High spatial and temporal resolution observations of an impulse-driven field line resonance in radar backscatter artificially generated with the Tromsø heater

Abstract

Abstract. 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.

Statistical study of global modes outside the plasmasphere

“…We expect global modes to exhibit coherent wave activity over large spatial scales (Waters et al ). Unlike standing Alfvén waves, which have radial spatial extents of roughly 0.5 Re when energy is provided by a monochromatic driver (Mann, ; Yeoman et al ) and exhibit a continuum of frequencies as radial distance changes (Anderson et al ), global modes should have coherent signals at the same frequency over several Re in radial distance. Thus, a test that examines the coherence between signals observed at spatially separated probes could be used to identify and further characterize global modes (Anderson and Engebretson, ).…”

SuperDARN observations of ionospheric heater–induced upper hybrid waves

“…Considering the Kodiak radar's 3.25° beamwidth and the 90 km wide pulse used in this experiment, returns from these beams and range bins are not inconsistent with the aforementioned horizontal extent of the heated volume. As is well known, backscatter near the radar's transmitted frequency is commonly observed during heating experiments and indicates the presence of strong heater‐induced field aligned irregularities or AFAIs [ Hedberg et al , 1983; Yeoman et al , 1997]. Figure 1 shows an average of all 125 spectra obtained for f c = f r (n = 0), beam 8 (directed over HAARP) and range bin 817–885 km.…”

SuperDARN observations of high‐m ULF waves with curved phase fronts and their interpretation in terms of transverse resonator theory

“…SuperDARN is a network of frequency‐agile HF coherent scatter radars [ Greenwald et al , 1995; Chisham et al , 2007]. A detailed description of the HF radar mode of operation for the interval under study here is given by Yeoman et al [1997]. In the SP‐UK‐OUCH mode, the Finland radar sounded on only 10 of its 15 beams (0–9), dwelling on each for 1 s, with a range cell length of 15 km, the first range gate being sampled at a range of 480 km.…”