High spatial and temporal resolution observations of an impulse-driven ®eld line resonance in radar backscatter arti®cially generated with the Tromsù heater

The CUTLASS Finland HF radar has been operated in conjunction with the EISCAT Tromsù RF ionospheric heater facility to examine a ULF wave characteristic of the development of a ®eld line resonance (FLR) driven by a cavity mode caused by a magnetospheric impulse. When the heater is on, striating the ionosphere with ®eld-aligned ionospheric electron density irregularities, a large enough radar target is generated to allow post-integration over only 1 second. When combined with 15 km range gates, this gives radar measurements of a naturally occurring ULF wave at a far better temporal and spatial resolution than has been achieved previously. The time-dependent signature of the ULF wave has been examined as it evolves from a large-scale cavity resonance, through a transient where the wave period was latitude-dependent and the oscillation had the characteristics of freely ringing ®eld lines, and ®nally to a very narrow, small-scale local ®eld line resonance. The resonance width of the FLR is only 60 km and this is compared with previous observations and theory. The FLR wave signature is strongly attenuated in the ground magnetometer data. The characterisation of the impulse driven FLR was only achieved very crudely with the ground magnetometer data and, in fact, an accurate determination of the properties of the cavity and ®eld line resonant systems challenges the currently available limitations of ionospheric radar techniques. The combination of the latest ionospheric radars and facilities such as the Tromsù ionospheric heater can result in a powerful new tool for geophysical research.