Belgian
Institute
for Space
Aeronomy

• Space physics



 

 

 

 

 

 

 

 


http://www.aeronomie.be

2.4.1 Solar cycle effects

2.4.2 Secular changes in the geomagnetic field

2.4.3 Low altitude trapped proton anisotropy

2.4.4 Magnetospheric conditions

 

2.4.1 Solar cycle effects

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 (7).

Figure 3. Variation of proton count rates in the 80-215 MeV channel of the MEPED detector aboard the TIROS/NOAA spacecraft over the solar cycle as a function of L (7). The dashed line shows the 13-month smoothed solar F10.7 flux.

The erosion effect increases with decreasing altitude and the recovery of the population shows a phase lag which also depends on altitude.

2.4.2 Secular changes in the geomagnetic field

The low altitude trapped particle population is also influenced by secular changes in the geomagnetic field (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 (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.

Figure 4. World map of the AP-8 MAX integral proton flux >10 MeV at 500 km altitude. Figure 5. World map of the AE-8 MAX integral electron flux >1 MeV at 500 km altitude.

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 (9).

 

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