By Kathleen Mandt, PhD, Senior Research Scientist, Southwest Research Institute
One of the main areas of cometary exploration is the study of how the solar wind, or the constant stream of energetic charged particles flowing from the Sun, interacts with the coma of a comet. The combination of the solar wind flowing toward the comet at high speed (close to a million miles per hour) and the neutral gas flowing away from the nucleus creates different interaction regions. These different regions have unique plasma density, composition and energy as well as magnetic and electric field properties.
The interaction of the solar wind with a comet’s coma can only be studied in situ, meaning that a spacecraft must visit the comet and take measurements of magnetic and electric fields and of particle properties at the spacecraft location. Prior to Rosetta, all of the spacecraft that studied this interaction flew through the coma one time at high speeds. Rosetta, however, has remained close to Churyumov-Gerasimenko (67P/CG) for more than a year, providing the observations of how boundaries close to the comet form and evolve with time.
Rosetta has five instruments that work together to observe this interaction. The Ion Electron Spectrometer (IES) measures the ion and electron flux as a function of energy. Protons in the solar wind appear in the IES spectra at around 1000 electron volts (eV) and were observed from the time that Rosetta arrived at 67P/CG in August 2014 until May 2015, when this signal disappeared because the amount of water outgassed from the nucleus increased as 67P/CG got closer to the Sun. What is even more interesting is that at the time when the solar wind disappeared the energy of the cometary ions showed signs of Rosetta moving between two different plasma interaction regions as the spacecraft moved closer to and away from the comet. The region farther from the nucleus had ions of varying energies with velocities greater than 5 km/s, while the region closest to the nucleus was much quieter. The ions in this region had such low energies that they were only observed by IES thanks to a negative electric charge on the spacecraft drawing them into the instrument.
The most exciting observation took place in September and October of 2015 during an excursion out to 1500 km from the nucleus. When Rosetta moved beyond 700 km from the nucleus it exited the inner region, where it had remained for several weeks, out to the outer region and remained in this outer region for several days until the spacecraft came back within 700 km of the nucleus again. This excursion showed that in addition to the change in ion velocity, the electron density and the magnitude of the magnetic field were different in the two regions. As the comet moved away from the Sun, and the water outgassing rate decreased, the boundary disappeared and the solar wind reappeared in January 2016.