Impact and modeling of the solar eclipse effects of 20 March 2015 on VLF measurements

Abstract

On 20 March 2015, a solar eclipse occurred over Northern Europe providing an excellent opportunity for studying the ionospheric response to well-defined changes of solar irradiation. Due to the obscuration of the Sun by the Moon, X-ray and EUV radiation were reduced affecting the plasma production. Thus, the ionization of the whole ionosphere decreased in close relation to the obscuration function. The study focusses on the response of the ionospheric D region by means of VLF measurements. Germany maintains two VLF instruments that are part of the International Space Weather Initiative (ISWI): the educational project named SOFIE (SOlar Flares detected by Ionospheric Effects) and the global, near real-time network named GIFDS (Global Ionospheric Flare Detection System). We analyzed the behavior of diverse VLF signals demonstrating different steps of solar obscuration. The observation results of VLF amplitude measurements varied in strength and time showing a strong relation to the obscuration function. VLF signal propagation was simulated according to the waveguide mode theory with the help of the Long Wavelength Propagation Capability Code (LWPC). By changing the boundary conditions of the waveguide, modelled VLF amplitudes were calculated for three exemplary paths. Therefore, an exponential electron density profile was used depending on the reflection height and the sharpness. A linear relation between the ionospheric reflection height / sharpness and the solar X-ray flux has been found. Though, a comparison with observations from the upcoming solar eclipse on 11 August 2017 over the United States may reveal quite different ionospheric behavior since different latitudes will be affected. This needs further investigation.