The team included researchers from the Tata Institute of Fundamental Research, Cochin University of Science and Technology, Osaka City University (Japan), Chubu University (Japan), Nagoya University (Japan), the University of Tokyo (Japan), and Hiroshima City University (Japan).
Before reaching Earth, some of these cosmic rays must navigate the Sun's magnetic field, which acts as a protective shield.
When this magnetic field is particularly strong, fewer cosmic rays get through.
Therefore, fewer muons mean a warmer upper atmosphere or a stronger solar magnetic shield.
To solve this, the research team compared their muon counts against NASA atmospheric temperature records and magnetic field data from spacecraft.
Ooty
At a high-altitude observatory in Ooty, Tamil Nadu, scientists have spent the last 22 years tracking billions of invisible particles raining down from space to better understand Earth's climate and solar weather. Using the massive GRAPES-3 muon telescope, researchers from India and Japan analysed data from 2001 to 2022 to measure how variations in the temperature of our upper atmosphere and the Sun’s interplanetary magnetic field influence the number of subatomic particles called muons reaching Earth's surface.
By applying mathematical filters to this massive dataset of over 4 billion daily particle detections, the team successfully deciphered the complex forces that alter this cosmic rainfall. The study demonstrates that ground-based particle detectors can provide highly accurate, real-time monitoring of atmospheric conditions and space weather. The team included researchers from the Tata Institute of Fundamental Research, Cochin University of Science and Technology, Osaka City University (Japan), Chubu University (Japan), Nagoya University (Japan), the University of Tokyo (Japan), and Hiroshima City University (Japan).
Galactic cosmic rays are high-energy particles constantly bombarding our solar system from deep space. Before reaching Earth, some of these cosmic rays must navigate the Sun's magnetic field, which acts as a protective shield. When this magnetic field is particularly strong, fewer cosmic rays get through.
The cosmic rays that reach Earth collide with oxygen and nitrogen in our upper atmosphere, creating a cascade of secondary particles. These secondary particles quickly decay into heavier, fast-moving particles called muons. Because muons live just long enough to reach the ground, the GRAPES-3 telescope can count them. Crucially, the rate at which these muons are detected changes depending on the weather high above us. When the upper atmosphere heats up, it expands outward. This expansion forces the precursor particles to travel farther before decaying. The longer distance increases the chance they break down too early, resulting in fewer low-energy muons reaching the ground detectors. Therefore, fewer muons mean a warmer upper atmosphere or a stronger solar magnetic shield.
Because the Sun operates on an 11-year cycle and Earth's seasons operate on a one-year cycle, their effects on the muon count overlap and blur together in the data. To solve this, the research team compared their muon counts against NASA atmospheric temperature records and magnetic field data from spacecraft. They used a mathematical technique called a Fast Fourier Transform to isolate the specific, distinct frequencies of these seasonal and solar variations. By expanding the timeline to over two decades and employing novel, automated algorithms to correct for minor glitches or ageing in the detector's sensors, the researchers were able to iteratively filter out solar influence to obtain a pure atmospheric signal, and vice versa.
However, the researchers noted that their calculations carry systematic uncertainties that depend on an assumed hadronic attenuation length, a complex measure of how far particles travel before interacting, which can vary with particle type and energy. Furthermore, tiny fluctuations in the solar magnetic field that happen to align with Earth's annual seasonal cycle can still subtly contaminate the temperature data, though the new iterative filtering method greatly minimises this issue.
By showing that ground-based telescopes can reliably and continuously measure both the invisible magnetic shielding of our solar system and the temperature profile of our high atmosphere, scientists have developed a novel observational tool. In an era increasingly defined by the need to understand global warming, integrating this real-time cosmic ray data could significantly enhance the accuracy of long-term climate models and help us better forecast the environmental shifts affecting our planet.
