Science.-Explanation to heavy elements in galactic cosmic rays

07-07-2021 The MMS mission is made up of four identical ships flying in formation. Data from NASA’s Magnetospheric Multiscale (MMS) mission have explained the presence of heavy energetic elements in galactic cosmic rays (GCR). RESEARCH AND TECHNOLOGY POLICY NASA


Data from NASA’s Magnetospheric Multiscale (MMS) mission have explained the presence of heavy energetic elements in galactic cosmic rays (GCR).

The new findings have significant implications for the composition of cosmic rays and the radiation spectra observed in astrophysical structures.

GCRs are made up of fast-moving energetic particles, primarily hydrogen ions called protons, the lightest and most abundant elements in the universe. Scientists have long debated how traces of heavy ions in GCRs are accelerated.

The supernova explosion of a dying star creates massive shock waves that propagate through the surrounding space, accelerating the ions on their way to very high energies, creating GCR. The way that heavy ions are energized and accelerated is important because they affect the redistribution of mass throughout the universe and are essential for the formation of even heavier and chemically complex elements. They also influence how we perceive astrophysical structures.

“Heavy ions are believed to be insensitive to an incoming shock wave because they are less abundant, and shock energy is overwhelmingly consumed by the preponderance of protons. Imagine standing on a beach as the waves move the sand under your feet, while you stay where you are, “said Dr. Hadi Madanian of SwRI (Southwest Research Institute), lead author of the Astrophysical Journal Letters article on this research, in a statement. “However, that classic view of how heavy ions behave in shock conditions is not always what we have seen in high-resolution observations of the near-Earth space environment with the MMS mission.”

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Shock phenomena also occur in the near-Earth environment. The Sun’s magnetic field is carried through interplanetary space by the supersonic solar wind flow, which is obstructed and deflected by the Earth’s magnetosphere, a protective bubble around our home planet. This region of interaction is called a bow shock because of its curved shape, comparable to the bow waves that occur when a ship travels through water. Earth’s arc of shock forms on a much smaller scale than supernova collisions. However, sometimes, the conditions of this small collision resemble those of supernova remnants. The team used high-resolution in situ measurements from the MMS spacecraft in the bow shock to study how heavy ions are accelerated.

“We observed an intense amplification of the magnetic field near the shock arc, a known property associated with strong shocks like supernova remnants. We then analyzed how different ion species behaved when they encountered the shock arc,” Madanian said. “We found that these enhanced fields significantly modify the trajectory of the heavy ions, redirecting them to the acceleration zone of the impact.”

While this behavior was not expected to occur for heavy ions, the team identified direct evidence of this process in alpha particles – helium ions that are four times as massive as protons and have twice the charge.

“The excellent resolution of the MMS observations has given us a much clearer picture of how a shock wave energizes heavy elements. We will be able to use this new understanding to improve our computer models of cosmic ray acceleration in astrophysical collisions,” said David Burgess, a professor of mathematics and astronomy at Queen Mary University of London and a co-author of the paper.

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