Theoretical study: Influence of different energy sources on the cusp neutral density enhancement

Simulations with the Global Ionosphere-Thermosphere Model (GITM) show that both Poynting flux and soft electron precipitation are important in producing neutral density enhancements near 400 km altitude in the cusp that have been observed by the Challenging Minisatellite Payload (CHAMP) satellite. Imposing a Poynting flux of 75 mW/m² in the cusp within the model increases the neutral density by 34%. The direct heating from 100 eV, 2 mW/m² soft electron precipitation produces only a 5% neutral density enhancement at 400 km. However, the associated enhanced ionization in the F-region from the electron precipitation leads to a neutral density enhancement of 24% through increased Joule heating. Thus, the net effect of the soft electron is close to 29%, and the combined influence of Poynting flux and soft particle precipitation causes a more than 50% increase in neutral density at 400 km, which is consistent with CHAMP observations in extreme cases. The effect of electron precipitation on the neutral density at 400 km decreases sharply with increasing characteristic energy such that 900 eV electrons have little effect on neutral density. Finally, the impact of 2 keV, 0.3 mW/m² proton precipitation on the neutral density is negligible due to a lowering of the altitude of Joule heating.