Solar flare gamma rays do indeed originate, at least in significant part, from interactions involving accelerated protons, but the process is more complex than just simple proton collisions. When a solar flare occurs, it is a sudden, intense release of energy in the Sun’s atmosphere, primarily driven by magnetic reconnection—a process where magnetic field lines break and reconnect, releasing vast amounts of energy. This energy accelerates charged particles, including both electrons and protons, to very high speeds.
The accelerated protons, which are high-energy ions, travel along magnetic field lines and can collide with the dense material in the lower solar atmosphere, such as the chromosphere. These collisions are hadronic interactions, meaning they involve protons and other ions smashing into atomic nuclei. When these high-energy protons collide with nuclei in the solar atmosphere, they produce secondary particles, including neutral pions. These neutral pions quickly decay into gamma rays, which are very high-energy photons. This pion decay process is a key mechanism behind the gamma-ray emission observed during solar flares.
In addition to proton collisions producing gamma rays via pion decay, accelerated electrons also contribute to gamma-ray production through a process called bremsstrahlung. Bremsstrahlung occurs when high-energy electrons are deflected by the electric fields of atomic nuclei, emitting X-rays and gamma rays as a result. However, the gamma rays produced by bremsstrahlung tend to dominate at lower energies compared to those from pion decay, which can reach much higher energies, up to several GeV (giga-electronvolts).
Observations from space-based instruments have confirmed that solar flares emit gamma rays extending from tens of MeV (million electronvolts) to several GeV. The spectral characteristics of these gamma rays often fit models that include pion decay, strongly indicating that proton acceleration and subsequent collisions are responsible for a significant portion of the gamma-ray emission. This is especially true for the highest-energy gamma rays detected during solar flares.
The timing and duration of gamma-ray emission also provide clues about their origin. Some gamma rays are emitted impulsively during the initial flare phase, while others, known as sustained gamma-ray emission (SGRE), can last for hours after the flare’s peak. The sustained emission is thought to be related to shock waves driven by coronal mass ejections (CMEs) that continue to accelerate protons, which then collide with the solar atmosphere to produce gamma rays over extended periods.
To summarize the chain of events in simple terms:
– A solar flare occurs due to magnetic reconnection, releasing energy.
– This energy accelerates protons and electrons to very high speeds.
– Accelerated protons collide with atoms in the Sun’s lower atmosphere, producing neutral pions.
– Neutral pions decay into gamma rays, which we detect as solar flare gamma-ray emission.
– Accelerated electrons also produce gamma rays through bremsstrahlung, but these tend to be lower in energy.
– Some gamma-ray emission lasts longer due to ongoing proton acceleration by CME-driven shocks.
This understanding highlights that solar flare gamma rays do come from proton collisions, but specifically from the complex nuclear interactions and particle decay processes triggered by those collisions, rather than from simple proton-proton collisions alone. The interplay of magnetic fields, particle acceleration, and nuclear physics in the Sun’s atmosphere creates the rich gamma-ray signals we observe during solar flares.