The coupling of ions and neutrals in the auroral thermosphere, during both active and quite auroral conditions is studied using three Fabry-Perot interferometers and the AMISR incoherent scatter radar. Three components (zonal, meridional, and vertical) of the thermospheric neutral wind are isolated in a mesoscale cell approximately 50km - 75km in size, within the AMISR coverage pattern. The goal is to quantify the mesoscale response to joule heating in the auroral thermosphere, and to evaluate ion-neutral coupling in the vicinity of auroral arcs. The measurements also provide the divergence and vorticity of the neutral wind. The physics-based Global Ionosphere-Thermosphere Model (GTIM) is applied with enhanced spatial resolution and measurement constraint to evaluate the importance of various forcing functions, including Lorentz drag, particle precipitation energy deposition, and Joule heating.
This award supported the operations of three Fabry-Perot interferometer systems in central Alaska from the initial observations obtained in October 2006 to the end of the award period, fall 2012. It became rather clear from the early observations that sustained upward vertical winds of 30 to 50 ms-1 in speed appeared whenever there was a substantive period of soft particle precipitation as characterized by enhanced 630 nm emission. The development of such large vertical winds is hard to understand because Joule heating, which is generally thought to be the major source causing such winds, is normally confined to altitudes below 140 km which is where the electrons remain magnetized but the ions through the ion-neutral coupling with the neutral atmosphere are driven to move in the direction of the neutral wind. Thus, a fourth FPI instrument was transported to Poker Flat so to observe continually with high temporal resolution the vertical winds. These results show strong indications also of thermospheric wave structure. Data for one such example were analyzed and published in a joint publication with the radar measurements of plasma motions in the thermosphere. These results taken together showed a strong indication of a gravty wave structure propagating toward the auroral activity moving into the zenith from the north. It is believed that this structure was generated as a result of auroral activity but this is not yet a firm conclusion. More work on auroral gravity wave production needs to be done. One interesting aspect of these observations was to compare the winds and temperatures observed with that of the Scanning Doppler Imager operated by Prof. Mark Conde (Univ of Alaska), which demonstrated that both instruments were reporting almost identical results in regard to temperature values. Vertical wind comparisons often showed considerable differences but this is likely to be a difference caused by the difference in the field of view for the two instruments. The FPI observing angle is narrow, 1.5 degrees, and the SDI angle is broad, ~25 degrees.
