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

Abstract

[1] The Hankasalmi SuperDARN radar in Finland, while operating in a high spatial and temporal resolution mode, has measured the ionospheric signature of a naturally occurring ULF wave in scatter artificially induced by the Tromsø Heater. The wave had a period of 100 s and exhibited curved phase fronts across the heated volume (about 180 km along a single radar beam). Spatial information provided by the radar has enabled an m-number for the wave of about 38 to be determined. It is demonstrated here that the curved phase fronts are a generic feature of nonstationary poloidal waves in a transverse resonator, caused by the common action of the field line curvature, the plasma pressure, and the equilibrium current. Some features of the observed event agree with the resonator in the vicinity of the ring current, where it is proposed that the wave is excited by a moving source in the form of a proton cloud drifting in the magnetosphere in the azimuthal direction.

Giant pulsations as modes of a transverse Alfvénic resonator on the plasmapause

“…6 and 8. Such structure was observed earlier with radars for a moderately high-m pulsation (Wright and Yeoman, 1999), and was interpreted in terms of transverse resonator theory (Yeoman et al, 2012).…”

“…Other analogies include the Kelvin ship waves and lee (mountain) waves in the atmosphere. This approach avoids some difficulties of the instability theory and has a firm observational basis (Zolotukhina et al, 2008;Mager et al, 2009a;Yeoman et al, 2010Yeoman et al, , 2012. Several cases were observed when the giant pulsations appeared at some azimuthal location at the same time as did a cloud of particles injected during substorm arrival at the same spot (Chisham et al, 1992;Wright et al, 2001).…”

Magnetohydrodynamic Oscillations in the Solar Corona and Earth’s Magnetosphere: Towards Consolidated Understanding

“…The phase fronts in the impulsively excited resonator must be curved. This behaviour was observed with SuperDARN radars in several cases of poloidal waves (Yeoman et al 2012). …”

Spatio-temporal structure of Alfvén waves excited by a sudden impulse localized on an L-shell

“…Furthermore, such sources should be moving in the azimuthal direction as they are related to magnetosheath flow, therefore the wave energy finally leaves the system. Klimushkin (2007, 2008), Zolotukhina et al (2008) and Yeoman et al (2012) considered ULF wave excitation by a moving source, but they supposed the source to be widely distributed across magnetic shells while localized in azimuth. A more general theoretical treatment is required here and will be the subject of future studies.…”