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Ann. Geophys., 23, 2281-2291, 2005
www.ann-geophys.net/23/2281/2005/
© European Geosciences Union 2005
Solar cosmic rays during the extremely high ground level enhancement on 23 February 1956
A. Belov1, E. Eroshenko1, H. Mavromichalaki2, C. Plainaki2, and V. Yanke1
1Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN), 42092, Troitsk, Moscow Region, Russia
2Nuclear and Particle Physics Section, Physics Department, Athens University Pan/polis-Zografos 15771 Athens, Greece
Abstract. The 23 February 1956 ground level enhancement of the solar cosmic ray
intensity (GLE05) is the most famous among the proton events observed since
1942. But we do not have a great deal of information on this event due to
the absence of solar wind and interplanetary magnetic field measurements at
that time. Furthermore, there were no X-Ray or gamma observations and the
information on the associated flare is limited. Cosmic ray data was obtained
exclusively by ground level detectors of small size and in some cases of
a non-standard design. In the present work all available data from neutron
monitors operating in 1956 were analyzed, in order to develop a model of the
solar cosmic ray behavior during the event. The time-dependent
characteristics of the cosmic ray energy spectrum, cosmic ray anisotropy,
and differential and integral fluxes have been evaluated utilizing different
isotropic and anisotropic models. It is shown that the most outstanding
features of this proton enhancement were a narrow and extremely intense beam
of ultra-relativistic particles arriving at Earth just after the onset and
the unusually high maximum solar particle energy. However, the contribution
of this beam to the overall solar particle density and fluency was not
significant because of its very short duration and small width. Our estimate
of the integral flux for particles with energies over 100 MeV places this
event above all subsequent. Perhaps the number of accelerated low energy
particles was closer to a record value, but these particles passed mainly to
the west of Earth.
Many features of this GLE are apparently explained by the peculiarity of the
particle interplanetary propagation from a remote (near the limb) source.
The quality of the available neutron monitor data does not allow us to be
certain of some details; these may be cleared up by the incorporation into the
analysis of data from muonic telescopes and ionization chambers operating at
that time.
Keywords. Interplanatary physics (Cosmic rays; Energetic
particles) – Solar physics, astrophysics and astronomy
(Flares and mass injections)
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