The scientists simulated solar eruptions and the subsequent acceleration of charged particles in the solar corona with state-of-the-art supercomputer simulations. They found that the acceleration mechanism requires specific conditions in the solar wind. Solar wind is a continual outflow of charged particles from the Sun.
When the solar wind speed is between roughly 500 and 650 kilometers (310 and 404 miles) per second and there are large areas on the Sun with strong magnetic fields poking through the surface, conditions are right for the acceleration mechanism to start operating.
The scientists' observations could lead to the development of novel space weather models that predict harmful space radiation with sufficient lead time to protect astronauts working outside the Earth's magnetic shield. Space radiation is a major health hazard for astronauts and poses a significant challenge for human missions to the Moon and Mars.
The research team's findings were published in the journal _Physical Review Letters_.
"When astronauts travel outside of Earth's protective magnetosphere, they are exposed to high levels of space radiation," said Dr. Vassilis Angelopoulos, Alumni Distinguished Undergraduate Professor in the Department of Physics at NC State and corresponding author of the study. "Much of this radiation takes the form of highly energetic protons. However, despite decades of research, we still don't fully understand the physical mechanisms that accelerate these protons to such high energies."
Astrophysicists believe the acceleration likely happens in the solar corona—the Sun's outer atmosphere—and that it must occur in stages because no single process can accelerate the protons to energies observed near Earth. The prevailing scenario is that the protons gain a significant amount of energy very close to the Sun through reconnection of magnetic field lines—a process termed magnetic reconnection—and are then further accelerated by an as-yet-unknown mechanism somewhere in the inner heliosphere—the region between the Sun and Earth.
Observations show that these energetic events seem to be associated with solar eruptions involving so-called coronal mass ejections (CMEs). However, CMEs are also ubiquitous phenomena happening all the time, yet very few of them—only about 1%—end up producing hazardous radiation.
"This shows that CMEs alone cannot be responsible for the acceleration," Angelopoulos said. "There has to be something more; some specific conditions that lead to the initiation of the particle acceleration process."
So, what are these specific conditions?
The research team performed an extensive series of physics-based simulations with state-of-the-art supercomputer models of solar eruptions, including CMEs. They found that the acceleration of the high-energy protons begins when the solar wind is at a specific range of speeds and there are large regions on the Sun with strong magnetic fields piercing through the solar surface.
"The solar corona is full of magnetic fields, and we have long suspected that magnetic fields play a critical role in the acceleration process," said Dr. Xiaowei Wang, former postdoctoral research scholar in the Department of Physics at NC State and lead author of the paper. "But magnetic fields have to be structured in just the right way—like a slinky fully stretched out across the Sun. Our numerical simulations show that when these conditions occur, the stage is set for the generation of high-energy protons."
When such favorable conditions are present, magnetic reconnection can become very fast. This, in turn, can rapidly restructure the magnetic fields in such a way that electric fields accelerate the protons to high energies.
Space weather models can potentially predict the occurrence and arrival times of high-energy proton events at Earth if they can provide information on solar wind conditions and large-scale magnetic field structure on the Sun. Developing space weather models with this capability is challenging but feasible, and research in Angelopoulos' group continues towards that direction.
