Ionospheric Signatures of ULF Waves: Passive Radar Techniques

A review is presented of passive coherent and incoherent radar observations of ultra-low-frequency (ULF) waves. Passive radars primarily observe the ionospheric signatures of the shear Alfvén field line resonance (FLR). Recent results include: identification of common discrete FLR frequencies at both high and low azimuthal wave numbers; a better understanding of magnetometer spatial integration effects and radar line-of-sight effects via radar-magnetometer comparisons and modeling; a new high-frequency (HF) radar data visualization technique and use of ground/sea scatter for ULF wave detection and analysis; and an observed dependence of the ionospheric plasma response to ULF waves on local time, azimuthal wave number, and altitude. An example ULF FLR wave event observed with the Super Dual Auroral Radar Network (SuperDARN) is presented and shows the simultaneous occurrence of high-m and low-m FLR wave modes at the same frequency but different local times within the afternoon-dusk sector. Some discussion of sources of discrete FLRs observed with HF radar is given.