Temporal evolution of neutral, thermospheric winds and plasma response using PFISR measurements of gravity waves

In this paper, we analyze the temporal variability of the propagation and dissipation of two southeast (SE)ward-propagating gravity waves (GWs) observed by the Poker Flat Incoherent Scatter Radar (PFISR) on 13 December 2006. We determine the GW vertical wavelengths as a function of altitude along each constant wave phase line, then extract the neutral, horizontal winds every ∼ 10 – 12 min (one-half of a wave period) along the direction of GW propagation as a function of altitude using an accurate, dissipative GW dispersion relation and MSIS temperatures. We find that the neutral wind in the northwest (NW) direction above PFISR was composed of a slowly varying “mean” of ∼ - 150 m / s plus a slowly moving, large-scale wave with a period of 3–5 h. These winds added at z ∼ 190 km , creating a large NW wind of ∼ - ( 200 – 250 ) m / s . This wind caused these two GWs to become evanescent or nearly evanescent some of the time, although their amplitudes increased up to z ∼ 210 – 240 km . We find that the winds accelerated in the SEward direction by ∼ 100 – 150 m / s in 30–40 min at z ∼ 190 km . We hypothesize that these accelerations are thermospheric body forces caused by the dissipation of SEward-propagating GWs excited by mountain wave breaking near the mesopause NW of PFISR. This hypothesis is supported by tropospheric winds and the consistency of the observed GW periods. Finally, we ray trace these GWs through the extracted winds, and using a simple single-ion plasma model, compare the theoretical and measured plasma responses. We find that theory agrees reasonably well with observations.