Solar flares produce gamma radiation through a complex process involving the acceleration of charged particles, mainly protons and electrons, to extremely high energies within the Sun’s atmosphere. When these energetic particles collide with other particles in the solar atmosphere, they can generate gamma rays by several mechanisms.
First, during a solar flare, magnetic energy stored in twisted magnetic fields near sunspots is suddenly released. This release accelerates protons and electrons to speeds close to that of light. These high-energy protons then slam into nuclei of atoms present in the Sun’s chromosphere or lower corona—regions just above the visible surface of the Sun. Although most atoms there are light elements like hydrogen and helium, some heavier elements such as carbon, oxygen, silicon, calcium, and iron are also present in small amounts.
When these accelerated protons collide with heavier atomic nuclei at very high energies (typically hundreds of MeV or more), they can cause nuclear reactions that produce unstable intermediate particles called pions—specifically neutral pions (π0). These neutral pions have an extremely short lifetime and quickly decay into pairs of gamma-ray photons. This process is known as **pion decay** and is one of the primary ways solar flares generate gamma rays at very high energies.
In addition to pion decay from proton collisions (hadronic interactions), accelerated electrons contribute by producing gamma rays through **bremsstrahlung** radiation. Bremsstrahlung occurs when fast-moving electrons are deflected by electric fields around ions or atomic nuclei; this sudden deceleration causes them to emit photons across a broad spectrum including X-rays and sometimes extending into low-energy gamma rays.
Another mechanism involves **nuclear excitation**: when energetic protons hit heavy nuclei but do not cause full nuclear reactions like pion production, they may excite those nuclei into higher energy states temporarily. As these excited nuclei return to their normal state (de-excite), they emit characteristic gamma-ray photons with specific energies corresponding to nuclear transitions.
The combination of these processes means that during intense solar flares:
– Protons accelerated by magnetic reconnection collide with heavy elements producing neutral pions which rapidly decay into high-energy gamma photons.
– High-speed electrons emit bremsstrahlung radiation contributing lower-energy X-rays up through soft gamma rays.
– Nuclear excitation followed by de-excitation emits discrete lines in the gamma spectrum from various isotopes present on or near the Sun’s surface.
This explains why normally quiet regions on the Sun do not emit significant amounts of gamma radiation—the necessary conditions for particle acceleration and dense targets for collisions only occur during flare events where explosive energy release drives particle acceleration.
Furthermore, cosmic ray interactions differ because cosmic rays hitting lighter elements like hydrogen mostly don’t produce excited states needed for strong direct emission; instead heavier elements provide better targets for generating observable gammas via nuclear processes. The Moon’s surface emits more steady-state gammas than quiet Sun regions because it contains abundant heavy elements bombarded continuously by cosmic rays—but only during solar flares does our star itself shine brightly in this extreme form due to its own accelerated particles colliding internally[1].
At even higher photon energies beyond typical bremsstrahlung ranges (~tens-hundreds MeV), pair production becomes relevant where a single very energetic photon converts its energy directly into an electron-positron pair near a nucleus’ electric field; however this is more about how such gammas interact after being produced rather than their initial creation inside solar flares[2].
In summary: Solar flares create intense bursts of charged particles energized by sudden magnetic changes; these particles then smash into atomic nuclei causing nuclear reactions including pion production plus electron bremsstrahlung emissions—all combining to flood space around Earth with powerful bursts of X-rays and true high-energy gamma radiation unique among normal stellar activity patterns[4].