Physical Processes in Magnetically Driven Flares on the Sun, Stars, and Young Stellar Objects

TLDR

Solar and stellar flares, first recorded 150 years ago, have long been enigmatic, with only secondary effects noted until recent high‑energy imaging revealed their primary products and showed that soft X‑ray emission and coronal dynamics are common across stars, including young stellar objects. Magnetic reconnection initiates the flare by reconfiguring the field to a lower‑energy state, converting most of the released energy into nonthermal particles that transport heat to denser plasma, raising it to soft‑X‑ray emitting temperatures. High‑energy electrons and ions, now spatially resolved in hard X‑ray and gamma‑ray images of solar flares, demonstrate that young‑star flares are orders of magnitude brighter and more frequent, profoundly ionizing protoplanetary disks and planetary ionospheres.

Abstract

The first flare on the Sun was observed exactly 150 years ago. During most of the long history, only secondary effects have been noticed, so flares remained a riddle. Now the primary flare products, high-energy electrons and ions, can be spatially resolved in hard X-rays (HXRs) and gamma rays on the Sun. Soft X-rays (SXRs) are observed from most stars, including young stellar objects. Structure and bulk motions of the corona are imaged on the Sun in high temperature lines and are inferred from line shifts in stellar coronae. Magnetic reconnection is the trigger for reorganization of the magnetic field into a lower energy configuration. A large fraction of the energy is converted into nonthermal particles that transport the energy to higher density gas, heating it to SXR-emitting temperatures. Flares on young stars are several orders of magnitude more luminous and more frequent; they significantly ionize protoplanetary disks and planetary ionospheres.