Sun emitted excess high-energy gamma rays – 03/11/2024 – Science

Due to its proximity and importance for maintaining life on Earth, the Sun is the star most researched by astrophysicists. This status as a preferred object of study does not mean that there is little to discover about the star. On the contrary. Some types of research, such as long-term research, can only be carried out because our planet is always in the vicinity of the Sun. Therefore, it is possible to observe it continuously and perceive details that cannot be seen in more distant stars.

It was precisely this particularity that led to a recent finding. An article published by Brazilian Bruno Arsioli and Italian Elena Orlando in February this year in the Astrophysical Journal reports that the Sun emitted an unexpected excess of high-energy gamma rays at its poles. The highest concentration of radiation occurred during its last most active period, the so-called solar maximum, in June 2014. Like the Earth, the Sun rotates around an axis, the ends of which define the poles. Rotation generates the magnetic field, so that the magnetic poles coincide with the ends of the rotation axis.

According to the authors of the work, what was expected was that, when there were variations in the level of gamma ray emissions, such fluctuations would manifest themselves with the same intensity in all areas of the Sun, in a more or less homogeneous way — instead of being exaggeratedly concentrated. in high latitude areas.

“This highest concentration of gamma ray emissions was observed at the moment when the inversion of the Sun’s magnetic poles occurred”, explains Arsioli, lead author of the study. “Therefore, we suspect that the magnetic reconfiguration is related to the excessive production of gamma radiation at the poles.” The switch causes the southern magnetic pole to migrate to the north of the solar disk and vice versa. Such an inversion occurs on average every 11 years, during solar maximum. Arsioli is currently a researcher at the Institute of Astrophysics and Space Sciences at the University of Lisbon, in Portugal, with funding from the European Marie Curie program.

According to Elena Orlando, from the University of Trieste, there is, for now, a 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”, comments Orlando, in an interview with Pesquisa Fapesp. Arsioli began the study with Fermi data in 2021, when he spent a year associated with the Italian group at the University of Trieste.

The unprecedented result was obtained from the analysis of data relating to 13 and a half years of observation of the Sun, between August 2008 and January 2022, by the Fermi space telescope. Operated by NASA (the North American space agency), in collaboration with the United States Department of Energy and European partners, Fermi is dedicated to recording emissions at frequencies of gamma radiation, the most energetic portion of the electromagnetic spectrum. It was also recently used to study a mysterious gamma-ray burst, the second most intense observed in space, probably caused by the rare merger of two neutron stars.

The work to analyze the Sun’s emissions was carried out in stages.

First, Arsioli and Orlando divided the data from almost 14 years of observation, which covered an entire solar cycle, into smaller intervals of 400 to 700 days.

Then, using data analysis tools they developed, they compared gamma ray emissions with energy above 5 gigaelectronvolts (GeV) from each subperiod in all regions of the solar disk.

In this way, they noticed the concentration of high-energy emissions production in polar zones during solar maximum. The finding is supported by statistical tests, described in the work, which indicate the relevance of the observed signs.

Considered a common star among the more than 100 billion stars in the Milky Way, the Sun was formed around 4.5 billion years ago. Unlike the Earth and the Moon, it is not a solid body. It is 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, sometimes 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 dots associated with colder areas on the 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 fast pace. The Sun’s energetic dynamics are also associated with other phenomena, such as the occurrence of flares (eruptions) and mass ejections.

Between the moment of greatest and least activity, the difference in brightness — that is, energy production — of the Sun is very small, at most 0.1%. Therefore, climatologists rule out that variations in solar activity could significantly influence the increase in global warming.

According to NASA calculations, over the last two centuries, the accumulated weight of greenhouse gas emissions from 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 operating regime produce clear 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 real impacts that the star can have on different aspects of daily life on Earth. By emitting more radiation and matter towards the Solar System, the star can affect terrestrial navigation systems, such as GPS, and 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 article, the data from Arsioli and Orlando’s article are important to improve understanding of the functioning of the surface of the Sun. “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 produced by black holes. “Some research groups have already tried to use Fermi to systematically study emissions from other stars, but have not been successful.”

One of Arsioli and Orlando’s challenges is to try to observe again the peak of gamma-ray emission in the polar regions of the Sun during the next solar maximum, which is expected to occur in 2025. If the star behaves again as it did in June 2014, the idea of that the excessive production of gamma rays arises from the periodic inversion of the magnetic poles becomes more robust. “There is no other star similar and as close as the Sun on which we can test our hypothesis”, says Arsioli. “We have to repeat the observations on himself.”