An Extreme Solar Millimeter Burst Induced by Two Successive Flares

How flares accelerate electrons efficiently remains a major puzzle in solar and stellar astronomy. Crucial information is carried by microwave bursts that are generated as energetic electrons spiraling around magnetic field lines. Here we report the eruption process during the strongest solar microwave burst (>10,000 solar flux units above 20 GHz, associated with an X1.7 flare) in Solar Cycle 25. We found that the interaction between postflare loops of a preceding M2.6 flare and rising southern loops generates abundant superhot plasma ( n e ∼ 2.0 × 10 9 cm −3 , T e ∼ 40 MK), which gets involved (by providing heated seed particles) in the subsequent three successive episodes of reconnection processes of the X1.7 flare. Using the gyrosynchrotron mechanism to fit the obtained spectra for the three subsequent microwave peaks, we found that energetic electrons are progressively abundant, with a hardening spectrum with the index reaching up to ∼2. We highlight the importance of preheated plasma and persistent multiepisode acceleration of energetic electrons in generating such extreme microwave bursts.