Influence of an inertia-gravity wave on mesospheric dynamics: A case study with the Poker Flat Incoherent Scatter Radar

A case study of mesospheric winds and waves observed by the Poker Flat Incoherent Scatter Radar (PFISR) on 23 April 2008 is presented. Active auroral precipitation created sufficient ionization for nearly 12 h of continuous incoherent scatter measurements of the D region ionosphere from similar to 60 to 90 km altitude. PFISR utilized a multilook-direction mode which permitted measurements of vector winds, in addition to high precision vertical velocities, at high temporal resolution. A large-amplitude coherent wave packet (appearing superficially to be a single wave) with a downward phase velocity and a long period (tau similar to 10.5 h) was observed. Vertical wavelengths were measured directly to be lambda(z) similar or equal to 4-10 km, increasing with altitude. The proximity of tau to the local inertial period in addition to its large horizontal wavelength are suggestive of a coherent inertia-gravity wave (IGW) packet. Using polarization analyses, we find that the IGWs are propagating mainly southward. The waves were observed to saturate at z similar to 70-85 km, and have their largest amplitudes in the first 8 h of the measurements (before 2000 UT). A stability analysis confirms that the waves were likely dynamically unstable at these altitudes and times. In conjunction with this observation, the background wind is found to be southward of HWM winds by 10-20 m/s until similar to 2000 UT, consistent with the horizontal background wind acceleration created by the saturation of these IGWs. After 2000 UT, the background wind relaxes to the north by 10-20 m/s, consistent with a significant decrease of the IGW amplitudes. The IGWs may have originated from a jet stream adjustment at z similar to 10 km in northern Russia about 5 days prior to the observation in Alaska.