Ionospheric Electron Density Response to Solar Flares

Solar flares are explosive events on the Sun that release energetic particles, X-rays, EUV and radio emissions that have an almost immediate impact on Earth's ionosphere-thermosphere (I-T) system and/or on operational systems that are affected by I-T conditions. In terms of impacts on the global positioning system (GPS), solar flare radio emissions in the GHz range have a severe impact by contaminating receiver inputs, causing the system to “lose lock” on GPS satellites. Fidelity of the GPS system is also degraded in the presence of significant plasma gradients, so the question arises as to what effects might occur in connection with solar flares? A first step in pursuing an answer is to specify the spatial-temporal response of the ionosphere to the changing EUV flux, with subsequent use of a radio propagation code to understand how the GPS signal is affected. In this paper we pursue an observational approach to this problem by using Digisonde Global Ionospheric Radio Observatory (GIRO) data to reveal the E- and F-region responses to several flares in a height vs. time framework. We furthermore interpret these responses in light of recent first-principles model results, and the anticipated dependences of flare response on solar zenith angle, latitude, and flare location on the solar disk.