field reversed, gamma rays were particularly concentrated at the poles. This is when the dipole of the Sun's magnetic field switches its two signals, a peculiar phenomenon that is known to happen at the peak of solar activity, once every eleven years.
“We found results that challenge our current understanding of the Sun and its environment,” says Elena Orlando of the University of Trieste, INFN and Stanford University, and co-author of this study.
“We demonstrated a strong correlation of asymmetry in solar gamma-ray emission coinciding with the exchange of the solar magnetic field, which revealed a possible link between solar astronomy, particle physics and plasma physics.”
The data used comes from 14 years of observations with the satellite in gamma rays Fermi Large Area Telescope (Fermi-LAT), between August 2008 and January 2022. This period covered a complete solar cycle, from one minimum to the next, with the peak in 2014. One of the challenges was separating solar emissions from the numerous other sources of gamma rays in the background sky , crossed by the apparent path of the Sun.
Bruno Arsioli and his colleague Elena Orlando have produced a tool to integrate all solar gamma ray events into a time window on the order of 400 to 700 days, with this window being able to slide over a period of 14 years. Through this visualization, moments of polar excesses became obvious, as well as the energy discrepancy between north and south.
“The study of gamma ray emissions from the Sun represents a new window to investigate and understand the physical processes that occur in our star’s atmosphere,” says Arsioli. “What are the processes that create these excesses at the poles? Perhaps there are additional mechanisms that generate gamma rays that go beyond the interaction of cosmic rays with the surface of the Sun.”
However, if we stick to cosmic rays, they can act as a probe of the inner solar atmosphere. The analysis of these Fermi-LAT observations also motivates a new theoretical approach that should consider a more detailed description of the Sun's magnetic fields.
The possible link between the Sun's production of gamma rays and its spectacular periods of solar flares and more frequent coronal mass ejections, and between these and changes in our star's magnetic configuration, could be an ingredient for improving physical models that predict the activity of the Sun.
These are the basis of space weather forecasts, essential for protecting satellite instruments in space and telecommunications and other electronic infrastructure on Earth.
“In 2024 and next year, we will record a new solar maximum, and another inversion of the Sun's magnetic poles has already begun. We hope, at the end of 2025, to reevaluate whether the inversion of magnetic fields is followed by an excess in gamma ray emissions of the poles”, says Bruno Arsioli.
Elena Orlando adds: “We have found the key to unlocking this mystery, which suggests future directions that should be taken. It is essential that the Fermi telescope works and observes the Sun in the coming years.”
But solar gamma rays likely have even more to reveal and require more attention. This now published study will reinforce the scientific argument in favor of continuous monitoring of the Sun by the next generation of space gamma-ray observatories.
“If it is established that high-energy emissions actually contain information about solar activity, then the next mission should be planned to provide real-time data on gamma-ray emissions from the Sun,” says Arsioli.
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