HF doppler sounder measurements of the ionospheric signatures of small scale ULF waves

Abstract

Abstract. An HF Doppler sounder, DOPE (DOppler Pulsation Experiment) with three azimuthally-separated propagation paths is used to provide the first statistical examination of small scale-sized, high m waves where a direct measurement of the azimuthal wavenumber m, is made in the ionosphere. The study presents 27 events, predominantly in the post-noon sector. The majority of events are Pc4 waves with azimuthal m numbers ranging from −100 to −200, representing some of the smallest scale waves ever observed in the ionosphere. 4 Pc5 waves are observed in the post-noon sector. The fact that measurements for the wave azimuthal m number and the wave angular frequency are available allows the drift-bounce resonance condition to be used to hypothesise potential particle populations which could drive the waves through either a drift or drift-bounce resonance interaction mechanism. These results are compared with the statistical study presented by Baddeley et al. (2004) which investigated the statistical likelihood of such driving particle populations occurring in the magnetospheric ring current. The combination of these two studies indicates that any wave which requires a possible drift resonance interaction with particles of energies > 60 keV, is statistically unlikely to be generated by such a mechanism. The evidence presented in this paper therefore suggests that in the pre-noon sector the drift-bounce resonance mechanism is statistically more likely implying an anti-symmetric standing wave structure while in the post-noon sector both a drift or drift-bounce resonance interaction is statistically possible, indicating both symmetric and anti-symmetric standing mode structures. A case study is also presented investigating simultaneous observations of a ULF wave in ground magnetometer and DOPE data. The event is in the lower m range of the statistical study and displays giant pulsation (Pg) characteristics.

Bistatic observations of large and small scale ULF waves in SPEAR-induced HF coherent backscatter

“…Subsequently Baddeley et al (2005b) were able to demonstrate a statistically significant relationship between such population inversions and ionospheric observations of high-m waves. Baddeley et al (2005a) additionally provided evidence that in the pre-noon sector the drift-bounce resonance mechanism was statistically more likely whereas in the post-noon sector both a drift or drift-bounce resonance interaction was statistically possible. The analysis of magnetospheric ion distribution functions was extended to a wider range of invariant latitudes by Wilson et al (2006).…”

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

“…Chisham (1996) suggested that the second mechanism may provide an explanation for the rarity of Pgs and their occurrence during geomagnetically quiet times. Several studies reported simultaneous observations of the bump-on-tail ion distribution functions and the poloidal pulsations very similar or identical to Pgs (Hughes et al, 1979;Baddeley et al, 2005).…”

Alfvén ship waves: high-<I>m</I> ULF pulsations in the magnetosphere generated by a moving plasma inhomogeneity

“…One of them is the coincidence of the directions of the azimuthal phase velocity of the high-m pulsations and the proton drift velocity (Fenrich et al, 1995;Yeoman et al, 2000;Baddeley et al, 2005b;Glassmeier, 1980). Besides, both velocities depend on the radial coordinate almost in the same way (Allan et al, 1982(Allan et al, , 1983.…”