No abstract

Abstract. The cosmic ray ground level enhancement (GLE) of December 7-8, 1982, occurred when the interplanetary magnetic field (IMF) direction measured near Earth indicated significant distortion from the nominal form. We have modeled the ground level response to this event to derive rigidity spectra and pitch angle distributions centered on apparent arrival directions. A deficit region associated with a near-Earth magnetic cloud-like structure has been modeled during the initial phase of the GLE. Using this modified model, we find agreement between the derived particle arrival directions and the measured IMF directions. We find a significant contribution from pitch angles greater than 90 ø throughout the event. This is probably due to scattering in the turbulent region behind the cloud-like structure.

The cosmic ray ground-level enhancement (GLE) of 24 October 1989 was the last of a series of GLEs associated with the same solar active region. Intensity enhancements were observed by at least 31 neutron monitors in the worldwide network, with the largest increase (~200%) observed at South Pole, Antarctica around 20:30 UT. Using a least-squares model fit to all available neutron monitor data, spectra, apparent source directions and particle pitch angle distributions have been derived. The effect of disturbed geomagnetic conditions has also been taken into account.

During the solar maximum of 1989-91 an un-recedented sequence of 13 cosmic ray ground-level ELECT E I--enhancements (GLEs) was observed by the world-wide MAR 3 01994 neutron monitor network. Of particular interest Were two OLEs observed by the Australian network. The 1989 , September 29 event was the largest GLE in the space era F while the October 22 GLE included an highly anisotropic precursor peak. Analysis of both these OLEs. calking into account disturbed geomaeonetic conditions, shows that the oarTicle arrivals at the earth were unusual. The September 29 GLE had significant particle propagation in the reverse direction and as the particle flux decreased following the peak the spectrum also softened. In contrst. the 1989 October 22) precursor exhibited extreme anisotropy while the particles involved in the main OLE showed a complex temporal structure possibly indicating multiple particle injection at the solar acceleration region.