Episodic energy release during the main- and post-impulsive phase of a solar flare
arxiv(2024)
摘要
When and where the magnetic field energy is released and converted in
eruptive solar flares remains an outstanding topic in solar physics. To shed
light on this question, here we report multi-wavelength observations of a
C9.4-class eruptive limb flare that occurred on 2017 August 20. The flare,
accompanied by a magnetic flux rope eruption and a white light coronal mass
ejection, features three post-impulsive X-ray and microwave bursts immediately
following its main impulsive phase. For each burst, both microwave and X-ray
imaging suggest that the non-thermal electrons are located in the
above-the-loop-top region. Interestingly, contrary to many other flares, the
peak flux of the three post-impulsive microwave and X-ray bursts shows an
increase for later bursts. Spectral analysis reveals that the sources have a
hardening spectral index, suggesting a more efficient electron acceleration
into the later post-impulsive bursts. We observe a positive correlation between
the acceleration of the magnetic flux rope and the non-thermal energy release
during the post-impulsive bursts in the same event. Intriguingly, different
from some other eruptive events, this correlation does not hold for the main
impulse phase of this event, which we interpret as energy release due to the
tether-cutting reconnection before the primary flux rope acceleration occurs.
In addition, using footpoint brightenings at conjugate flare ribbons, a
weakening reconnection guide field is inferred, which may also contribute to
the hardening of the non-thermal electrons during the post-impulsive phase.
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