Charge exchange in cometary coma: Discovery of H(−) ions in the solar wind close to comet 67P/Churyumov‐Gerasimenko

As Rosetta was orbiting comet 67P/Churyumov‐Gerasimenko, the Ion and Electron Sensor detected negative particles with angular distributions like those of the concurrently measured solar wind protons but with fluxes of only about 10% of the proton fluxes and energies of about 90% of the proton energies. Using well‐known cross sections and energy‐loss data, it is determined that the fluxes and energies of the negative particles are consistent with the production of H(−) ions in the solar wind by double charge exchange with molecules in the coma.