The Role of Molecular Ions in the Overall Ionic Composition of Polar Wind Outflow
Date:
The talk is selected to present in the virtual Cold Plasma Workshop, hosted by Gian Luca Delzanno (Los Alamos National Lab) and Joseph E. Borovsky (Space Science Institute).
Abstract: Albeit limited, several observations show the importance of ionospheric N+ and molecular ions, including NO+, N2+ and O2+, in the high-altitude ionosphere and magnetosphere, opening up the question how these cold ion outflows acquire sufficient energy to transport upward through the polar wind. The transport of H+ in the polar wind has been well explained by the “classical polar wind theory”; however, this theory fails to account for additional acceleration mechanisms required to explain the escape of heavy ions. Moreover, to allow molecular ions to escape from the ionosphere, they are required to obtain the sufficient energy in a short time scale, in order to overcome the gravity and the dissociative recombination lifetime. Therefore, tracking the outflow of all relevant ions could provide new insights into the mechanisms responsible for accelerating the ionospheric heavy ions from eV to keV energies, and their abundances in the polar wind could reveal the link between the lower thermosphere and the ionosphere.
Developed from the Polar Wind Outflow Model, the Seven Ion Polar Wind Outflow Model (7iPWOM) solves the gyrotropic transport equations for all the relevant ion species (e-, H+, He+, N+, O+, N2+, NO+ and O2+) along open magnetic field lines. The 7iPWOM includes numerical schemes that account for the suprathermal electron production and ion-electron-neutral chemistry and collisions, and therefore has the capability to assess the role of all heavy ions in the supersonic ionospheric outflow. In addition, for altitudes above the collision-dominated region where the hydrodynamic solution becomes increasingly inadequate, the 7iPWOM applies a kinetic particle-in-cell (PIC) approach that enables the inclusion of wave-particle interactions (WPI) and Coulomb collisions.
Numerical simulations using the 7iPWOM suggest that N+ and the molecular ions play important roles in the polar wind solution during the quiet time in the solar maximum, both from the hydrodynamic or kinetic solutions. In the low altitude polar ionosphere, N+ is the second most abundant ion species in the polar wind, while still exceeds the abundances of He+, even at higher altitudes. The presence of N+ and molecular ions in the polar wind alters the ion-electron-neutral chemistry, leading to a redistribution of ion composition into different species. Moreover, the molecular ions outflow is considered to be associated with the energization of wave-particle interactions, which is also quantified here in an idealized simulation case. This implies that molecular ions can introduce additional plasma instability, and possibly affect the large-scale transport properties. Therefore, the inclusion of N+ and molecular ions leads to an overall improvement of polar wind solution as compared to observations, suggesting that the study of these outflowing “hidden heavy ion” species is needed in order to appropriately interpret observations, and truly understand atmospheric escape.
Recommended Citation: M-Y. Lin, R. Ilie, A. Glocer, The Role of Molecular Ions in the Overall Ionic Composition of Polar Wind Outflow, Virtual Cold Plasma Workshop 2020