  Where and how flares efficiently accelerate charged particles remains an
unresolved question. Recent studies revealed that a "magnetic bottle"
structure, which forms near the bottom of a large-scale reconnection current
sheet above the flare arcade, is an excellent candidate for confining and
accelerating charged particles. However, further understanding its role
requires linking the various observational signatures to the underlying coupled
plasma and particle processes. Here we present the first study combining
multiwavelength observations with data-informed macroscopic
magnetohydrodynamics and particle modeling in a realistic eruptive flare
geometry. The presence of an above-the-loop-top magnetic bottle structure is
strongly supported by the observations, which feature not only a local minimum
of magnetic field strength but also abruptly slowing down plasma downflows. It
also coincides with a compact hard X-ray source and an extended microwave
source that bestrides above the flare arcade. Spatially resolved spectral
analysis suggests that nonthermal electrons are highly concentrated in this
region. Our model returns synthetic emission signatures that are well matched
to the observations. The results suggest that the energetic electrons are
strongly trapped in the magnetic bottle region due to turbulence, with only a
small fraction managing to escape. The electrons are primarily accelerated by
plasma compression and facilitated by a fast-mode termination shock via the
Fermi mechanism. Our results provide concrete support for the magnetic bottle
as the primary electron acceleration site in eruptive solar flares. They also
offer new insights into understanding the previously reported small population
of flare-accelerated electrons entering interplanetary space.
