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Radiation Environments and Damage in Silicon Semiconductors 329
of the Sun is the same and ii) the same charge state when the Sun magnetic-field
polarity is reversed.
The charge-drift effect can be best observed when the solar activity is at mi-
nimum [Potgieter, Le Roux, McDonald and Burlaga (1993)]. For instance, the so-
lar modulation dependence on the sign of CR particles was observed by Garcia-
Munoz, Meyer, Pyle, Simpson and Evenson (1986). The charge dependence becomes
gradually ineffective with increasing solar activity, as observed using IMP8 satel-
lite data during the period 1973–1995 by Boella, Gervasi, Mariani, Rancoita and
Usoskin (2001); the effect exhibited its maximum strength at the solar minima: in
Fig. 4.10, it is shown the variation of proton and helium flux (in percentage)
1 − R = 1 −
Φ
−
Φ
+
=
Φ
+
− Φ
−
Φ
+
between two subsequent solar minima as a function of the kinetic energy
∗∗
(in
MeV/amu); Φ
+
and Φ
−
are the particles fluxes
k
for A > 0 and A < 0, resp ecti-
vely. The observed particle flux depletion decreases with increasing particle energy.
As discussed above, drift effects are expected to affect the modulation, helio-
centric gradient and energy change of CRs
‡‡
in the inner solar system [Jokipii and
Levy (1977); Garcia-Munoz, Meyer, Pyle, Simpson and Evenson (1986); Reinecke
and Potgieter (1994); Bieber and Matthaeus (1997); Boella, Gervasi, Mariani, Ran-
coita and Usoskin (2001)] (see also references in [Potgieter (1998)]). For instance,
when the Sun magnetic-field lines in the northern hemisphere are directed outwards
(e.g., for A > 0), the inclusion of drift effects [Jokipii and Thomas (1981)] are rel-
evant to account that positively charged particles drift inwards the polar regions
and outwards along the current sheet (see page 311); the sense of drift is reversed
when the magnetic-field polarity of the Sun is reversed (A < 0) (see [Isenberg and
Jokipii (1979); Jokipii and Thomas (1981)], Section 7.7.2 of [Kallenrode (2004)] and
references therein). As a function of the solar activity, these effects can be accounted
and, partially, estimated by means of computer simulations of CRs propagation in-
side the heliosphere (e.g., see [Potgieter, Le Roux, Burlaga and McDonald (1993);
Bobik et al. (2003, 2006a)]).
In addition, the actual requirements of simulating the space radiation environ-
ment are addressed by means of empirical or semi-empirical dynamical models of the
GCR modulation like, for instance, the Nymmik–Panasyuk–Pervaja–Suslov model
(e.g., see [Nymmik, Panasyuk, Pervaja and Suslov (1992); Nymmik and Suslov
(1996); ISO-15390 (2004)]). In this model at 1 AU beyond Earth’s magnetosphere,
the effective modulation potential of the heliosphere is the main parameter and
is calculated, at the time t, using the expression {see Equation (9) in [Nymmik,
∗∗
The reader can find a definition of kinetic energies per amu in Sect. 1.4.1.
k
These fluxes are observed at the solar minima and normalized to the same value of neutron
monitor counting rate registered at Climax Station (for a discussion about the normalization
procedure, the reader can see [Boella, Gervasi, Mariani, Rancoita and Usoskin (2001)]).
‡‡
Effects on modulated spectra can be relevant for particles with rigidities as large as 4 GV [Bieber
and Matthaeus (1997)].