increase in high-energy cosmic rays
Thanks to the sensitivity of HAWC, it’s possible to detect high- energy cosmic gamma ray explosions at the edge of the universe. (Photo: HAWC)

The High-Altitude Water Cherenkov (HAWC) Observatory lies at the foot of the Sierra Negra volcano, in the state of Puebla. This powerful instrument studies the most energetic astrophysical objects in the universe and the origin of the highest energy cosmic rays.

Dr. Alejandro Lara Sánchez’s team at the Institute of Geophysics of the National Autonomous University of Mexico (UNAM) has spent more than a decade monitoring the cosmos through HAWC’s eyes.

In October 2016, something out of the ordinary happened while they were fine-tuning the observatory: an increase in high-energy cosmic rays (CR) (associated with solar flares).

“These data jumped out at us quickly. We have more or less similar increases when there are thunderstorms, but this was different. It couldn’t have been a pressure effect. It wasn’t an electric field effect,” says Lara.

Once the team had corroborated that it wasn’t an error, they began to investigate what had happened in the interplanetary environment. First, they looked for the trace of a solar flare, but there was nothing. “Then, we saw an interplanetary magnetic cloud that matched the timing perfectly,” explains the doctor.

increase in high-energy cosmic rays
Representation of the finding, model of a MFR. (Courtesy: Elsa Noemi Sánchez)

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A series of interplanetary coincidences

Cosmic rays have been observed and studied for nearly 100 years. They’re the consequence of highly energetic events in the universe, such as a supernova explosion. They can be described as jets of very high-energy particles that continuously bombard the Earth.

“It isn’t possible to see the sources of these rays directly because they’re usually diverted by the magnetic fields they encounter along the way. When seen from Earth, they come in from all directions of space. But HAWC’s high sensitivity allowed us to detect an increase in CR flow during the passage of a “magnetic cloud” coming from the Sun,” explains the researcher.

Once the magnetic cloud was detected, the team searched for its origin. It was a non-explosive event. “Sometimes, these clouds just radiate from the Sun very slowly, and that was the case here,” Lara said.

The team noticed through simulations that the cloud didn’t interact with anything in the interplanetary environment. The October 2016 cloud didn’t interact with anything, so the team was able to simulate the CR trajectory within the cloud. “The geometry is beautiful: it’s helical (like DNA)”, says the head researcher.

What allowed for the discovery was the coincidence of the continuous monitoring, HAWC’s sensitivity, and the exposed shape of the cloud. In addition, there is, of course, the large research team that includes 50 researchers and master’s and doctoral students.

increase in high-energy cosmic rays
Cosmic rays are the consequence of very energetic events in the universe, for example the explosion of a supernova. (Illustration: Special)

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The answer to a century-old enigma

“Sporadic increases in cosmic rays like those of October 2016 have been observed for a century, but they weren’t understood. Their origin was a mystery and was put aside. Sometimes they were seen, sometimes not. The increases weren’t as obvious as those we saw now,” the researcher says.

Most cosmic rays are thought to come from supernova explosions. When a massive star explodes, it ejects most of its material into space. However, other unknown cataclysmic phenomena may also be at play, especially in the case of cosmic rays with higher energy, such as the CRs detected by the team.

Through a detailed analysis of the observations and with the help of simulations, the UNAM team discovered that the increase was due to the “ordered” deviation of CRs caused by the helical configuration of the magnetic field in the cloud that reached Earth in October 2016.

What impact does this finding have?

After a decade of interpreting what was going on in the interplanetary environment, Lara’s team proposes a clear explanation of the origin of the 100-year-old mystery of the increases in cosmic rays observed on Earth. This finding is of global importance, so the researchers have published their work in one of the most prestigious scientific journals: The Astrophysical Journal.

Dr. Alejandro Lara Sánchez, head of the research team, has dedicated his life to understanding the physical phenomena that occur in the Sun. Here’s how he feels about the finding: “I’m happy, first for the team we’ve formed, and second for all our effort, starting with construction of the instrument through to handling the data, understanding it, and assimilating it. Fine-tuning an instrument to detect something that it wasn’t made for was a massive task, and it’s starting to bear fruit.”

This work gives rise to more concrete studies into the fluctuation of CRs in the galaxy and beyond. In addition, it’s a very important indicator for future space missions. “When we talk about space weather, we talk about highly energetic events in the Sun, from gusts of solar wind to incredibly strong explosions, and you need to give a complete weather forecast to make sure they’re fully prepared on the International Space Station,” he says.

The magnetic fields that interact with cosmic rays are everywhere in the universe, so this finding clears away some of the boundaries that existed to studying them. It will also open up new lines of research, be an important indicator of space weather for astronauts, and above all will help us understand the energies surrounding our galaxy and the universe.

The research was published on December 15, 2020 in The Astrophysical Journal, under the title: Interplanetary Magnetic Flux Rope Observed at Ground Level by HAWC.