Sporadic VLF amplitude perturbations in the high latitude D-region ionosphere

The propagation of very low frequency (VLF) radio waves permit us to study the response of the lower ionosphere (60-90 km) to sporadic space weather events such as solar proton events (SPEs) and energetic electron precipitation. In this study, we use the 37.5 kHz VLF signal transmitted from NRK (Iceland, L=5.5) recorded at Sodankylä (Finland, L=5.5) since 2010 till 2016. We compare the observed variations in the VLF amplitude measurements with changes in AE index, solar wind velocity, solar proton events and nitric oxide (NO) concentration. The analysis was divided in to positive and negative variations of the VLF perturbations with respect to the quiescent level. We found that the positive variations are mainly associated with SPEs and NO concentration enhancements. The observed large negative daytime perturbations are currently unexplained