Thermospheric Control of the Auroral Source of O+Ions for the Magnetosphere

Linear theory, model ion-density profiles and MSIS neutral thermospheric predictions are used to investigate the stability of the auroral, topside ionosphere to oxygen cyclotron waves: variations of the critical height, above which the plasma is unstable, with field-aligned current, thermal ion density and exospheric temperature are considered. In addition, probabilities are assessed that interactions with neutral atomic gases prevent O+ ions from escaping into the magnetosphere after they have been transversely accelerated by these waves. The two studies are combined to give a rough estimate of the total O+ escape flux as a function of the field-aligned current density for an assumed rise in the perpendicular ion temperature. Charge exchange with neutral oxygen, not hydrogen, is shown to be the principle limitation to the escape of O+ ions, which occurs when the waves are driven unstable down to low altitudes. It is found that the largest observed field-aligned current densities can heat a maximum of about 5×1014 O+ ions m−2 to a threshold above which they are subsequently able to escape into the magnetosphere in the following 500s. Averaged over this period, this would constitute a flux of 1012 m−2 s−1 and in steady-state the peak outflow would then be limited to about 1013 m−2 s−1 by frictional drag on thermal O+ at lower altitudes. Maximum escape is at low plasma density unless the O+ scale height is very large. The outflow decreases with decreasing field-aligned current density and, to a lesser extent, with increasing exospheric temperature. Upward flowing ion events are evaluated as a source of O+ ions for the magnetosphere and as an explanation of the observed solar cycle variation of ring current O+ abundance.

Ionospheric origin of magnetospheric O+ ions

Flows of thermal atomic oxygen (O+) ions are deduced from topside ionospheric plasma density profiles. The mean flux within most of the polar cap is of the order of 10^12 m^{−2} s^{−1}, a figure which is consistent with both theoretical and experimental estimates of the light ion polar wind at greater altitudes. Larger flows (up to 6 × 10^12 m^{−2} s^{−1}) are observed near the poleward edge of the night-side statistical auroral oval, a feature not reproduced in the light ion flux. The implication is one of a low altitude acceleration mechanism, acting upon the O+ ions at these latitudes and at heights above that at which the fluxes are observed. Such a process would enable ions to escape from the ionosphere because they do not exchange charge with neutral hydrogen. The observations are in general agreement with energetic O+ ions as previously observed in various parts of the magnetosphere.

Ions in the atmosphere

In Earth’s atmosphere, an ion is a cluster of molecules carrying an overall charge, known as a molecular cluster ion. Such cluster ions, with dimensions of approximately one nanometre, have usually been referred to as small ions, and their motion in air constitutes a small electric current. Large ions (or Langevin ions), by comparison, are physically larger (tens to hundreds of nm) and consequently electrically less mobile. Usage of the term “ion” to represent these molecular clusters originates from the early history of atmospheric electricity, which spans the discovery of the electron and the elucidation of the structure of matter. The distinction between large and small ions originates from distinguishing ions that could be accelerated by atmospheric electric fields (and therefore directly contribute to the conductivity of air), and those (the large ions) which were insufficiently electrically mobile to contribute to electrical conduction in air.