  The SOL2001-12-26 moderate solar eruptive event (GOES importance M7.1,
microwaves up to 4000 sfu at 9.4 GHz, CME speed 1446 km/s) produced strong
fluxes of solar energetic particles and ground-level enhancement of cosmic-ray
intensity (GLE63). To find a possible reason for the atypically high proton
outcome of this event, we study multi-wavelength images and dynamic radio
spectra and quantitatively reconcile the findings with each other. An
additional eruption probably occurred in the same active region about half an
hour before the main eruption. The latter produced two blast-wave-like shocks
during the impulsive phase. The two shock waves eventually merged around the
radial direction into a single shock traced up to $25R_\odot$ as a halo ahead
of the expanding CME body, in agreement with an interplanetary Type II event
recorded by the Radio and Plasma Wave Investigation (WAVES) experiment on the
Wind spacecraft. The shape and kinematics of the halo indicate an intermediate
regime of the shock between the blast wave and bow shock at these distances.
The results show that i) the shock wave appeared during the flare rise and
could accelerate particles earlier than usually assumed; ii) the particle event
could be amplified by the preceding eruption, which stretched closed structures
above the developing CME, facilitated its lift-off and escape of
flare-accelerated particles, enabled a higher CME speed and stronger shock
ahead; iii) escape of flare-accelerated particles could be additionally
facilitated by reconnection of the flux rope, where they were trapped, with a
large coronal hole; iv) the first eruption supplied a rich seed population
accelerated by a trailing shock wave.
