An analytical study of neutral expansion, ionization, and charged particle acceleration in a neutral lithium release for the parallel electric field measurement

Abstract

The acceleration of lithium ions produced from a neutral lithium release experiment is analytically described to study a method of measuring the electric field parallel to the magnetic field in the auroral region. The particles photoionized by the UV radiation from the Sun are accelerated by the parallel electric field and the measurement of their energy at an observation point at a higher altitude than the release point gives information on the electric field structure. Ion flux estimates at several observation points are calculated as a feasibility study of the experiment. We have modelled the neutral cloud expansion, ionization, and subsequent charged particle acceleration during a high‐altitude (∼3000 km) neutral lithium release. In the case of a lithium release the photoionization time constant is larger than the characteristic expansion time of the release cloud. The equations describing the ionization and acceleration of particles are given in detail. The effect of a uniform and constant parallel electric field, gravity, and the mirror force of the Earth's dipole magnetic field are included in the calculation. The calculation results of a particle release from a satellite at 3000 km altitude are presented. It is shown that when the satellite at the observation point moves in the same direction as the satellite from which lithium is released, there is enough ion flux for the detection at the observation point for more than 400 s. The lithium energy‐time spectrum obtained during the observation window provides information on magnitude and structure of the parallel electric field.