Solar flares and coronal mass ejections (CME) are among the most powerful phenomena in the solar system, unleashing vast amounts of highly energetic plasma gas towards Earth’s magnetic field. This intense activity poses a risk to power grids, satellites, and communication networks. Understanding the particle acceleration process behind these events, particularly in large solar energetic particle (SEP) events, has been a focal point in heliophysics research.
Dr. Gang Li, a professor at The University of Alabama in Huntsville (UAH), has made a significant breakthrough in this field. In a paper titled “Modeling Solar Energetic Neutral Atoms from Solar Flares and CME-driven Shocks,” published in The Astrophysical Journal, Dr. Li presents a novel approach that utilizes energetic neutral atoms (ENAs) to investigate the acceleration process in large SEP events. Moreover, this method can help distinguish between the two potential acceleration sites: large loops in solar flares and areas downstream of CME-driven shocks.
Dr. Li believes that his work will stimulate the heliophysics community to delve deeper into the generation and propagation of solar ENA particles. The paper demonstrates, for the first time, that ENAs can be utilized to differentiate between CME/Flare SEP acceleration. This achievement lays the essential theoretical foundation for potential future measurements of solar ENAs.
Dr. Gary Zank, the director of UAH’s Center for Space Plasma and Aeronomic Research and the Aerojet Rocketdyne chair of the Department of Space Science, commends Dr. Li’s groundbreaking research. He emphasizes that this innovative approach expands the ongoing efforts of the Department of Space Science, which employ ENAs to explore remote regions of the heliosphere. By leveraging ENAs generated at the distant boundaries of the heliosphere and neighboring interstellar medium, scientists can investigate the plasma physics in those regions.
Dr. Li’s ultimate objective is to obtain various physics parameters at the acceleration sites using ENAs. Scientists have long debated which location, either solar flares or CME-driven shocks, is more efficient in accelerating particles and capable of achieving higher energies. These questions remain unanswered, and Dr. Li’s work paves the way for future advancements in understanding the particle acceleration process and its associated sites.
The primary challenge in unraveling the mysteries surrounding solar energetic particle (SEP) events is the inherent difficulty in directly observing the sun and its near-sun conditions. This hampers our understanding of the physical processes involved in SEP production and the dynamics near the acceleration sites.
Energetic neutral atoms (ENAs) present a promising avenue for gaining insights into these processes. ENAs are neutral particles formed from protons through charge exchange reactions. Their neutrality allows them to remain unaffected by magnetic fields, unlike charged particles such as protons, ions, and electrons.
Dr. Li emphasizes the significance of this characteristic, as it enables ENAs to propagate from the sun to observers without distortion caused by the solar wind’s magnetohydrodynamic turbulence. Consequently, ENAs carry valuable physics information about the acceleration site, making their observation a novel opportunity to constrain the underlying particle acceleration process.
Additionally, ENAs can disclose their secrets even when observed at a distance of 1 astronomical unit (approximately 150 million kilometers) from the sun. At this distance, the flux of ENAs is measurable by dedicated ENA detectors. The pursuit of capturing this data could drive the development of a new NASA solar mission dedicated to understanding these particles and the origins of large SEP events that impact Earth’s magnetosphere.
Dr. Li highlights that their simulations establish a theoretical foundation for interpreting future ENA observations. These observations are likely to be a priority for NASA in the form of a future mission, such as the NASA SMEX mission, specifically focused on solar ENA studies. A dedicated ENA mission that selectively measures ENAs while filtering out the more abundant charged SEPs could provide new insights into SEP acceleration near the sun and help resolve longstanding questions that have perplexed the scientific community.
In fact, Dr. Zank, a co-investigator on the NASA mission IMAP (Interstellar Mapping and Acceleration Probe), reveals that ENA instruments will be deployed at 1 astronomical unit within this mission. These instruments will be capable of measuring ENAs originating both from the distant boundaries of the heliosphere and from the sun itself.