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1. Solar Energetic Particle and Cosmic Ray Cutoffs During Geomagnetic Storms II B. T. Kress, Dartmouth College, Hanover, NH Average polar cap flux Cutoff latitude on Earth’s surface SAA SEP event r M λ Forbidden South cap Allowed North cap South cap… South exit South enter (need to modify search so we are here) 4/17 6:09 UT SAA Cutoff 4/18 6:08 UT 4) Case Studies of Cutoff Variations During Storms: 1) Introduction 3) Geomagnetic cutoffs The Earth's magnetic field usually shields latitudes below ~60o from direct penetration by solar energetic particles (SEPs). During geomagnetic storms, a suppression in geomagnetic shielding can lower the cutoff latitude up to ~15o, increasing the area of the polar cap region to which particles have access by a factor of two to three (Labrador et al., 2002). It has been shown that this reduction in geomagnetic shielding is correlated with the depression and recovery of the disturbance storm time (Dst) index (Leske et al., 2001), and can thus be attributed to a reduction in field strength in the inner magnetosphere due to ring current buildup. Observations and model results show that changes in solar wind conditions may also significantly modify SEP cutoffs on a timescale of minutes well before the main phase of a storm (Kress et al., 2004); however, the effect of solar wind dynamic pressure and IMF on the Earth’s energetic particle cutoffs remains poorly understood. In several case studies we here examine the role of changes in solar wind conditions on SAMPEX energetic particle cutoffs. In future work, a systematic comparison will be made between SAMPEX observations and SEP cutoffs, modeled by computing energetic particle trajectories in magnetospheric model fields from the coupled CISM (Center for Integrated Space Weather Modeling) codes. The envelope of the green trace in Figure 2 shows the intensity of SEPs in the solar wind (measured in the polar cap regions). The rapid variations are due to SAMPEX’s ~90 min. orbital period (in low Earth orbit), during which it passes through the North and South polar cap regions. While SAMPEX is at lower latitudes the energetic ion fluxes are cut off due to geomagnetic shielding. Figure 2. SAMPEX 19-27 MeV protons during an SEP event In Figure 3 we expand the time scale of Figure 2 to see just a few North and South polar cap passes. In this work, the cutoff latitude is taken to be where the SEP flux falls below ½ its mean polar cap value, shown by the black trace. 2) Background Summary Using conservation of energy and the azimuthal component of the generalized momentum of a charged particle in a dipole magnetic field, Störmer (1955) showed the existence forbidden and allowed regions for particle orbits. 5) A Global View • An algorithm has been developed for extracting cutoffs from SAMPEX energetic particle data. Visualization tools have also been developed, for studying and comparing variations in Geomagnetic cutoffs. • During severe geomagnetic storms the cutoff latitude typically varies up or down ~10o between 50o and 70o magnetic latitude. • The relationship between geomagnetic cutoff and solar wind dynamic pressure is complex, with the cutoff latitude sometimes increasing and sometimes decreasing with an increase in solar wind dynamic pressure. Future study is warranted. Acknowledgments Figure 3. Expanding time scale in Figure 2, several polar cap crossings are shown Figure 4 shows cutoff latitude vs. time for the 18-19 Apr 2001 event. Note also the appearance of the SAA near ~20o Lat. SAMPEX/PET energetic particle data is provided by Glenn Mason (JHU/APL); ACE Magnetic Field data is from N. Ness (Bartol Research Institute); ACE/SWEPAM Solar Wind data is from D. J. McComas (SWRI); Dst index from the WDC-C2 KYOTO Dst index service. This material is based upon work supported in part by the STC Program of the National Science Foundation under Agreement Number ATM-0120950. Additional funding is from NASA LWS grant NAG5-12202. References Figure 1. where M is the dipole moment, λ is the latitude and r is the radial distance from the center of the dipole. Note that for a constant dipole moment M, the size of the shielded region is determined by the rigidity (mvc/q) of the particle. As the rigidity is increased, the size of the forbidden region becomes smaller and the cutoff latitude is lower. Although Störmer's analytic result is derived in a pure dipole, well defined cutoffs are observed in geospace. The intersection between the boundary of the forbidden region and the Earth’s surface is at the cutoff latitude. Kress, B. K., M. K. Hudson, K. L. Perry and P. L. Slocum, Dynamic Modeling of Geomagnetic Cutoff for the 23-24 November 2001 Solar Energetic Particle Event, Geophys. Res. Lett., 31, L04808, 2004. Labrador, A. W., R. A. Leske, S. Kanekal, B. Klecker, M. Looper, J. Mazur and R. A. Mewaldt, SAMPEX Measurements of Geomagnetic-Cutoffs during the April 21, 2002 Solar Energetic Particle Event, Storms 2 Workshop, APL August 19-21, 2003 Leske, R. A., R. A. Mewaldt, and E.C. Stone, Observations of geomagnetic cutoff variations during solar energetic particle events and implications for the radiation environment at the Space Station, J. Geophys. Res. 106, 30,011--30,022, 2001. Störmer, C., "The Polar Aurora", Oxford University Press, 1950. Figure 5. Red stars indicate observed cutoff latitudes for 19-27 MeV protons as SAMPEX exits the North polar cap region on 17 & 18 Apr 2001. Blue contours show several IGRF L-shells. Figure 4. SAMPEX energetic particle cutoffs vs. time.