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Space Radiation Effects and Microelectromechanical Systems 85

Characterizing space radiation environment requires knowledge of charge states
and energies of the particles emitted by the Sun. In addition, the degree to which
interactions between particles alter their charge states and energies as they travel
through space must be determined. Electrons and ions spiral in opposite directions
around the Sun’s magnetic field lines in their journey away from the Sun. The
resulting helical orbits are a function of the ions masses, charges, and velocities as
well as the Sun’s magnetic field strength. The particles emitted by the Sun form
‘‘solar wind.’’ Solar wind is not constant, varying with both time and location.
Temporal variations are due to changes in solar activity, whereas spatial variations
are due to a number of factors, such as distance from the Sun, the effects of local
magnetic fields, and to a lesser extent, interparticle scattering. Although the solar
magnetic field decreases in strength with increasing distance from the Sun, the total
magnetic field in the vicinity of certain planets, such as Earth and Jupiter, may be
significantly greater because they contribute their own magnetic fields. Most of the
particles streaming towards the Earth are deflected by the Earth’s magnetic field.
However, some become trapped in belts around the Earth where their densities are
many times greater than in interplanetary space.
As already pointed out, the solar wind is not constant, fluctuating in intensity as
a result of variable solar activity. Figure 5.1 shows that solar activity, as measured
by the number of solar flare proton events, exhibits both long-term and short-term
variations. Long-term variations are fairly predictable, consisting of periods of
approximately 11.5 years. For 7 years the Sun is in its active phase characterized
by an enhanced solar wind and an increase in the number of storms on the Sun’s
surface. Solar storms are either ‘‘coronal mass ejections’’ or ‘‘solar flares,’’ both of

Event Fluences for Cycles 20−22
Cycle 20 Cycle 21 Cycle 22
10 11 200
8
> 10 MeV; Φ 10 p/cm 2
7
> 30 MeV; Φ 10 p/cm 2 180
Zurich Smoothed Sunspot Number
160
10 10
140
Protons/cm 2 10 9 120 Zyrucyh Smoothed Sunspot Number
100
80
60
10 8
40
20
10 7 0
1965 1970 1975 1980 1985 1990 1995
Year
* Sunspot Maximum: Cycle 20: 11/1968, Cycle 21: 11/1979, Cycle 22: 11/1989 (Ref. Feynman et al. 1993) NASA/GSFC-1996
FIGURE 5.1 Large solar proton events for cycles 20 to 22. The number of sunspots is
2
superimposed on the graph. (From J. Barth, Modelling Space Radiation Environments,
IEEE, 1997.)

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