Solar Wind Speed Inferred from Cometary Plasma Tails using Observations from STEREO HI-1

The high temporal and spatial resolution of heliospheric white-light imagers enables us to measure the propagation of plasma tails of bright comets as they travel through the interplanetary medium. Plasma tails of comets have been recognized for many years as natural probes of the solar wind. Using a new technique developed at the University of California, San Diego to measure the radial motion of the plasma tails, we measure the ambient solar wind speed, for the first time in situ at comets 2P/Encke and 96P/Machholz. We determine the enhanced solar wind speeds during an interplanetary coronal mass ejection encounter with 2P/Encke and compare these to previously modeled values, and also present solar wind speeds covering a range of latitudes for 96P/Machholz. We here apply this technique using images from the Sun-Earth Connection Coronal and Heliospheric Investigation Heliospheric Imagers (HI-1) on board the Solar TErrestrial RElations Observatory-Ahead spacecraft.