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Ann. Geophys., 20, 1247-1252, 2002
www.ann-geophys.net/20/1247/2002/
© European Geosciences Union 2002
Letter to the Editor
Forbush precursory increase and shock-associated particles on 20 October 1989
A. Struminsky
Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN), Troitsk, Moscow Reg., 142092, Russia, and Equatorial Geophysical Research Laboratory, Indian Institute of Geomagnetism, Krishnapuram, Tirunelveli 627011, India
Correspondence to: A. Struminsky
(astrum@izmiran.rssi.ru)
Abstract. Strong interplanetary
disturbances may affect cosmic ray protons tremendously with energies less than
1 GeV, increasing their intensity by hundreds of percents, but they are not so
effective for protons of higher energies. This energy limit is crucial to
understand processes of cosmic ray propagation and acceleration in the
heliosphere. The Forbush pre-increase and the effect of shock-associated
particles observed on 20 October 1989 illustrate the problem. This is a rare
event, when the energies of shock-associated particles measured by the GOES-7
satellite spread continuously to the neutron monitor energies. The Forbush
pre-increase could be attributed to a single reflection of galactic cosmic rays
from the magnetic wall observed at 12:00 UT. It had a very hard spectrum with
maximum energy of modulation more than 10 GeV. The spectrum of shock-associated
particles was soft and their maximum energy was less than 1 GeV. The problem of
shock acceleration versus trapping is discussed for the 20 October 1989 event.
It is argued that the shock-associated particles were accelerated near the
flare site and then propagated to the Earth inside the trap between two
magnetic walls at 12:00 UT and 17:00 UT.
Key words. Interplanetary physics
(cosmic rays; energetic particles; interplanetary magnetic fields)
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