Polarization and m $m$ -Number Characteristics of Mid-Latitude Pc5 ULF Waves Observed by SuperDARN Radars

Polarization and propagation characteristics of ultra-low frequency (ULF, $\simeq 1-1000$ mHz) waves are conventionally studied using arrays of ground-based magnetometers. However, the ground magnetometer observations are subject to distortions due to polarization rotation and spatial integration effects caused by the transition of the magnetohydrodynamic wave into an electromagnetic wave at the lower ionospheric boundary. In contrast, high-frequency (3–30 MHz) radars, like those comprising the Super Dual Auroral Radar Network (SuperDARN), are capable of direct observations of the ULF wave characteristics at ionospheric altitudes via measuring plasma drift velocity variations caused by the wave's electric field. In this work, we use multi-beam data from SuperDARN Hokkaido East, Hokkaido West, and Christmas Valley West radars to identify the dominant polarization modes as well as azimuthal wave numbers of evening-night-side-morning ULF waves in the Pc5 frequency band (1.67–6.67 mHz) propagating over sub-auroral and mid-latitude regions. The observed statistical characteristics of these waves point at the solar wind dynamic pressure variations and Kelvin-Helmholtz instability at the magnetopause as their potential principal sources, although the drift-bounce resonance with trapped energetic ions may contribute to the small-scale part of the observed Pc5 wave population.