8.1 Effects in the magnetosphere

High-energy ions trapped in the Earth’s magnetic field can cause significant effects in spacecraft systems. While malfunctions such as memory upsets can occur almost anywhere, a vast majority of the events are observed in the southern hemisphere over the South Atlantic Anomaly region where the Earth’s magnetic field is weakest and the energetic particles thus have best access to altitudes where the low-Earth-orbiting spacecraft reside (300 – 1000 km altitude). Furthermore, the high energies above tens of MeV make galactic cosmic rays highly penetrating into the satellite systems, where they can cause a wide variety of harmful effects (Baker et al., 2001).

Solar energetic particles arising from active events on the Sun cause degradation and failure of space-borne systems. Especially, single event upsets can occur in electronic components when a charged particle (e.g., a heavy ion) ionizes a track along a sensitive part of the circuit and causes the circuit to change state. Single event upsets are commonly detected in transistors and spacecraft memory devices. Error-correcting software solutions have been developed to decrease the damages to the satellite operations.

High-energy electrons in the outer Van Allen radiation belts can penetrate through spacecraft walls and through electronics boxes and become buried in dielectric materials (Baker et al., 1987). The excess negative charge can give rise to potential differences, which in turn can lead to intense voltage discharges and surges of electric energy deep inside the electric circuits of the spacecraft causing severe damage to various subsystems. The discharges can produce short-lived (fractions of a microsecond) but intense (several Amperes) current pulses. As the anomalies tend to occur only during relatively long-duration events, it is not only the peak intensity of the electron flux but also the duration of the exposure that determines the amount of excess charge accumulation.

Moderate-energy electrons associated with substorms cause spacecraft surface charging (Garrett, 1981). During a surface charging event, insulated surfaces may charge to several kilovolts potential (typically negative relative to the ambient plasma). In hot, tenuous plasmas, the incident electrons and ions and the secondary photoelectrons and scattered electrons are not in current balance, which leads to potential buildup at the spacecraft. Differential charging of spacecraft surfaces can lead to harmful discharges, which introduce noise to the system and may interrupt normal spacecraft operations and/or represent false commands for the spacecraft. The discharge breakdown can cause physical damage which can change the satellite conductivity or thermal, chemical or optical properties.

Besides being a threat to technological systems, energetic particles pose a hazard to astronauts on space missions. The atmosphere effectively shields the Earth from high-energy particles such that only the highest-energy particles can gain access to the lower layers of the atmosphere, and only in the polar regions where the geomagnetic field is weakest. However, astronauts in orbit are relatively unprotected especially during the high-latitude parts of the spacecraft orbit. For example, the International Space Station reaches latitudes above 50, which is sufficiently high to increase the health risks especially during extra-vehicular operations. Furthermore, as the atmospheric shielding is significantly reduced above 10 km altitude, aircraft crews and passengers on transpolar routes are subjected to increased radiation doses from the energetic particles. In practice, this problem is dealt with by limiting the number of transpolar flights the crewmembers can take in a given time period. As the fluxes of energetic particles are strongly modulated by the solar activity, these problems are largest during sunspot maxima.


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