The solar wind carries the energy of violent events from the sun to the Earth and can drive extreme space weather, such as the aurora. Some of this energy is contained in the turbulent flow of the solar wind, much like a section of rapids on a swiftly flowing river. Exactly how the energy of this turbulence is deposited in plasma— an ionized gas of protons and electrons—has long been debated by scientists. Understanding this process in near-Earth space is essential for the development of models needed to protect our space-borne technology from the ravages of extreme space weather, which can disrupt or destroy communication and GPS navigation satellites or cause severe damage to the electrical power grid on Earth. For the first time, researchers at the University of Iowa, University of Arizona, University of New Hampshire, and NASA have directly shown that proton cyclotron damping can dissipate turbulence in space.

Cyclotrons were invented in the 1930s by Ernest Lawrence at University of California, Berkeley’s Radiation Laboratory—now known as the Lawrence Berkeley National Laboratory—to accelerate subatomic particles (protons) to very high energies. This achievement earned him a Nobel Prize in Physics in 1939. Lawrence originally referred to his invention as a “proton merry-go-round,” because the cyclotron acceleration process is akin to two parents pushing their daughter faster and faster on a merry-go-round, with each push propelling the child in the same direction as she rotates around. 

NASA’s Magnetospheric Multiscale (MMS) mission consists of four spacecraft orbiting the Earth to probe the turbulent solar wind plasma that surrounds the Earth. MMS provides unprecedented measurements of the protons, electric fields, and magnetic fields. Research Scientist Arya Afshari of the University of Iowa led a team that used the MMS data and unequivocally identified that the turbulent waves were causing cyclotron acceleration of the protons in the solar wind plasma. These findings are reported in a new article, “Direct observation of ion cyclotron damping of turbulence in Earth’s magnetosheath,” published in the journal Nature Communications. This new study also demonstrated a viable means to use spacecraft measurements to account fully for how the energy removed from the turbulence is split between the protons and electrons in the plasma via an innovative analysis method developed by researchers at the University of Iowa. 

When a proton is moving across a magnetic field, the Lorentz force causes it to travel in a circle about the magnetic field direction. Since magnetic fields pervade the solar wind plasma that fills interplanetary space, protons in space are constantly undergoing this circular motion. Under appropriate conditions, the electric field associated with the turbulent waves in the solar wind plasma can accelerate protons by the same merry-goround process. In this process, the amplitude of the turbulent waves diminishes, removing energy from the turbulence. (See Figure 1 for a depiction of how the protons are accelerated, causing them to spin in a circle of ever-increasing radius).

Figure 1. Cyclotron Acceleration of a Proton: The diagram shows the trajectory (blue) of a proton (blue circle with “+” sign) as it undergoes cyclotron acceleration due to alternating kicks by the electric field E (red) in opposite directions every half rotation about the magnetic field B (green). The cyclotron acceleration causes the proton rotation speed to increase; thus, the proton spirals outward while gaining energy. Image Courtesy of Gregory G. Howes, University of Iowa.

“Providing a clear demonstration of how to use existing spacecraft instrumentation to determine how the energy of space plasma turbulence differentially heats the plasma protons and electrons represents a major step forward in the scientific community’s longterm goal of being able to predict how space weather affects the evolution of the Earth’s near-space environment,” said Professor Gregory Howes of the University of Iowa, a coauthor of the study. 

This research was funded by NASA and the National Science Foundation.

See the article "Direct observation of ion cyclotron damping of turbulence in Earth’s magnetosheath plasma" in Nature Communications 15, published Oct. 7, 2024