Model predictions of subionospheric VLF signal perturbations resulting from localized, electron precipitation‐induced ionization enhancement regions

Abstract

Whistler‐induced precipitation of energetic electrons produces transient ionospheric conductivity variations that perturb the amplitude and phase of VLF signals propagating in the earth‐ionosphere waveguide (i.e., Trimpi events). This study uses a waveguide mode theoretic propagation model to predict the effect of localized ionization enhancement (IE) regions on nighttime long‐path VLF signals from the U.S. Navy navigational transmitters NSS, NAA, NLK and NPM to Antarctic receiving stations, principally the U.S. station at Palmer. These predictions are then compared with the observed signal behavior in Trimpi events from which inferences can be drawn about an IE region's approximate location, size, and particle characteristics. The propagation model considers multiple modes, variable ground conductivity, and mode conversion. IE regions of variable length (typically 50–150 km) and distance from the receiver (up to 600 km) were considered. The conductivity change within an IE region was modeled by exponential electron density profiles modified by precipitated monoenergetic fluxes of 50‐ to 150‐keV electrons. Our calculations show that some signal paths are more sensitive than others to perturbation by IE regions and that the size of an IE region is less important than its distance from the receiver. For a specified flux, higher‐energy electrons produce larger effects than lower energy electrons; for a specified energy, larger fluxes will produce larger signal changes. For the particular case of 150‐keV electrons precipitating into an ionosphere with VLF reflection height of 87 km, the received signal was largely insensitive to increasing the electron flux above ∼10−5 erg cm−2 s−1. This flux is equivalent to ∼3×10−3 erg cm−2 s−1 for E > 40 keV and an exponential energy spectrum with e‐folding energy in the range 40–100 keV. The measured electron fluxes precipitated by lightning are of the same order. Higher saturation flux levels would apply in situations where the normal VLF reflection height was lower over that portion of a path where the whistler‐induced precipitation was occurring. This study also examines the effects of transmitter‐induced electron precipitation on the VLF signals. Such precipitation is known to occur and, as shown here, may on occasion have important effects on certain signals.