Earth's trapped radiation environment
The motions of charged particles entering the magnetosphere from the solar wind and undergoing acceleration, or resulting from the decay of neutrons produced by cosmic ray interactions with the neutral atmosphere, are dominated by the magnetospheric magnetic field. The motion of these energetic charged particles consists of three components:
The resulting trajectories lie on toroidal surfaces, called drift shells, centred on the Earth's dipole centre. Particles confined to a drift shell can remain there for long periods, up to years for protons at altitudes of a few thousand kilometers, whence the term "trapped particles".
The population of charged particles stably trapped by the Earth's magnetic field consists mainly of protons with energies between 100 keV and several hundred MeV and electrons with energies between a few tens of keV and 10 MeV. There is also evidence for the existence of a narrow region centred around altitudes of about one Earth radius containing trapped heavy ions which are believed to be decelerated anomalous cosmic ray ions; the intensities of these heavy ions are several orders of magnitude below the intensities of trapped energetic protons in this region.
The energetic (above 10 MeV) trapped proton population is confined to altitudes below 20,000 km, while lower energy protons cover a wider region, with protons below 1 MeV reaching geosynchronous altitudes.
Figure 1 shows the distribution of trapped protons with energies above 10 MeV, as predicted by the NASA AP-8 MAX model (Reference 4), in invariant coordinate space.
The region of space covered by higher energy protons diminishes with increasing energies and the location of the highest intensities moves inward.
Figure 2 shows the AE-8 MAX (Reference 5) trapped electron population above 1 MeV in invariant coordinate space. The population distribution is characterised by two zones of high intensities, below altitudes of one Earth radius and above two Earth radii in the magnetic equatorial plane, respectively, which are separated by a region of low intensities, called the slot region.
The location and extent of the inner and outer belts and of the slot region depends on electron energy, with higher energy electrons confined more to the inner belt, and lower energy electrons populating the outer belt to altitudes beyond geosynchronous orbit. Note that at high latitudes the outer electron belt reaches down to very low altitudes.
The general description of the radiation belts in the above sections 2 and 3 represents what could be called the average particle distributions based on the static NASA models AP-8 and AE-8 (Reference 6). However, it has long been established that the actual population is very dynamic over different time scales.
The variation of solar irradiance with the 11-year solar cycle induces a periodicity of the low altitude trapped proton and electron fluxes: during solar maximum the Earth's neutral atmosphere expands compared to solar minimum conditions, so that the low altitude edges of the radiation belts are eroded due to increased interactions with neutral constituents.
Figure 3 shows the variation of the low altitude trapped proton flux over the solar cycle (Reference 7).
The erosion effect increases with decreasing altitude and the recovery of the population shows a phase lag which also depends on altitude.
The low altitude trapped particle population is also influenced by secular changes in the geomagnetic field (Reference 8): the location of the centre of the geomagnetic dipole field drifts away from the centre of the Earth at a rate of about 2.5 km/year (the separation currently exceeds 500 km), and the magnetic moment decreases with time.
The combined effect is a slow inward drift of the innermost regions of the radiation belts. The separation of the dipole centre from the Earth's centre and the inclination of the magnetic axis with respect to the rotation axis produce a local depression in the low altitude magnetic field distribution at constant altitude.
As the trapped particle population is tied to the magnetic field, the lowest altitude radiation environment (below about 1,000 km) peaks in the region where the magnetic field is depressed (Reference 1). This region is located to the south east of Brasil, and is called the South Atlantic Anomaly (SAA). Figures 4 and 5 represent a world map at 500 km altitude of the trapped proton (>10 MeV) and trapped electron (>1 MeV) distributions, respectively.
The SAA shows up clearly in both maps. Proton fluxes are negligible outside the SAA, but electron fluxes can be very high at high latitudes where field lines from the outer electron belt reach down to low altitudes. A further effect of the secular change in the geomagnetic field is a slow westward drift of the SAA at a rate of 0.3 deg/year (Reference 9).
At low altitudes (typically below 2,000 km), trapped particles interact with the neutral atmosphere. The gyroradii of trapped protons with energies above 1 MeV are comparable to the atmospheric scale height, which means that during a gyration motion they encounter different atmospheric densities.
As a result, proton fluxes depend on their arrival direction in the plane perpendicular to the local magnetic field vector (as well as on their pitch angle). The resulting anisotropy is called the East-West effect, and can cause differences of a factor three or more in fluxes arriving from different azimuths. The effect is illustrated in figure 6, which shows the angular dependence of the AP-8 MAX integral proton flux >10 MeV, averaged over an 800 km geosynchronous orbit.
Besides the long term variations in the trapped particle population described in Solar cycle effects and Secular changes in the geomagnetic field, variations on much shorter time scales occur as well. Outer zone electrons can vary in intensity by orders of magnitude over periods of a few hours.
Measurements with instruments onboard the Combined Release and Radiation Effects Satellite (CRRES) have shown that there are also major changes in the spatial distributions of outer zone electrons (Reference 10). Gussenhoven et al. (Reference 11) have shown that the changes in flux and spatial distribution can be ordered by level of magnetospheric activity, i.c. the fifteen day running average of Ap.
Figure 7 shows omnidirectional electron flux profiles on the magnetic equator as a function of McIlwain's L (Reference 12) for six ranges of Ap15.