Ionospheric HF radars Pasha Ponomarenko

Outline Conventional radars vs ionospheric radars Collective scatter processes Aspect angle effects HF propagation features SuperDARN

Radars RAdio Detection And Ranging Remote sensing tool based on EM wave emission and its reflection/scatter from a target Main target parameters: range – time of flight direction – antenna directivity velocity – Doppler shift

Antenna beam echo Radar Ground signal Distance “Normal” radars target The target is usually smaller than the spatial pulse length. Doppler velocity

Antenna beam Radar Ionosphere with irregularities Ground signal echo ~300 km Ionospheric radars Refractive index fluctuations The scattering volume is larger than the spatial pulse length.

k λ Collective scatter theory λ/2 irregularities k λ Constructive interference (Bragg condition) Spatial array with l = λ/2

Ionospheric turbulence Continuous spatial spectrum of irregularities ln S ln k

Bragg scatter still works! ln S ln k k λ k λ λ/2 Constructive interference condition (Bragg condition)

Anisotropic turbulence E and F region irregularities are aligned with the background magnetic field

Dipole antenna with l >> λ

Aspect conditions ψ ψ phase front

HF Propagation Ionospheric scatter Ground scatter Radar n <1 n = 1 h x HF: f N ~ f MHz

SuperDARN Super Dual Auroral Radar Network

Dual Radar Network

Things to remeber: Backscatter signals are produced by plasma structures with Bragg scale sizes, l = λ/2 Most of the backscatter power comes form areas where the radio wave propagates orthogonally to the geomagnetic field, k  B 0 HF signals are capable of over-the-horizon propagation due to consecutive reflections from the ionosphere and the ground