Coronal-Mass-Ejection-Driven Shocks and Sustained Gamma-Ray Emission Events

Sudden explosive eruptions are observed to occur occasionally in the outermost part of the Sun’s atmosphere called solar corona. Huge amounts of energy are released in the coronal solar eruptions. A fraction of the released energy is deposited at the eruption site and in the regions of the lower solar atmosphere.

The deposited energy makes the regions appear very bright in extreme ultraviolet (EUV) light and emit X-rays and less frequently gamma rays. The sudden flashes of radiation, commonly known as EUV, X-ray, and gamma-ray flares or events, can be observed only using special detectors, because human eye is not sensitive to these wavelength bands. Because solar eruptions occur quickly, the associated flares are also relatively short-lived.

Especially, the duration of the gamma-ray flares is typically quite short, around 10–100 seconds. However, some gamma-ray flares are observed to last up to several hours, much longer than the concurrent EUV and X-ray flares. The purpose of this 3-year research project is to better understand how the long-duration gamma-ray emission is produced at the Sun.

The primary scientific objective of the project is to determine whether shocks driven by coronal mass ejections (CMEs) are the main source of high-energy protons producing the sustained gamma-ray emission (SGRE) events observed at energies >100 MeV by using combined analysis of gamma-ray, white-light, EUV and radio observations together with in-situ solar energetic particle (SEP) observations. Observations by the Large Area Telescope (LAT) on the Fermi satellite have shown that SGRE events are rather common.

Interactions of >300 MeV protons with the material in the low solar atmosphere are believed to produce neutral pions, which promptly decay into the >100 MeV gamma-rays, but the source of the high-energy protons is debated. Suggested sources include both flares and CME-driven shocks. Recent discovery of a relationship between decameter-hectometric type II radio bursts and SGRE events has provided support for CME-shock as a source of high-energy protons.

The question whether the CME-shock model is a valid description of SGRE events will be investigated by performing statistical analyses of the kinematics and spatial extent of CME-driven shocks and the properties of the associated type II radio bursts and SEP events observed during the SGRE events. These analyses will be carried out by comparing events that seem to agree well with the CME-shock model with those that do not seem to comply with the model, and by identifying the differences in the circumstances of the eruption and the properties of the associated CMEs and shocks.

The secondary scientific objective is to determine the characteristics of the associated CMEs and SEP events, which could provide observational constraints and parameters for numerical modeling efforts of particle acceleration and transport during SGRE events. The research and EPO agenda of this project supports the Strategic Goals of the AGS Division in discovery, learning, diversity, and interdisciplinary research.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.