Time response of O+ to a weak transverse ion heating event in the polar ionosphere

Abstract

Time response of O+ ions to transverse ion heating in the polar ionosphere is studied by means of a particle code. Here we deal with relatively low levels of heating over a few minutes yielding superthermal ions energized up to a few eV. Under the influence of the upward mirror and downward gravitational forces, most of the heated ions are trapped between the topside ionosphere and some high altitude depending on their perpendicular energies. A few ions with sufficiently large perpendicular energy escape the gravitational pull, and the flux of such ions increases with the heating level. The trapped ions bounce back and forth. During the first upward transit of the heated ions, the transient outflux from the topside ionosphere is as large as 107 ions Cm−2S−1 and it subsequently decays to quite small values in a few tens of minutes. A large transient outflux (∼ 3 × 108 ions Cm−2S−1) consisting of unheated ions develops even below the heating region, lasting over just a few minutes. As the ions having relatively large energies continue their upward journey, and the less energetic ones begin to fall down, the O+ drift velocity ranges from a relatively large negative value (downward) just above the dense O+ plasma in the topside ionosphere to a large positive value at high altitudes. Substantial flux of downward moving ions is also seen. During subsequent upward motions, the bouncing ions form a well‐defined, quite sharp expansion front. The bounce motion generates a succession of plateau formation over a period of several hours in response to a brief heating period of just a few minutes. This leads to nearly periodic oscillations in local densities, with the oscillation period increasing with altitude. In a time of about one day, the oscillations in the density cease due to the spatial dispersion of the heated ions having differing initial energies. Eventually the O+ density profile becomes nearly stable but remains extended with significantly enhanced O+ density at high altitudes. A consequence of trapping of the ions by the downward gravitational force is that the kinetic energy of the ions decreases with the increasing altitude; the coldest ions appear near the top of apex of the trapped ion trajectory, while the warmest ions appear near the bottom of the flux tube. Such cold ions are likely to contribute to the thermal O+ population observed at altitudes of several thousand kilometers.