Due to its proximity and importance for maintaining life on Earth, the Sun is the star astrophysicists have researched the most. However, the Sun’s status as a preferred object of study doesn’t mean there is little left to be discovered about our closest star, quite the contrary. Some types of research, such as long-term studies, can only be conducted for the very reason that our planet is always in the Sun’s vicinity, making it possible to observe it continuously and perceive details that cannot be seen in more distant stars.
It was precisely this unique vantage that led to an important recent finding. An article published by Brazilian astrophysicist Bruno Arsioli and Italian astrophysicist Elena Orlando in February this year in the Astrophysical Journal reports that the Sun emitted an unexpected increase of high-energy gamma rays at its poles. The highest concentration of radiation occurred during its last most active period, known as the solar maximum, in June 2014. Like the Earth, the Sun rotates around an axis, the ends of which define its poles. This rotation generates the Sun’s magnetic field, such that the magnetic poles coincide with the ends of the rotation axis.
The study’s authors had expected that when there were variations in the level of gamma-ray emissions, such fluctuations would exhibit the same intensity in all areas of the Sun, in a more or less homogeneous way, instead of being found highly concentrated in higher-latitude areas.
“The highest concentration of gamma-ray emissions was observed at the moment when the inversion of the Sun’s magnetic poles occurred,” explains Arsioli, the study’s lead author. “For this reason, we suspect that magnetic reconfiguration is related to the exorbitant production of gamma radiation at the poles.” Arsioli is currently a researcher at the Institute of Astrophysics and Space Sciences at the University of Lisbon, in Portugal, with funding from Europe’s Marie Curie fellowship program.
The polar flip causes the southern magnetic pole to migrate to the north of the solar disk and vice versa. Such an inversion occurs on average every eleven years, during solar maximum. Elena Orlando, from the University of Trieste, says that for now there is no detailed explanation of how the reversal of the magnetic pole would lead to excess emission of gamma rays at the extremes of the solar disk. “We think that the star’s magnetic field is somehow involved in this anomaly, but we still don’t have a physical explanation for it,” Orlando explained in an interview with Pesquisa FAPESP. Arsioli began the study with data from Fermi in 2021, when he spent a year associated with Orlando’s research group at the University of Trieste.
The unprecedented result was obtained through analysis of data from thirteen and a half years of solar observations conducted from August 2008 through January 2022, made by the Fermi space telescope. Operated by NASA (the American space agency), in collaboration with the United States Department of Energy and its European partners, Fermi is dedicated to recording emissions of gamma radiation, the most energetic frequencies of the electromagnetic spectrum. It was also used recently to study a mysterious explosion of gamma rays, the second most intense ever observed in space, which was probably caused by the rare merger of two neutron stars (see report on page 62).
NASA / SDO / AIA / LMSALArtist’s illustration using a real solar-image background shows the lines of the complex magnetic field on the Sun’s surfaceNASA / SDO / AIA / LMSAL
The work to analyze the Sun’s emissions was carried out in distinct phases. First, Arsioli and Orlando divided the data from almost 14 years of observations, covering an entire solar cycle, into smaller intervals of 400 to 700 days. Then, using data analysis tools of their own design, they compared the gamma-ray emissions with energy above 5 gigaelectronvolts (GeV) from each subperiod in all regions of the solar disk. With this method, they noted the concentration of high-energy emissions produced in polar zones during solar maximum. The finding is supported by statistical tests, described in the study, which indicate the relevance of the radiation observed.
Considered a common star among the more than 100 billion stars in the Milky Way galaxy, the Sun was formed around 4.5 billion years ago. Unlike the Earth and the Moon, our nearest star isn’t a solid body; it’s a ball of hot plasma (ionized matter with electrically charged particles), made up of hydrogen and helium gases. The level of solar activity (energy production) varies over time in a more or less regular way, in cycles. Sometimes the star is less active, and other times more. The average duration of a solar cycle is 11 years, but it can vary between 9 and 14 years.
The formation of sunspots, black spots associated with cooler areas on the Sun’s surface, is a thermometer of solar activity. Occasionally, the largest spots are visible from Earth without the need for telescopes. More spots signal that the star is working at a faster pace. The Sun’s dynamic energies are also associated with other phenomena, such as the occurrence of flares (eruptions) and mass ejections.
However, the difference in brightness between times of the Sun’s greatest and least activity—i.e., energy production—is very small, at most 0.1%. This is why climatologists rule out the idea that variations in solar activity could be a significant contributor to the increase in global warming.
According to NASA calculations, the accumulated effect of greenhouse gases emitted over the last two centuries by human activities on the Earth’s average temperature is at least 270 times greater than the possible influence of any change in the Sun’s luminosity. Even so, changes in its functioning conditions produce evident impacts on the star’s appearance and behavior.
GPS Disturbances
In addition to generating basic knowledge about stellar physics, studies on solar activity are useful for understanding the actual impacts that our star can have on different aspects of daily life here on Earth. By emitting more radiation and matter into the Solar System, the star can affect terrestrial navigation systems such as GPS, as well as telecommunications, on the planet.
For astrophysicist Rodrigo Nemmen, from the Institute of Astronomy, Geophysics, and Atmospheric Sciences at the University of São Paulo (IAG-USP), who did not participate in the study, the data from Arsioli and Orlando’s article are important for improving our understanding of the functioning of the Sun’s surface. “The main source of gamma rays that Fermi observes is the Sun,” says Nemmen, who uses data from the NASA satellite to study emissions of this type of radiation in the jets of matter generated by black holes. “Other research groups have already tried to use Fermi to systematically study emissions from other stars but weren’t successful.”
One of Arsioli and Orlando’s challenges is to try to observe the coming peak of gamma-ray emissions from the Sun’s polar regions during the next solar maximum, which should occur in 2025. If the star behaves again as it did in June 2014, the concept that the increased production of gamma rays arises from the periodic reversal of the magnetic poles becomes more robust. “There is no other similar star as close as the Sun on which we can test our hypothesis,” says Arsioli. “We have to repeat the observations on the Sun itself.”
Scientific article
ARSIOLI, B. & ORLANDO, E. Yet another sunshine mystery: Unexpected asymmetry in GeV emission from the solar disk. The Astrophysical Journal. Feb. 7, 2024.
