Presentation #401.04 in the session Drivers and Dynamics of the Coupled Ionosphere-thermosphere-mesosphere-atmosphere System I.
In this paper, we will introduce the lower thermosphere circulation and its impact that has received little attention. This winter-to-summer circulation at ~100-120 km is located above the well-known summer-to-winter circulation. Recently this cell was observationally confirmed by gas concentration measurements like CO2, CO, and H2O: convergence in the summer polar mesopause region and divergence in the winter polar region. Similar to the mesopause circulation, this lower thermosphere circulation is mainly driven by gravity wave forcing and tidal dissipation. Because this cell is located between the source region of atomic oxygen (O) at 130 km and the chemical sink region below 100 km, it serves as an effective transport mechanism to remove O from its source to sink and alters the O budget in the entire thermosphere. We will quantify the transport term analysis of O in SD-WACCM-X including vertical advection that is associated with circulations, tide, eddy diffusion and molecular diffusion. O density in WACCM-X will be compared to the corresponding SABER measurement at 95 km and GUVI measurement at 130 km. Most importantly, this lower thermosphere circulation plays a vital role in the annual oscillation (AO) and semiannual oscillation (SAO) that has been observed in the thermosphere mass density and ionosphere plasma density for decades. Our work provides a new mechanism to explain the AO and SAO that is critical for satellite drag and radio communication, in addition to known mechanisms. At last, we will discuss how Joule heating in the polar region may interact with this circulation. Because this circulation is driven by the waves from the lower atmosphere, it illustrates the important role that the lower atmosphere controls the baseline of the ionosphere and thermosphere.