The creation of ionized tail rays in comets

Abstract Energetic electrons at a bow shock surface profusely ionize easily ionizable cometary molecules. They then accelerate them along interplanetary magnetic field lines to the highly anisotropic velocity distributions the cometary ions are observed to have. The density of ionizable molecules (CO, N 2 in particular) is critical. The electrons escape along the field lines unless enough heavy ions are produced to electrostatically trap the electrons and enable them to complete their ionization up to 300 ions cm −3 . The ambipolar diffusion of the electrons along the field lines accelerates the ions along the field lines up to energies of hundreds of electron volts. The energy of the ions perpendicular to the field lines remains only a few eV, at most, as is evident from the observed extreme narrowness of the rays.