| Abstrakt: |
Polarization and propagation characteristics of ultra‐low frequency (ULF, ≃1−1000 $\simeq 1-1000$ 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. Plain Language Summary: Ultra‐low frequency waves (ULF, ≃1−1000 $\simeq 1-1000$ mHz) are large‐scale hydromagnetic waves, which are generated by different mechanisms and propagate in the near‐Earth's plasma environment. They contain important information about Space Weather phenomena and have been intensively studied since the late 1950s. ULF waves are routinely studied using ground‐based magnetometers, which provide data with relatively low spatial resolution and geographic coverage restricted to landmasses. In this work, we mitigate these problems by using high‐frequency (3–30 MHz) ground‐based radars comprising the Super Dual Auroral Radar Network, which provides superior spatial resolution and covers inaccessible geographic regions like the sea or mountains. Using more than 10 years of mid‐latitude data from the Hokkaido East and Hokkaido West radars (Japan) and the Christmas Valley West radar (US), we performed detailed statistical studies of ULF wave propagation characteristics in the Pc5 frequency range (1.67–6.67 mHz). These studies provide important information about the possible generation and propagation of Pc5 ULF waves in the near‐Earth's plasma environment. Key Points: Statistical characteristics of Pc5 ultra‐low frequency (ULF) waves at mid‐latitude and sub‐auroral ionosphere was studied using Super Dual Auroral Radar Network radarsObserved occurrence rate, frequency, polarization, and azimuthal m $m$‐number point at a common source of these wavesSolar wind dynamic pressure variations and Kelvin‐Helmholtz instability at the magnetopause may be the principal generation mechanisms [ABSTRACT FROM AUTHOR] |
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