If solar scientists aren’t already eager to observe the sun at solar maximum in 2024, a newly discovered gamma-ray puzzle will intensify that desire.
Examining 14 years worth of data from NASA’s Fermi space telescope, a team of researchers found that during the last solar maximum in 2013 and 2014, the sun’s polar regions burst with high-energy gamma-ray radiation as high as 10 .times higher than expected.
The sun is known to shine in all wavelengths of electromagnetic radiation, including gamma rays, but this was expected to be evenly spread across the face of the sun. The team that found high activity in the polar regions at the moment cannot explain this imbalance.
“The study of gamma-ray emissions from the sun opens a new window to investigate and understand the physical processes that occur in the atmosphere of our star,” team leader and Institute of Astrophysics and Space Sciences researcher Bruno Arsioli said in a statement.
“What processes 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 sun’s surface.”
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Each solar cycle describes periodic changes in the sun’s magnetic activity, lasting about 11.5 years and showing a period of intense activity known as a solar maximum that sees more sunspots spread across the face of the sun, more solar flares, and massive outflows of plasma known as coronary mass ejections. (CME).
The upcoming solar maximum of the current solar cycle that began in December 2019, solar cycle 25, may help explain why the sun became brighter in gamma rays at its poles during the peak activity of the last series.
A better understanding of the sun’s behavior at these maximum levels could allow scientists to better predict space weather driven by solar flares and outflows, which can affect power and communications infrastructure here on Earth as well as damaging satellites and threatening astronauts.
Observing a complete solar cycle with NASA’s Fermi space telescope
The data used by Arsioli and his colleagues were collected at Fermi’s gamma-ray capture instruments between August 2008 and January 2019, spanning an entire solar cycle from the solar minimum of one cycle through the solar maximum and to the solar minimum of the first another.
To make sense of the data, the team created a tool that could disentangle gamma rays from the sun from this high-energy radiation from other sources in the background sky. This resulted in the integration of solar gamma-ray events into a 400- to 700-day window, which can be slid over the 14-year observation period to focus on gamma-ray activity at specific times.
Although the sun shines across all wavelengths of light, 99% of its light comes in the form of ultraviolet, visible and infrared radiation. Much higher energy gamma-ray radiation emitted by our star comes from the solar flare, the solar corona, and, to a lesser extent, the photosphere, which is loosely thought of as the surface of the sun.
“The sun is bombarded with particles close to the speed of light coming from outside our galaxy in all directions,” said team leader and researcher of the Institute of Astrophysics and Space Sciences Bruno Arsioli in the statement. “These so-called cosmic rays are electrically charged and deflected by the sun’s magnetic fields. The ones that interact with the solar atmosphere produce a gamma-ray shower.”
Scientists theorized that these gamma-ray showers would be produced evenly across the sun. Contrary to this, the results of Arsioli and colleagues suggest that something is happening with the sun’s magnetic field at higher altitudes that is boosting the gamma-ray output at the poles.
The researchers also detected another gamma-ray asymmetry, this time between the two poles and related to the energy of the gamma-ray photons produced.
“In the south pole, there is an excess of higher energy emissions, of photons with 20 to 150 gigaelectronvolts (GeV), and most of the less energetic photons come from the north pole,” said Arsioli.
The gamma-ray emissions from the poles were particularly concentrated in June 2014, when the sun’s magnetic poles reversed, a phenomenon that occurs every 11.5 years or so at solar maximum.
“We have obtained results that challenge our current understanding of the Sun and its environment,” said team member and University of Trieste scientist Elena Orlando. “We showed a strong correlation of the asymmetry in solar gamma-ray emission coinciding with the solar magnetic field flip, which indicated a possible link between solar astronomy, particle physics, and plasma physics.”
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Understanding the connection between how and where the sun emits gamma rays and the activity of our star during periods of more frequent solar flares and CMEs could lead to better space weather forecasts.
“In 2024 and next year, we will experience a new solar maximum, and another inversion of the sun’s magnetic poles has already begun,” said Arsioli. “We expect to reassess by the end of 2025 whether there will be an excess of gamma-ray emissions from the poles following the inversion of the magnetic fields.”
Orlando also points out that the team’s findings make a strong case for paying more attention to gamma rays from the sun.
“We have found the key to unlocking this mystery, which suggests the future directions that should be taken. It is fundamental that the Fermi telescope will operate and observe the sun in the coming years, ” she said.
“If it is determined that high-energy emissions do indeed carry information about solar activity, the next mission should be planned to provide real-time data on gamma-ray emissions from the sun,” Arsioli said.
The team’s research has been published in The Astrophysical Journal.