MLT Wind Measurements Using Sunlit Trimethyl Aluminum Trails

Liquid trimethyl aluminum (TMA) has long been used as a vapor tracer for measuring winds in Earth's mesosphere and lower thermosphere (MLT). Chemiluminescent oxidization of TMA at high altitude produces an easily visible glow, that allows motion of the tracer clouds to be tracked optically (to measure winds) whenever the sky is dark. However, in a number of recent missions, we have deployed trails of TMA at twilight, when the sky is still dark at ground level, but MLT heights are sunlit. Resonant scattering of sunlight by these trails greatly increases their visibility compared to chemiluminescence alone. The scattering is extraordinarily efficient: Even though we only release roughly 400 ml of liquid TMA per deployment, this is easily able to form a persistent and easily visible trail spanning many tens to even hundreds of kilometers in length. Here we present examples of continuous wind profiles measured in the height range 85-160 km using four TMA trails deployed at locations forming a square grid of side length approximately 80 km horizontally. Results show the now-familiar strong vertical shears in the 95-115 km height range. Of more timely interest, these four trails also allow us to derive height-resolved estimates of horizontal gradients in the MLT winds, which are useful diagnostics for understanding Joule heating, Lorentz forcing, and wave perturbations.