Geomagnetic cutoff Penumbra structure: approach by transmissivity function
Abstract
Numerical tracing of cosmic ray trajectories in the model magnetospheric field is widely used to understand the "magnetospheric optics" of the Earth's magnetosphere for primary cosmic rays and their access to ground based observation sites. Recent review on the subject can be found, e.g. in (Smart et al., 2000). Penumbra structure is usually described as a system of allowed and forbidden trajectories (A,F) between low, RL , and upper, RU , cutoff rigidities. The probability of a particle in a given rigidity interval within (RL,RU) to access the position of a cosmic ray station, the transmissivity function TF, is deduced from the (A,F) structure which is dependent on the elementary step in rigidity for computations, on local time (if the external field is included) and on the geomagnetic activity level. We study TF using Tsyganenko'89 field model with rigidity steps ∼R=10-5 GV - 10-1 GV, for the high latitude Oulu site (65.05o N 25.47o E). We illustrate the penumbra structure in terms of the divergence of asymptotic direction of neighbouring allowed trajectories. The TF function weighted statistically by the Kp distribution over long time may serve as a tool for CR transparency characteristic at a particular station.
- Publication:
-
International Cosmic Ray Conference
- Pub Date:
- August 2001
- Bibcode:
- 2001ICRC...10.4056B