Abstract
Abstract. The CUTLASS Finland HF radar has been operated in conjunction with the EISCAT Tromsø RF ionospheric heater facility to examine a ULF wave characteristic of the development of a field line resonance (FLR) driven by a cavity mode caused by a magnetospheric impulse. When the heater is on, striating the ionosphere with field-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 field lines, and finally to a very narrow, small-scale local field 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 field line resonant systems challenges the currently available limitations of ionospheric radar techniques. The combination of the latest ionospheric radars and facilities such as the Tromsø ionospheric heater can result in a powerful new tool for geophysical research.
Highlights
The transient magnetohydrodynamic response to the impulsive excitation of the magnetospheric cavity has been the subject of a number of observational studies at Correspondence to: T
Sutclie and Yumoto (1989; 1991), Yeoman and Orr (1989), Yumoto (1990), Yumoto et al (1990), Lin et al (1991), Yeoman et al (1991) have all proposed that low- and midlatitude Pi2 pulsations are driven by compressional cavity resonances
Each panel presents parameters derived from ®ve sub-intervals of the wave packet at 1601±1603, 1606±1608, 1611±1613, 1616±1618, and 1621±1623 UT most readily be compared with the simulation of an impulse driven ®eld line resonance in VHF coherent radar data presented by McDiarmid and Allan (1990). They used the output of the time-dependent numerical model of Allan et al (1986b) to model the development of a FLR driven by a cavity mode caused by a magnetospheric impulse
Summary
The transient magnetohydrodynamic response to the impulsive excitation of the magnetospheric cavity has been the subject of a number of observational studies at Correspondence to: T. Ruohoniemi et al (1991) and Samson et al (1992) have demonstrated that, on occasions, nightside and early morning data from the high latitude Goose Bay HF radar, Canada, showed structured spectra with distinct spectral peaks at a discrete set of frequencies These were interpreted as ®eld line resonances (FLRs) driven by magnetospheric cavity/wave guide modes. A high-latitude observation of a wave signature characteristic of a coupled cavity/wave guide ± FLR is presented This wave has been detected at very high spatial and temporal resolution by the Cooperative UK Twin Located Auroral Sounding System (CUTLASS) radar, running a SuperDARN discretionary mode experiment (Greenwald et al, 1995).
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