In-flight upset - 154 km west of Learmonth, WA, 7 October 2008,

Factors affecting SEE exposureAn aircraft is constantly exposed to galactic and solar radiation, but the amount ofexposure varies depending on a number of factors, including the following: 219• The flux of particles varies significantly with altitude and, for neutrons, is about300 times higher at 40,000 ft than at ground level. At moderate latitudes (seebelow), the neutron flux peaks at about 60,000 ft, dropping to about5,600 neutrons per cm 2 per hour at 40,000 ft, and less than 20 at the Earth’ssurface. The flux is somewhat lower at altitudes above 60,000 ft due to thereduced number of nuclear interactions with the Earth’s atmosphere, althoughparticles tend to possess higher energies at higher altitudes.• Particle flux is higher at the Earth’s magnetic poles than at equatorial latitudesdue to the influence of the Earth’s magnetic field. At 40,000 ft, the neutron fluxis less than about 2,100 neutrons per cm 2 per hour in the tropical zone 220 , and isabout 8,400 between the 60° latitudes and the poles.• There are natural variations in solar activity, which predominantly follow anapproximately 11-year cycle (Dyer et al. 2003). In general, this cycle hasminimal effect on SEE rates except during the occurrence of ‘solar flares’,which mostly occur during the solar maxima phase of the cycle. Solar flares canlast from a few hours to several days, and in very rare cases can increase fluxesby a factor of up to 300 in polar regions (Dyer and Lei 2001). 221The low-energy neutron flux within an aircraft is higher than outside due tointeractions with the aircraft’s structure and contents (Normand et al. 2006, Dyer etal. 2006). These very low-energy neutrons (called thermal neutrons) are more likelyto react with certain isotopes commonly used in the manufacture of silicon chips,resulting in an increase in the SEE rates.Types of SEEA particle collision within an integrated circuit can have one of four types of effecton a system:• No effect. Not all collisions have an effect on the state or operation of a deviceand this is not characterised as an SEE.• A non-destructive, temporary or recoverable change is known as single eventupset (SEU), or a soft error. This is the case where the problem can be resolvedwithout cycling the power (turning the device OFF and ON).219220221The neutron fluxes in this section are based on the approximate levels given in IEC TS 62396 forneutrons with energy levels of greater than 10 million electronvolts, which is the energy rangemost likely to be associated with SEEs.Between about 23° north and south of the equator.Not all solar flares produce particles. With the exception of solar flares that produce particles, therate of SEE at aircraft cruise altitudes decreases during the solar maxima because the solar activityreduces the presence of high-energy neutron particles from outside the solar system. However, thereduction is small. During a solar minima (last occurred in late 2008), the neutron flux is about1.2 times higher than during a solar maxima.- 261 -