Erdfernerkundung - Numerische Physik: Modellierung

266 KAPITEL 5. DIE ERWEITERTE PERSPEKTIVE: CAWSES
Abbildung 5.34: Monte Carlo simulation: Tracks of a 50 MeV electron (right) and a 300 MeV
proton (left) in a model atmosphere [217]
interaction (for instance, ionization, hadronic interaction, production of Bremsstrahlung)
depending on the relative interaction cross sections. In our simple model, we adhere to
the Bethe–Bloch equation and the only energy loss mechanism is ionization. Thus the
second throw of the dice can be omitted and the interaction cross section for ionization
also becomes the total interaction cross section. If no interaction occurs, the next time
step starts. If an interaction happens, we have to throw the dice again to determine the
energy loss from the energy loss distribution. The particle properties (energy, eventually
also direction of motion) will be updated and the particle’s energy loss is added as an energy
gain to the volume. Alternatively, it is also possible to just count the ionization processes
in each volume element (or along each line element of the travel path). if fast secondaries
are produced, they are treated as additional particles and also have to be tracked by a
Monte Carlo simulation.
The Model
§ 879 Our model atmosphere is plane-parallel because the height of the atmosphere is small
compared to Earth’ radius. Up to a height of 100 km it is divided into 29 equidistant layers;
its remaining mass is condensed into a 30 th layer, 10 km thick: thus details of the energy
deposit above 100 km are lost.
§ 880 The composition of the atmosphere is homogeneous with 23.3 wt% O2, 75.5 wt% N2
and 1.3 wt% Ar. For numerical studies, pressure, density and temperature height profiles are
taken from the equatorial June atmosphere in the SLIMCAT/TOMCAT model [35]. This
approach ignores the pronounced seasonal variability of the polar atmosphere and gives an
average ionization profile instead [189]. For individual SEP events the corresponding polar
atmosphere is used.
§ 881 During a SEP event protons, electrons, and α-particles are accelerated (for a recent
summary see e.g. [119]). Electron to proton ratios as well as energy spectra depend on the
parent flare, in particular whether it is impulsive or gradual, the properties of the coronal
mass ejection, and the geometrical relation between observer and solar activity.
§ 882 Particle precipitation is assumed to be isotropic from the upper hemisphere: particle
distributions in interplanetary space tend to be isotropic for electrons and become isotropic
for protons during the time course of the event (e.g. examples in [122]). Observations by the
MEPED (Medium Energy Proton and Electron Detector) instrument on board POES (Polar
2. Juli 2008 c○ M.-B. Kallenrode