Articles | Volume 33, issue 10
Regular paper
30 Oct 2015
Regular paper |  | 30 Oct 2015

Self-consistent electrostatic simulations of reforming double layers in the downward current region of the aurora

H. Gunell, L. Andersson, J. De Keyser, and I. Mann

Abstract. The plasma on a magnetic field line in the downward current region of the aurora is simulated using a Vlasov model. It is found that an electric field parallel to the magnetic fields is supported by a double layer moving toward higher altitude. The double layer accelerates electrons upward, and these electrons give rise to plasma waves and electron phase-space holes through beam–plasma interaction. The double layer is disrupted when reaching altitudes of 1–2 Earth radii where the Langmuir condition no longer can be satisfied due to the diminishing density of electrons coming up from the ionosphere. During the disruption the potential drop is in part carried by the electron holes. The disruption creates favourable conditions for double layer formation near the ionosphere and double layers form anew in that region. The process repeats itself with a period of approximately 1 min. This period is determined by how far the double layer can reach before being disrupted: a higher disruption altitude corresponds to a longer repetition period. The disruption altitude is, in turn, found to increase with ionospheric density and to decrease with total voltage. The current displays oscillations around a mean value. The period of the oscillations is the same as the recurrence period of the double layer formations. The oscillation amplitude increases with increasing voltage, whereas the mean value of the current is independent of voltage in the 100 to 800 V range covered by our simulations. Instead, the mean value of the current is determined by the electron density at the ionospheric boundary.

Short summary
In a simulation study of the downward current region of the aurora, i.e. where electrons are accelerated upward, double layers are seen to form at low altitude and move upward until they are disrupted at altitudes of ten thousand kilometres or thereabouts. When one double layer is disrupted a new one forms below, and the process repeats itself. The repeated demise and reformation allows ions to flow upward without passing through the double layers that otherwise would have kept them down.