The solar flare of June 3, 2012 behaved like an orchestra where, after the main performance, a second soloist unexpectedly steps in. First, instruments recorded a sharp burst of gamma-ray emission with energies above 100 megaelectronvolts — as if billions of tiny bullets struck the detectors at once. Then, after a brief pause, the emission intensified again and persisted for more than eight minutes. Previously, this was thought to be unique to this flare, but it turned out that a similar 'second act' had been observed as early as 1982 and subsequently in more than forty solar eruptions.
The second wave of emission is known as late gamma-ray emission. It is produced when fast protons — hydrogen nuclei accelerated by the flare — catch up with the expanding plasma bubble (coronal mass ejection) and collide with its shock wave. The shock wave acts as a natural accelerator: it picks up particles and hurls them with even greater force, causing them to generate gamma-ray photons. The delay between the first and second waves exactly matches the time needed for protons to catch up with this bubble.
This result links two phenomena previously studied separately: solar flares and particle streams arriving at Earth from shock waves. It turns out that during powerful events, the Sun triggers both mechanisms, and late gamma-ray emission is the bridge between them. It is like a single furnace that first quickly warms the room, then continues to heat it through pipes for a long time.
as written by the authors arXiv:2602.10284
A recent paper on SOL2012-06-03 reported the detection for the first time of two distinct phases of $$>$$100 MeV $$\gamma$$-radiation indicating separate acceleration processes. But such two-phase emission has been seen before and was first observed in SOL1982-06-03. The second phase is known as Late Phase Gamma-Ray Emission (LPGRE) and was cataloged for $$>$$40 solar eruptions, including SOL2012-06-03. Here we provide evidence that the second SOL2012-06-03 $$\pi$$-decay peak is the onset of LPGRE that lasted for $$>$$8 min. Its delay from the impulsive X-ray peak is consistent with the time it would take flare-produced suprathermal protons to overtake the expanding CME and be accelerated by its shock. The high accelerated ion-to-electron ratio in SOL2012-06-03 and other LPGRE events is consistent with the ratio observed in gradual SEP events produced by shocks and is inconsistent with ratios typically found in impulsive flares and solar energetic particle events produced by reconnection.
Late gamma-ray emission from solar flares was first noticed in 1982, but at the time it was not recognized as a distinct phenomenon and was attributed to interference.