High-latitude HF Doppler observations of ULF waves: 2. Waves with small spatial scale sizes

Abstract

Abstract. The DOPE (Doppler Pulsation Experiment) HF Doppler sounder located near Tromsù, Norway (geographic: 69.6°N 19.2°E; L = 6.3) is deployed to observe signatures, in the high-latitude ionosphere, of magnetospheric ULF waves. A type of wave has been identi®ed which exhibits no simultaneous ground magnetic signature. They can be subdivided into two classes which occur in the dawn and dusk local time sectors respectively. They generally have frequencies greater than the resonance fundamentals of local ®eld lines. It is suggested that these may be the signatures of high-m ULF waves where the ground magnetic signature has been strongly attenuated as a result of the scale size of the waves. The dawn population demonstrate similarities to a type of magnetospheric wave known as giant (Pg) pulsations which tend to be resonant at higher harmonics on magnetic ®eld lines. In contrast, the waves occurring in the dusk sector are believed to be related to the storm-time Pc5s previously reported in VHF radar data. Dst measurements support these observations by indicating that the dawn and dusk classes of waves occur respectively during geomagnetically quiet and more active intervals.

Van Allen Probes Observations of Symmetric Stormtime Compressional ULF Waves

“…If the waves have a horizontal scale size at ionosphere height (∼100 km) that is comparable to or shorter than the height, they cannot be detected with ground magnetometers (Hughes & Southwood, 1976;Yeoman et al, 2000). This screening effect explains why magnetospheric ULF waves are detected by ground magnetometers only when |m| ≤ 50 (Takahashi et al, 2013;Wright & Yeoman, 1999;Yamamoto et al, 2018).…”

Eastward Propagating Second Harmonic Poloidal Waves Triggered by Temporary Outward Gradient of Proton Phase Space Density: Van Allen Probe A Observation

“…Poloidal waves excited through the wave-particle interaction generally have high-m number (|m|~100) and propagate westward (m < 0), while toroidal waves excited by the solar wind have low-m number (|m| < 10) (e.g., Olson & Rostoker, 1978;Sarris et al, 2013). Since high-m waves cannot be observed by ground stations due to the ionospheric screening effect (Hughes & Southwood, 1976), these waves are observed by spacecraft or high frequency radar (e.g., Wright & Yeoman, 1999;Yeoman et al, 2012). The m number of the high-m waves can be determined from two neighboring spacecraft aligned in the azimuthal direction (e.g., Le et al, 2017;Rubtsov et al, 2018).…”

Multipoint visualization of ULF oscillations using the Super Dual Auroral Radar Network

“…However, since ground magnetometers measure the magnetic field due to ionospheric currents integrated over a radius of about 100 km, they are limited to the study of ULF pulsations with relatively large azimuthal scale size only. Pc5 pulsations with small azimuthal scale size are thought to be generated by drift and drift-bounce resonance interactions of particles trapped in the magnetosphere [e.g., Wright and Yeoman, 1999;Baddeley et al, 2005]. High-frequency (HF) radars provide a more direct means of observing ULF waves and are capable of detecting pulsations of either large-or small-scale size [Fenrich et al, 1995].…”