1. Spring 2004 AGU Meeting, Montreal SH33A-02 UNUSUAL FEATURES OF THE OCTOBER 28, 2003 GROUND LEVEL ENHANCEMENT John W. Bieber 1, Paul Evenson 1, Roger Pyle 1, David Ruffolo 2, and Alejandro Sáiz 2,3 1. University of Delaware, Bartol Research Institute, Newark 2. Mahidol University, Department of Physics, Bangkok 3. Chulalongkorn University, Department of Physics, Bangkok Supported by NSF grant ATM-0000315 Visit our Website: http://www.bartol.udel.edu/~neutronm/

2. Spaceship Earth Spaceship Earth is a network of neutron monitors strategically deployed to provide precise, real-time, 3- dimensional measurements of the cosmic ray angular distribution: 11 Neutron Monitors on 4 continents Multi-national participation: U.S.A., Russia, Australia, Canada 9 stations view equatorial region at 40-degree intervals Thule and McMurdo provide crucial 3-dimensional perspective

3. COSMIC RAYS IN CYCLE 23 13 Ground Level Enhancements (GLE) to date Bastille Day 2000 and Easter 2001 GLE were biggest 3 small GLE during Oct-Nov 2003 activity Oct 29, 2003 Forbush decrease was largest of cycle

5. UNUSUAL FEATURES OF THE OCTOBER 28, 2003 GLE First particles arrive from anti-Sunward direction Initial spikes at some stations Exceptionally slow decay

6. A Closer Look at the Initial Spikes Most stations saw gradual increase of 6% 4 stations saw initial spikes of much larger size Earliest spike was observed in Norilsk, which was viewing approximately anti- Sunward along the nominal Parker spiral at the time

7. Spaceship Earth Viewing Directions October 28, 2003 11:30 UT “O” and “X” denote the Sunward and anti- Sunward Parker direction, respectively. Squares show viewing direction for a median rigidity particle, and lines encompass the central 50% of detector response. Station codes: CS Cape Schmidt, Russia; IN Inuvik, Canada; FS Fort Smith, Canada; TH Thule, Greenland; PE Peawanuck, Canada; MC McMurdo, Antarctica; NA Nain, Canada; MA Mawson, Antarctica; AP Apatity, Russia; NO Norilsk, Russia; TA Terre Adelie, Antarctica; TB Tixie Bay, Russia

8. Spectral Evolution of the October 28, 2003 GLE South Pole station has both a standard neutron monitor (NM64) and a bare counter lacking the usual lead shielding (Bare). The Polar Bare responds to lower particle energy on average. Comparison of the Bare to NM64 ratio provides information on the particle spectrum. As shown at left, the ratio held fairly steady at 1.3-1.4 in the October 28 event. Assuming a power law momentum (P) spectrum, this implies the differential spectrum varies as ~P -4.2, which is slightly harder than the average GLE.

9. A GLE IN A CME? Were the particles injected into a loop geometry created by a prior unrelated event? Earth is in a magnetic loop created by a prior CME GLE particles are injected into far leg of loop and are first observed in anti-Sunward hemisphere GLE particles mirror in near leg of loop and are now observed in Sunward hemisphere (perhaps supplemented by direct injection on near leg) After multiple mirroring, particles are isotropic or display trapped distributions. Event decay is extremely slow because of trapping.

10. Modeling Results (Preliminary) Based upon Boltzmann Equation Model Assumptions Loop length: 4 AU Scattering Mean Free Path: 3 AU First injection on far leg of loop Second injection 40 min later on near leg

11. Magnetic Loop Geometry (A GLE in a CME) SUMMARY Transport of solar energetic particles often takes place in a very disturbed background. Bastille day GLE probably included reflection from a downstream magnetic bottleneck. See ApJ, 567, 622, 2002. Unusual features of the October 28, 2003 GLE may be explained if particles were injected into a pre-existing magnetic loop. October 22, 1989 may have been another “GLE in a CME” event. See Ruffolo et al., poster SH31A-04, this conference Magnetic Bottleneck on Flanks of CME