Geomagnetic cutoff refers to a cutoff rigidity (momentum per unit charge) below which energetic particle fluxes in the magnetosphere are cut-off due to geomagnetic shielding. Each location in the Earth's magnetosphere has a corresponding cutoff rigidity. A global map of cutoff rigidities provides a measure of the shielding of solar energetic particles (SEPs) and cosmic rays by the Earth's magnetic field. An estimate of the total SEP and cosmic ray flux in the inner magnetosphere can be directly obtained from the cutoff rigidity and a near-Earth interplanetary spectrum.
The movie accessed with the link below shows the time evolution of the geomagnetic cutoff rigidity at 100 km above the Earth's surface during the 28-31 October 2003 solar-geomagnetic event. The movie was produced by computing cutoff rigidities in a series of static field snapshots from the Tsyganenko & Sitnov (2005) geomagnetic field model, driven by solar wind input parameters obtained from the ACE and Geotail spacecraft. The cutoff rigidities were computed using code developed by the Center for Integrated Space Weather Modeling (CISM) at Dartmouth College. Cutoff rigidities are determined by computing time-reversed Lorentz trajectories. Particles of different rigidity are launched and each trajectory is followed backwards in time. If a particle escapes the magnetosphere then a viable inward trajectory has been found indicating that the particle's rigidity is above the cutoff. An efficient search algorithm is used to locate the cutoff rigidity at each point on a latitude-longitude grid to produce the series of global maps shown.
To view the movie click here (66MB).
Revised -- 10/9/09