Parametric study of shock‐induced transport and energization of relativistic electrons in the magnetosphere

Abstract

The sudden appearance of a new radiation belt consisting of electrons with energies greater than 13 MeV near 2.5 Earth radii (RE), was observed by CRRES (Combined Radiation and Release Experiment Satellite) on 24 March 1991. Li et al. (1993) showed that the cause was an electromagnetic pulse within the Earth's magnetosphere caused by an unusually strong, fast shock in the solar wind. Sudden shock‐induced injections of electrons with energies above 10 MeV to equatorial distances within 2.5 RE are extremely rare because of the intensity of the shock required. In the current study, the propagation velocity parameter and electric field amplitude of pulses within the magnetosphere in the Li et al. model were varied from 750 to 2500 km/s and 70 to 400 mV/m, respectively. It was found that a stronger electric field shifted the peak of the resultant relativistic electron population toward the Earth. Doubling the electric field amplitude from 120 to 240 mV/m moved the peak of the injected electrons with energies above 13 MeV from 2.8 to 2.4 RE. However, as the electric field pulse becomes even larger, the increase in response diminishes. This asympotic behavior shows that it is extremely difficult to produce energetic electron injections inside two Earth radii. The nominal propagation velocity (velocity parameter) is compared to the radial propagation velocities that would have been measured under this model and others and compared to observation. It is found that although the model radial velocity is smaller than the velocity parameter and decreasing with radial distance, it is faster than MHD results and observations. Decreasing the nominal propagation velocity of the pulse within the magnetosphere from 2500 km/s to 1400 km/s also moved the peak of the injected electrons with energies above 13 MeV slightly closer to the Earth. However, at velocities smaller than approximately 1200 km/s the number of electrons injected within 2.5 Earth radii with energies above 13 MeV greatly decreased. Halving the velocity from 2000 to 1000 km/s shifted the peak of electrons with energy greater than 13 MeV from L = 2.6 to L = 2.4 but produced a count rate reduced by a factor of more than 250, resulting in no significant new radiation belt. These results show that the typical large electromagnetic impulses caused by interplanetary shocks, with amplitudes of the order of 10 mV/m, are more than an order of magnitude too small to produce any significant new radiation belts within 2.5 Earth radii with energies of the order of 10 MeV. Contributing to the difficulty in producing such a new belt is the need for an already relativistic electron population with adequate flux beyond geosynchronous orbit. These results thus also help explain the rarity of events such as the 24 March 1991 injection.