The time evolution of the ring current population during the recovery phase of a typical moderate magnetic storm is studied, using a newly developed kinetic model for H+, He+ and O+ ions which includes nonequatorially mirroring particles. The bounce-averaged distribution function is defined for variables that are accessible to direct measurement, and some useful formulas for calculating the total energy and number density of the ring current are derived. The bounce-averaged kinetic equation is solved, including losses due to charge exchange with neutral hydrogen and Coulomb collisions with thermal plasma along ion drift paths. Time-dependent magnetospheric electric fields and anisotropic initial pitch angle distributions are considered. The generation of ion precipitating fluxes is addressed, a process that is still not completely understood. It is shown that both the decrease of the distribution function due to charge exchange losses and the buildup of a low-energy population caused by Coulomb collisions proceed faster for particles with smaller pitch angles. The maximum of the equatorial precipitating fluxes occurs on the nightside during the early recovery phase and is found to be of the order of 104–105 cm−2sr−1s−1keV−1. The mechanisms considered in this paper indicate that magnetospheric convection plays the predominant role in causing ion precipitation; Coulomb scattering contributes significantly to the low-energy ion precipitation inside the plasmasphere.

Collisional losses of ring current ions

The role of ion drift in the formation of ionisation troughs in the mid- and high-latitude ionosphere—a review

Since observed velocity distributions of particles in the magnetosphere generally have a suprathermal tail instead of an exponential one, we propose to recalculate the density and temperature distributions in a nonrotating ion exosphere with a Lorentzian velocity distribution function (VDF) instead of a Maxwellian. The number density, the flux of particles, parallel and perpendicular pressures, and energy flux of the different classes of particles in the exosphere have been determined for any value of the index κ characterizing the Lorentzian VDF. The barometric density and temperature distributions for a Maxwellian VDF and for a Lorentzian VDF are compared. It is shown that for particles in an attractive potential, the barometric density decreases more slowly with altitude for the Lorentzian VDF. Furthermore, the temperature increases with altitude in this case, while for a Maxwellian VDF, it is independent of altitude. It is suggested that positive gradients of the ion and electron temperatures observed between the topside ionosphere and the outer plasmasphere can be explained by this effect, that is, a non-Maxwellian VDF with an enhanced suprathermal tail.

Lorentzian ion exosphere model

The fluxes and energy spectra of picked-up planetary O+ ions incident on the dayside atmospheres of Venus and Mars are calculated using the neutral exosphere models of Nagy and Cravens (1988) and the Spreiter and Stahara (1980) gasdynamic model of the magnetosheath electric and magnetic field. Cold (∼10 eV) O+ ions are launched from hemispherical grids of starting points covering the daysides of the planets and their trajectories are followed until they either impact the dayside “obstacle” or cross the terminator plane. The impacting, or precipitating, ion fluxes are weighted according to the altitude of the hemispherical starting point grid in a manner consistent with the exosphere density models and the local photoion production rate. Maps of precipitating ion number flux and energy flux show the asymmetrical distribution of dayside energy deposition expected from this source which is unique to the weakly magnetized planets. Although the associated heating of the atmosphere and ionosphere is found to be negligible compared to that from the usual sources, backscattered or sputtered neutral oxygen atoms are produced at energies exceeding that needed for escape from the gravitational fields of both planets. These neutral “winds,” driven by pickup ion precipitation, represent a possibly significant loss of atmospheric constituents over the age of the solar system.

Dayside pickup oxygen ion precipitation at Venus and Mars: Spatial distributions, energy deposition and consequences

The decay of the major ion species which constitute the ring current is studied by solving the time evolution of their distribution functions during the recovery phase of a moderate geomagnetic storm. In this work, only equatorially mirroring particles are considered. Particles are assumed to move subject to E x B and gradient drifts. They also experience loses along their drift paths. Two loss mechanisms are considered: charge exchange with neutral hydrogen atoms and Coulomb collisions with thermal plasma in the plasmasphere. Thermal plasma densities are calculated with a plasmaspheric model employing a time-dependent convection electric field model. The drift-loss model successfully reproduces a number of important and observable features in the distribution function. Charge exchange is found to be the major loss mechanism for the ring current ions; however the important effects of Coulomb collisions on both the ring current and thermal populations are also presented. The model predicts the formation of a low-energy (less than 500 eV) ion population as a result of energy degradation caused by Coulomb collision of the ring current ions with the plasmaspheric electrons; this population may be one source of the low-energy ions observed during active and quiet periods in the inner magnetosphere. The energy transferred to plasmaspheric electrons through Coulomb collisions with ring current ions is believed to be the energy source for the electron temperature enhancement and the associated 6300 A (stable auroral red (SAR) arc) emission in the subauroral region. The calculated energy deposition rate is sufficient to produce a subauroral electron temperature enhancement and SAR arc emissions that are consistent with observations of these quantities during moderate magnetic activity levels.

Decay of equatorial ring current ions and associated aeronomical consequences

Observations of thermal and energetic ion populations by the Dynamics Explorer 1 satellite in the magnetospheric energy source region as well as nearly simultaneous Langmuir probe measurements of enhanced electron temperatures by Dynamics Explorer 2 within the stable auroral red (SAR) arc at F region heights are used to examine the role of heavy ions in the formation of SAR arcs. It is found that sufficient energy is transferred to the electron gas at high altitudes via Coulomb collisions between the observed ring current ions and thermal electrons to support the enhanced F region electron temperatures measured on these field lines. The latitudinal variation in the electron heating rates calculated using observed ion populations is consistent with the observed variation in electron temperature across the SAR arc. In all cases, ring current O(+) is the major source of energy for the SAR arcs.

E. G. Shelley

Papers

The role of ring current O+ in the formation of stable auroral red arcs