Solar Cycle Variation of Radiated Electric Field and Ionospheric Reflection Height Over NWC Transmitter During 2005–2009: DEMETER Spacecraft Observations and Simulations

Besides the significant effects of a variety of naturally occurring magnetospheric waves on the electron dynamics in the magnetosphere, the important contribution of ground‐based very‐low‐frequency (VLF) transmitter waves also has been gradually discovered. The VLF transmitter's wave penetrating into the topside ionosphere is its energy source injected into the magnetosphere and has been extensively investigated. In the VLF wave trans‐ionospheric propagation, the main energy attenuation occurs in the lower ionosphere which is controlled by solar short‐wave radiation. However, the investigation on the variation of the VLF transmitters' energy in the topside ionosphere and ionospheric reflection height with solar activity is lacking. We use 4 years electric field measurements performed by DEMETER satellite and full‐wave simulations to address these concerns. The results show the electric field radiated from NWC was relatively similar from May 1 to July 31 in 2006, 2008, and 2009 in daytime and nighttime, stronger than that in 2005, because the solar activity was similar and extremely low in these years compared with that in 2005. The nighttime and daytime ionospheric reflection heights are also relatively similar in these 3 years, with about 6 km higher than that in 2005. The difference in the simulated electric field based on the electron density profile from the IRI‐2016 model between 2005 and 2009 is lower compared with the observation results. However, considering 6 km added in ionospheric reflection height in 2009, the simulation results are much more consistent with the observation results both in daytime and nighttime. Key Points The radiated electric field from NWC was weaker in 2005 compared with that in the other 3 years when solar activity was lower The reflection height overhead NWC transmitter in 2005 was about 6 km lower than that of the other 3 years The simulation demonstrates the lower reflection height is responsible for the weaker radiated electric field in 2005