First Survey of Lightning Effects in the Lower Ionosphere Over Arecibo, Puerto Rico

The ionosphere plays a crucial role in radio communications, as electromagnetic waves can reflect, refract, or pass through it. Possible types of interactions depend on the wave frequency, the vertical electrical conductivity profile as well as the electron density. The electron density in this region is highly variable, and the sources of variability are understood to come from above, such as photoionization from solar UV radiation, electron precipitation from the radiation belts, meteor ablation, etc; and below from gravity waves and tropospheric and stratospheric severe changes. However, strong electrostatic fields caused by lightning in underlying thunderstorms can also locally modify the electron density content in the lower ionosphere, affecting long-range radio communications. In this work, we present a systematic survey of lightning effects in the lower ionosphere over the Arecibo Observatory (AO), in Puerto Rico. This location is ideal for this type of study because Arecibo hosts the most comprehensive suite of instruments to probe the ionospheric electron density, and also because of the high occurrence of tropical storms and hurricanes in the Caribbean region. Such convective systems are usually accompanied by lightning. A detailed analysis of lightning activity around the island, as probed with Vaisala's GLD360 lightning detection network, shows a lightning occurrence hotspot just 36-km West of the Arecibo Observatory. Furthermore, there is also an unusually high percentage of positive lightning flashes in this region; 35% compared to the commonly reported fraction of 10%. Here we analyze vertical profiles of electron density in the altitude range between 60 and 400 km, measured with the AO's 430 MHz Incoherent Scatter Radar, and present a cross correlation of its variability with nearby lightning activity measured by the GLD360 network.