Saturation Effects in the VLF Scattering off Strongly Heated Ionosphere
Nikolai G. Lehtinen1, Timothy F. Bell1, Umran S. Inan1,2
1. Electrical Engineering, Stanford University, Stanford, CA, USA, 2. Koç University, Istanbul, Turkey

The HAARP ionospheric HF heating facility modifies the conductivity of the lower ionosphere, creating a perturbation in the propagation of VLF transmitter-generated waves. The changes in the VLF amplitude may be detected by ground-based receivers and satellites with VLF instruments. In order to quantify these effects, we introduce a novel computational technique which combines the Stanford FWM (full-wave method) with the method of moments (MoM) to calculate the scattering on localized strong perturbations in stratified media. The Born approximation, which was previously used for similar calculations [Lehtinen et al., 2010], neglects the scattered field inside the perturbation region and therefore becomes invalid when the scattered field is of the order of the incident field, which happens when the perturbation is strong (i.e., the relative change of the conductivity is ~1). On the contrary, the use of MoM allows us to calculate the effects of the total field and thus may be applied to situations with any level of perturbation. This technique is applied to the VLF scattering off ionospheric disturbances created by HF heating. We show how the field measured on the ground and at a satellite deviates from that calculated previously in the Born approximation by demonstrating saturation effects for strong heating.